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Message started by DragBikeMike on 09/12/21 at 17:02:50

Title: Evolution of a HotRod - Part 8 - High Compression
Post by DragBikeMike on 09/12/21 at 17:02:50

This is the eighth in a series of reports intended to document the results of progressive modifications to the LS650 engine.

Part 1 outlined the project, set the rules, and established the baseline performance values for a box-stock LS650. You can find Part 1 here.

http://suzukisavage.com/cgi-bin/YaBB.pl?num=1620523526

Part 2 examined the airbox. We improved performance dramatically with a few simple and inexpensive modifications. It was a lot of bang for the bucks. You can find Part 2 here.

http://suzukisavage.com/cgi-bin/YaBB.pl?num=1621150483

Part 3 took a hard look at the exhaust system. We tried a Dyna muffler and a hybrid muffler (home-built). We also tried the larger Mac header pipe. The exhaust modifications provided significant improvements in acceleration and fuel economy. You can find Part 3 here.

http://suzukisavage.com/cgi-bin/YaBB.pl?num=1623048749

Part 4 installed a performance carburetor and revisited the air filter. The larger carb and filter extended the upper rpm limit. Overall performance was improved. You can find Part 4 here.

http://suzukisavage.com/cgi-bin/YaBB.pl?num=1625732492

Part 5 installed a new camshaft with higher lift and duration. The results were gratifying and informative. Big power gains were evident. Now, with some valve overlap, the exhaust header took on a new role. You can find Part 5 here.

http://suzukisavage.com/cgi-bin/YaBB.pl?num=1626391255

Part 6 installed a lighter flywheel. The results were surprising. Acceleration in the lower gears was improved, while top speed suffered a bit. You can find Part 6 here.

http://suzukisavage.com/cgi-bin/YaBB.pl?num=1626921647

Part 7 installed a ported cylinder head. The Stage II head flowed a lot better than the stock head. More airflow equals more power. It was an eye opener. You can find Part 7 here.

http://suzukisavage.com/cgi-bin/YaBB.pl?num=1627891507

Part 8 will bump up compression. The stock compression ratio (CR) is only 8.2:1. That’s great if you want to run low-octane fuel, but if you want more power, crisper acceleration, better fuel economy, and a tire twistin cruiser, you’ll be needin more squeeze. Getting that CR up around 10:1 or better makes a huge difference in performance and efficiency.

What I am about to describe worked good for me. If you decide to try this stuff on your own, you assume responsibility for the outcome. If you don’t have the skills, don’t do it. If you don’t understand something, STOP and get help. Get a manual. Read up. Comply with ALL the safety requirements outlined in the manual. Make sure you know what you are doing before attempting any of these modifications.

Let’s get started.

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by DragBikeMike on 09/12/21 at 17:05:08

Up to this point, we have increased power and economy by improving volumetric efficiency. The exception would be the lighter flywheel (Part 6) which reduced rotating mass. That was a mixed bag. Now we are going to explore the benefits of improving thermal efficiency.

Bumping up the CR improves thermal efficiency by increasing the initial pressure and temperature of the charge and raising the expansion ratio. It gets more work out of a given amount of fuel. Hotrodder’s have used this trick since the advent of the internal combustion engine.

There are several ways to increase the compression ratio. The standard methods are milling the head, shortening the cylinder, increasing displacement by overboring and/or stroking, or a high compression piston. Some of those options are difficult to accomplish on the Savage. Milling the head, shortening the cylinder, and increasing the stroke are off the table for the average Savage wrench turner. But a high compression piston is well within the capabilities of a decent shade tree mechanic.

Lucky for us, Lancer has two different Wiseco pistons. If you want a modest boost in performance and economy, the 9.2:1 pop-top variety fits the bill. Its super-easy to install and provides a gratifying boost in acceleration and fuel economy. If you are serious about getting from point A to point B in a hurry, and want to do that in a miserly way, the 10.4:1 flat-top is the piston for you.

Both pistons are available in a standard 94mm size, so if your cylinder is not chewed up or worn out, a simple deglaze should be all that’s required. These pistons position the rings higher in the bore, so any sort of ridge is unacceptable. I have four cylinders in varying degrees of age and abuse, and none have any discernable ridge. But to be safe, it’s advisable to use a rigid hone and torque plates to deglaze the jug. I suspect many folks will simply hit the hole with a spring hone and have good results.

