Head gasket thickness

[MCfan]

A question or two for you experienced engine builders, if I may:

 

I want to reduce the compression ratio on my 402 (actually 413 with .060 overbore) because I am weary of fighting detonation with 93 pump gas plus 108 Octane Booster and conservative timing. I am wondering if installing a thicker head gasket can/will reduce my compression ratio enough to make it worthwhile (without a major rebuild). I know it's impossible to answer that question without more data so here's what I know:

 

Unfortunately, I am not certain what the current CR actually is, but here are some clues from the S and S Speed Center's shop tickets of the engine's major rebuild in 1974. The TRW forged pistons (L2337F-60) that were installed are listed at 11.9:1 CR with 114cc heads (although I don't know the deck height spec). I also know that both heads were replaced with new "GM heads" (I still need to check their casting number) plus "resurface heads .010" and "install 16 oversized valves" (V2562X intakes; S2692 exhausts). I assume new head gaskets were also installed but none are listed on the shop tickets so no way to know their thickness.

 

The invoiced machine work also included "Deck top of block" but no dimension was given (the date code and partial vin were machined off so I know some decking was done). All of these parts and modifications could add up to a relatively high compression ratio and certainly one higher than I want or need for street use. I also ran a compression check of all eight cylinders with a new compression tester about a year ago and got consistent readings of 320 +/- 5 psi which seemed high, at least to me.

 

A thicker head gasket should help but may not be enough to make much difference. Is there any way I can determine, in either an absolute or relative sense, how much CR relief I could get with a thicker head gasket? A related question is whether longer push rods would be needed if a thicker head gasket were used? I am considering replacing the Edlebrock Torker II single plane with a dual plane Air Gap (for better street behavior) so pulling the heads at that time shouldn't be that much more work.

 

I would like to tame this engine down enough to safely use today's pump gas since the nearest E85 station is 30 miles away and racing gasoline seems too expensive to be practical. The thicker head gasket idea may be too ineffective or impractical, also. Any other ideas? Many thanks.

[monte70car]

Dennis your 402 sounds almost like my 396(bored .040 over). My motor was done almost the same way with the block being decked and then getting the intake shaved to match the new mounting area(.010) was taken off on the intake to head area. I figured my motor to have 10.25:1cc and run 87 octane.

[Sam (Bones)]

head gaskets are not going to help much with almost 12:1 compression, don't try to run E85 with your standard carb either, it will be WAY! lean and could burn up your engine, you need to run approx 30% more fuel thru your carb for E85

[MCfan]

Thanks, Sam, I appreciate your input.

[Mike 57]

Thicker head gaskets would probably make it worse because they would decrease the "quench" area between the piston and head.

The pistons you have are an older design that is no longer available. They are a heavy design with a big dome, 47cc I believe. The best way to get where you want to be is replacing the pistons and rebalance. Kind of a costly proposition. Your 402 may be just .030 over. The pistons were originally designed for a 396. A .060 over 396 is a .030 over 402.

[wallaby]

There are a number of tricks that can be used to deal with detonation, but with 12:1 compression they won't ammount to a hill of beans. Going conservative on camshaft will make matters worse. If you chose a cam with more overlap, it will bleed off some of the pressure, but at a loss of low-end power and mileage. Going to a dual-plane intake might make things worse as well, being they are designed to get maximum cylinder filling at lower speeds (compared to a single plane).

Make sure you aren't running lean on your fuel mixture.

 

When do you experience the detonation? At light loads, or just under heavy throttle?

[CrazyDavey]

Dennis first you need to get the casting number from the heads so you know which ones you have and the chamber sizes. The easiest way to knock down the compression without going into the short block is using a head with a larger chamber. GM made many, many versions of the BBC head so there are some maybe not so expensive possibilities there. As Mike said the thick gasket is not a good idea because of the quench increase which would actually make it worse off. Once the heads are off you can measure the deck height and see what you have, this is information you need anyway to select the correct gasket thickness when you put it back together. The quench (deck height + gasket) should be in the .035 - .045 range...

[MCfan]

Thanks for your feedback, Mark.

 

This engine was completely rebuilt in 1974 with quality components that are now technologically dated. The cam, for example, is a Sig Erson Hi-FLOW-IH #120421 hydraulic flat tappet single pattern with 286* duration, 64* overlap and .540 lift. It originally had a Holley 750 cfm carb on the Edlebrock Torker II manifold but it was replaced by the PO with an Edlebrock 650 cfm AVS Thunderer (austensibly for better mileage - not so much ).

