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# Cylinder pressure, boost or ignition Advance?

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Hello all,

What makes more cylinder pressure, boost or ignition Advanced?

My logic says that if a oem NA engine is seeing about 200psi CP (Cylinder Pressure) if i add 20psi of boost, the CP will be 220psi..?

Hope it makes sense..

Compression is a multiplier not additive with intake pressure. You will have approximately 200×((14.5+20)/14.5) = 476PSI with boost applied.

That is [pressure generated by compression at ambient intake pressure]×[(ambient pressure+boost pressure)/ambient pressure] = generated pressure.

That is all before ignition. Ignition then increases pressure significantly, timing being critical to where and how high it peaks.

Thank you both,

What will make more cylinder pressure?

Just for an example, theoretically Same engine different ignition timing (not knock limited)

500hp@20psi (mbt)

Or

500hp@30psi

Thx

Not enough information to answer that question.

I would guess the 500 hp@30 psi would have greater cylinder pressure, since the timing isn't optimized, and it is probably making increased pressure at the wrong time (so it can't be used to produce additional torque/hp).

Hi david, thanks for the insight.

I think the opposite, because the timing is not at MBT it's not reaching it's pick cylinder pressure..

Now who can rule who is right?

(;

You have to measure it -- that would require in-cylinder pressure measurement (either an instrumented cylinder head, or special spark plugs).

MBT does not mean you necessarily have more cylinder pressure, only the best average pressure over the desired crank angles. A non-optimal combustion could produce more peak pressure, but less at the crank angles of maximum leverage from the connecting rod.

Thx, i know..

I was hoping to get an answer from someone that saw/measured it, to get a broad answer..

I would have guessed cylinder pressure would be very comparable between the two. Same hp=same torque=same pressure acting on the piston? Granted there would be some variance, as the other cylinders would have increased pumping losses, meaning the cylinder firing must have slightly higher cylinder pressure to compensate (because it's making the same hp). Actually now I don't know... the more I think about it, the less I understand

That last sentence Ben..

That's how i felt when i was trying to get my head around it..

But i believe there has to be an answer from someone here.. no?

Ben is absolutely correct - same power in both cases tells us that the torque is also the same. As torque is the function of cylinder pressure forcing on top of the piston the same torque means the same pressure ( providing the same rpm at the same power)...

You should be able the make the same power with denser charge with significantly retarded timing, overall average pressure is higher due to initial charge density but peak less due to later ignition. This is how people fudge a bit past the conventional power limits of engines with known rod or piston limitations, but requires a lot of excess fuel or auxiliary injection to avoid burning valves/seats/turbo.

Uh, peoples - no mention of RPM with the power - it's important...

As for lower boost matching a higher boost number, it's certainly possible to match, even exceed it, as 'boost' is only part of the equation.

Well, i assumed that the question was about the same engines with same power at same rpm. Otherwise that qustion will have no answer at all as it will be the subject to specific engine. Some engines will make more cylinder pressure with more boost and less ignition some will make more power with less boost and MBT ignition timing - it all comes to the engine geometry, compression ratio, head airflow, fuel octane rate and intake air temperature...

Thx,

The question is on the same rpm.. for the discussion it's at 7000rpm..

Is there a definite answer? Or it's on a grey area?

I saw slides answer.. looks good (;

Same power at the same rpm equals same turque thus same cylinder pressure.

Georg1970,

So the conservative approach of "less timing more boost"

Is relatively to knock only?

Regardless of cylinder pressure?

The way the question is asked does not involve any other detailes of how exactly the power was achieved. It is very simple - same power at same rpm - same torque - same pressure.

If the question is what would be more effective in terms of making cylinder pressure that depends. For instance if you are using a small turbo there is no sense to raise boost too much as it will start producing very hot air which in its turn will lower the knock threshold big time - so in this case you would try lower boost level and maximum ignition timing ( how big will depend on fuel octane ratio and intake air temperature).

Thx Georg,

That last input i know (;

And you are right, the question was asked like that for a reason, got my answer.

Thanks!