SRT_DSTRHOLC
Well-Known Member
Was still going off of this OP.
Max cyln pressure on pump gas ?
- Thread starter Mo-Par
- Start date
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Was still going off of this OP.
moper
Well-Known Member
In my opinion that builder that dropped 120psi with a cam only ended up with one heck of a turd as an engine. Can it be done? Probably.. But you're talking about an engine that had a cam that was in the .500+ lift range with a high static ratio going to something like .650 lift with a wider lobe seperation angle...lol. It's the wrong approach.
My "guides" for various combos on pump mid grade (89 octane, 10% ethanol) are these:
Iron head - open chamber - no quench - max dynamic of 8.25:1
Iron head - closed chamber - with quench tighter than .040 - max dynamic of 8.5:1
Aluminum head - closed chamber - with quench tighter than .040 - max dynamic 8.75:1.
There's not much difference, but there is some. Also because dynamic compression ratio is calculated with 100% filling of the cylinder and cylinder filling on a good engine will exceed 100% before peak torque is reached - the cylinder pressure will rise with rpm to a point. You can be fine on pump swill with 9:1 dynamic while tooling around at 3K rpm. But when you hammer down and run it up to 6500, it more than likely is detonating whether you can hear it or not. So I build for lower dynamic. As long as you design it into the build there is no real compromise or drop in power from it.
My "guides" for various combos on pump mid grade (89 octane, 10% ethanol) are these:
Iron head - open chamber - no quench - max dynamic of 8.25:1
Iron head - closed chamber - with quench tighter than .040 - max dynamic of 8.5:1
Aluminum head - closed chamber - with quench tighter than .040 - max dynamic 8.75:1.
There's not much difference, but there is some. Also because dynamic compression ratio is calculated with 100% filling of the cylinder and cylinder filling on a good engine will exceed 100% before peak torque is reached - the cylinder pressure will rise with rpm to a point. You can be fine on pump swill with 9:1 dynamic while tooling around at 3K rpm. But when you hammer down and run it up to 6500, it more than likely is detonating whether you can hear it or not. So I build for lower dynamic. As long as you design it into the build there is no real compromise or drop in power from it.
BoredandStroked
Well-Known Member
Had to chime in on this one after reading, a few things.
Did a wet and dry compression run on an engine a few days ago for a guy with some of the highest readings I have ever seen (on pump gas) at 245 psi (dry) on all cylinders with a variable of only 5+/- psi. Engine had 3,000 miles, ran on 87 octane, but had fully adjustable intake and exhaust cam timing. I say "but" because cam timing/design has a lot of effects of cylinder pressure and dynamic compression.
Aluminum heads can help in raising cylinder pressures, in theory. Aluminum dissipates heat quicker, and we all know heat causes detonation. So if our combustion chambers are not as hot as with iron heads we can bump up the compression/cylinder pressure. This holds true if the heads are exact copies with the only difference being iron vs aluminum, but when we change chamber design, quench point and area, and a ton of other things we can fiddle with the outcome and it can be either positive or negative.
Did a wet and dry compression run on an engine a few days ago for a guy with some of the highest readings I have ever seen (on pump gas) at 245 psi (dry) on all cylinders with a variable of only 5+/- psi. Engine had 3,000 miles, ran on 87 octane, but had fully adjustable intake and exhaust cam timing. I say "but" because cam timing/design has a lot of effects of cylinder pressure and dynamic compression.
Aluminum heads can help in raising cylinder pressures, in theory. Aluminum dissipates heat quicker, and we all know heat causes detonation. So if our combustion chambers are not as hot as with iron heads we can bump up the compression/cylinder pressure. This holds true if the heads are exact copies with the only difference being iron vs aluminum, but when we change chamber design, quench point and area, and a ton of other things we can fiddle with the outcome and it can be either positive or negative.
That was strictly posting a question whether or not it was possible. I was not stating you could do it. Just a question. I personally wouldn't feel comfortable with pump gas at that CR. Although I know a guy who ran pump gas in his 14:1 440 challenger to drive it to a show because he had no race fuel. He mixes pump gas and 110 race fuel to run the car. Do I agree with running the mix? No. Pump fuel, that was dumb imo. He ran the mix workout pinging. On pump gas it would ping if he gave it much throttle. So he said.
OldmanRick
Well-Known Member
One of the best posts i've seen in a while......i agree completely. I've always been conservative with compression. Either build a street friendly engine or go big on a track car. Why push the limit too much...... Back in the 70's, a local builder that i looked up to, wouldn't even build a 11 to 1 comp. engine. 9-10ish for the street, or step up for the track in the 12-13+ range.
Holy cow! That's about what my engine compression tested at and I had to run 112 octane. Some thing just don't make sense sometimes.
SRT_DSTRHOLC
Well-Known Member
I agree. Just trying to help, not putting you down or anything. :cheers:
Mo-Par
Well-Known Member
On the 410 I am building,10.25 compression with an intake valve closing at 43 deg's ABDC will give me 189.54 psi of cylinder pressure according to the RSR advanced dynamic compression ratio calculator I am using.
Lonewolf3165
Living the Dream!
By "pump gas" are we talking 87 or 92^?
SRT_DSTRHOLC
Well-Known Member
Seems like 91 octane to me
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