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Message started by DragBikeMike on 07/11/24 at 23:53:47

Title: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/11/24 at 23:53:47

I recently tore down my engine for a 20,000 mile inspection. I had done extensive modifications to the lube oil system, and I wanted to do an inspection to see how things were holding up. The engine had a 97mm pop-top with the compression ratio bumped up a little (9.5:1 to 10.8:1). It was a test bed for my oil system mods. It made excellent power and lots of torque; the perfect motor for testing the drive train.

Once the cylinder was removed, I noticed that my second piston ring (the oil scraper ring) was a little tight in the groove. Not stuck, just snug. I attributed that to carbon accumulation. The rings looked fine. I continued with the tear down. With the cylinder out of the way, I had good visual access to the input gears.

All the stuff related to the lube oil system modifications looked great. Gear teeth were in good condition, oil spray tube was intact, no evidence of main bearing failure, cam and rockers looked fine (same small pits I have been monitoring for several years), high-speed pump drive looked fine.

The oil system mods seemed to have solved my bearing and gear problems, so I was ready to add a bit more steam. I got busy with other projects and put the Savage on the back burner for a while. I was happy with the inspection, and was looking forward to building a new motor with a 97mm flat-top piston.

Several weeks later, I was reviewing photographs of the teardown and inspection. I noticed an unusual dull grey condition on the rear of the piston. It was on the circumference at the very top, just above the top piston ring. It appeared as if the surface had been sand blasted with a course abrasive media. The grooves around the circumference were badly eroded. Those grooves are referred to as “Anti-Detonation Grooves” or “Detonation Suppression Bands”. They are supposed to protect the ring lands from damage resulting from extreme pressure (i.e. detonation).

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/11/24 at 23:54:45

I figured that tight second ring might not be carbon accumulation. A close look at the piston was in order. Sure enough, measurements of the groove heights showed that the top ring groove was .002” taller across the intake side, and the second ring groove was .002” shorter across the intake side. The ring land was obviously deformed. It had been forced down onto the 2nd ring. That’s why the 2nd ring was tight across the intake side.

I also noted that the top of the piston had no carbon buildup on the intake side. It wasn’t pitted or sand blasted; it looked good. I have always assumed this was a good thing, but now I’m not so sure.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/11/24 at 23:56:05

I inspected the cylinder head and there was a trench blasted into the head. This trench looked like a cluster of tiny pits right in line with the crevice formed by the piston outside diameter and the cylinder bore.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/11/24 at 23:57:30

It didn’t take too long to search the internet and find examples of this type of detonation damage. Apparently, fuel and air accumulate in the crevice formed by the piston O.D., cylinder I.D., and the top ring. As the flame propagates across the combustion chamber the pressure and temperature rise to the point where the trapped fuel and air auto-ignite and explode. Most of the examples I found were confined to the intake side of the piston (just like mine). Most of the web articles attributed the problem to compression ratio. Start getting the squeeze up past 10:1 on an old-school engine (carbureted, no ignition retard, etc.) and pump gas becomes a problem.

I have no experience with the dreaded spark knock. The little I do know has been gleaned from books & magazines, YouTube vids, bull sessions, etc. I’ve always kept my ear to the ground, taking note of any unusual knocking, rattling, pinging, etc. This engine had none of that. At idle, it had the usual clatter, but under load there was no unusual noise. The spark plug always looked good; no metallic balls, no melted cement, no fractured insulator, no burned off ground electrode. It looked normal to me, certainly on the rich side.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/11/24 at 23:59:11

This particular engine was an attempt to see if the 97mm Wiseco pop-top could perform as well as the 97mm Wiseco flat-top. I wanted to see if the pop-top could match the performance and economy of the flat-top. When installed in a standard height cylinder, the pop-top only increases compression to 9.5:1, but the flat-top can take compression all the way up to 11.4:1 (depending on quench clearance). To level the playing field, I installed the pop-top in a shortened cylinder. It was set up at .117” quench clearance which provided 10.8:1 compression ratio.

