pump gas cranking compression?

[blwilliams]

On a 10.8 CR 434 SBC .039 quench cam is 246 @.050 110 LS 110 LS Cam is straight up.
Heads are aluminum 225 & 64 CC
Cranking compression is 195-210
This seems quite high to me for 91 octane pump gas.
I ran the figures through Pat Hales Engine pro program and it says cranking compression will be in that area.
So I guess my question is will 210 cranking compression be pump gas friendly?

[redliner]

On a 10.8 CR 434 SBC .039 quench cam is 246 @.050 110 LS 110 LS Cam is straight up.
Heads are aluminum 225 & 64 CC
Cranking compression is 195-210
This seems quite high to me for 91 octane pump gas.
I ran the figures through Pat Hales Engine pro program and it says cranking compression will be in that area.
So I guess my question is will 210 cranking compression be pump gas friendly?

I remember Big Joe saying try to stay right at 200lbs.

[Newold1]

What is your concern at 210 psi cranking compression with respect to 91 octane fuel? Why do you think its a problem? Have you calculated your dynamic compression and compared that number to your static cranking compression as that is probably more important.

[PackardV8]

It depends. With aluminum heads, blocked heat risers, cold air intake, downshifting as appropriate, good tune, including ignition advance curve, that 210 cranking compression is still in the danger zone. Some will tolerate it, some won't. Pinging isn't just an unpleasant noise. Run it long enough and the piston tops will look like they've been hammered with little ball peins.

[groberts101]

It depends. With aluminum heads, blocked heat risers, cold air intake, downshifting as appropriate, good tune, including ignition advance curve, that 210 cranking compression is still in the danger zone. Some will tolerate it, some won't. Pinging isn't just an unpleasant noise. Run it long enough and the piston tops will look like they've been hammered with little ball peins.

Fully agree. I've run 200+ and up to about 235 cranking on pump gas many times but the gears and vehicle weight must not allow longer rev times or ANY engine lugging whatsoever. You'll probably also be running a compromised ignition tune with 91. Tough to have your cake and eat it too. Or at least have to deal with a smaller piece.

And my guess is if detonation gets out of hand, you'll end up losing or closing up a ringland long before the ballpeen does the trick to force a teardown.

[blwilliams]

What is your concern at 210 psi cranking compression with respect to 91 octane fuel? Why do you think its a problem? Have you calculated your dynamic compression and compared that number to your static cranking compression as that is probably more important.

Newone that is old.
I would think that cranking compression is a pretty good representation of dynamic compression. Just just FYI the dynamic is 9.3.
My concern of the 210 psi is detonation. This is a street car that can and will be lugged as already suggested is not going to be good.
I must admit that a 10.8 static has this much compression at cranking speed.
Just trying to get some real world first hand information before this becomes a ball peen expedition.

Thanks for asking

[MadBill]

Obviously many other factors come into play, e.g. altitude, head materials, spark advance, operating engine and intake temperature, manual vs. auto, driving habits, actual spec of the '91 octane' fuel and whether the pressure was arrived at with a huge cam and 15:1 CR or a short one with 10:1, but a 200 psi target comes up quite often, including by David Vizard.

[David Redszus]

Assume we have built an engine with 211 psi cranking pressure at 80F deg.

That would result in 451F deg at 32 deg at BTC and 687F deg at TDC.

If inlet temp goes to 120F deg, then we have 519F deg @ 32 deg BTC and 772F deg @ TDC.

If inlet temp goes to 140F deg, then we have 553F deg @ 32 deg BTC and 815F deg @ TDC.

If cranking pressure were increased to 225 psi, it would result in an increase of about 9F deg @ 32 deg in all cases.

Clearly, compression temperature is much more critical than is compression pressure.

Now the real question becomes: At what temperature does our fuel auto-ignite?

And when does the unburned end gas reach 1800F deg at which point detonation will occur, regardless of fuel octane?

[blwilliams]

Assume we have built an engine with 211 psi cranking pressure at 80F deg.

That would result in 451F deg at 32 deg at BTC and 687F deg at TDC.

If inlet temp goes to 120F deg, then we have 519F deg @ 32 deg BTC and 772F deg @ TDC.

If inlet temp goes to 140F deg, then we have 553F deg @ 32 deg BTC and 815F deg @ TDC.

If cranking pressure were increased to 225 psi, it would result in an increase of about 9F deg @ 32 deg in all cases.

Clearly, compression temperature is much more critical than is compression pressure.

Now the real question becomes: At what temperature does our fuel auto-ignite?

And when does the unburned end gas reach 1800F deg at which point detonation will occur, regardless of fuel octane?

Oh Mr. Redszus. You make my head hurt trying to think about all of this in a new light, but thank you anyway.
Per my quick google search I see gasoline auto ignites in the 477-566* range.
So if my feeble mind can wrap around your numbers your first example of 451* @ 32*BTDC quickly reaches above the auto ignite figure as it heads to TDC. Presuming the timing is at 32* this engine is right at the edge.
Correct?
Here in sunny AZ as our summer quickly approaches the inlet temps will be near auto ignite standing still.

After studying this some more, it raises another question.
If these compressed cylinder pressures quickly exceed the auto ignition point how is it that race gas with its higher octane prevents auto ignition?
It must be quite late for my feeble mind to wrap my head around this.
Please explain.

Now my head is really hurting.

[GARY C]

I have run about 205 on an iron head 355 sbc with 92 octane, it had 20* initial, 40* part throttle and 36* total timing.

Although some will disagree I have found this calculator and info helpful.
http://cochise.uia.net/pkelley2/DynamicCR.html

[David Redszus]

After studying this some more, it raises another question.
If these compressed cylinder pressures quickly exceed the auto ignition point how is it that race gas with its higher octane prevents auto ignition?

Higher octane does not affect auto-ignition temperatures. Pre-ignition can and will occur regardless of octane if the temperature is sufficiently high.

The evidence of support is reflected by the fact that adding TEL to low octane fuel does raise the octane but does not affect auto-ignition temperature.

Fuel octane (for a given operating condition and chamber design) avoids detonation by allowing more time for the flame front to complete its task and complete (well, about 50%) of the burn. Octane buys you more time; higher temp and pressure gets you less time.

Remember that even after combustion has begun, a secondary ignition point can result due to local auto-ignition. If a substantial volume of unburned mixture reaches a critical temperature the result is detonation instead of conflagration.

[MadBill]

To slightly re-state David's words as I understand them (been wrong before...): once the required conditions for auto ignition/detonation are met in a region of the combustion chamber, a short time period called the ignition delay must pass before the actual event occurs. If the normal flame front arrives before this delay times out, the event is cancelled. Higher octane fuels produce a longer delay. lower RPM increases the available time and thus increases the chances for detonation.

[Orr89rocz]

Solid roller cam or hyd roller? Or flat?

[Orr89rocz]

Pipemax is telling me cranking is much lower than 200. Depending if thats a solid cam with lash or hyd. If hyd its even lower. 158-186 psi depending

Imo theres no reason why that combo shouldnt handle pump gas

[pdq67]

I have run about 205 on an iron head 355 sbc with 92 octane, it had 20* initial, 40* part throttle and 36* total timing.

Although some will disagree I have found this calculator and info helpful.
http://cochise.uia.net/pkelley2/DynamicCR.html

No, Pat's DCR calculator is good!

Just input advertised and not duration at .050" numbers and go.

I do figure that I would consider more like 8.2 to 1 DCR as being optimal, but I am conservative.

Off topic, but rod length plays a small roll here also...

pdq67