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by DragBikeMike on 09/12/21 at 17:06:53

The pop-top has a compression height that is only .025” taller than the stock piston. The boost in compression ratio comes predominantly from the raised pop-top that sticks up about .060” above the basic top of the piston.

Here you see the stock and pop-top pistons sharing a wrist pin. They have almost identical compression heights, but the pop top has an area that sticks up about .06”.

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by DragBikeMike on 09/12/21 at 17:07:52

With the pop-top, you end up with a deck height that is well shy of the cylinder head (around .143”). There is little risk of valve-to-piston interference problems. Even the most aggressive cam available (Web 340b) doesn’t present an assembly problem. That doesn’t mean you don’t have to check clearances; it simply means there is almost no risk of having to enlarge the valve reliefs in the piston. The pop-top is pretty much a bolt-in slug.

You can see there is lots of room between the piston and the cylinder head at TDC.

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by DragBikeMike on 09/12/21 at 17:09:13

The flat-top is a different story. Here’s the flat-top sharing the same wrist pin with the stock piston.

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by DragBikeMike on 09/12/21 at 17:10:40

The flat-top’s compression height is .150” taller than the stock piston. You MUST check clearances on the flat-top piston. That sucker runs right at the top of the cylinder. It is NOT a bolt-in part. It requires a standard performance setup. You must play with the cylinder base gasket and head gasket thicknesses to achieve .035” to .045” quench clearance, and you must do clay checks to make sure that all four valves have sufficient vertical and radial clearance with the piston. Get careless with these checks and you might regret it.

The valves get very close to the piston at TDC, and the top of the piston gets very close to the cylinder head. It’s easy to achieve a condition where an immovable object meets an irresistible force. The quench clearance must be setup, the cam must be perfectly timed, and the valve-to-piston clearances must be checked. No shortcuts.

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by DragBikeMike on 09/12/21 at 17:14:11

Rather than give you a blow-by-blow on piston installation, I am going to provide links to old posts. I just didn’t have it in me to tear it down and do all the associated work and testing for the two high compression pistons. I had already done it once for each piston and the posts were decent, so I figured they would be good enough.

This post provides a good summary of the pop-top installation and testing. The test regimen is not the same as the regimen I have been using on this project, but it will give you a good idea of what’s involved and what to expect. You will notice that when I did the post, I was worried about cranking pressure. At the time, I felt that the pop-top was better suited for street applications. It kept cranking pressure below 200 psi. You can disregard those comments. I have since learned through personal experience that the Savage will tolerate cranking pressures well over 200 psi. You can expect about 155 psi from the stocker, 195 psi from the pop-top, and about 235 psi from the flat-top.

http://suzukisavage.com/cgi-bin/YaBB.pl?num=1585106199

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by DragBikeMike on 09/12/21 at 17:17:14

This post provides a thorough summary of the flat-top installation and testing. It’s detailed and it used the same testing regimen that I currently use on this project (Evolution of a HotRod). In addition, I still have the videos of the test runs so I was able to review and compare them. Later on, I found that it ran better with the Keihin PWK40 (actually 38mm), so the test results I list in this current post (Part 8) are using the PWK40.

http://suzukisavage.com/cgi-bin/YaBB.pl?num=1613022271/0

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by DragBikeMike on 09/12/21 at 17:18:00

If you read both of those old posts, you will see that the flat-top piston performs a lot better than the pop-top. It vibrates less, runs cooler, gets better fuel economy, and is significantly faster. The boost in compression combined with the tight quench work together to improve combustion and thermal efficiency. The flat-top is way better. The only advantage the pop-top has is ease of installation.

Let’s look at the acceleration test results.

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by DragBikeMike on 09/12/21 at 17:21:01

This was the setup for the 94mm pop-top engine. Stage II ported head, Web 340b cam, 3” flywheel, stock bore & stroke, increased compression ratio (9.2:1 for pop-top), modified airbox with K&N RD-0710 cylindrical filter element, VM38 carb, Mac 1.79” exhaust header with LCGP high flow muffler. Except for the high compression piston, Web cam, and VM38 carb, everything is the same as it was for the Part 7 ported head test.

