Compression question with head gaskets??
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Senior User
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From: North Carolina
Compression question with head gaskets??
It's been a while since I have been on here and you've helped me in the past so here I am again. Got a question about compression ratio pertaining to Head Gaskets. I have a 410 motor and it spark knocks badly if I don't run a 60/40 mix 93/110 octane gas. We have played with the timing and can't get it to run on pump gas for nothing. I had everything figured up right until the machine shop resurfaced my heads and took more off them then expected and now it knocks. I used the Mr. Gasket .038 head gaskets. Would it make a noticeable difference if I went to a .042 gasket? Also could the timing being set lower be causing it to bog down in 2nd gear under heavy acceleration? Thanks for any help.... Josh
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I don't think that difference will do much. Enter your variables here:
RSR Static Compression Ratio Calculator
I ran one set of numbers and it was less than half a point.
RSR Static Compression Ratio Calculator
I ran one set of numbers and it was less than half a point.
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Senior User
Joined: Dec 2005
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From: North Carolina
Guess I basically got two options then. Either turn the timing up and have to run racing gas or pull the heads off and get some different ones... Either way it's going to be expensive... errr Well guess I got three options. Could pull the pistons and put some lower comp pistons in it.
Postmaster
Joined: Jan 2009
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From: Chesapeake, Va.
Guess I basically got two options then. Either turn the timing up and have to run racing gas or pull the heads off and get some different ones... Either way it's going to be expensive... errr Well guess I got three options. Could pull the pistons and put some lower comp pistons in it.
Keep trying and let us know what you do to cure the problem. In reality the shop should replace the heads with the exact same ones because it's their screw up.
Laughing Gas
Joined: Aug 2008
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Some questions:
1/ What's your timing at idle with vacuum advance disconnected?
2/ Was your vacuum advance connected properly after the engine work? Easy to get it hooked to manifold when it needs ported, or vice-versa.
3/ Just exaclty how much did the Machine Shop take off? Usually it's only a skim, so maybe the problem lies elsewhere?
4/ Standard or manual transmission? (I know you said 2nd gear, but that could be an automatic)
5/ Do you happen to know the max spark advance? I know you said you tried it all over, but don't forget, increasing advance to get the bottom end to perform, may make the higher rpm range knock unless you limit the dist advance.
6/ Stock camshaft or performance since rebuild?, or in other words, did you do anything that might require different ignition timing?
The more you can let us know, the more chance of some one here knowing what's wrong. You are making the same mistake I make when posting a problem....you are leading us towards a CR problem when in fact it may not be related at all.....or it might.....LOL. So all facts need to be adressed.
1/ What's your timing at idle with vacuum advance disconnected?
2/ Was your vacuum advance connected properly after the engine work? Easy to get it hooked to manifold when it needs ported, or vice-versa.
3/ Just exaclty how much did the Machine Shop take off? Usually it's only a skim, so maybe the problem lies elsewhere?
4/ Standard or manual transmission? (I know you said 2nd gear, but that could be an automatic)
5/ Do you happen to know the max spark advance? I know you said you tried it all over, but don't forget, increasing advance to get the bottom end to perform, may make the higher rpm range knock unless you limit the dist advance.
6/ Stock camshaft or performance since rebuild?, or in other words, did you do anything that might require different ignition timing?
The more you can let us know, the more chance of some one here knowing what's wrong. You are making the same mistake I make when posting a problem....you are leading us towards a CR problem when in fact it may not be related at all.....or it might.....LOL. So all facts need to be adressed.
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Senior User
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From: North Carolina
Well that's a lot of questions and off the top of my head I can't answer all of them. lol I sent the dizzy to a racing shop and they set it up to be full advanced at 2800 rpm. he told me not to run the vac advance. It has the Petronix 2 conversion with MSD coil and ignition box if that matters. The timing question is something I will have to go check again. We were getting weird readings from it. IF I remember right it said I had like 32 degrees of timing at idle and moved up to around 50 at 2500+/-. The machine shop didn't take off much but we had it figured for just the right amount of quence with those gaskets, or so I thought. It's an automatic C6, with 3000 stall, heads were ported and polished and matched to the headers, cam is a Comp 292H 292 dur and .560 lift with roller rockers and dual billet roller timing chain. Not sure of the max spark advance off the top of my head. I'm running a Edelbrock 750 carb if that matters.