With quench clearance that large, I was concerned about detonation from the start. My understanding of the detonation problem was that it was associated with large voids far away from the spark plug. Fuel and air hide in these voids, and as pressure and temperature build and migrate toward the void, the hidden charge auto-ignites and explodes. Anything to the inside of the quench zone can aggravate the condition by interfering with flame propagation. For instance, the pop-top gets in the way; it can prevent even propagation of the flame and also trap fuel and air. Given the very large quench clearance, and the pop-top configuration, I assumed it would be prone to detonation. But the absence of any telltale indicators led me to believe that detonation was not occurring on this engine. As I mentioned, there was no unusual noise, knocking, pinging, etc., no visual indications on the spark plug.

I decided to look at all my old photos and parts to see if I had missed anything on my other engine projects. I had built six different engines with Wiseco pistons. Did any of those engines exhibit similar sandblasting on the circumference of the piston?

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/11/24 at 23:59:50

The 94mm pop-top looks great. Absolutely no evidence of sandblasting. The detonation suppression bands are in good shape. This piston was set up with a 9.2:1 compression ratio. It was run with my Stage 2 head, a DR650 cam, a Web 340b cam, several different carburetors & airboxes, and several different exhaust systems. The piston has 5023 miles on it.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/12/24 at 00:00:20

I guess you could say that there wasn’t any sort of clean area on the top of the 94mm PT.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/12/24 at 00:01:08

The 94mm flat-top looks great. Absolutely no evidence of sandblasting. The detonation suppression bands are in good shape. This piston was set up with a 10.4:1 compression ratio. It was run with my Stage 2 head, a DR650 cam, a K&N RD-0710 air filter, and several different carburetors and exhaust systems. The piston has 16,680 miles on it.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/12/24 at 00:01:42

The top of the 94mm flat-top piston was clean as a whistle on the intake side. Is that possibly a precursor to detonation problems?

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/12/24 at 00:02:52

The first 97mm pop-top is a mystery. I’m sorry, I don’t have any photos that show the area in question on this particular piston. I don’t recall anything unusual about that engine other than chronic oil leaks. It was a strong engine. This piston was set up with a 9.5:1 compression ratio. It was run with my Stage 3 head, a Web 402 cam, and several different carburetors and exhaust systems. The piston had 1583 miles on it.

I don’t think it had any sort of detonation issue. Unfortunately, I wasn’t aware of the sandblasting problem when I tore it down. I have no idea where this piston ended up. I’ve got so much of this stuff layin around I can’t keep track of it all. I suspect that if there had been any sand blasting I would have eventually stumbled on it. It would be a valuable nugget of info. The years are takin a toll.

I do have a photo of the piston top. It had the clean area across the intake side. It also seemed to have a lot of carbon accumulation for only 1583 miles.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/12/24 at 00:03:29

The 97mm flat-top has evidence of sandblasting. The detonation suppression bands are pitted across the intake side, but not as severely as my latest 97mm pop-top. This piston was set up with a 11.2:1 compression ratio. It was run with my Stage 3 head, a DR650 cam, a PWK40 carburetor, a 1.79” Mac header with LCGP HiFlow muffler. The piston has 6390 miles on it.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/12/24 at 00:04:03

There’s that clean area across the top on the intake side.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/12/24 at 00:04:44

The first 97mm pop-top with boosted compression suffered a seizure. I attribute the seizure to inadequate running clearance. Wiseco specifies .0019” clearance, but that’s simply not enough for this air-cooled dinosaur, especially when you jack up the compression ratio even more. This piston was set up with a 10.8:1 compression ratio (short cylinder). It was run with my Stage 3 head, a DR650 cam, a PWK40 carburetor, a K&N RD-0710 air filter, and a 1.79” Mac header with LCGP HiFlow muffler. There is no evidence of sand blasting on the detonation suppression bands. The piston has 2532 miles on it.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/12/24 at 00:05:17

The top was very clean across the intake side.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/12/24 at 00:06:06

After the seizure, I restored the engine with another 97mm pop-top and continued testing. It had the same configuration, I just added a bit more running clearance. That build went through a main bearing and it was restored again, but I continued to run the same top end, so ultimately the current piston has about 27,000 miles in total.

