How much hp does a head shim rob from the engine? 5% 10% 20% 30%???? I've read numerous posts of guys saying no way on the shim. Is it really that noticeable? I have one and honestly I can't really tell the difference. So really, what is the drawback? I guess if I was racing and wanted an advantage against the guy next to me but for trail riding with my buddy?
mbarryracing
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Basically any naturally aspirated engine with a lower compression ratio will make less Torque than one with more CR at the same ignition timing, with all things being equal (except CR)... how much depends on the engine and state of tune so I could only guess here.
Consider it's the same when your boosted engine is NOT on the boost (aka: on-off-on throttle response on trail, or off idle throttle stab, etc). Torque is what provides throttle response and low end snap...
The difference is that a lower CR allows you to run higher relative boost levels which in turn will produce higher HP under boost. Then again, you have to run higher boost to gain same or more HP... that carries it's own drawbacks.
Where things get really get scientific is the difference in area between the piston dome and the outer edges of the combustion chamber at TDC, called the squish band. 2 stroke guys know the greater the clearance, the more prone to detonation because it holds more residual hot gas from the previous exhaust cycle on the next intake / combustion cycle. With a head shim or thicker head gaskets, you are literally increasing this area and allowing more room for that residual hot exhaust gases to linger and flirt with detonation... correctly designed lower CR pistons are dished in the middle but maintain a desired squish band. The reason I say correctly designed pistons, because some brands were known to just have the wrist pin bore moved up to lower the whole piston crown at TDC.
I'd think the higher risk of detonation is more the drawback, then. Especially if you have a setup that doesn't have a very efficient intercooler so the intake temps are naturally going to be higher, and add in the risk of suspect fuel quality out on the trail.
In reality, the difference in HP might be so minimal you might not even really know the difference, so for the ease of installing a shim that is why it's been the preferred method by most.
Consider it's the same when your boosted engine is NOT on the boost (aka: on-off-on throttle response on trail, or off idle throttle stab, etc). Torque is what provides throttle response and low end snap...
The difference is that a lower CR allows you to run higher relative boost levels which in turn will produce higher HP under boost. Then again, you have to run higher boost to gain same or more HP... that carries it's own drawbacks.
Where things get really get scientific is the difference in area between the piston dome and the outer edges of the combustion chamber at TDC, called the squish band. 2 stroke guys know the greater the clearance, the more prone to detonation because it holds more residual hot gas from the previous exhaust cycle on the next intake / combustion cycle. With a head shim or thicker head gaskets, you are literally increasing this area and allowing more room for that residual hot exhaust gases to linger and flirt with detonation... correctly designed lower CR pistons are dished in the middle but maintain a desired squish band. The reason I say correctly designed pistons, because some brands were known to just have the wrist pin bore moved up to lower the whole piston crown at TDC.
I'd think the higher risk of detonation is more the drawback, then. Especially if you have a setup that doesn't have a very efficient intercooler so the intake temps are naturally going to be higher, and add in the risk of suspect fuel quality out on the trail.
In reality, the difference in HP might be so minimal you might not even really know the difference, so for the ease of installing a shim that is why it's been the preferred method by most.
Last edited:
kinger
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Maybe 5-8 hp.
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Here are the dyno sheets for my 14 Viper XTX with low compression pistons at 9#'s and 12#'s of boost and my 15 viper XTX with stock motor and no head shim at 7#'s and 10.5#'s
The stock motor made 129.02 hp at 7#'s to 113.03 at 9 #'s on the low compression motor That is track hp
The stock motor made 129.02 hp at 7#'s to 113.03 at 9 #'s on the low compression motor That is track hp
mbarryracing
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Yamadog,
Whoa, a picture is worth 1000 words... so the stock CR motor made more HP at less boost.
The only question I have is what boost level / HP level is each capable of on pump gas, what it the deto threshold.
Can the low CR motor make more HP on pump gas running higher boost?
Whoa, a picture is worth 1000 words... so the stock CR motor made more HP at less boost.
The only question I have is what boost level / HP level is each capable of on pump gas, what it the deto threshold.
Can the low CR motor make more HP on pump gas running higher boost?
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No.
Well the 15 does have an EVO flash to retard the timing several degrees at WOT but the turbo, fuel pump, 4th injector and intercooler is the one out of the 14 in it. we did upgrade to a billit turbo wheel but everything else is the same. I have been hitting 8#'s of boost on 91 octane with maybe a bit of 110 mixed in for safety but did the 30 mile pump gas shoot out ride and drag races at St Germain 1-1/2 weeks ago with no problems.
Allen set it up to run 7#'s on pump gas and 10.5 on race gas the head studs are the limit on the high boost. my 14 at 13#'s was wicked but this thing at 7-8 on the trail is much snappier at low end and instant on when coming through a corner. With the XTX even with the Hygear linkage still carries the skis when ever on the gas so you have to dump the throttle to get it to bite but even with 144 studs and the 1.6" track I can break the track loose and sweep the rear through a corner like the old 121" sleds.
Whatever it is it sucks! At 6.5lbs it takes all its got to stay even with a stock Apex!
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It's probably only making 20 more HP then a stocker. But let me guess, by the time it comes alive the stocker has pulled away from you and then you are always playing catch-up?
That's why there has always been a perceived "turbo lag" reputation.
That's why there has always been a perceived "turbo lag" reputation.
If you are having trouble with an apex at 6.5#'s you need to have someone set it up that knows what they are doing!!! Last Friday night I was running against Bikeman 1100 Turbos and only losing by 3 sled lengths in 600 ft at 7#'s. I have had over 5 RX-1's even a turbo and 4 Apex and 2 attacks and none of them could see my tail light! Your description says MPI sc I did not even know they were making a sc for a viper what model Viper are you running?
At 6.5 he should be making at least 50 more HP (dyno proven) and there is absolutely no turbo lag!!!!!!!!!!!!
This is my 15 at 7#'s before the motor was broke in. My buddy is riding my 13 Pro R 800
Sorry forgot this was the turbo section not the Viper section you are on a Nytro?? I have ridden a few S/C Nytros and there is a definite difference on what you can get those to do compared to the viper but I would think at 6.5 #'s you should be able to take on a stock Apex.
mbarryracing
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Ah good observation.
Although a SC robs even more HP being it's crank driven, I'd think it should respond like SC's are noted for, regardless if lowered CR?
Although a SC robs even more HP being it's crank driven, I'd think it should respond like SC's are noted for, regardless if lowered CR?
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A .066" head shim costs about 5-6 hp compared to stock compression, same way with your retarded timing (so about 10-12 hp total over a stock compression, stocking timing motor). You'd need almost 2 lbs more boost on the supercharger to make the same horsepower as previously. Now the nice thing is, even with the compression lowered, timing reduced, etc you could damn near run 87 octane with 6.5 lbs of boost. 8.5 lbs of boost is still safe on 91 octane fuel with that setup, I've had plenty of guys run 10 lbs of boost, maybe a little on the edge, you have to decide where you're happy at.
