Compression and Quench

[CTX-SLPR]

Howdy,

So on my 4.1 I ordered custom Diamond pistons and though a mixup with my machine shop they are a bit too short and sit down in the bore more than I'd like using stock 5.960in rods. Since the motor is coming out and at least partially apart this winter I've been debating getting a set of 6.0in rods and having the pin's reamed out to the Buick pin size.

This will get me my quench back (-0.030 to +0.010) and up my compression from 7.9 to 8.3. The thing is I'd need to buy the rods, get the crank rebalanced, and go through reassembling the whole engine again. The compression I'm happy to get back but the quench is really what I'm after. But all that expense ($500 for the rods, $300 for the machine work on the rods and crank at least) and trouble; is it worth it? While getting new pistons is only marginally more expensive it does still require a crank rebalancing.

Opinions please?

 

[novaderrik]

can you just deck the block and run thinner head gaskets to make up the difference? that would be cheaper than getting the new rods and getting everything rebalanced.
you'd need to machine the intake to match the heads and get shorter pushrods, tho.

 

[bison]

So your machine shop measured the deck height incorrectly? Tell them next time to measure twice and cut once. You should have left it up to them to order the pistons since they will likely say they werent the ones who ordered them wrong. Your quench still wont be bad with the pistons .030 in the hole but id still want more CR than 8.3:1. Unless you are going to run it hard it wont really matter. You will lose a little bit of scavenging on the ex stroke with the low CR. Get the ex valve close asap. Advance the cam if needed.

 

[CTX-SLPR]

The deck isn't getting any thinner, it's a 84 4.1L block which has enough deck issues to begin with.
I think I told George I wanted the pistons 0.015in in the hole and an 8.3 compression but we ended up with something mixed up. He did order the pistons for me though Diamond and did all the machine work short of redoing the radius on the early Eagle crank's flexplate flange. It was originally setup for stock rods but was rebalanced and ran with 5.960in K1's already. This is a heavy car (~4000lbs) so more compression would be nice but I'm really looking for more low rpm chamber motion with the tighter quench so theoretically I'll have less trouble with detonation with low rpm's and getting into the boost to get up a hill from a stop.

Right now I'm running a Comp 212/212 roller straight up. More advice is fine.

 

[novaderrik]

how about offset grinding the crank to a smaller diameter to get a little bit more stroke out of it? you'd need to rebalance, but wouldn't need any new parts.
this would make the engine a little bigger and help out with torque just a little bit.
might be able to find some CR with thinner head gaskets and by decking the heads a little bit.. unless they've already been decked.

 

[Chuck Leeper]

Offset bush the rods...

 

[CTX-SLPR]

how about offset grinding the crank to a smaller diameter to get a little bit more stroke out of it? you'd need to rebalance, but wouldn't need any new parts.
this would make the engine a little bigger and help out with torque just a little bit.
might be able to find some CR with thinner head gaskets and by decking the heads a little bit.. unless they've already been decked.

Don't want to turn down the crank to 0.020 under, I think I'd just live with the quench rather than do that. Thinner gaskets would be fine with me but I'm already running FelPro 1000's so I'm not sure what is thinner without going to something hugely expensive like a custom gasket. Remember I have a 3.995in bore. Heads are Champion CNC Irons.

Offset bush the rods...

Cost associated with this? I like the idea as it's 6 rods worth of machine work vs 6 new rods (offset some by selling the old ones) and balancing machine work. I am running floating pins though.

Thanks,

 

[it's just a six]

I always say, if you can get close to a zero deck the better. Unless the heads will not give you any quench.
Not sure if Buick 3.8 or 4.1 can run a zero deck?

Part of the reason why the newer LS engines can run more compression is because they have a positive deck height.

I run a positve deck of .002-.004 on my inline 250 L6 & ran pump gas w/12.0:1 compression on 91 octane,normally aspirated,ran great w/no detonation & the cranking compression was 220-230 psi..

 

[CTX-SLPR]

I always say, if you can get close to a zero deck the better. Unless the heads will not give you any quench.
Not sure if Buick 3.8 or 4.1 can run a zero deck?

Any engine can run 0 deck with proper measurements. Mine wasn't supposed to be when I was specing out the pistons but now I'm not so concerned. If you do the math on what I was proposing with the 6.0in rods, I'd be 0.010in out of the hole with the pistons.

 

[dr_frankenstein]

its my opinion, that quench isnt as important in a turbo motor as a NA motor.

theres plenty of turbulence in the intake to atomize everything completly.

I had tried both 0.0 deck and -.050 on my turbo SD Pontiac, and I have not noticed any major change in performace.

I found the most improvement, by focusing the compression dutys to the head. the smaller the head chamber had better effect on performance, then a dome or 0.0 to + piston to deck height.

in my crazy mind, I have theorys that its easier to push a slug thats slightly down the hole than to try and push one thats slightly out, as you dont have combustion gasses loading the crown of the piston from all angles rather than just down.

just kinda a thought I had.

Plus, have you ever measured a Stock 3.8 piston to deck height? there all over the place usually.

A.j.

 

[BlownV6]

its my opinion, that quench isnt as important in a turbo motor as a NA motor.

theres plenty of turbulence in the intake to atomize everything completly.

I had tried both 0.0 deck and -.050 on my turbo SD Pontiac, and I have not noticed any major change in performace.

I found the most improvement, by focusing the compression dutys to the head. the smaller the head chamber had better effect on performance, then a dome or 0.0 to + piston to deck height.

in my crazy mind, I have theorys that its easier to push a slug thats slightly down the hole than to try and push one thats slightly out, as you dont have combustion gasses loading the crown of the piston from all angles rather than just down.

just kinda a thought I had.

