SuzukiSavage.com /cgi-bin/YaBB.pl General Category >> Rubber Side Down! >> Deep-Dish Detonation /cgi-bin/YaBB.pl?num=1777693740 Message started by DragBikeMike on 05/01/26 at 20:48:59

Title: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 20:48:59 In July of 2024, I tore down my engine for a 20,000-mile inspection.  I was testing the modified lube oil system, and it was time for a look inside the engine.  At that time, my odometer was reading just a little over 67,000 miles.  All the components affected by the lube oil system mods looked great.  No problems with gears, bearings, cam, rockers, or valve stems.  The lube oil system mods seemed to be a success.  But the piston was another story.  It had significant detonation damage. I was running a 97mm Wiseco Pop-top piston.  It was set up with extra-high compression.  The cylinder was shortened to achieve a higher compression ratio (10.8:1 vs 9.5:1). To keep my test program going, I restored the engine with a Wiseco 97mm Flat-top piston.  To reduce the compression ratio, I milled out the top of the piston to achieve a deeper pocket.  The deeper pocket increased the volume from 5cc to 10cc (the Deep-Dish Piston mod).  I used a standard-length cylinder and set up the quench at .038”.  Those changes lowered the compression ratio to 10.6:1. I was hoping that the slight reduction in squeeze along with the tight quench would solve the detonation problem. This old post provides details on the failure, my evaluation, and recovery. https://suzukisavage.com/cgi-bin/YaBB.pl?num=1720767228

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 20:50:34 In March of 2026 I hit the 100,000-mile mark.  The engine had been running great.  It was time for another look.  I wanted to put the final seal of approval on the lube oil system mods and check to see how the Deep-Dish Piston was holding up.  It was a repeat performance.  The lube oil system mods were still doing a great job, but the Deep-Dish Piston had detonation damage.   Looks like it’s back to the drawing board on the detonation problem. This recent post provides the gory details on the Deep-Dish detonation damage.  You can pick it up starting on page 12, reply #172. http://suzukisavage.com/cgi-bin/YaBB.pl?num=1695617698

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 20:51:17 It’s time to put this problem to bed.  The 97mm pistons seem to be prone to detonation when you jack up the compression over 10:1. The 94mm pistons don’t seem to mind, but the 97mm pistons aren’t happy with cranking pressure above 195 psi.  I decided to make three changes to my engine; seriously reduce compression, install a cam that closes the intake valves later, and take out two-degrees of ignition advance.  I anticipate a slight increase in cylinder head temperature resulting from the decrease in compression and retarding the ignition timing.  This post will document the changes. Let’s start by figuring out how to reduce compression.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 20:51:59 I didn’t have any more 97mm Pop-Top pistons, but I had two new 97mm Flat-Top pistons.  These new Flat-Tops already have a 9.5cc pocket.  Great, no milling involved.  Unfortunately, to increase the volume Wiseco machined the spherical pocket deeper.  That removes most of the quench surface.  I don’t see that as a big hurdle.  I had detonation problems with both types of pistons, Pop-Top with no effective quench, and Flat-Top with good tight quench.  At this point, I just wanna get the compression ratio down. The new piston (on left) has a narrow flat surface around the circumference, while the Deep-Dish (on right) has a significantly wider flat area.  Not gonna be able to squeeze out much end gas with the new piston.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 20:52:47 OK, so no tight quench.  I can’t use tight quench to mitigate the detonation issue, but I can easily reduce compression.  I used the compression calculator on the Wiseco website to figure out how much negative piston deck I would need to bring the compression ratio (CR) down to 9.8:1.  If the piston is situated .012” below the cylinder deck at TDC, and I install a .062” head gasket, I should have 9.8:1 CR.   A .036” base gasket combined with a stock length cylinder results in -.012” piston deck. BTW, that calculator on the Wiseco website is sweet.  This is the data you will need if you use the tool.  The values will vary depending on your build.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 20:53:34 The 97mm cylinder I had been running was ready for the junk yard.  It was .0035” above finished size and had numerous scratches and gouges from previous gear and bearing failures.  I needed a replacement cylinder. This old 94mm eBay beater looked like the perfect candidate.  Lots of rust on the cast iron sleeve but who cares, it’s getting bored out.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 20:54:22 The cylinder deck needed a kiss to make it flat.  I always find these things with an irregular head gasket surface when I check them on the surface plate.  That small area by the right-rear corner will be ok (I plug the oil passage).   I wanna keep the material removal to a minimum, and you can see the tool bit was just touching, so it can’t be more than a few ten-thousandths low.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 20:55:09 Boring the cylinder was tricky.  I could not seem to get a decent finish, and my tools just didn’t want to cut.  I had poked out two of these jugs with no problems.  