Here are the results for the Wiseco pop-top. As I mentioned previously, I don’t have a full set of test data because I didn’t run this setup through the same regimen. The data shows the pop-top is faster than the stock piston, but I don’t think it’s a fair comparison without the 2nd & 5th gear results, and of course the cam is different. The pop-top runs a lot better than these results suggest.

Second Gear 4K to 7K: N/A Did not record

Third Gear 4K to 6.5K: 4.65 seconds 0.17 seconds faster than stock piston

Third Gear 4K to 7K: 6.45 seconds 0.15 seconds faster than stock piston

Fifth Gear 3.5K to 5K: N/A Did not record

Fifth Gear 3.5K to 5.5K: N/A Did not record

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by DragBikeMike on 09/12/21 at 17:22:44

This was the setup for the 94mm flat-top engine. Stage II ported head, Web 340b cam, 3” flywheel, stock bore & stroke, increased compression ratio (10.4:1 for flat-top), modified airbox with K&N RD-0710 cylindrical filter element, PWK40 carb (actually 38mm), Mac 1.79” exhaust header with LCGP high flow muffler. Except for the high compression piston and Web cam, everything is the same as it was for the Part 7 ported head test.

Here are the results for the Wiseco flat-top. It’s killer fast.

Second Gear 4K to 7K: 2.41 seconds 0.27 seconds faster than stock piston

Third Gear 4K to 6.5K: 4.19 seconds 0.63 seconds faster than stock piston

Third Gear 4K to 7K: 5.61 seconds 0.99 seconds faster than stock piston

Fifth Gear 3.5K to 5K: 4.10 seconds 0.66 seconds faster than stock piston

Fifth Gear 3.5K to 5.5K: 6.45 seconds 0.74 seconds faster than stock piston

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by DragBikeMike on 09/12/21 at 17:24:09

Fuel Economy

Fuel economy for the stock piston averaged about 55 mpg.

Fuel economy for the pop-top averaged about 63 mpg.

Fuel economy for the flat-top averaged about 71 mpg. It was astonishing, and it wasn’t a fluke. I logged a little over 2200 miles on the flat-top and the fuel economy never ceased to amaze me. It is a super-efficent engine. With either carb (VM38 or PWK40), it averaged above 70 mpg.

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by DragBikeMike on 09/12/21 at 17:24:55

Operating Temperatures

Cylinder head temperature (CHT) was always higher on the engines with the stock piston or the Wiseco pop-top. With both pistons, CHT ran around 310°and would routinely approach 330°-340° CHT. The flat-top piston always ran around 280°-300° CHT. We’re talkin cruise, not stuck in traffic.

Oil temperature followed the trend. The stock piston and pop-top would motor along with oil temp around 200°-220°. The flat-top runs around 180°-200°oil temp.

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by DragBikeMike on 09/12/21 at 17:26:13

Noise & Vibration

Audible noise was highest on the flat-top piston. It had this rattle at light load. Nothing alarming. Just a faint rattle. The pop-top had a hint of the same rattle. The forged pistons need more clearance and it’s not uncommon to get a bit of slap. The fins on the air-cooled engine amplify the noise. Inspections on teardown showed that both pistons were in perfect condition. The noise is not a problem, it comes with the territory.
The stock piston makes the least audible noise.

The vibration levels on the stock piston and flat-top piston are very close. That makes sense since they are almost the same weight. The Wiseco pop-top is significantly lighter than the stock piston. As a result, the pop-top shakes a bit. It’s not obnoxious, but after an hour or so on the freeway you are gonna know you were riding a thumper. Your fingers go numb.

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by DragBikeMike on 09/12/21 at 17:28:26

Durability

All three pistons held up well. There was no evidence of wear or distress. Oil consumption throughout all the testing with all the pistons was zero. I never had to add oil on any of the projects. My breather catch can never collected more than a drop or two of oil (it was usually bone dry).

The stock piston was a used eBay special. I have no idea how many miles were on it when I installed it. For this project, I logged 2917 miles and at least 180 WOT test pulls. Visually it looked exactly the same as when I installed it.

