Using 3.4 iron head gaskets to raise compression?

[ForcedFirebird]

As weak as the ring lands are (especially under boost), do you think it's safe to weaken them further?

[Sidewinder]

Sidewinder:

I have brought that piston shaving idea to the table before, i think it would work well. We'd just need to get the piston CC'd after the .020" was taken off to calculate the compression.

I have a few 3400 pistons laying around, someday i might take them in for a shave..

Yeah I know you did, I actually got it from you :P

Well anyway it would be interesting to see what it does, and I was curious because I'm still trying to understand engines better. Looks like I may be getting a 3400 shortblock soon though!

[TazMan]

Just a thought. Could I have the heads shave for even more compression? If so would it be worth the extra power? Also would the manifolds still line up okay with the heads shaved and would the pushrods now be too long?

[Superdave]

I think that depends on the thickness of the gen2 gasket. I don't know how thick they are but they could already be .040".

they are .060".. the gen2 has like 8.8:1 compression.



Sidewinder:

I have brought that piston shaving idea to the table before, i think it would work well. We'd just need to get the piston CC'd after the .020" was taken off to calculate the compression.

I have a few 3400 pistons laying around, someday i might take them in for a shave..

[TazMan]

Ok so if I'm understanding this correctly, that means that I already have more compression than a gen2 3.1 that uses the same gaskets because my pistons actually stick out more.

I think that depends on the thickness of the gen2 gasket. I don't know how thick they are but they could already be .040".

[Sidewinder]

Ok so if I'm understanding this correctly, that means that I already have more compression than a gen2 3.1 that uses the same gaskets because my pistons actually stick out more. That means that if you have a gen2 3.1 you can benefit from using the iron head gasket, but it would be pointless on a gen 3 wouldn't it?

The gen3 already has a .040" quench as it was called, but the gen2 3.1L has 0.60"? The reason for it being pointless:

Using iron head gaskets give .020" quench, but you want to keep .040". So if you shaved off .020" off the pistons to get .040" quench, wouldn't that have made using the iron head gaskets pointless because you would still have the same compression ratio?

[Superdave]

So what does a high compression actually do for a N/A engine? This is one of those areas I don't fully understand. Does it increase horsepower and torque?

How much of a gain would using the iron head gaskets actually give you? Would using them even be worth it?

Higher compression alone typically gains you torque (which by default gives slightly more HP).

I don't fully understand this stuff either but I figure if the 3x00 pistons stick out of the heads .020" and the gaskets are .060", that would make the distance from the top of the piston to the top of the gasket .040". Since my 3.1 pistons come flush with the block I would think I'd actually be loosing compression with the stock 3100 gasket. Using the .040 would make up for that.

As far as a HP and Tq. go, I have absolutley no idea how much of a noticable difference there would be, if any, but I'd love to know if anybody else does.

you are correct, in a gen2 3.1 where the pistons come to 0 deck it's perfectly fine to use the thinner gaskets to raise compression, however in a gen3 where the pistons pop out you'll run the rick of piston to head contact...


remember, .020" is about the thickness of a business card

[TazMan]

I don't fully understand this stuff either but I figure if the 3x00 pistons stick out of the heads .020" and the gaskets are .060", that would make the distance from the top of the piston to the top of the gasket .040". Since my 3.1 pistons come flush with the block I would think I'd actually be loosing compression with the stock 3100 gasket. Using the .040 would make up for that.

As far as a HP and Tq. go, I have absolutley no idea how much of a noticable difference there would be, if any, but I'd love to know if anybody else does.

[Sidewinder]

So what does a high compression actually do for a N/A engine? This is one of those areas I don't fully understand. Does it increase horsepower and torque?

How much of a gain would using the iron head gaskets actually give you? Would using them even be worth it?

[ForcedFirebird]

Ive seen some that almost completely fill the cc at tdc.

