Anyone know the head gasket bore required for 389 - Pontiac GTO Forum
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post #1 of 14 (permalink) Old 07-20-2011, 01:42 PM Thread Starter
 
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Anyone know the head gasket bore required for 389

So, I have a 64 389 rebuilt 30 over. i had a set of 4.160 cometics on and they stuck into the combustion chamber (may have explained this in another post), I called SCE and their ICS titan gaskets are perfectly round too. They said a 4.250 or 4.320" bore will work for the 64 389 with 345 heads. Anyone done this before? Cometics or SCE? Cometic sold me a set that weren't right so i may try SCE. Anyone with experience using these on pre 1965 engines please help. thanks
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post #2 of 14 (permalink) Old 07-20-2011, 02:02 PM
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Well, bore size on a +.030 389 would be 4.0925 so if the Cometic 4.160's were too small the problem must have been in the heads, not the bores (unless maybe your block has the valve relief chamfers at the tops of the bores?)

Do you need a compressed gasket thickness that is different from "stock" for some reason? I hear good things about the stock replacement FelPro head gaskets, and they're way less expensive than the "customs" like Cometic or Titan.

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Well, bore size on a +.030 389 would be 4.0925 so if the Cometic 4.160's were too small the problem must have been in the heads, not the bores (unless maybe your block has the valve relief chamfers at the tops of the bores?)

Do you need a compressed gasket thickness that is different from "stock" for some reason? I hear good things about the stock replacement FelPro head gaskets, and they're way less expensive than the "customs" like Cometic or Titan.

Bear
Right now i have a compression ratio of 10.3:1 and i would like to put it down to 9.5-9.7:1 SCR. You are correct, the combustion chambers are closed type and oval shape 1.92/1.66. putting an .062 thick gasket will lower cr to mid 9's. Because parts of the combustion chamber sit outside of the cylinder, i was just wondering if anyone had encountered this and what their solution was.
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post #4 of 14 (permalink) Old 07-20-2011, 03:09 PM
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Right now i have a compression ratio of 10.3:1 and i would like to put it down to 9.5-9.7:1 SCR. You are correct, the combustion chambers are closed type and oval shape 1.92/1.66. putting an .062 thick gasket will lower cr to mid 9's. Because parts of the combustion chamber sit outside of the cylinder, i was just wondering if anyone had encountered this and what their solution was.
Cool - if you're trying to solve a detonation/pinging problem, please keep this in mind. By going with thicker gaskets you'll also be giving up quench pad area and the resulting chamber turbulence it causes. Doing this can make a motor more prone to detonation than it was before, even with lower compression ratio. So if that's the goal, there's a chance it might not work or even make the problem worse. The safer solution would be to D-dish the pistons to get the increased volume and lower compression while keeping a good quench area, and/or go with a longer duration on the intake side of the cam to delay the intake valve closing event to reduce dynamic cylinder pressure.

If you've got other constraints you're working with that are forcing you to try thicker gaskets, I understand - just be aware it's a gamble.

I'm using Cometics in my 400 stroker and I love them. Perhaps someone else will chime in with a specific recommendation on gasket bore size.

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post #5 of 14 (permalink) Old 07-20-2011, 03:40 PM Thread Starter
 
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Cool - if you're trying to solve a detonation/pinging problem, please keep this in mind. By going with thicker gaskets you'll also be giving up quench pad area and the resulting chamber turbulence it causes. Doing this can make a motor more prone to detonation than it was before, even with lower compression ratio. So if that's the goal, there's a chance it might not work or even make the problem worse. The safer solution would be to D-dish the pistons to get the increased volume and lower compression while keeping a good quench area, and/or go with a longer duration on the intake side of the cam to delay the intake valve closing event to reduce dynamic cylinder pressure.

If you've got other constraints you're working with that are forcing you to try thicker gaskets, I understand - just be aware it's a gamble.

I'm using Cometics in my 400 stroker and I love them. Perhaps someone else will chime in with a specific recommendation on gasket bore size.