The common denominator here seems to be compression ratio. Looks like running the CR past 10.5:1 results in detonation trouble. I’m gonna stick my neck out and say that I think you can get away with a little higher CR on the flat-top, but the safe bet would be to keep it below 10.5.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/12/24 at 00:07:03

My plan to build a new 97mm flat-top motor was in the toilet.

I needed to clean up the cylinder head to eliminate the crevice caused by the detonation. If I didn’t get rid of that crevice, I was just settin myself up for more trouble. Bad enough that there’s already an inherent crevice between the piston and cylinder wall.

I took a .010” cut on the head surface. That didn’t completely eliminate the crevice, but it came close. Unfortunately, it also reduced the combustion chamber volume by 1cc. Now it was 54cc.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/12/24 at 00:07:44

I tried to increase the volume by unshrouding the valves and opening up the chamber where feasible, but I could only squeak out 1cc. So now it was back to 55cc.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/12/24 at 00:08:20

The only other option was to do a proper trench cut on the piston to increase the volume of the dish. I needed to get that dish to about 10cc to achieve a reasonable CR. Lucky I had the old 97mm flat-top to practice on.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/12/24 at 00:08:59

It worked out pretty good. The trench cut turned out well. The burette said it was 10cc on the money. With quench clearance set to .038”, the CR should be right at 10.6:1. As a bonus, the piston was now 15 grams lighter, so the assembly is closer in weight to the stock piston assembly. It should run a bit smoother.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/12/24 at 00:09:36

It pumped 210 psi. That’s more reasonable, but still pretty high for pump gas. Best I can do over here is 92 octane. As you can see, I got a new gage. One of these days I will see if I can salvage the old one. Have to find some stout hose.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/12/24 at 00:10:11

I’ve been breaking it in over the past few days. Just rolled over 500 miles this morning. Taking things slow and easy. It seems to run great; quiet, really crisp, loads of torque, no oil consumption. Time will tell if 10.6:1 is safe with 92 octane fuel. I plan to log another 20K and then take a look. See you when I reach 87,000 on the odo.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/12/24 at 23:42:51

I had mentioned this detonation issue in another post regarding oil system mods. Michael Moore and Zevenenergie had pertinent comments. To keep things a bit more cogent I am pasting those comments here along with my response.

On 7/9/24 Michael Moore Responded:

Mike, I think the erosion on the piston and the slight groove on the head have the same cause since they are both in the same area.

I'd expect detonation to not be localized like that, but instead taking place all around the edge of the chamber. But there might be some very slight differences in the piston/chamber that aren't readily visible that caused the detonation to be limited to those areas. Maybe detonation starts there but is extinguished before it travels to the rest of the bore?

On 7/9/24 Zevenenergie Responded:

I'm no expert, but when droplets of gasoline are deposited on the cylinder wall, they are collected in the space above the upper piston ring.
This can cause corrosion above the piston ring and in the squis area of the head, just like you show.
A squis that is too small increases this effect because the gasoline evaporates more poorly during compression.

It doesn't have much to do with the compression ratio.

This effect can be caused by the mist from the carburetor being incorrect.
You may have an optimal mixture adjustment, but the mist may have too large droplets.

Also the valve seats of the inlet valve can be too fluid in shape, which prevents further atomization. They must always have sharp corners for both a 3 corner and a 5 corner valve job.

The exhaust valve seats can, be rounded.

My response to those comments:

Michael Moore, thanks for your comment. I also was surprised that the damage is confined to the intake side. I assumed that would be the cooler area and less prone to detonation. But I guess detonation must occur where the fuel is, and fuel certainly will be on the intake side. The ignition source will be the rapid rise in pressure, and that pressure will be uniform throughout the combustion chamber. I was able to find some YouTube vids of a Ducati that had almost exactly the same condition as mine. I have no idea if the fellow that made the vid is an authority, but what he said made sense.