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Laughing Gas
Joined: Aug 2008
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Ah hah!....your dist has been re-worked and you no longer use vacuum advance. We didn't know that before..............
You know I am laughing "with you", not "at you"......it's just funny all the new info you just "let slip".....
Anyway, as a starting place, I'd try an initial advance of 18deg BTDC. With your dist, that should give about 36 deg max at higher rpms.
Try that first and see what happens.
After that, I'd try to get the vacuum advance hooked up again. Leave it off for now until you get things sorted out.
At any rate, I'd be suspicious of the igntion timing and work with it a lot before tearing down the engine due to possible CR problems. Many machine shops don't even mill heads any more....they use a machine with a large sanding belt on it...typically taking off well under 10 thou.
You know I am laughing "with you", not "at you"......it's just funny all the new info you just "let slip".....
Anyway, as a starting place, I'd try an initial advance of 18deg BTDC. With your dist, that should give about 36 deg max at higher rpms.
Try that first and see what happens.
After that, I'd try to get the vacuum advance hooked up again. Leave it off for now until you get things sorted out.
At any rate, I'd be suspicious of the igntion timing and work with it a lot before tearing down the engine due to possible CR problems. Many machine shops don't even mill heads any more....they use a machine with a large sanding belt on it...typically taking off well under 10 thou.
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Thread Starter
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Senior User
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From: North Carolina
I need to get another timing light and try that. We tried that with the timing light I have and I think it is wrong or something. Cause at 18 BTDC it won't start or it kicks back on the starter. Might have to borrow my buddy's digital MATCO timing light and try it. Something a little better then my adjustable Craftsman. I know this thing has been driving me crazy. You think a stock or different dizzy would be worth a try? My dad has a Mallory dizzy on the shelf that is suppose to be like a stock one only without points.
Laughing Gas
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er....confusion....you stated earlier it runs about 32 deg at idle, but now you say at 18 it kicks back when you try to start it? Somthing ain't right here.
I was gogin to explain how to set it up without a light, but that's old school and may not work as you have an igntion box.
I think an MSD expert is needed here. If the igntion box is adding advance, and the dist is adding advance, how can anyone predict what will happen and if it will happen correctly without using a timing light throughout the rpm range?
I was gogin to explain how to set it up without a light, but that's old school and may not work as you have an igntion box.
I think an MSD expert is needed here. If the igntion box is adding advance, and the dist is adding advance, how can anyone predict what will happen and if it will happen correctly without using a timing light throughout the rpm range?
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Senior User
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From: North Carolina
Yeah I kind of wondered if something else wasn't messing with the timing or my light is totally screwed up. One thing I thought was the Petronix unit is the one that adjust the dwell for the best performance. I thought that might be causing it but not sure.
Postmaster
Joined: Jan 2009
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From: Chesapeake, Va.
You neeed to start over.
1. First bypass the MSD system you can re-use it after the truck is running.
2. Turn the engine over by hand to 12* btdc.
3. Make sure all power wires are correct and the plug wires are in the correct firing order.
4. Using a lighted probe on the wire to the distributor rotate the dist. until you get light when the rotor is pointing at the location of the #1 plug wire in the cap. The cap will be off the dist. for this.
5. Tighten the dist. clamp just enough to hold it but still be able to rotate it for adjusting the timing, reinstall the cap.
6. Start the engine.
Good luck with it and let us know how it works out.
Postmaster
Joined: Jan 2009
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From: Chesapeake, Va.
Copper Head Gaskets
mailto:copper@headgasket.com
Not in the Chicago area but they have the right products, head gaskets up to .125" thick. These guys should be able to cure your problem.
Let us know what the fix is..
Not in the Chicago area but they have the right products, head gaskets up to .125" thick. These guys should be able to cure your problem.
Let us know what the fix is..
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