 

When I bought the car it had a stock distributor with an Accel pointless conversion and a plugged vacuum advance. Initial (static) timing was 16* with 20* max mechanical. I decided to reconnect the vacuum advance (which was mechanically limited to 12* max) and reduced the initial to 4* per factory 402 specs (not having any other point of reference at the time). I then experiemented with different weight springs until the mechanical was all in at 3000. This combo avoided detonation during street driving and usually at WOT also (although I do very, very little of that around here). However, it seemed to struggle after 4000 rpm and required a faster idle which caused persistent run-on at shut down.

 

I recently replaced that distributor with a late 70's stock HEI (non-computer) with a 20* vacuum advance can). The engine responded very well to additional advance at idle (4* initial + 20* vacuum), allowing me to reduce the idle from 950 to 800 rpm which completely solved any run-on at shutdown. Unfortunately, it introduced persistent detonation at the transition between cruise and acceleration in any gear (4 speed manual) even with the stock weights and strong stock springs in the HEI.

 

I found and read a lot of tuning and "curving" advice for HEIs which suggested an adjustable vacuum can for solving transitional detonation. So, I tried one ... in vain. I finally went back to the stock vacuum can with a bushing that limits max vacuum advance to 16* and moved the initial up to 6* to preserve improved idle. The detonation at transition is much better but still there. It is simply impossible to go from cruise to WOT without some transitional pinging - so naturally, I've been avoiding that which is no fun, of course.

 

I suspect I need to throw the carb settings into the solution mix, but that is something I probably should leave to a professional old school tuner. I have the complete Edlebrock carb tuning kit of springs and rods for my specifc carb but the instructions said to get the timing set up right first. With everything I've tried so far, I now doubt that a solution can be found in the timing alone.

 

I've found a couple of old school speed shops in Ft. Myers that I need to check out now that my taxes are filed. It would be great to learn and know all of that engine tuning magic, but I can see that doing it by trial and error is probably not a very reliable or efficient approach for me.

 

Sorry for the long reply ... if it triggers any additional thoughts or questions, I would appreciate knowing.

[MCfan]

Thicker head gaskets would probably make it worse because they would decrease the "quench" area between the piston and head.

The pistons you have are an older design that is no longer available. They are a heavy design with a big dome, 47cc I believe. The best way to get where you want to be is replacing the pistons and rebalance. Kind of a costly proposition. Your 402 may be just .030 over. The pistons were originally designed for a 396. A .060 over 396 is a .030 over 402.

 

Thanks for sharing your insights, Mike.

 

I was having a hard time understanding how a thicker head gasket would decrease the quench area since neither the configuration of the piston nor the combustion chamber would change. But, I can see how increasing the distance between those surfaces where the quench takes place could reduce the effectiveness of the quench which some have characterized as "free octane". I can see your point, however, that the small decrease in the compression ratio (by raising the head a bit further from the piston tops at TDC) could be a bad trade-off because of reduced quench. Thanks for pointing that out.

 

That last piece of info solves a mystery in the shop tickets. The pistons are clearly specified as .060" over but the machining details listed a .030" overbore. I assumed the machining line item might have been an error but I see now how they match up since the pistons were originally for a 396. That would make my current displacement around 408, I suppose.

[Mike 57]

Water/methanol injection may be an option. there are many companies out there here is one Linky or you could make one yourself. Search online there are many out there. Basically water or a mix of water and methanol is sprayed into the carb raising the octane.

Your goal is to keep the combustion chamber cool. cooler plugs without extended electrodes. thermostat no higher than 160, a little richer on the mixture would help.

Do you have the casting numbers of the heads? It would be helpful to know. There are some later low performance heads out there with chambers as big as 122cc that would drop the compression to around 10.8 still high but it may help a little.

Just some ideas.

[400hpmonte]

Maybe Look into meth injection. It Is a option.

[Mike 57]

Have you tried going back to the setup without the vacuum advance? I don't run it, mine will ping on high vacuum conditions with it. I run 18 initial and 20 mechanical total of 38 all in at 2400. No detonation with pump gas on the street. but I probably have more cam, 11.6 compression, and aluminum heads.

[Its Just Me]

Dennis:

You say your not opposed to changing your heads since you're considering swapping on a dual plane intake anyway...Have you considered aluminum heads? I know they may be price prohibitive, but from everything I have read, you can effectivly reduce compression by 1 point just from the improved heat dissipation. Not to mention you can look for a set with a larger combustion area, further reducing CR.

A cheaper option (maybe) is if you're running a closed chamber head now (which was popular back then), maybe look for a set of cast iron open chambers. Open chambers typically have a larger combustion area than closed chambers, and they tend to breath better on the street. A good GM open chamber, iron head to consider would be an 049 or a 781 casting. The 781's are a newer version of the 049. They were produced from 1973-??? and are considered to be one of the best flowing, out of the box GM heads made. And can be made even better with some valve and port work (now they get expensive). I believe the 049's had a 122-126 cc combustion area, which is quite large, and would probably help reduce your compression. By how much would depend on what you have for heads and combustion chamber volume now.

