Compression ratio and Boost

[JTAB]

I have a couple of questions for you guy's. If you have an Apex with stock compression and running at 16 lbs boost and an Apex with a head shim or low comp pistons and running 16 lbs boost. What is the horsepower difference between the two sleds and is the motor with the stock higher compression ratio stressed more than the motor with the lower compression ratio? I have always wondered if boost is just boost no matter what the compression is?
I realize that octane is the biggest concern in higher boost applications.
So is this the main reason for lower comprssion?

Thanks Jason

 

[rotax]

Using a HP calculator from TY...I did this and found the difference is ~5 HP...calculated.

It may effect torque more though.

This is 11:1 to 9:1 (Nytro).


But the attainable BOOSTED power is FAR greater..

And you can run lower quality fuel at higher boost settings.

 

[reed4rmk]

Which sled had the higher HP, the lower compression one, or the higher? I'm getting ready to put a shim and kinda wondered the same thing. Sounds like at 12 lbs boost on either one you wouldn't notice the hp difference. But you would sure notice the difference in how much it cost to fuel it. that leaves more money for

 

[Sapling-Thrasher]

Good luck with 16 pounds of boost with stock compression. What does the calc tell you for octane level at that compression???


You would need to pull a ton of timing on the top or have some serious meth to run stock compression at a moderate altitude with any kind of reliability.

 

[Jetpilot]

What is the stock compression on the nytro?

 

[JTAB]

Good luck with 16 pounds of boost with stock compression. What does the calc tell you for octane level at that compression???


You would need to pull a ton of timing on the top or have some serious meth to run stock compression at a moderate altitude with any kind of reliability.

I run at 9-10,000 ft so I don't worry and I use a Parker Knock Box and have seen no problems. The lowest I run is 6-7500 ft and usaully run straight 110 turbo blue all the time unless I run lower boost.

 

[JTAB]

Using a HP calculator from TY...I did this and found the difference is ~5 HP...calculated.

It may effect torque more though.

This is 11:1 to 9:1 (Nytro).


But the attainable BOOSTED power is FAR greater..

And you can run lower quality fuel at higher boost settings.

What about stress to the engine any difference?

 

[zebiraross]

From what I've seen stock compression vrs head 9:1 compression it takes about 2-3 more psi for the 9:1 to come even with stock compression. At least with my rx1 with 10:1 took 3 more psi to keep up with my two buddies each running stock compression. I've also seen this with a head shim Apex vrs stock compression Apex. So you figure 8 hp per 1 psi so you loose 16-24 hp.

Yes, a higher compression engine is more prone to detonation. A head shim will give you a better cusion of safety from detonation, but if a guy is going to run GOOD GAS and only run 15 psi or so I wouldn't do a head shim. But if your running pump or half and half I would run a head shim. It's not like stock compression is putting more stress on the motor to a point of breaking rods or pistons unless you get detonation and it's game over. I mean if you can make the same horsepower at 15 psi with stock compression as someone who has to run 18 psi to make the same power I really don't see the point.

 

[ttyR2]

Low compression makes for soggy throttle response until the turbo kicks in (i.e. when you're lightly cruising around).

 

[woodies]

What is the stock compression on the nytro?

210 quote right from shop manual give or take a couple

 

[modsledr]

210 quote right from shop manual give or take a couple

210 is the cranking compression pressure, not the compression ratio (which is what I think Jetpilot was looking for).