Zevengerie, I am confident that this is detonation rather than corrosion. The ring land is deformed across the back side of the piston. You make a good point about fuel droplets and fuel hiding out in the crevice above the top ring. Smokey Yunik recommended reducing that crevice as much as possible without bringing the ring too close to the top of the piston. He felt that the fuel trapped in the crevice could be good for something on the order of 5% more power. Otherwise, it's simply wasted. I may be aggravating the condition with my port configuration. I increased the short side radius to improve flow on the bottom of the port. That in turn results in more charge being directed at the area in question. But I don't want to give up on the improved flow, so I'm gonna rely on the tried and proven recommendation to keep the quench as tight as possible, and reduce CR to a more appropriate level (as close to 10.5 as reasonably achievable).

I don't take any extra effort to blend in my seat angles. I cut them and leave the corners as they are cut. I do use four angles (75, 60, 46, & 30). Thankyou for your comments. They are very helpful and give me more to ponder.

See my most recent post on this detonation issue.

On 7/12/24 Zevenenergie responded:

Tanks Mike, I feel compelled to share a little more.
(Life is all about sharing)
I don't want to sound too decisive, like this is how it should be, but you can also think about this:

There is also some power to be gained if you round off the squis edge, so that the flam front can enter the squis and crevice more easily when the piston descends.

Detonation in the squis cleft is a bit strange. If the squis is not larger than 1mm, the piston and head cool the mixture sufficiently so that it does not self-ignite.
When you increase compression you have to look at the squish space and optimize it to prevent detonation.

You cannot make the gap too small because the connecting rod stretches.
And you also have to pay attention to the speed of the gases that are squished because they can blow the plasma away from the spark. Sometimes a tapered squis slit is therefore better.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/13/24 at 00:17:49

Zevenenergie, interesting that you mention rounding the edge. The flat-top pistons with the radius dish sort of incorporate what you suggest. When Wiseco cut the dish, they extended it well into the quench zone. That results in a tapered squish area, so at the edge you are referring to, the quench clearance is about .021" (0.5mm) greater that the region closest to the cylinder wall. It sort of forms a nozzle. On my first flat-top project, the quench clerance was about .040" at the cylinder wall, but about 061" at the edge you are referring to. I never liked that. I've always felt that you want squish as tight as reasonably achievable, and you want the edge abrupt. The idea is to drive the trapped mixture out, and create as much turbulence as possible. I have recently seen some articles and YouTube vids where the authors suggest a different approach, similar to what you suggest.

What I have now is sort of a mixed bag. I've reduced the CR to something more appropriate for pump gas, tightened the quench to .038", but have the tapered quench clearance I just described. I honestly don't know which approach is better. There seem to be plenty of advocates for each method. I guess we will find out.

I used to run my dragbike with quench so tight that you could actually see where the piston lightly contacted the head surface (just a kiss). It's not a good example for this particular project since it was race only. That engine had a large bore (3.812") and a long stroke (4.250"). The head had a very small combustion chamber (bathtub) and I ran very tight quench (somewhere around .030" as I recall). The shift light was set at 7500 rpm. The piston speed was off the hook, and YES the rod gets longer. I don't recall what the CR was, but it was way past 11:1. It never had a detonation issue, but I was running VP 116 octane race fuel.

Although I love my hotrod, I don't want to live with a motorcycle that needs a special brew for fuel. I need to be able to fill it up at any available station. That means 92 octane. I could probably adapt to octane booster, but I would prefer to be able to run 92.

Let's see how the current combination works out. Thanks for the advice.

Title: Re: 97mm Wiseco Piston Detonation
Post by zevenenergie on 07/13/24 at 12:31:33

I have another question Mike, I see on your pistons those parts that have no carbon residue (on the intake side).

http://https://i.imgur.com/9nIM3eDm.jpg

My question is what does the piston look like on the inside?

Is the piston blacker there as you would expect ?

Because detonation makes the piston hotter and causes burn marks on the inside?

Title: Re: 97mm Wiseco Piston Detonation
Post by verslagen1 on 07/13/24 at 13:21:36

65636C10121511210 wrote:

I would have to assume this was due to localized heating. And hot enough to burn off the carbon deposits as well.

I would also guess that the detonation suppression bands work by reducing the pressure in the way that labyrinth seals work by creating volumes of air and restrictions to flow. So each groove will have a lower pressure than the previous one.

So the problem seems to be localized heating, can the oil flow be improved?
Does the reduced squish area increase the chance of detonation?