Plus, have you ever measured a Stock 3.8 piston to deck height? there all over the place usually.

A.j.

No expert here but I will throw out my opinion. I agree with the above quote. Im using pistons that were ordered for 6.325 long rods and put the top of the piston down in the hole .050" , now Im using 6.200 rods and cut the top of the pistons .o50" so basically Im at .225" down in the hole trying to lower the compression. Unless your looking for a slightly more efficent burn because your after beter gas mileage I say leave it alone. Any lack of power lost by lower compression can be easily made up with a pound+- more boost or a bit more timing. Just my 2 cents!! Mike

 

[bison]

in my crazy mind, I have theorys that its easier to push a slug thats slightly down the hole than to try and push one thats slightly out, as you dont have combustion gasses loading the crown of the piston from all angles rather than just down.

A.j.

Your forgetting about the ex stroke. Thats where added CR helps you the most. You would have to run a really small <20cc dish to get any appreciable CR on a stock stroke engine if the pistons are sitting down in the hole.

 

[bison]

Any lack of power lost by lower compression can be easily made up with a pound+- more boost or a bit more timing. Just my 2 cents!! Mike

Not if you plan on making any appreciable power. You will have the EGR effect so bad the engine wont pick up chit as you turn up the boost.

 

[BlownV6]

Not if you plan on making any appreciable power. You will have the EGR effect so bad the engine wont pick up chit as you turn up the boost.

I respectfully disagree. I believe that 2 basically identical engines except compression have different potential HP!! The one with less compression is capable of receiving much more boost than a higher comp engine , so with more boost comes more fuel , more fuel will make more power. Higher compression limits the boost potential and the power obtainable without lifting the heads. Thats the way I see it!! Mike

 

[bison]

I respectfully disagree. I believe that 2 basically identical engines except compression have different potential HP!! The one with less compression is capable of receiving much more boost than a higher comp engine , so with more boost comes more fuel , more fuel will make more power. Higher compression limits the boost potential and the power obtainable without lifting the heads. Thats the way I see it!! Mike

So what is the boost potential of my 9.3:1 hyd cammed engine? Its a lot more than ill ever run i know that for sure. I have it set to run under 30psi even though with the correct fuel and tune it could run over 30psi . Why not run 6:1? If your trying to make huge power your limitation is not lifting the heads. Plenty of 10:1 30+psi gasoline engines out there. Try running high backpressure with a low CR and see how much power drops off per psi of boost when you start turning it up. There is a sweet spot for a hyd cammed 3 bolt turbo application thats trying to extract the most out of the combo and from what ive experienced its over 9:1. It may actually be close to 10:1 with the boost under 30psi. I wonder what Jason White's CR is in his TSM engine? We could only guess at that. Id bet a bunch its not 8:1.

 

[it's just a six]

So Bison,
what is your take on having a zero to positive deck giving you a better quench?

I think the LS guys dont put there pistons down the hole when they turbo there rides.

 

[BlownV6]

So what is the boost potential of my 9.3:1 hyd cammed engine? Its a lot more than ill ever run i know that for sure. I have it set to run under 30psi even though with the correct fuel and tune it could run over 30psi . Why not run 6:1? If your trying to make huge power your limitation is not lifting the heads. Plenty of 10:1 30+psi gasoline engines out there. Try running high backpressure with a low CR and see how much power drops off per psi of boost when you start turning it up. There is a sweet spot for a hyd cammed 3 bolt turbo application thats trying to extract the most out of the combo and from what ive experienced its over 9:1. It may actually be close to 10:1 with the boost under 30psi. I wonder what Jason White's CR is in his TSM engine? We could only guess at that. Id bet a bunch its not 8:1.

Let me simply say that I consider you an expert compared to me , in that I have never owned or tuned any type of turbo engine. However I do still stand on the simple fact that the original question was, should he do back flips and empty his wallet to change his compression ratio from 7.9 to 8.3. I say no!! Just add a bit more boost or timing to make up the difference if you would even know the difference?? I dont think he is limited by rules to a certain turbo , mabe his turbo is such that it would not work , I dont know the answer I simply gave my opinion!! Mike

 

[it's just a six]

If you look @ the first post,he is not worried about the compression weather he should run 7.9 or 8.3, he is concerned about how his piston is down the hole & lost his quench.

 

[dr_frankenstein]

Your forgetting about the ex stroke. Thats where added CR helps you the most. You would have to run a really small <20cc dish to get any appreciable CR on a stock stroke engine if the pistons are sitting down in the hole.

absolutly and I agree, compression needs to be correct no matter where the location of the slug in the bore is. I myself run 9.2/3:1 compression in my motors as boost isnt always a friendly way to make up for a lack of compression, example being the Draw-thru crowd.

I read exactly what your saying Brian, were just slightly caught up in the details as were blower guys and not subjected to the same problems that you are.

So I think were all correct based on application. :biggrin:

LMAO.... now this was fun. we need more serious engine questions like this.

A.j.

 

[bison]

Let me simply say that I consider you an expert compared to me , in that I have never owned or tuned any type of turbo engine. However I do still stand on the simple fact that the original question was, should he do back flips and empty his wallet to change his compression ratio from 7.9 to 8.3. I say no!! Just add a bit more boost or timing to make up the difference if you would even know the difference?? I dont think he is limited by rules to a certain turbo , mabe his turbo is such that it would not work , I dont know the answer I simply gave my opinion!! Mike

I agree that changing the CR in his instance wont do much of anything. Now if he was making 800hp and running a 3 bolt at 8:1 and went to over 9:1 he would see a big difference in power.