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by DragBikeMike on 09/12/21 at 17:29:23

The Wiseco pop-top had the most mileage on it. I logged 5023 miles on the pop-top and it had been used for the carburetor shootout, the muffler shootout, and all sorts of other projects. It had been subjected to countless WOT pulls and all sorts of grossly rich and lean fuel mixtures. The skirt still looked new. No significant wear on this thing. Not even a scratch.

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by DragBikeMike on 09/12/21 at 17:30:08

The Wiseco flat-top was the baby of the crowd. Just 2249 miles on it’s odometer. As expected, it looked great.

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by DragBikeMike on 09/12/21 at 17:32:04

Combustion

Each piston exhibited clues that might be useful in evaluating how well the combustion process was working. The three pistons had unique deposits and burn patterns. These deposits and patterns might aid in evaluating efficiency. Since the flat-top performed best, maybe the location of the deposits and burn pattern might be something to work toward. I recall reading something about this but don’t remember what book. If my memory serves me correctly, that expert was of the opinion that if there were areas with no deposits then the burn wasn’t complete. That would seem to be contrary to my own experience since the flat-top had the best performance and efficiency but the least amount of deposits. The area around the intake valves had almost no carbon deposits.

This shows the three piston tops, stock on left, flat-top in center, pop-top on right.

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by DragBikeMike on 09/12/21 at 17:33:01

The pop-top was downright nasty looking. The deposits were thick and looked to me as if they would eventually become a problem. All these pistons were setup using the same techniques to establish proper air/fuel ratio. A wide band air/fuel ratio meter was used to achieve optimum air/fuel ratio at cruise and WOT, and timed performance runs were used to verify that the best power mixture was achieved at a ratio of about 12:1. Oil consumption was zero for all three. Where’s all this carbon coming from? Maybe all the carburetor setups.

The build-up on the pop-top was crazy thick. I got no answer for this one.

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by DragBikeMike on 09/12/21 at 17:33:49

In contrast, the flat-top was clean. I prefer this. Note the bright silver area along the back side of the intake valve relief. I find that interesting.

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by DragBikeMike on 09/12/21 at 17:37:21

Driveability

This is the area where the high compression pistons really shine. Unfortunately, it is also difficult to measure. It’s subjective rather than objective, seat-o-da-pants stuff.

All three setups have excellent manners. They all start right up, idle smooth, have good throttle response, and broad useable power bands. But the high compression setups are crisper. The higher the CR the better it gets. The high compression changes the character of the engine from that of a workhorse to a thoroughbred. It changes from an engine that’s always willing and able to one that is chomping at the bit. Sure, the WOT tests show conclusively that the high compression setups are faster, but the real benefit is at part throttle operation. This is especially evident on the flat-top with its associated tight quench. Part throttle response is instant and crisp.

Where the part-throttle to WOT transition is good with the stock piston, it is much better with the pop-top, and excellent with the flat-top. The transition from part throttle to WOT is smooth and predictable on all three pistons, but the flat-top is off the hook. It will kill in the twisties, probably allowing a good rider to run one or possibly two gears higher through turns. It’s makin good power way down low.

At part throttle, the venturi vacuum is highest. That results in much better atomization and homogenization of the fuel droplets and air. Throw that brew into a combustion chamber that is more turbulent due to the higher compression and the burn is more uniform and complete. Add the flat-top’s tight quench and things really get stirred up. You get more power from less fuel. More heat is used to do work, less heat is lost to the cooling system, CHT and oil temps go down.

I can’t say enough about the part-throttle performance. Yes, both these high compression pistons are faster at WOT, but it’s the part-throttle performance that makes them worth the entry fee.

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by DragBikeMike on 09/12/21 at 17:38:49

This completes the compression portion of the project. The Wiseco pop-top adds a nice boost to overall performance and the Wiseco flat-top really kicks it up. What’s not to like about big improvements in power and economy.

Installing a high compression piston is relatively easy and inexpensive. The increased compression will provide a measurable improvement in performance and fuel economy. If you go one step further and tighten up the quench (Wiseco flat-top), you will bring your operating temperature down significantly, get even better fuel economy, and run with the bigboys. The flat-top is a little harder to set up but it’s well worth the effort.

