Mo-Par
Well-Known Member
What is the max cylinder pressure (psi) you can run using pump gas (92 octane) without experiancing detonation using alum cyln heads ? 190 psi cylinder pressure is the highest I have run without issue so far.
Max cyln pressure on pump gas ?
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OldmanRick
Well-Known Member
I think your just about there. The timing curve will end up dictating the final result, but any more psi then that, will probably require pulling out enough timing that it would be a wash to increase the comp. any more then where you already are. Jmo, but i tend to be conservative in this regard.
On my Magnum headed LA 360 with iron heads I had pressures that were all in the 190 to 195 range. 16 initial timing and 34 total. It worked real good.
Jack
Jack
fishy68
Tyr Fryr's Inc.
My iron headed 408 has 186-190 cranking compression and it's as quiet as a church mouse on 92. The heads are closed chambers (with a good chamber polish) and I got it set up for .038" quench so it's pretty efficient and only takes 32 degrees total timing for max power so I'm sure that helps. I usually figure 190 psi max for a iron head engine built right (good quench and a fair size cam) and 200 for aluminum heads. If the heads are open chambers and have no quench you better figure on 20 psi less
To give an example of how chamber design and quench affects things my old 360 only had 160-165 psi cranking but it required at least 92 octane with 34 degrees total timing so it didn't spark knock. It had stock open chambered X heads with no quench. It actually responded good to more timing (it liked 36 the best) but I had to mix in some 105 to keep it quiet.
I definetly agree with Rick on being conservative.
To give an example of how chamber design and quench affects things my old 360 only had 160-165 psi cranking but it required at least 92 octane with 34 degrees total timing so it didn't spark knock. It had stock open chambered X heads with no quench. It actually responded good to more timing (it liked 36 the best) but I had to mix in some 105 to keep it quiet.
I definetly agree with Rick on being conservative.
SRT_DSTRHOLC
Well-Known Member
Same here.. Loved my magnum heads because of this. 91 CA gas
Good information offered here so far...but the OP never made any qualification about how much fuel in this equasion.
So as I understand it, the limiting factor on how much compression you can run is how much gas you can flow into the cylinder...and timing (as has been mentioned).
If you can maintain the appropriate air:fuel ratio, you can squeeze in more and more air until you run out of the ability to accommodate it with timing and/or component strength.
So I think that in order to answer your question, we need another limiting factor.
So as I understand it, the limiting factor on how much compression you can run is how much gas you can flow into the cylinder...and timing (as has been mentioned).
If you can maintain the appropriate air:fuel ratio, you can squeeze in more and more air until you run out of the ability to accommodate it with timing and/or component strength.
So I think that in order to answer your question, we need another limiting factor.
moper
Well-Known Member
I don't agree there. At least with gasoline or alcohol as a fuel. Even oxygenated gasoline that's incorrect. You need an oxidizer to make that true - that's what nitromethane is: a fuel and an oxidizer mixed into one compound.
I think what you might be getting at is that the amount of a/f mix isimportant to it. I'd argue that even then it's not the case. It's not how much of the cylinder is filled - it's a combination of five things:
1. How well the contents that enter through the intake cycles are kept mixed (swirl/squish/tumble)
2. The physical volume and shape of the chamber, valves, and piston (surfaces, shapes, and pressure)
3. The thermal properties of the head, valves, and piston
4. The fuel component of that mixture
5. The quality and timing of the ignition spark.
I've run 185psi with open chambered iron and no quench. With quench and aluminum I've run around 200 but I usually plan for no more than 190-195. Like Forrest and his chocolates - with pump gas quality "you never know what you're gonna get".
I think what you might be getting at is that the amount of a/f mix isimportant to it. I'd argue that even then it's not the case. It's not how much of the cylinder is filled - it's a combination of five things:
1. How well the contents that enter through the intake cycles are kept mixed (swirl/squish/tumble)
2. The physical volume and shape of the chamber, valves, and piston (surfaces, shapes, and pressure)
3. The thermal properties of the head, valves, and piston
4. The fuel component of that mixture
5. The quality and timing of the ignition spark.
I've run 185psi with open chambered iron and no quench. With quench and aluminum I've run around 200 but I usually plan for no more than 190-195. Like Forrest and his chocolates - with pump gas quality "you never know what you're gonna get".
I don't think we're in disagreement here... you're just being more precise/specific.
You make a good point about the "swirl"- I was just assuming a well mixed/uniform AFR.
I don't think the volume of the chamber is relevant since we're just talking pressure.
Good point about the thermal properties of the components. A poor design (or one with carbon build up) might develop hot spots more readily and therefore require a richer mixture to stay cool...which would introduce a bottleneck on the fueling/timing end.
Good point about the quality of the ignition system. IIRC, you will need more punch from your spark plug at some point.
Still, it comes down to maintaining Air:Fuel and the strength of the components as the primary limiting factors.
Lonewolf3165
Living the Dream!