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/13/24 at 22:28:51

Zevenenergie: "My question is what does the piston look like on the inside?"

The underside of the piston is totally sweet. Absolutely zero coking. That is one of the parameters I always inspect. I want to know how hot things are getting. I view this sandblasting as a relatively minor failure. It's not catastrophic. I think it indicates that I have been running the thing right on the edge; that fine line between OK and total destruction. I believe the reduction in CR was the right thing to do.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/13/24 at 22:54:29

Versy, I'm not so sure the lack of carbon accumulation is due to heat. There is no indication of elevated temperature on the underside of the piston. Maybe shock. I suspect that the sandblasting is a result of shock as well. Nothing indicates that it was excessively hot. My CHT readings have always been very low as well, but I admit that the instrument is on the exhaust side of the cylinder. All of the Wiseco pistons that I ran above 9.5:1 had clean tops on the intake side, and they all are sweet on the underside (no coking).

Regarding additional oil to cool, don't know what more I can do. Already increased the pressure by a lot, and added a cooler. Maybe sub-cool the oil; don't wanna do that. I don't think there's enough reserve in my current oil system to add an oil spray to the underside of the piston.

I think reducing the CR to a more reasonable level is the most practical approach. It runs really good, but I have to admit my fuel economy seems a bit lower. Have to wait until I'm done with the breakin routine to verify. The CHT seems a bit higher now but so is the ambient. Oil temp might also be a bit higher. Will know more after I log 1K on the odo.

"Does the reduced squish area increase the chance of detonation?"

That seems to be the $64,000 question. All the old-school guidance seems to advocate the tightest quench reasonably achievable to surpress detonation, but a lot of the latest guidance seems to contradict that. In the past, I've always run it as tight as possible. It's the driving concept behind the flat-top piston. I never intended the flat-top to exceed 10.5:1. Wiseco chose to do the radius cut on the top. I asked for 10cc but they provided 5.2cc. That resulted in a bit more squeeze. Seems to work fine on the 94mm, but the 97mm pushes it over the edge.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/13/24 at 23:03:32

Dave, regarding the dual plugs. That setup on the DR is strange; both plugs on the same side of the combustion chamber. That engine only has 9.5:1 CR. I guess it shows the lengths the OEM has to go to in order to deliver a reliable motorcycle. I see from the spec sheet that it had a CDI ignition. That probably means the primary windings on the coil will have significantly lower resistance. The kiss of death for TCI. But I bet I could find a Dual-Fire Dyna coil with compatible primary resistance. I think I could probably pull off the second spark plug in the location shown. The increased compression should in itself require a little less spark lead. I might eventually pull two degrees out.

Thanks for the pics of the DR head. Do you have that in your possession?

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/13/24 at 23:07:08

Ruttly, I agree, no sharp edges anywhere in the combustion chamber. That's why I don't try and run OEM head gaskets in these Big Bore motors. I always break my edges after resurfacing the head. The photo doesn't do the edge justice. However, it probably wouldn't hurt to massage it a bit more. Thanks for the reminder.

Title: Re: 97mm Wiseco Piston Detonation
Post by zevenenergie on 07/14/24 at 01:35:17

61676814161115250 wrote:

"Does the reduced squish area increase the chance of detonation? That seems to be the $64,000 question."

I'm new to 4 tack tunning but with two strokes there is no doubt about it. NO
The piston and the head have a cooling effect on the mixture and this prevents detonation where it first occurs.

The entire combustion chamber form is based on this principle in a two-stroke engine.

This is what comes to mind:

Since your piston shows no heating at all on the inside, I don't think there was any detonation going on.

The light spots are caused by a blast of evaporating fuel. Is my conclusion.
The cylinder head gets the full blast because the blast media comes from the crevis.

A smaller squis gap counteracts this effect because the pressure is higher in a smaller squishy gap.

but also cooler because it is cooled by the piston and cylinder head.
I know counter is intuitive, but
I would increase the compression ratio rather than decrease it in this situation.

Anyway I would opt for as small a squis slit as possible.
But in my opinion the real cause lies with the carburetor.

