Low Speed Cylinder Pressure

There were some recent posts I checked but none answered my question which is, is there a number that's considered the upper safe limit when it comes to cylinder pressure for a short block with good forged components? I understand it has alot to do with cam timing but I'm trying to find a cut off point like 300psi, 350psi, etc... Also low speed would be defined as 2000-3000 rpms. Thanks.

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94 Formula M6
Last year 10.98 @ 126
355 stock heads, P600

In the works:
383 worked heads, D1, th400

I don't know about 2000-3000 RPMs becuase there's no way to spin the motor that fast without actually running it, and at that point cylinder peak pressures will be far above 300 PSI, even at light part throttle (high manifold vacuum).

On a cranking compression test (ignition disconnected, throttle blocked wide open, turned over with the starter) I would say that anything over 200 PSI starts to get marginal on pump gas. At 220 you're usually right at the upper limit and when you get over 240 or so, you've almost certainly gone too far. I'm talking about a limit for detonation here using pump gas on a N/A engine, since any engine will die a quick death if it is run in detonation on a regular basis- forged components or not.


[This message has been edited by Damon (edited September 10, 2002).]

<font face="Verdana, Arial" size="2">Originally posted by N20Dave:
There were some recent posts I checked but none answered my question which is, is there a number that's considered the upper safe limit when it comes to cylinder pressure for a short block with good forged components? I understand it has alot to do with cam timing but I'm trying to find a cut off point like 300psi, 350psi, etc... Also low speed would be defined as 2000-3000 rpms. Thanks.

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Dave: I am not sure what you are asking. If you mean the cylinder pressure that the components can tolerate without breaking, that is not rpm dependent. It seems you are thinking about the effect of nitrous, where for a given amount of nitrous injected, the resulting increase in cylinder pressure will be greater the lower the rpm. IOW: for a given power output, the BMEP will be higher the lower the rpm at which that level of hp is achieved.

In terms of pressure tolerance I also think you have to differentiate peak cylinder pressure from average pressure. Hopefully, some of the ME types here will jump in, but I think that a short pressure spike would be better tolerated than an equivalent pressure over a longer time.

In any case, race motors may have a BMEP up to 400psi. I have heard that peak pressures in these motors might be in the range of 2000psi. These would only be achievable with race gas because the detonation limit would be exceeded at far lower BMEP with pump fuel.

BTW, this is the formula for estimating BMEP: 792,000 X BHP/(engine displacement in cubic inches X RPM). For supercharged motor, you need to consider how much hp is used to drive the blower. For my motor with N20 the claculation comes out to ~337psi if you figure the SC takes ~60hp and there is ~10% driveline loss.

In terms of cams, cam timing will obviously effect peak pressures and to a lesser extent BMEP. But I don't see how the cam would greatly influence the ability of the motor to tolerate a given pressure.

Here's hoping an ME or someone else will jump in.

Rich Krause

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'95 Z-28 383: Vortech, nitrous, etc.
AIM: rstevenkrause

Some estimated BMEP's based on 792,000 X BHP/(engine displacement in cubic inches X RPM)

Top fuel = 792K*6,000/(600*10,000) = 792psi but they only run for 5sec at WOT at a time on nitromethane.

Modern F1 = 792K*800/(183*18,000) = 192psi but they run 2h races up to 70% of the time at WOT on 100octane unleaded!

Turbo F1 = 792K*1000/(91*12,000) = 725psi in "qualifying" trim (higher boost) running on exotic gas for a few minutes.

I'm not good at math, so those results may be all screwed up!

Rich Krause

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'95 Z-28 383: Vortech, nitrous, etc.
AIM: rstevenkrause

Rich, you are closing in on what I'm looking for. I have been messing with some engine simulation software that has the capability to model superchargers and nitrous. Some literature I've read indicates that while on nitrous BMEP greater than 300psi should be avoided at rpm's less than 3000 or so. I was wondering if the same holds true for boost. I would agree that cam dynamics plays a big roll in this with overlap, etc...

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94 Formula M6
Last year 10.98 @ 126
355 stock heads, P600

In the works:
383 worked heads, D1, th400