Now the tooling seemed to be rubbing more than cutting and I had little control over the depth of cut.  I contacted Fast650 for a tech-assist.  The Fastman suggested I try playing with my tool height.  That seemed like a good idea, but I chickened out.  I was only .011” away from the finished size.  I didn’t want to risk it, so I used the rigid hone to take it home.  That was a job. Next time I bore one of these, I’m gonna triple check my tool height before I start.  Look at that lousy finish.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 20:56:04 The rigid hone saved my bacon.  I ended up with a great surface finish, and the cylinder was round and straight within .0002”.  I was very happy.  The Wiseco Website has some excellent guidance on honing.  I followed that guidance using three grades of stones and it turned out great. The rigid hone has a micrometer feed, two abrassive stones, and two felt wipers.  The wipers help wipe and wash away debri.  The wipers also stabalize the hone.  This Lisle hone is a bargain.  It will do a great job if you are patient and meticulous.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 20:56:47 I like to use lots of honing lubricant.  I fill a large plastic jar with 8-parts kerosene to 1-part oil, then periodically dip the entire hone into the mixture to keep the felt wipers soaked.  No skimping on this, keep those wipers soaked so they can do their job.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 20:57:26 I used 80 grit stones to bring the bore diameter within .003” of finished size.  Then 180 grit stones to bring it within .001” of finished size.  Then 280 grit stones to hit the mark.  The surface finish and cross hatch turned out great.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 20:58:04 I have a cheap dial bore gage, but I don’t like it.  It is difficult to keep centered and I never trust the readings.  I prefer using an inside micrometer in conjunction with an outside micrometer.  The outside micrometer has a vernier, so I can achieve accuracy within .0001”.  It takes a lot of time, but I am only checking one cylinder.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 20:58:42 I always do a second check with feeler gages to verify that I didn’t screw up.  It’s a quick & dirty way to check piston clearance.  The feeler reading agreed with my micrometer readings (.0025” on the money).  I am confident I got the piston clearance right.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 20:59:30 I run a copper head gasket on these big bore cylinders.  The copper gasket is prone to oil leaks.  I got tired of always having oil weeping at the right-rear corner of the head gasket, so I plugged the oil feed passages in the cylinder and head cover and ran an external hose to the head cover.  The feed passage in my cylinder head cover is already plugged but this replacement cylinder needed to be plugged.   To prep the oil passage for the plug, I cut M6 x 1.0 threads.  The passage is just the right size for the tap, so no drilling is required.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 21:00:17 Then I make a threaded plug from an M6 x 1.0 bolt or screw.  Just cut off the head of the fastener and slot the end with a hacksaw.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 21:00:59 Mix up some JB Weld and coat the threads on the plug.  Then screw the plug all the way in and cover up the end with JB Weld.  After a hot bath the cylinder will be ready to install.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 21:01:41 I like to run plenty of end gap on my piston rings.  I used Wiseco’s recommended gap for “Street-Moderate Turbo/Nitrous”.  For a 3.820” bore you end up with .019” for the top ring and .021” for the second ring.  Better safe than sorry, it’s an air-cooled engine.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 21:02:39 I installed a .036” cylinder base gasket. The piston deck was -.012” to -.013”.  With my current 55cc combustion chamber and a .062” head gasket, I will have 9.8:1 compression ratio.  My experience with this engine shows that you can expect lower head temperature with an increase in compression.  So, I expect reducing the compression this much should result in higher head temperature.  We shall see. The cylinder work was finished.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 21:03:30 Installing a cam that closes the intake valves later will help reduce cylinder pressure.  The cam holds the valves open as the piston starts up on the compression stroke.  At lower speeds, this allows some of the pressure to bleed off through the open valves.  As engine speed increases, the inertia of the incoming charge prevents this bleed-off.  Since detonation is more prevalent at lower engine speeds, a cam that allows some bleed-off helps to mitigate detonation. The OEM DR650 cam I was running closes the intake valves at about 44° ABDC.  I have a Web 402 grind that is supposed to close the intake valves at 55° ABDC.  It also has significantly more lift.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 21:04:09 I reviewed old graphs that compared the OEM DR650 cam to the Web 402.  The plot of the intake lobe didn’t agree with the Web specs.  Looks like I had better check this cam with a degree wheel to make sure I get the results I am looking for.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 21:04:54 Web doesn’t specify performance springs for the 402, but with .395” lift it takes the stock valve springs to the point where coils are touching (the early stages of coil bind).  