3.6 to 3.8 - combustion chamber size vs larger cylinder

[stanglife]

Hey guys - I'm not on the same budget as some of you but I do have champagne dreams so I do want to buy nicer parts.. Anyway, I sometimes need to buy parts that need a little 'help" to work together. This is one of those cases, I think. I have a set of 3.6 heads that have been completely rebuilt and I also have a set of cylinders for a 3.8. The picture attached is from the cylinder sitting on the head and looking through to see the combustion chamber. You can see that the cylinder is a little larger than the combustion chamber - the little shiny edge there is the flat surface of the head. Do I need to just have the head machined/beveled slightly at that edge or can I roll as-is? I know I'd need to check the piston/deck height. Ideas?

[stanglife]

[stanglife]

[stanglife]

Now you guys get to enjoy my MS paint skills.

[targa72e]

I believe many pistons are designed to accommodate that issue with there design like one in picture.





john

[stanglife]

I was hoping that was the case but I've had a hard time seeing any guidelines about this. So is it a given that oversized pistons for these applications retain the appropriate dome shape but the increase in diameter is purposely flat as pictured above?

[dannobee]

Not an issue at all. It's called "quench area" and it's actually desirable.

[stanglife]

Originally Posted by dannobee

Not an issue at all. It's called "quench area" and it's actually desirable.

You mean the flat area between the oversized piston and the head? That space would not be there with a stock cylinder and piston is why Im curious

[dannobee]

We've learned a few things about combustion in the 50+ years since these were designed.

https://www.enginelabs.com/engine-tech/quench-unlocking-performance-squished-between-the-piston-and-head/

[JoeMag]

I ran 964 heads shaved about 0.010” on 3.8 Mahle rsr 11.7 pistons ( look very similar to ones in above pic) with no issues. 0.040” deck height.

[stanglife]

thx guys. Finally getting all sorted to send off for balancing...and just getting my head on straight. I havent measured static compression but can extrapolate from common combustion chamber volume and dome spec. I'd of course double check but it's coming out at about 11.5:1. I have a LITTLE concern that this will be problematic in summer heat on 93. Thoughts?

[winders]

That compression is too high for 93 octane pump gas.....

[safe]

Originally Posted by stanglife

thx guys. Finally getting all sorted to send off for balancing...and just getting my head on straight. I havent measured static compression but can extrapolate from common combustion chamber volume and dome spec. I'd of course double check but it's coming out at about 11.5:1. I have a LITTLE concern that this will be problematic in summer heat on 93. Thoughts?

They came stock with 11.3:1 didn't they? Assuming they measured the same way.

[winders]

Originally Posted by safe

They came stock with 11.3:1 didn't they? Assuming they measured the same way.

That was the claim....but 10.6:1 to 10.8:1 is the actual I have heard from engine builders.

[stanglife]

So the minimum off the shelf JE piston is 11.5 and the lowest compression on an off the shelf Mahle piston is 11.4. This is assuming 90cc chamber volume. I cant imagine every street-driven car with aftermarket pistons is running race gas. Guess I need to get to measuring my heads.

[stownsen914]

Correct that it's worth checking the volume of the combustion chambers. With twin plugs the volume can easily be greater than 90cc, and a couple cc make a significant difference in the CR.

[Aircooled4evr]

Originally Posted by dannobee

We've learned a few things about combustion in the 50+ years since these were designed.

https://www.enginelabs.com/engine-tech/quench-unlocking-performance-squished-between-the-piston-and-head/

Hi Dannobee.
This would come into play, if one were to build an engine with original 964 heads, and 100mm CP pistons (11.5: 1 cc) ... Hmmm ..
Since they must be specially ordered as they are for 70.4mm strokes, and 22mm cross spin. So one has to look a little at the piston crown as well. Exciting...

[Sboxin]

Originally Posted by targa72e

I believe many pistons are designed to accommodate that issue with there design like one in picture.





john

Another bit of information - - - we built a 3.8L with 964 heads and rebuilt cylinders by EBS and JE 102 pistons that are 12.5CR ( not measured) and Webcams 20/21 cams . . . the engine was then tuned by Steve Wong on a dyno and chip built for 100 octane fuel (to cool the heads for race car) . . . Steve could have tuned it for 93 if we asked, but chose to go with the 100 and are willing to pay the price for the expensive gas . . . Steve tuned for best HP/torque along with durability also as a goal . . . after 8 years on the track the heads/pistons/cylinders are doing just fine (with one minor rebuild of rod bearings and cams/rockers) . . . Steve advised that the 964 heads were the weak link and if pushed to far would fail (he had this experience from a client that pushed him to over tune an engine) . . . attached a photo of our new (8 years ago) JE 102 pistons with the proper crown design (IMHO of course) . . .

Regards,
Roy T

JE 102 12.5 Piston and EBS rebuilt cylinder -



964 cylinder head new valves etc. . . .

[JoeMag]

You can run 11.5 on pump gas, just can not try to get every ounce of power out of it with heavy advance. Some may recall Geoffrey Ring who was tuner and MoTeC dealer... He had 3.8 race motor 12.0:1 and ran it on pump gas. He tuned my motor (long time ago) on the mahle domed rsr 3.8's with 964 heads on pump gas - they're around 11.5:1.

[JoeMag]

Stanglife - are your heads 964 or 993?