So what are we talking about in terms of compression ratio & cam duration etc to achieve 190 psi?
SRT_DSTRHOLC
Well-Known Member
I used magnum heads usually 59-62cc..KB107s 10.7-10.9 compression with .039 head gasket and XE274...Dynamic compression I believe was 8.5-8.7
moper
Well-Known Member
"I don't think the volume of the chamber is relevant since we're just talking pressure."
Yeah - we're heading in the same direction...lol. But- assuming "pressure" is result of the total volume of the engine's cylinder compressed, that total volume includes the gasket and chamber. Agreed?
Lone - It's different in every engine. Intake valve closing event, static commpression ratio, chamber and gasket volume, and rod to stroke ratio all play a role on the final pressure. I use the dynamic compression ratio as my guide rather than actual cylinder psi. It works for me...lol
Yeah - we're heading in the same direction...lol. But- assuming "pressure" is result of the total volume of the engine's cylinder compressed, that total volume includes the gasket and chamber. Agreed?
Lone - It's different in every engine. Intake valve closing event, static commpression ratio, chamber and gasket volume, and rod to stroke ratio all play a role on the final pressure. I use the dynamic compression ratio as my guide rather than actual cylinder psi. It works for me...lol
I talked to an engine builder a while back about this but only in slight detail so I don't have much to add here, but I will raise a question. I wish now that I would have talked to the guy more in depth about the subject. So this builder had a race engine that he claimed by changing the cam in, he changed the cylinder pressure from 300 psi to 180 psi. All by changing the cam (with more overlap or something I assume). So here's my question, say if you have an engine that is STATICALLY 12:1 or 13:1, but your cylinder pressure is only around 180 psi with the camshaft combo, could you really run 91 or 93 octane?
jimmyray
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Tyr playing with this calculator, and you can see the difference between Static and Dynamic compression, and see the calculated effect of cam changes, specifically the effect of changing the intake duration (closing point in degrees ABDC - After Bottom Dead Center).
http://www.kb-silvolite.com/calc.php?action=comp
SRT_DSTRHOLC
Well-Known Member
NO, cams only bleed off pressure up to a certain point. But they will chime in more.
jimmyray
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Below is the calculations with my current stroker, using the Hypertectics from Keith Black, and the same Edelbrock heads. I run the car on 87 octane, it never pings, timing is set at 18 before at idle, 34 all in. Best quarter time with 3.73's and thje 727 is 12.003 @ 111.
SRT_DSTRHOLC
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9.8 is straight race gas...I will be running 8.8 DCR with magnum heads and Brian at IMM said 91 should be fine.
jimmyray
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Aluminum heads must make a huge difference, eh? I'm at 8.7 with 87 octane.
SRT_DSTRHOLC
Well-Known Member
Really??? I believe there was a test done and it showed no "real" difference between aluminum and iron..Are your closed chambers? By all means try it, but I know I would be looking at race gas with 9.8 DCR. I searched and searched and when I looked guys with EFI camaros and mustangs were worried about running 8.8dcr with 91 octane and said 93 would work, so 9.8 just seems crazy...
jimmyray
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I am actually running the combo I posted above, with the KB 2024's, 0.039 head gaskets, and the closed chamber Edelbrock aluminum heads. 236-242 @ 0.050 at .544 lift. 34 degrees total mechanincal timing, and a dynamic compression of 8.7, as per above. Cylinder pressures average around 180.
I am running an aluminum 3 core radiator, which keeps temps down. Heads have been moderately polished in the chmaber.
I literally run 87 octane most of the time. I can't tell any difference in times with 93, and no difference in drivability.
I am running an aluminum 3 core radiator, which keeps temps down. Heads have been moderately polished in the chmaber.
I literally run 87 octane most of the time. I can't tell any difference in times with 93, and no difference in drivability.
SRT_DSTRHOLC
Well-Known Member
Being able to run that I would believe is more the closed chambers then the aluminum..Try it, but I wouldn't run it with 91
Cool. I'll play around with that a little. I wish I knew my cam specs so I could see what my DCR is. I would love to drop my CR considerably from 13.9:1 where it's at now. Now that the engine is apart, my engine builder will get the cam specs soon so we'll see what's going on. Pump gas would be nice, but if we can't make it through milling the piston tops and combustion chambers, E85 will be the route I'll be going.
SRT_DSTRHOLC
Well-Known Member
Once again...just because you run 13-1 compression and a big cam that makes your DCR 8.5 doesn't mean you can run 91 octane. SMH:violent1::banghead:
SRT_DSTRHOLC
Well-Known Member
I guess you did not read my post correctly. That's not at all what I said. I simply stated that I have 13.9:1 now, and by opening up the combustion chambers some, dishing the pistons, whatever I can do to get the CR down to MAYBE run pump gas. MAYBE. I never said I was going to run 13:1 compression and run pump gas. I had hopes that I could get down in the 10s to run pump gas. I doubt i can get there without custom pistons though. No need to get all bent out of shape about what you thought I said.
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