Next stop on the Power Train? Displacement. There’s no replacement for displacement. In the next installment (Part 9), we’re gonna bump up the cubic inches with the Wiseco 97mm pop-top and the Wiseco 97mm flat-top.

I hope some of you find this project informative and can use the data I collect to help make decisions on your own project. If you have suggestions or comments on my test methods, post a reply so we can discuss. As mentioned earlier, if you have a particular component or modification that you would like to see included, let me know and we can collaborate.

Best regards, Mike

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by Armen on 09/12/21 at 20:20:06

Awesome as always!
Thanks!

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by verslagen1 on 09/12/21 at 21:05:46

On 1 of my stockers, it was burning a lot of oil and the carbon deposits were very thick all over the piston. But not bubbly like your pop top.

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by sparktfxr on 09/25/21 at 16:42:45

Hi Mike, would it be a ridiculous idea to press fit some type of metal slug into the centre of the gudgeon pin when using these lighter pistons to bring the reciprocating weight up to that of the original to help smooth out the vibes.
Thanks for all your great research & write ups

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by DragBikeMike on 09/25/21 at 19:21:21

Sparktfxr, I assume you are interested in the Wiseco 94mm pop-top piston since it is the only assembly that is significantly lighter than the stock piston.

I have thought about installing a plug and decided it wouldn't be wise. You would have to set it up with a lot of interference to have confidence that the plug would stay put. The wrist pin has very little clearance with the rod and piston. A plug with an interference fit would most likely increase the wrist pin diameter and cause clearance problems. I think it would be better to just procure a heavy-wall wrist pin, like the pin that comes with the flat-top Wiseco.

The vibration on the 94mm pop-top isn't that bad, but you will notice it. Long freeway rides at a constant speed take a toll. The 94mm Wiseco flat-top is only a few grams heavier than the stock piston so vibration levels are about the same as stock.

I'm currently testing the 97mm Wiseco flat-top. It's about 35 grams heavier than the stock piston. It increases vibration at a lower rpm. IMO, that's easier to live with because the vibration is in a range where you don't typically ride at a steady speed. Get it up around 4K and it smooths out nicely. So when I'm on the freeway running steady state, the vibes are pretty low. The 97mm flat-top is killer.

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by sparktfxr on 09/26/21 at 16:01:54

Hi Mike, I perhaps wrongly thought the wiseco's were generally lighter, I am currently running a 96mm pop top but after reading your article re the HC flat tops last week have been in touch with Lancer to try to buy one of the 97mm. Being 35gms heavier if there was enough meat in the crown could the underside of the crown be milled or drilled to bring the weight down. Just a thought!

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by DragBikeMike on 09/26/21 at 17:10:29

It might be an option. I didn't take any measurements to see how thick the crown is. It would certainly be easier to lighten the piston assembly than to add weight.

That said, I don't think it's worth the effort. As I mentioned, the 97mm flat-top vibrates more at lower rpm. Get it on the freeway and it smooths out. I went for a long ride this morning and paid particular attention to the vibration levels at 4000 rpm. The mirrors are clear and the hand grips seem to be buzzing about the same as the stock engine. These are subjective tests but I don't have any sort of vibration analyzer to collect hard data.

The vibes at lower rpm are higher but you typically drive through the range, so you aren't subjected to the constant steady-state vibration. It's a non-problem.

I discuss the vibration levels in my posts because I think its important. To some folks, it might be a big deal, or it might cause concern over reliability. None of the Wiseco pistons I have tested exhibit vibration levels that are unacceptable to me. The improvement in performance is well worth the small increase in vibration. I think you will be very pleased with that 97mm flat-top.

One thing I failed to mention in this post on the 94mm Wisecos. The stock clutch can handle the 94mm pop-top, but the 94mm flat-top might overwhelm the stock clutch. It will depend on what else has been done to the engine. Here is a post that provides some options for the clutch.

http://suzukisavage.com/cgi-bin/YaBB.pl?num=1615547049

Title: Re: Evolution of a HotRod - Part 8 - High Compress
Post by Bear151556 on 09/27/21 at 09:19:13

Carbon deposits are the result of incomplete combustion. So the more efficient engine will build up less deposits. And your results really show that, thanks!