Bear
Agreed. My quench would end up being between .085 and .09. I guess i could also carve about 4 cc's out of the old 345 heads. Problem is i just had her rebuilt and she is in but had valve train issues so i am trying to take care of my cr at the top end rather than turning dishes into the pistons. The cam is a 275DEH with intake closing at 63 ABDC so dynamic cr is down currently with .039 felpros to 8.2-8.3:1 still a touch high for 94 octane. I liked the cometic gaskets too but i am a little angry that cometic did not inform me that the gaskets i was ordering did not work with my engine (i was on the phone with the guy at cometic).
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the valve train issue should have been resolved during the rebuilt, as this is not a new issue with 389 and their smaller heads. The change with just a gasket and opening up the heads will not be successful for a detonation free operation. you will be plagued by gasket failures as well as valve recession in the head..
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post #7 of 14 (permalink) Old 07-20-2011, 05:46 PM Thread Starter
 
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the valve train issue should have been resolved during the rebuilt, as this is not a new issue with 389 and their smaller heads. The change with just a gasket and opening up the heads will not be successful for a detonation free operation. you will be plagued by gasket failures as well as valve recession in the head..
Not sure I understand what you are saying. How can lowering compression to use hi octane pump gas not prevent knock?
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post #8 of 14 (permalink) Old 07-20-2011, 07:13 PM
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When it comes to knock, compression is only part of the equation. Heat and flame travel/combustion efficiency also play a big part. Amuminum heads run much cooler than iron and that allows them to remain "knock free" at higher compression ratios than iron can. Getting good turbulence in the chamber helps in several ways: it helps keep a nice homogenous mixture and the circulation helps prevent isolated hot spots. Both of these guard against getting multiple ignition sources with resultant colliding flame fronts leading to knock. The hotter things get in the chambers the more likely you are to get detonation and also soften up the exhaust seats so the action of the valves can pound them into the heads. Good chamber turbulence mostly comes from having a good tight quench pad area: where the piston top comes very close to the flat part of the head as it approaches TDC creating a pressure wave that spreads across the whole chamber keeping things nice and "shook up". Fatter gaskets hurt this - a lot - especially on a closed chamber head like the 670 for example. Couple that with the fact that closed chamber heads tend to need more ignition advance because they already aren't as combustion-efficient as open chamber heads are, and you can get into trouble really quick. A D-shaped piston dish gets you the addtiional chamber volume you need to drop compression ratio without sacrificing the quench pad area.

If you lower compression with fat gaskets and in the process sacrifice "too much" quench, it's possible to find yourself in a situation where the motor is still detonating as much (if not more) than it was before.

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post #9 of 14 (permalink) Old 07-21-2011, 07:54 AM Thread Starter
 
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When it comes to knock, compression is only part of the equation. Heat and flame travel/combustion efficiency also play a big part. Amuminum heads run much cooler than iron and that allows them to remain "knock free" at higher compression ratios than iron can. Getting good turbulence in the chamber helps in several ways: it helps keep a nice homogenous mixture and the circulation helps prevent isolated hot spots. Both of these guard against getting multiple ignition sources with resultant colliding flame fronts leading to knock. The hotter things get in the chambers the more likely you are to get detonation and also soften up the exhaust seats so the action of the valves can pound them into the heads. Good chamber turbulence mostly comes from having a good tight quench pad area: where the piston top comes very close to the flat part of the head as it approaches TDC creating a pressure wave that spreads across the whole chamber keeping things nice and "shook up". Fatter gaskets hurt this - a lot - especially on a closed chamber head like the 670 for example. Couple that with the fact that closed chamber heads tend to need more ignition advance because they already aren't as combustion-efficient as open chamber heads are, and you can get into trouble really quick. A D-shaped piston dish gets you the addtiional chamber volume you need to drop compression ratio without sacrificing the quench pad area.

If you lower compression with fat gaskets and in the process sacrifice "too much" quench, it's possible to find yourself in a situation where the motor is still detonating as much (if not more) than it was before.

Bear
Well, that sucks. It is my understanding that quench should be in the area of .045 to .065 is comfortable and .080 is the outside. And I was thinking making 72cc chambers with .065 quench and 160 tstat could keep my detonation issues at bay. I really don't wanna take this engine apart again to dish the pistons so any solution outside of that is the solution I want to go with. There has to be a top end solution that works. Thanks for all your input bear.
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post #10 of 14 (permalink) Old 07-21-2011, 12:44 PM
 
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