Web installation instructions specify a minimum of .010” clearance between coils with the valve at max lift.  Stock springs aren’t gonna work with the Web 402.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 21:06:15 I have a set of SuperTech performance springs that can handle the lift, but they are also a lot stiffer.  I would prefer springs that apply pressure similar to the stock springs.  That would be easier on the cam and rockers.  To compromise, I can install the SuperTech springs at 1.350” instead of the stock height of 1.300”.  That will reduce the spring pressure about 12 lbs at max lift.  The SuperTech springs will still apply more pressure than stock, and they can easily handle the lift. That sounds like a good plan.  Install the Web 402 to close the intake valves about 13° later, and install the SuperTech springs to accommodate the increased lift. First order of business was to install light checking springs so that I could verify the intake valve closing points.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 21:06:58 Then install the degree wheel and get it adjusted correctly at TDC.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 21:07:58 The valve timing is checked with zero valve lash at .050” of valve lift.  I needed to set up a dial indicator to determine .050” lift.  Not a problem of the intake side, but the exhaust side is a bit harder.  There’s more stuff in the way (like the frame).  Not a problem.  I’m not worried about the exhaust timing.  So, this exercise will be confined to the intake.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 21:09:26 I don’t have factory specifications for the OEM DR cam.  All I have is data from checks I did many years ago.  The DR cam agreed with my previous data.  Intake opened at 10° BTDC and closed at 44° ABDC.  Max lift was .380”. The Web 402 opened the intake valve at 10° BTDC and closed the intake at 50° ABDC.  Max lift was .395”.  The cam was well shy of the advertised 248 duration (240 vs 248).  Oh well, I want that intake valve closing significantly later.  Solution, retard cam timing about 5°.  Armen figured out a long time ago that there are 22 splines in the cam drive sprocket but there are only 19 teeth for the chain.  So, if you move the sprocket one-spline clockwise and the chain one tooth counterclockwise you should retard cam timing by 2.6°.  If you move the sprocket two-splines clockwise and the chain 2 teeth counterclockwise you retard the cam timing by 5.2°.   It gets a little more complicated if you want any more than a 5° change.  This simple table might prove helpful to some of you.  I’m sure there are more possible combinations, I’m just too lazy to figure them all out.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 21:10:23 I repositioned my cam drive sprocket 2-splines clockwise, and moved the chain 2-teeth counterclockwise.  That retarded the cam 5°.  We’re talkin 5° on the crank.  The cam rotates at half crank-speed, so the cam is repositioned 2.5° counterclockwise.  This is what 2.5° retarded looks like on the timing marks.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 21:11:10 Now the intake valve opened at 4° BTDC.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 21:12:09 And the intake closed at 56° ABDC.  So, I should be closing the intake valve about 12° later than I was with the OEM DR cam.  I hope this helps mitigate the detonation problem.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 21:12:50 While the checking springs were still installed, I did clay checks to verify that the valves weren’t getting too close to the piston.  It’s a bigger cam with more lift & overlap, and I have retarded the timing.  Best make sure that the valves have plenty of room.  No problem here, the valves didn’t even touch the clay.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 21:13:37 I removed the checking springs and installed the SuperStreet performance springs.  My Stage III head was already set up for high lift cams.  The guides had been shortened and the Kibblewhite titanium retainers afford additional travel.  There’s no need to worry about a spring retainer crashing into a guide seal.  Then the head went back onto the engine.  Almost done.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 21:14:32 While I was messing around with the cam timing, it became painfully evident that my cam chain tensioner was a problem.  I have been using a system with a floating plunger (no rack & pawl) and a jack bolt that bucks up the whimpy rear chain guide.  I adjust the jack bolt periodically to maintain a maximum of .06” back-travel on the tensioner plunger.  That seemed reasonable to me.  After observing all this intake valve closing action, I can see that the floating plunger still allows the intake to close earlier than intended.  The plunger strokes back as the cam lobe goes past the nose.  The valve springs force the cam to overrun the chain drive. See that polished area on the plunger.  That can only happen if the plunger is stroking in & out.  It’s been letting my intake valve close early.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 21:16:21 So, I’m goin back to the stock cam chain tensioner with the pawl & rack.  That contraption locks the plunger as it advances toward the chain.  From a valve control perspective, you can’t get much better.  Let’s give a shoutout to the Verslagy.  I still have the jack bolt so I can periodically adjust the system to bring the plunger back to this original setting (9.5mm).  What the heck, I inspect this thing almost every service anyway.