engine gurus I need help!
12-13-2010, 03:29 PM
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engine gurus I need help! I recently built a 355 and I think I might have gotten the comression ratio a little too high. I'm using some old 041 casting heads w/ 64 cc combustion chambers. I'm running some flat tops I scored from the local machine shop. I got the heads form a buddy who didn't know much about em but upon closer inspection it seems that they have been milled quite a bit. I would say at least .020. I've heard that 180 psi is pretty much the limit on pump gas and I'm getting about 205-210 psi on a compression check. seems a little too high too me.
the cam is a comp 272 4x4 cam. my machinist assured me that the compression ratio would be "around" 10:1 but I have some doubts about it.
anybody think My compression is too high for pump gas?
the cam is a comp 272 4x4 cam. my machinist assured me that the compression ratio would be "around" 10:1 but I have some doubts about it.
anybody think My compression is too high for pump gas?
12-14-2010, 06:56 AM
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I would bet the compression is right around 10.5:1 (or a little less). That cranking compression isn't too high, but you do have iron heads. I run 10.8:1 compression with aluminum heads and pump gas with no problems (210psi cranking compression). If I were you, I'd get the highest pump grade available, and crank your timing advance back a little. Drive it and watch the plugs closely for signs of detonation. I would probably start with 28* total advance and then creep up slowly from there.
12-14-2010, 11:55 AM
#4
If your motor has a 0 deck and you have a completely flat-top (no valve relief piston and use a .039 head gasket and a 64cc chamber your motor would be 11.08:1. So just get a thicker gasket and I doubt your block is 0 decked and you should be in the 10 range.
12-16-2010, 12:33 PM
#5
180 psi on a cranking compression check is the general rule of thumb, if it's that and lower then 91-93 octane should be all you need. Between 180-200 psi chances are you will require 93 octane fuel, and above 200 psi is usually too much compression for gasoline with 93 octane and lower. This is general rule of thumb, you have to take a few other things into account to really know what you have and what's likely to happen.
Like was said static compression rato which is calculated mathematically via volume of piston TDC / piston BDC, versus cranking compression pressure measured with a compression gauge, versus dynamic compression pressures which vary with rpm, are all different. What matters is the actual cylinder pressure at a given rpm and load, which I believe is dynamic compression or ratio. It's the pressure in the cylinder that creates the heat which detonates the fuel. While it depends greatly on your static compression ratio, your cam profile also plays a vary big part in dynamic compression numbers and what allows engines with high CR numbers nearing 11:1 to not detonate and it can also causes motors with low CR numbers such as 9.5:1 to have high cylinder pressures and have > 180psi cranking compression and be prone to detonation.
one other thing is piston to head clearance, the tighter that is the less likely the chances for detonation. When you have too much compression, the typical solution is to use a thicker head gasket to lower the compression ratio but an unwanted effect of that is it increases piston to head clearance (also known as quench) and can increase the chances of detonation more so than the thicker head gasket lowering CR prevented it. If you do need to lower the compression, the head gasket is the cheapest and easiest way and if it works great. But if you find you have detonation problems after that it may be because of too much piston to head clearance in which case the best way to go is with pistons with some relief, or larger cc heads. The quench distance numbers I've seen thrown around are you want 0.040-0.050". Above 0.060" and you may be looking at lost performance and increased chances of detonation.
Like was said static compression rato which is calculated mathematically via volume of piston TDC / piston BDC, versus cranking compression pressure measured with a compression gauge, versus dynamic compression pressures which vary with rpm, are all different. What matters is the actual cylinder pressure at a given rpm and load, which I believe is dynamic compression or ratio. It's the pressure in the cylinder that creates the heat which detonates the fuel. While it depends greatly on your static compression ratio, your cam profile also plays a vary big part in dynamic compression numbers and what allows engines with high CR numbers nearing 11:1 to not detonate and it can also causes motors with low CR numbers such as 9.5:1 to have high cylinder pressures and have > 180psi cranking compression and be prone to detonation.
one other thing is piston to head clearance, the tighter that is the less likely the chances for detonation. When you have too much compression, the typical solution is to use a thicker head gasket to lower the compression ratio but an unwanted effect of that is it increases piston to head clearance (also known as quench) and can increase the chances of detonation more so than the thicker head gasket lowering CR prevented it. If you do need to lower the compression, the head gasket is the cheapest and easiest way and if it works great. But if you find you have detonation problems after that it may be because of too much piston to head clearance in which case the best way to go is with pistons with some relief, or larger cc heads. The quench distance numbers I've seen thrown around are you want 0.040-0.050". Above 0.060" and you may be looking at lost performance and increased chances of detonation.
