okokok... Compression Question

[Earlsfat]

Anyone know what PSI relates to what compression ratio?

I was told my 454 was running 11:1 compression when I bought it, but after looking up the pistons, etc I'm guessing it's probably closer to 10:1 maybe even 9.5:1.

All I know is that when I ran the compression test intially, the good cylinders (heh) all read 180 psi. Not sure if that is 11:1 or what.

ALSO - From what I understand (which may not be much) the higher the compression the more susceptible the engine is to detonation, etc... is that right?

[Earlsfat]

Hang on... Pulled this:

Pull the sparkplug from an engine (like a chainsaw). Put your finger over the sparkplug hole, and pull the starter rope. Can you feel the compression? If you can get a compression tester, test the pressure in the cylinder. Some compression gauges give pressure but don’t indicate the ratio. If a cylinder has 105.8 psi, what is the compression ratio?

Atmospheric pressure is approximately 14.7 psi. Round it off to 15 psi. 105.8 divided by 15 is about 7.


From here : http://www.ankn.uaf.edu/publications/VS/TEcompression.html

Is this right? If so 180 psi / 14.7 = 12.24 Compression Ratio - WTF???

[wrightz28]

Jim, when you're doing your test do you have all plugs out or just the one your testing? And how many puffs?

Plus that reasoning is on the bunk side, according to that formula my Buick v6 had a 11.57;1 ratio

[Mr. Luos]

Cam also effects compression. Not the static, but the dynamic.

Do you know the specs on the heads and block?? Deck height, piston cc, chamber cc, etc?
If so, check this out. Just change around what you need to make your set-up fit.
Should give you a rough estimate...even though it is designed for LSx motors.
http://www.c5frc.com/calculators/Com...Calculator.htm

[Earlsfat]

Both ways. I also looked at a couple other sites that said pretty much the same thing... the "accepted" compression ratio designation is less than wht the formula produces. Saw 5 other sites that used that same formula.

That ain't right... is it?

Could the CR being that high be what ate the valves? (In addition to them being ground / cut.)

[Morley]

When I did a compression check on my original engine I got an average of 160-170 on each cylinder and the engine was 9.5:1. I would guess you are closer to 10:1. At 11:1 on a 454 with carb you'd almost HAVE to be running 110 octane race gas all the time.

[Earlsfat]

Thanks Curtis... It'll take me some time to figure out what all those are but I'll see if I can figure it out and try that calculator.

[Earlsfat]

Originally Posted by Morley

When I did a compression check on my original engine I got an average of 160-170 on each cylinder and the engine was 9.5:1. I would guess you are closer to 10:1. At 11:1 on a 454 with carb you'd almost HAVE to be running 110 octane race gas all the time.

Back to my other question though... could that be why the valves burnt out?

If I was running (and the guy before me) 93-94 octane and the CR is that high, isn't that a problem???

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Just did another search (searched "compression ratio PSI") ... and found this http://www.geocities.com/terminalvelocitycoaster/adr/

I don't know... maybe it is right. If so, that means I gotta have this thing dialed in otherwise I'm gonna have some problems, right?

[1986Z28]

what do you pistons look like flat top, dish, dome, etc...

[Mr. Luos]

Originally Posted by Morley

At 11:1 on a 454 with carb you'd almost HAVE to be running 110 octane race gas all the time.

One of the reasons I like fuel injection and aluminum heads.
I run 11:1 no problem on pump gas. 91 octane.

[Earlsfat]

Domed.

I think this is the one... http://www.gmpartsdirect.com/perform...CATID=507.html

(Forget off the top of my head - have it at home. Pretty sure they were stamped "3999295")

[Morley]

Originally Posted by Jimsfat

Back to my other question though... could that be why the valves burnt out?

If I was running (and the guy before me) 93-94 octane and the CR is that high, isn't that a problem???

----------

Just did another search (searched "compression ratio PSI") ... and found this http://www.geocities.com/terminalvelocitycoaster/adr/

I don't know... maybe it is right. If so, that means I gotta have this thing dialed in otherwise I'm gonna have some problems, right?

A burned valve by itself would more likely be a valve open part way at all times, valve edges ground too thin.
If the compression were too high for the gas you were running you'd have detonation and holes in the pistons to go along with the burned valves.

[Morley]

Originally Posted by Mr. Luos

One of the reasons I like fuel injection and aluminum heads.
I run 11:1 no problem on pump gas. 91 octane.

Yep, you can't beat FI for making up for deficencies in octane.

[Earlsfat]

Originally Posted by Morley

A burned valve by itself would more likely be a valve open part way at all times, valve edges ground too thin.
If the compression were too high for the gas you were running you'd have detonation and holes in the pistons to go along with the burned valves.

That's what the head guy said, they ground the edges too thin, AND the vales were going up into the sleeve, so he had to ream out the valve hole (whatever the hell its called) and go up a size on the exhaust valves.


EW, EW, EW... (like Arnold Horshack) ... 2 or 3 pistons had some pitting... I'm no engine guy, but it didn't look THAT bad, and the guy helping me said that was a common thing, but if the pitting was on the same cylinders as the burnt valves...???


ANOTHER QUESTION... If I were to use that race gas... I think it's like 105 - 110 octane, do I have to do anything special? Does it affect the fuel mixture (IE make it more lean or rich) or anything that would make using that a bad idea? I have no problem putting 2-3 gallons of that in from time to time - or should I look at using octane boosters?

[Morley]

Originally Posted by Jimsfat

EW, EW, EW... (like Arnold Horshack) ... 2 or 3 pistons had some pitting... I'm no engine guy, but it didn't look THAT bad, and the guy helping me said that was a common thing, but if the pitting was on the same cylinders as the burnt valves...???


ANOTHER QUESTION... If I were to use that race gas... I think it's like 105 - 110 octane, do I have to do anything special? Does it affect the fuel mixture (IE make it more lean or rich) or anything that would make using that a bad idea? I have no problem putting 2-3 gallons of that in from time to time - or should I look at using octane boosters?

Depends on how deep the "pitting" is. But I was talking about actual holes through the piston tops.
Valves going up into the sleeves? You mean up into the guide? That would mean you sunk the valves, another sign of detonation. I would definately get the engine checked to see what compression ratio you are running. If it is over 10:1 you'd need to run race gas all the time to keep from ruining the engine.