Check out "racingjunk.com" there is a set of 781's listed for, I think, $500, and already have the larger valves installed.

As others have said, you need to consider the "whole package" though. Changing the heads may reduce compression a bit, but if you change the cam & intake at the same time, you may lose what you gained. Can be a tough game to play. Talk to a reputable machine shop & enjoy the ride.

 

Scott

[Sam (Bones)]

049 heads have a combustion chamber of approx 113-115 CC's, the 781 can be up to 122 CC's, my set of 1973 049's were 113 until I shaved them down some to 110 CC's, with a 47 CC dome on the piston the best way to do it is replace the pistons and re-balance it like Mike suggested, even using a 122 CC chamber might drop the compression 1 point, that's still over 10:1, and with iron heads detonation could still be a issue, even with a conservative timing curve

I know my 454, w/049 heads @ approx 9.6:1 comp and a cam with a bunch of overlap is ok on 93 octane, and doesn't like anything less, I'm not running a vacuum advance either

[MCfan]

Thanks, guys (Davey, Sam, Mike, Scott, Mark, Norris, et al) for your feedback and suggestions.

 

I pulled a valve cover and located the casting number and date code on my 402 heads today. The casting number is 3999241 and the date code is E23 72 (May 23, 1972).

 

According to the the BBC Head Casting list on the Chevy Legends website, the 3999241 head was used only in '72 on 402 and 454 engines. It has 113cc Open chambers with oval ports.

 

 

 

The shop tickets say the heads were milled .010" which would reduce the combustion chamber a maximum of .7cc. Larger valves were installed but not sure if or how much that might have changed the combustion chamber volume. Assuming a typical deck height of .040" and a stock head gasket thickness of .039", the available static volume should be around 117cc to 118cc. The displaced volume is 835.75cc for a .030 over 402 (4.155" X 3.76"). For a 11.9:1 CR, the non-displaced volume is 70cc so the piston dome displacement must be around 47cc to 48cc just as Mike remembered.

 

Using these ball park numbers each increase in head gasket thickness of .010" would only reduce the compression ratio by sligtly less than .1 (i.e. an 11.9 CR would only improve to 11.82 CR using a .010" thicker head gasket. Clearly, that would be futile and it might even make matters worse as Mike and Davey have suggested.

 

Replacing the current heads with different heads having a larger combustion chamber would certainly be more effective. A 119cc head could theoretically reduce the CR to 11.0:1, but that's probably still too high and no telling what it might do to quench and other factors relevant to detonation.

 

So ... it looks like I've got my answer on increasing head gasket thickness and probably also on swapping heads (since I'm too cheap to go to aluminum heads ) . Guess it's time to followup on the other suggestions of getting rid of the vacuum advance and exploring the merits and costs of water/methanol injection systems. Of course, it's entirely possible that I'm missing something obvious here, also. So please let me know if you see some other possible solutions.

 

I think I will take the car to one or more local professional tuners and get their opinions and advice on what I can/should do to improve my current situation. That's not to say I don't value all of the advice and suggestions from club members - I surely do and it's very valuable.

 

So, thanks again, guys... and don't be bashful about sharing any further thoughts or suggestions.

[CrazyDavey]

well your curmudgeon cheapness aside.. .... sense this is kinda your "fun" car (and you can save the old parts) I would start collecting quarters in a mayonnaise jar and do it right. First some aluminum heads will get about 80 lbs off the nose of the car and second they are a lot more tolerant when it comes to detonation issues. Next I would get with Chris Straub and have him design you a nice roller cam specifically for your application. Even at 11 to 1 pump gas should not be an issue with properly designed valve/combustion timing events. Dart make a 121cc oval with a 275 intake runner that would be perfect. Brodix makes a 119cc with a 270 intake runner...many choices out there....I could help you with the Dart and Straub stuff....

[MCfan]

Great idea, Davey ... I could basically replace the top of the engine to get the better behavior I want and bring it into the 21st century at the same time! I have been wanting to replace the intake manifold and get new headers anyway. The new 650 Edelbrock carb may or may not have to go.

 

I'd better figure out how big my jar of quarters needs to be. Guess I'll have some time to do that this summer because it's too close to my seasonal migration north to start such an ambitious project now.

 

Thanks for your recommendations and encouragement!

[CrazyDavey]

well let me know what you are thinking and I can help you with some budget numbers...I can get with Chris and discuss the whole car and we can come up with a good plan for you to maximize everything....and yes...change the intake and ditch the Eddy carb....we can probably add a 100 hp to that thing and make it drive nicer...that might be fun...