But the ignition time could also be looked at.
In a two-stroke, an ignition with a variable ignition time has a lot of influence on the power band. It gives more pulling power over the entire speed range instead of just peak power at top.

And optimal ignition timing at all speeds, contributes to optimal combustion and prevents complicated situations associated with the complicated pattern of evaporating fuel in the cylinder, combustion chamber and crevis.

In my opinion, the crevis above the upper piston ring should be kept as small as possible.
This also shows that a bumper due to wear of the cylinder should never be removed.

My knowledge comes from boosting small two-stroke engines. a compression ratio of 1:14.5 with petrol station is possible. (effective compression ratio 1: 8.4)

But at the same time, a 2-stroke has pressure filling due to the resonance in the exhaust. So the pressure in the combustion chamber is higher as the engine enters its powerband.

So that makes the situation about the same as in a 4 stroke.

But because the fress mixture in a four-stroke comes into direct contact with the cylinder wall, the problem of condensation is much greater and this is clearly visible on the piston image on the intake side.

So the size of the droplet in a four-stroke is of decisive importance for the power developed. (lets say more than whit a 2 stroke)
Experimenting with different carburetors and their setup is worth it.

You can transfer the $64.000 to my bank account that I mentioned in the PM. ;)

Title: Re: 97mm Wiseco Piston Detonation
Post by ThumperPaul on 07/14/24 at 05:14:58

Maybe Zevenenergie is on to something here about fuel atomization and timing. Rather than another carb shootout, how about trying fuel injection with an ECU! Just kidding…

Zevenenergie’s mention of 2-strokes does make me think about the PWK carb. Weren’t the PWK carbs initially designed for 2-stroke engines and subsequently adapted and experimented with in 4-stroke engines? Just thinking out loud.

Title: Re: 97mm Wiseco Piston Detonation
Post by zevenenergie on 07/14/24 at 05:57:21

http://https://i.imgur.com/Bqg6VDxm.jpg

This is the biggest difference between a 2-stroke and a 4-stroke carburetor. A 2-stroke has a shield at the gas needle. This is because a 2-stroke has the intake gases flowing back and forth in the carburetor. The shield ensures that the mixture does not become too rich, and the air only gets fuel when it flows towards the engine.

You also need to replace the atomizer.
http://https://i.imgur.com/gHknKLHm.jpg
If you convert your carburetor to 4 stroke.

The change from 2 to 4 stroke in the dirtbike world has of course produced very good 4 stroke carburetors, I would look for a carburetor there.
You will always have to search for the right atomizer,
but that is a relatively simple search for power.

Converting to an injection system is something I have never looked into.
But sensors capture many variables where the carburetor cannot.
Also adjusting with a laptop is of course much easier.

Think about what you can do with your laptop if you have a programmable ignition and an injection system. :)

Title: Re: 97mm Wiseco Piston Detonation
Post by Dave on 07/14/24 at 10:04:47

13151A66646367570 wrote:

Dave, regarding the dual plugs. That setup on the DR is strange; both plugs on the same side of the combustion chamber.
Thanks for the pics of the DR head. Do you have that in your possession?

Back in the day of dual plug heads on the Porsche cars - they only had 2 valve heads so the spark plugs were on each side of the head with the valves in between. On a 4 valve head there is 1 spark plug in the center and 1 spark plug on the side. The side plug is over on the accessible side.........the side opposite the cam chain.

I deleted those photos in this thread as I didn't feel it was adding anything - nobody is likely going to go the dual plug route. Those photos were taken from eBay listings. What I did find interesting is that the hump in the combustion chamber between the Savage valves is no there in the Dr650 head.

Title: Re: 97mm Wiseco Piston Detonation
Post by LANCER on 07/14/24 at 13:58:25

Interesting comment you made Dave about the DR head not having the little bump in the center of the chamber, while the LS does. It’s been a long time since I had a DR head in my hands, but I have no memory of noticing the difference. But, the memory is not what it use to be either.

