I need enlightenment!! Could someone please explain to me how a restricted silencer can cause a lean condition with engine detonation. I installed an MBRP trail silencer on my 700 XCsp on my stock pipe. Full throttle engine rpm went from 8000ish to 8300 or so. I have EGT's and I saw higher exhaust temps, assuming the can leaned it out a little. An extra 2 grams on the primary weights took care of the rpm problem. I ran the sled for 300 miles and liked the sound of the silencer, but I decided to put an SLP single pipe on the sled, too. The performance shop I bought the pipe from said I'd be the first guy he knew of to run this combination. I have two SLP flow rite's and a high flow air horn kit on the sled, and I followed SLP's jetting sheet and ran 560 main jets. I took the sled out Sat and it ran good until I nailed it. Within 200 ft. I had cratered the top of the mag side piston. The highest temp on the run was 1125 degrees f, over 100 degrees cooler than what I usually run when jetted dead on. I've had an SLP tech and the performance shop guy tell me that the MCRB silencer was too restrictive, causing the severe detonation. They said they are sure this was the problem. My problem is, I can see how more air / less gas makes a lean condition. How does a restricted exhaust cause this?

If you reduce the diameter of the stinger, in an expansion chamber, like they do on drag pipes, it increases the wave action of the pipe. This makes the pipe work better. I guess you could say its more restictive. It quicky get the pipe up to operating temp, and also increases the heat that is kept in the combustion chamber, which will evetually lead to detonation. This is why only drag race engines can get away with using small pipe outlets. I would think that the MCRB can would be less restrictive. If this thing detonated so bad, it holed a piston, you should have heard it. Detonation is LOUD. Once an engine (cylinder) starts to detonate, the exhaust gas temp actually drops. Does the inside of the can look like it would be more restictive? Got any pics of the pistons? Are you runnig just one probe?

with 560 jets I dont think you have a fuel problem, you may have had a water problem that caused it to run lean but thats a lot of jet ! take a fuel sample out of the carbs and down tubes look at if in a clear glass? If you have water drain the tank 100% fix the top end and start over! if your EGT probe is more then 100mm away from the piston its to far! you also need a probe for each hole ? 100 mm is about 1.25~1.50 from the jug on the y pipe. If you can post a picture of both piston tops this will help us give you GOOD info not just guesses!

So it is mostly a higher temp issue, or does the stronger return pulse do something to lean out the mix too?
The piston just had a crater in it, I stopped before it was holed. It looks like somebody took a sandblaster and ate away at the top of the piston in a teardrop shape with the large part of the teardrop under the sparkplug, and the smaller washed away part toward the exhaust port. I didn't get to run it enough to see the temp drop, it was just always low. I've got a probe in the Y pipe for each cylinder. (EGT's have been on sled all year, working fine, and I had gauges on my old sled, too) I was out on a lake, and I only ran for 15 seconds or so and that was it. I heard some engine rattling when I restarted it, but I just replaced the plug who's electrode was gone and I just limped it home. I thought the MBRP can would be less restrictive too, but with that as the only mod to the sled I saw an increase in both temp and WOT engine rpm. It is physically much smaller, but I didn't look inside it. The shop that sold me the silencer tries to keep track of what mods work and what mods don't, (he does performance mods, too) and he said he has heard from some people that he sold them to that they were a little more restrictive. The SLP tech guy said you can actually gain a couple hp that way. Obviously I should have worked my way up slowly. I used SLP's pipes etc before, and their setups were pretty darn close, so I just took it for granted SLP's jetting/clutching setups for this pipe would work fine with the MBRP silencer. I learned an expensive lesson. (Cylinder had a couple of small cracks on the bridge between the main exhaust port and the little exhaust ports on each side of the main one, so I'm replacing it too)

xc-mark,
fuel was ok, I ran over half of this tank with no problem before having this trouble.
EGT placement is at 2 inches from flange on engine, which is where manufacturer told me to put them. I've been running them all year in this location, (with plug and piston wash readings to verify before I trusted them) and when they read 1225 to 1250, I had the most beautiful shade of light coffee brown on the plugs. I'll take a pic of the piston when I get home, and post it. I only cooked one cylinder, the other side looks great.

The stronger return pulse puts more of the wasted air, and fuel back in the cylinder. Its kind of like supercharging. Detonation blows away the thin boundry layer of air that protects the piston top, causing it to rapidly overheat, and melt. A broken, or melted plug tip is a classic sign of detonation. A very restrictive exaust, or too far advanced timing will cause this. I'm not sure about what size jet you need for your application, but maybe you were way too lean, and thats why the temps never came up. Were both sides reading about the same? Also what octane was the fuel?

I had already run over 1/2 tank of this fuel in much colder outside air temperatures and the sled ran great. When I had the new pipe on, both cylinders were running cool. I think the guy from SLP hit it on the head, I just hadn't heard much about an exhaust restriction causing a problem, but he would know how much more air their pipe will move over my stock one. I never dreamed that ANY aftermarket silencer would be more restrictive than stock. I appreciate the info.

I have egt also its a pyro 2000. I have never had mine even into the 1200's never mind 1250! But that not a big problem! about how long does it take to get the egt's up over 1200? Is there any signs of det in the good hole? any flacking of carbon burnt tip on the plug? is the carbon dry looking on the good or the burnt hole meaning dry not oily?what side pto/mag?

I only have one EGT probe on mine, and its right after the Y pipe. WOT it hits 1250 within about 5 seconds. The probe is a fast response, exposed tip. On a 2 into 1 setup like mine, they say its reading about 50 deg. higher because its being heated by two cylinders.

Sherm,
I'm no expert here, but I don't think the can was your problem in this case. I think you were too lean either in the mid-range or top end and that is what caused your detonation. I have done the same thing on my AC with a SLP single with their can. I was jetted up 4 from stock the last time I ran the SLP set-up and in the mid-range I also melted the electrode of a plug. The SLP tech will blame you for mixing the can and their pipe, but I don't believe that is the problem. I melted a piston with their pipe and my EGT's were under 1100 when this happened. Fix the sled, go back to the stock pipe and ride.

ZR8Rider and the rest of you guys--Thanks for your input. I ran a set of SLP pipes on my old sled, and their setup instructions were good, so I think I'll give the pipe one more shot. I don't think brand competitiveness influenced the info I got from SLP tech. He didn't care that I was running a different companies silencer. He didn't think it was a problem until I told him how the silencer affected the sled with my stock pipe on it. (leaned it out a bit, and raised WOT rpm vs. stock silencer) That was when he said he knew what caused the detonation, and the silencer would be too restrictive to work with their pipe. I'll post back after I try it again.

Just in case any of you guys are still checking on this, I've put 350 miles on the new top end and pipes. It's running strong, there is a noticeable hp increase out of the hole. SLP's jetting chart seems to be pretty close. I need to richen up the low end so that it is approximately as rich as it is on top with the 560 main jets. Then I'll be able to use my Vari-Flow to lean it out to where it needs to be on any given day. I can't find any info on the series 9 needles the Mikuni 40 mm flatslides are using, and I don't like running the e-clip in position 5. I'm still playing around with it, but I'm close. Thanks again for the help.

Are you running a different pipe setup now? What about the silencer? Just curious.

I'm running SLP's single pipe and silencer now. It's not much louder than stock, but I can notice the hp boost out of the hole.