This most important step consists of measuring the distance from the assembled piston tops to the surface of the block deck (deck clearance), and milling as necessary. The general feeling is that the total quench or squish distance should be about .040". The quench distance is the compressed thickness of the head gasket plus the deck clearance. The quench area is the flat part of the piston that would contact a similar part of the head if you had .000 assembled quench height. In a running engine, the .040 quench height decreases to a close collision between the piston and the cylinder head. The shock wave from the near collision drives air at high velocity through the combustion chamber. This movement tends to cool hot spots, averages the chamber temperature, reduces detonation and increases power. The shock wave also provides better fuel/air mixing, and this allows the fuel to ignite better and burn faster. A faster burning fuel charge means less timing is required for optimum power output. An example of this--a running 462 had a factory deck height of about .020, the deck was set to 0. There were no other significant changes to the engine (new rings and bearings, but same cam, heads, intake and exhaust systems). The optimum timing setting prior to the change was 34 degrees - that provided the fastest MPH and quickest ET. After the change to 0 deck, the optimum timing using the same Amoco gas changed to only 30 degrees total mechanical. Not only did the lowering the deck raise the CR by several tenths of a point, but by retarding the timing 4 degrees, it was possible to increase the CR even higher due to the optimum lower timing setting.

I did notice that the car pings under load with low grade gas

Perhaps with a higher lift camshaft there would be issues though.

It will work, it's just not ideal. When blueprinting an engine, all the professionals use this number.

[FallenZ24]

Isaac is right. The car has a complete '98 3100. At the time of those pics it had the 3500 uim. Now it just has the large port 3100 upper. It ran very hard with the old intake setup and large tb. The car did need a little tuning for optimum performance though.

The car runs great now aside from a little tunning needed. It runs a little rich at idle and lean in the upper rpms. I've verified this with wb 02.

What I don't understand is why you guys say .040" is the "optimum" quench. Why? Is this just a universal # that someone came up with? Has anyone tested this theory? All engines are different and until testing and dyno #s are posted I don't think it should be spread around. Have you guys ever seen hi-comp sbc pistons? How far do the domes go into the combustion chambers? Ive seen some that almost completely fill the cc at tdc.

Also the "belief" that the thinner gaskets will cause problems is misinformation. I've tested it on my engine and have no clearance problems in upper rpms due to expansion of parts. Perhaps with a higher lift camshaft there would be issues though.

-Joseph

[rsisbetter]

interesting, i need a calculator. hahaha, that way i can picture it in my head easier, right now, just reading it, is hard to see, (compression raise)...

thanks
cody

[Superdave]

this is all very interesting, since, i have a 3400, and would like to do a 3500 top end swap. in which i would like thinner head gaskets, (3.4 iron head, .040) as opposed to the stock .060. So what i have gathered from this thread, is that, a 3.4 iron head, .040 head gasket can be used on a 3400, (with 3x00 top end), but the quench height will only be .020? as opposed to .040 quench, with .060 head gaskets?

thanks
cody

correct, which is not ideal and most likely will cause problems...


you could just have .020" shaved off the tops of your pistons and be good to go.. smaler dish, thinner gaskets should bump the compression as well.


i have .045" compressed ROL gaskets but the 3500 pistons don't pop out as far as the 3400, iirc it's .010" so my quench is .035"

[IsaacHayes]

It's a complete 3100 in there. The 3x00 swap is common for J-bodies. The thread he posted shows he then upgraded it with different head gaskets, and 3400 lower manifold and 3500 upper manifold and a 4.3l 70mm throttle body.

[rsisbetter]

this is all very interesting, since, i have a 3400, and would like to do a 3500 top end swap. in which i would like thinner head gaskets, (3.4 iron head, .040) as opposed to the stock .060. So what i have gathered from this thread, is that, a 3.4 iron head, .040 head gasket can be used on a 3400, (with 3x00 top end), but the quench height will only be .020? as opposed to .040 quench, with .060 head gaskets?

thanks
cody