hi guys. and gals,
one quick question for ya...
can anyone tell me what the approximate peak psi is in the chamber of a stock sbc 350 just after tdc on the power stroke when all the fuel in the cylinder has ignited and expanded to its max.
thanks for the info all!
A-Saalt

 

depends on the power, what the max cylinder pressure is.

 

Also on rpm, right?

Rich Krause

 

An engine simulation showed about 1100+ psi at peak torque point, and lower on either side. That makes sense to me because that's also peak VE.

As an example, a high end 350 with 200 more hp and 150 more lb-ft showed about 1800 psi in a similar simulation.

I'm not convinced that " just after tdc on the power stroke when all the fuel in the cylinder has ignited and expanded to its max." is a true statement. In some engines at some rpms burning continues most of the way thru the power stroke. In the extreme case of a Top Fuel engine, burning continues well into the exhaust stroke!

My $.02

 

Typically peak cylinder pressure is around 15 ATDC. That's regardless of rpm if your engine is burning the charge efficiently.

Just curious, does your computer program predict peak cylinder pressures, or a peak average over 100 or more cycles? I've found that difference can easily be 200 psi. 1800 psi sounds like an awful lot. The turbo engines I've worked with making about 100 HP/liter are usually in the 1400 psi range peak, with a peak average in the 1200 psi range.

 

Also, it depends on octane of fuel, cylinder volume, compression ratio, chamber shape, etc.

Flame travel and combustion are odd things and there are way, WAY too many factors to be simplified into this type of question.

 

Good question. I don't know, but my guess is that it is the final result for all the inerative calculations it makes for each rpm point. It is the max pressure during combustion in psi absolute. I'm guessing that would be a difficult number to accurately measure in a running engine.

The NA engine quoted made nearly 90 lb-ft/l and 95 hp/l with a fairly high dynamic compression ratio of 9.5:1. Static was 11.7:1. Yes, octane, cyl. and chamber volume, flame travel, etc. is factored into the calculations.

When the actual engine was run, it matched the simulation quite well, and was on the verge of detonation most of the way from about 3000 to the 5000 torque peak, so I'd agree that those peak pressures were very high.

 

Bret To add to this question how would one calculate Dynamic CR?

 

Try this from Jason (Soma07) originally written by Pat Kelley:

http://cochise.uia.net/pkelley2/DynamicCR.html




Jon (OldSStroker and Bret's ol'man)