3400 engine, 3500 heads, 10:1 compression?

[woody90gtz]

I did some searching and found some approaches, but not exactly what I'm looking for.

http://60degreev6.com/forum/showthread.php/45554-CR-Question-3400-w-3500-heads?highlight=3500+hybrid

Ideally, I'd like to throw a pair of 3500 heads on my 3400 with the end result being in the 9.5-10.5:1 range. 10:1 would be perfect!

I did this with my old SBC a few years ago and really woke up the engine with a head swap & compression bump by using a .015" head gasket. Is anything similar available for this situation, or does the quench get screwed up?

[SappySE107]

You can't run a thinner gasket with the stock pistons, so no, not possible. You have to change the pistons to get the compression you want.

[3400-95-Modified]

He was talking about shaving the heads once before to make the combustion chambers smaller to up the compression, and I thought this wouldnt work because you would have to shave off way too much. Was I correct on that one? I'm not sure how much the CC would have to be changed to bring it to 10:1, but I'm only assuming you'll end up with major intake alignment issues later, or have to mill that to shit as well.

[Superdave]

That's a lot of work for ~5HP.

[SappySE107]

Yeah you can't take that much off the chambers. 32.4cc to 28.6cc just brings you back to stock. No way. Id go with 10.5:1 minimum for the effort.

[ForcedFirebird]

He was talking about shaving the heads once before to make the combustion chambers smaller to up the compression, and I thought this wouldnt work because you would have to shave off way too much. Was I correct on that one? I'm not sure how much the CC would have to be changed to bring it to 10:1, but I'm only assuming you'll end up with major intake alignment issues later, or have to mill that to shit as well.

The intake and heads would have to be milled, and the pistons for valve clearance.

As a rule, static compression is not as beneficial as better flow. The 3500 heads with less compression on a 3400 bottom end will out perform the higher compression with a 3400 and 3400 heads. Good case in point, I gained almost 30%hp at the wheels when dropping my compression .7:1 with better flowing heads.

[woody90gtz]

Ok, well this is not the news I wanted to hear, but it is what I needed to know!

I hate to drop compression since I know compression is one key to efficiency, but if the LX9 head flow makes the swap worthwhile even considering the compression drop, that is encouraging.

Does anyone have before/after dyno numbers for a stock 3400 vs a 3400/3500 top end?

[Superdave]

I have dyno numbers of my 3500 with 10.4:1 static VS 9.8:1.. It made more power at 9.8:1.. lol

[mfuller]

I have dyno numbers of my 3500 with 10.4:1 static VS 9.8:1.. It made more power at 9.8:1.. lol

Trying to wrap my head around this one......can you elaborate/spectulate as to why it made more power with less compression? All other factors being equal, I'd expect a smidge more power with higher compression.

[SappySE107]

Same quench?

[bob442]

So in a N/A application, stock compression E.G stock pistons is the way to go now? LOL, these 60dv6 defi basic engine building rules still!

[3400-95-Modified]

What they are saying is not that less compression makes more power... Its more the point that when your changing from a lower flowing head to a better flowing one and loosing 0.35 points of compression its NOT worth trying to get that back for how little the power increases.

With Daves setup 10.4 to 9.8... I'm not sure what was changed there, but that may have been something else that helped the efficiency of the motor in turn freeing up more power.

High compression will make power, but if you get there via an inefficient way then you will not see good gains.

All other factors being equal, I'd expect a smidge more power with higher compression.

I agree with this, but with what he would have to do to go from 9.15:1 to 10:1 without changing pistons then you would compromise something else that would negate any power you will gain from the compression change.

[woody90gtz]

Interesting.

Well I'm not sure if I'll do a 3500 top end or not, but I guess if I do I'll just throw it on and not do anything special about compression. Thanks for the input guys.

I still kinda want to see if I can get a stock long block 3400 in the 13s!

[RedBird]

not the same quench, he was using 3.4 ironhead HG's @ 10.4:1 and stock 3500 HG's @ 9.8:1

[Superdave]

Different head gaskets. I had .035 quench before and am down to stock now. The actual power difference was minimal on the dyno but my trap speeds went from 102 to 108. Of course lots of other variables involved there. My dynamic compression is like 8.5:1


Yes a change from 9.1:1 to 10:1 static should increase power by a little but in the end the increased port flow of the 3500 top end is going to gain you far more.