Title: Re: 97mm Wiseco Piston Detonation
Post by Michael Moore on 07/14/24 at 14:34:28

The Aprilia RSV1000 (Rotax) Vtwin heads have both twin plugs and the chamber filled with extra quench material.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/15/24 at 01:10:15

Oh boy Michael I can dig those heads. Look at the size of those valves. I bet they are at least 38mm. And look at the angle of that intake. That sucker is steep. About as straight a shot as you can get. What is the HP and redline on that badboy.

Dave, I pondered eliminating that bridge between the valves when I was trying to find more combustion chamber volume. I was reluctant. When you start staring at the thing, it just sort of yells at you "Don't do it". Don't know why, can't justify my reluctance, no sound technical basis for the aversion, but it's one of those things that sort of looks like you will regret. It just looks like it needs to be there. It does add additional quench surface.

Zevenenergie, I am confused a bit. Please help me understand what you are trying to share. I want to be sure I understand exactly what you suggest.

First, I believe you are telling me that I should tighten up my quench. Yes, or no?

Second, it sounds to me like you suggest I either leave the CR where it was, or even increase the CR. Yes or no?

Third, I think you are suggesting that my problem lies solely in the carburetion. Yes, or no?

If I understand correctly, you think that the sand blasting has nothing to do with detonation. Yes, or no?

I can see how the sand blasted appearance could be the result of some sort of liquid blasting. All sorts of stuff is going on in that crevice, and my cheap knock-off carb certainly could stand some improvement. It wouldn't surprise me if the fuel droplets are waaaay too big for satisfactory performance in a four-stroke engine. But any sort of blasting will require a motive force, like an explosion of some sort. What is that motive force? What propels the blast media?

How does this trapped fuel deform my ring land? Why is my ring land deformed? Why did the ring land pinch the second ring? Wouldn't that also require tremendous force, like an explosion, like detonation? Are you suggesting hydraulic lock?

I agree that the carburetion is less than ideal, and I agree that the carburetion could be contributing to this problem, but I am reluctant to say that detonation is not in play. All that blasting and deformation requires a motive force. I don't think fuel condensing and flashing off provides enough energy to deform the ring land, and corrosion certainly won't deform the ring land. The deformation of the ring land requires BIG TIME FORCE. Extreme pressure all at once; a hammer. Detonation.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/15/24 at 01:18:47

Fast650 reminded me that my first tight quench engine had a piston failure and that too was predominantly on the intake side of the piston. I had not included that particular failure because it was not a Wiseco piston, but the deeper we get into this problem it seems pertinent.

I had attributed that failure to insufficient ring gap. I arrived at that conclusion because the top of the piston was ripped upward. I didn't think it was a detonation issue because I thought detonation would deform the piston downward, not upward.

This piston was a stock piston used in a cylinder that was shortened 3mm. It was set up with .070" quench clearance. The quench was very loose in an effort to try and keep the CR somewhere close to like 11.5:1. This thing had way too much CR.

Note that there is no sandblasting on the OD of the piston, but there is a lot of missing piston.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/15/24 at 01:20:01

Here is another view so that you can clearly see there is no sandblasting on the OD.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/15/24 at 01:26:40

This is a good view of the lifted piston top. It is evident that the ring pulled the top of the piston upward. I had those gaps waaaay too tight, and I ran the guts out of the thing with the mixture on the lean side. I shoulda known better. But could detonation also have been in play? The CR was way up there, at least 11,5:1. Cranking pressure was about 245 psi. Pump gas was in the tank, and my A/F ratio meter was tellin me no. It was like 13 at WOT. I was on a mission and sustained WOT was in order.

The results.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/15/24 at 01:31:10

I pulled that old piston out of the junk box today. I wanted to see if the ring land was deformed. It is. Just like the Wiseco the ring land is deformed .002" on the intake side. That would require downward force, not upward force. I don't think the tight ring gap lends itself to the deformed ring land, especially not on the intake side where things should be much cooler.