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 21:17:56 Now it was time to address the ignition timing.  There isn’t a lot you can do with the stock LS ignition system.  The advance curve is fixed and it is controlled by the igniter unit.  There is no feature that automatically retards ignition timing under load.  The curve is fixed at 25°.  Your ignition pickup is set to trigger at 5° BTDC.  Up to 2000 rpm the system fires the spark plug at 5° BTDC.  As the speed increases, the igniter unit advances the timing gradually until the engine speed reaches 4000 rpm.  At that point, the timing will be fully advanced at 30° BTDC. It’s easy to adjust the pickup about 2° to 3°.  Any more than that will require a bit more innovation.  I know FinnHammer found some sort of fancy aftermarket unit that is manufactured in Czechoslovakia (I think).  That’s way past my pay grade.  I just moved the pickup.  Every .05” of movement changes the timing 1°. You start by elongating the mounting holes in the pickup.  I wrap the stator in a plastic bag to keep the metal filings from getting into the coils.  I also apply a layer of blue tape to all the surfaces on the pickup.  The pickup has a strong magnet inside, so metal filings have a tendency to accumulate on the pickup.  The tape makes it easier to clean up.  Then I mount one ear of the pickup on scrap wood and elongate the opposite ear with a small round file.  Reverse the mount ear and do the other side.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 21:18:43 The pickup has an airgap between the magnetic button on the pickup and the raised metal bar on the alternator rotor.  That airgap should be maintained, but it’s difficult to measure.  I made a special tool to allow me to maintain the factory air gap.  The tool fits into the bore of the stator core.  When I install the pickup, I make sure that the tip of the tool just touches the magnetic button when the pickup has been set in the location desired.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 21:19:23 I relocated the pickup .10” in the direction of rotation.  That will retard my ignition timing 2° across the board.  Hopefully that will be enough.  You can see from the photo that there’s not much leeway in this system.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 21:20:13 After reassembling the engine it was time to see what I had for cranking pressure.  I was shooting for 185-195 psi.  Lucky me, 190 psi.  Let’s hope that does the trick.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 21:26:59 The finished setup was as follows: 97mm Wiseco FT piston with CR set to 9.8:1 (.036” base gskt, .062” head gskt) Stage III cylinder head, 55cc combustion chamber, 34mm int vlvs, SuperStreet springs Web 402 cam retarded 5° Ignition timing retarded 2° Mac 1.79” header LCGP modified muffler with 1.87” baffle PWK 40 knockoff carb, 150MJ, .075” MAB, 35PJ, #8 slide, EGK needle 2nd groove K&N 3”x5” cylindrical air filter with modified airbox KrankVent crankcase ventilation Modified lube oil system with high-speed pump drive and gear spray circuit 4-Speed transmission with late model primary drive 13-Plate clutch She ain’t pretty but this ain’t a beauty pageant.  It’s clean and put together right.  It’s solid.

Title: Re: Deep-Dish Detonation Post by DragBikeMike on 05/01/26 at 21:34:35 It fired right up.  Start the clock with zero miles on the odometer. The audible noise has been reduced dramatically.  Amazing what correct running clearance will do for the old rattle trap.  It runs well.  I’m still breaking it in, so I don’t have data on performance. I can’t discern any notable decrease in torque or acceleration.  The vibration is low, and the carburetion seems to be spot on.  I didn’t touch the carb, no adjustment whatsoever.  Zero oil leaks, weeps, or seeps. It has a little over 400 miles on it.  I will wait until I hit the 1K mark before I start beating it up. I can tell you that my cylinder head temp has increased about 10° to 15°.  I figured that would happen since the retarded ignition timing increases EGT and my temperature sensor is situated just below the exhaust port.  I also know the lower compression ratio reduces thermal efficiency, so I’m using less heat from the combustion process to do work, and more heat must be rejected by the cooling system. My fuel mileage has also decreased a fair amount.  Again, I expected that the reduced compression and retarded ignition timing would rob a few mpg.  I don’t think the cam plays much of a role in the fuel economy; it’s a tame cam.  I was averaging about 58 to 60 mpg before the changes, now I’m averaging about 52 to 54 mpg.  A small price to pay for reliability. I’m not planning on tearing this thing apart unless something goes wrong.  I’m getting too old for these projects.  At this point, I just wanna ride it.  So, going forward, I plan to monitor the results by doing endoscope inspections of the piston, keeping track of compression, and monitoring oil accumulation in the catch can.  If the top of the piston doesn’t start to accumulate some carbon on the intake side, I will be concerned. I think the carbon accumulation indicates detonation-free operation.  If it detonates a little, but the compression remains steady and I don’t see a lot of oil accumulation in the catch can, then I figure it’s ok.  I would prefer to see all three conditions; carbon accumulation, steady compression, minimal blowby. I will update this report after the first service.

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