If you look at the skirt you can see it was seizing on the intake side too.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/15/24 at 01:34:27

And the exhaust side was also seizing. Stands to reason, that piston was gettin bigger so both sides were being pinched. If you look at the OD above the top ring there is no sand blasting. There is no sand blasting anywhere on the piston.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/15/24 at 01:39:28

And what about all the missing aluminum? This huge chunk of aluminum was just plain gone. It was flame sprayed all over the place. It was on the exhaust valves, and the intake valves, and the combustion chambers. That sucker had to have been extremely hot. It's a lot of aluminum. You would think that I would have found a chunk or two, but it was all sprayed onto the surrounding parts, sorta like paint. That thing had to have been really hot.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/15/24 at 01:49:50

The underside of the piston? Clean. No coking, discoloration, nothin. So I'm gonna take the position that this mess that I originally thought was solely the result of insufficient ring gap was more a colaboration between tight rings, too much squeeze, inadequate octane, and too little fuel.

The point here is that you can get one of these things so hot that it melts away, but the underside of the piston can remain pristine. I don't think the underside of the piston is a good indicator to use to differentiate between detonation issues and liquid blasting issues. The underside of the piston might be useful to evaluate how things are running during sustained operation, but for catastrophic failures it can only give you a small piece of the picture. There's no doubt that this piston got really hot, but it was a brief excursion and didn't result in coking.

Title: Re: 97mm Wiseco Piston Detonation
Post by DragBikeMike on 07/15/24 at 02:01:57

I appreciate your comments and suggestions regarding the carburetor. I have been wanting to improve on the carburetion for a long time now. I am running a Chinese knock-off PWK. There is no emulsion tube, it's just a needle jet with an air correction port. I have a new PWK knockoff that I have been wanting to try. It's an upgrade. It incorporates a bona fide emulsion tube. That should dramatically improve atomization. I was mainly interested in flattening out the fuel curve, but the improved atomization seems like a bonus.

Why do you feel that 2-stroke engines require the hood over the needle jet to mitigate overly rich mixtures resulting from reversion? Shouldn't the reed valves prevent any reverse flow back through the carburetor? BTW, don't think for a second that 4-strokes don't have the same problem. Most Savage cams close the intake valve just after BDC, so they don't have much intake reversion. But if you run a super-hot cam with lots of overlap and a late closing intake, you can form a little rain cloud of fuel over the mouth of the carburetor. That charge can run back & forth through the carb several times before the intake finally closes. It's especially problematic with drag pipes.

Title: Re: 97mm Wiseco Piston Detonation
Post by zevenenergie on 07/15/24 at 04:11:02

That's a lot of questions Mike I have to think about it especially about that piston damage

About the squid gap
I was talking about as small as possible.
So with a Savage I would do deflection tests, especially if I were to mount a heavier piston. I also wonder what the bearing clearance is.
I would like to come out around 1 mm if possible.
With two strokes you can only handle higher compressions without detonation if the squid gap has approximately that value. 0.5 mm to 0.7 mm is often used in smaller engines.

Two strokes with reevalves still have the hood in between because at higher revs a reed valve starts to behave like a piston-driven intake. There is even a point where the intake only sucks you can leave the reed valve out completely at a certain revs.
It is all much more complicated with a two stroke.

The dynamics of evaporating and condensing mixture in a 4-stroke is complex. In a two-stroke it hardly plays a role.

I don't have enough insight into it to adequately answer your question What is that motive force? What propels the blast media?

I will certainly delve into it because this whole topic has made me think about what I do and don't know.

Title: Re: 97mm Wiseco Piston Detonation
Post by Michael Moore on 07/15/24 at 13:51:29

Don't forget that you may not be able to hear detonation taking place especially with a loud exhaust, and heavy detonation can do a lot of damage in a very short time.

I'd think that char on the underside of the piston is going to be a factor of general chamber temps -- detonation doesn't normally take place in the middle of the chamber, but high temps from CR/lean mixture running will be experienced by the piston there.

https://help.summitracing.com/knowledgebase/article/SR-05007/en-us

mentions: How is it indicated?
Knocking or pinging sound
Drop in Exhaust Gas Temperature (EGT)
Broken piston rings and/or spark plugs
Damage to the piston and/or valves

You've sure got broken piston and rings covered!

There are some photos of detonation damage here:

https://www.lycoming.com/tips/detonation

The Aprilia/Rotax valves are nominal 36/31 with 6mm stems. Power is roughly 125hp @ 9500 RPM.