How to Increase CR W/out Replacing Pistons?

[fast0911]

I would like increase my CR from 9.5 to 10.5 with out replacing my 9.5:1 JE Pistons.
Of course I will have to twin plug the motor plus all that goes along with doing this. Before I contemplate the idea of I would like to know if I can reuse my pistons if at all possible.....

My understanding is that there is a maximum amount one can machine the heads because if it is overdone it will mess up the timing since the cams will now be bit closer to the centerline of the crank. This makes perfect sense... No Problem there.

However, if one can machine the heads but maintain the same centerline distance between Cams and Crank will this be OK to do???

Again, I am novice so it may be a stupid question for those of you in the know to contemplate, but I just needed to ask anyway.

Here is my thought:

Is it possible to remove about .5mm off the heads to reduce the Volume in the combustion chamber to increase CR (This will of course bring the cams closer to the crank centerline). But what if one was to add .5mm of gaskets between the Engine Case and the Cylinders to still maintain the same centerline distance between the cams & crank as before...

Engine config:
3.2L Short Stroke
DC-40 Cams
46mm PMO
98mm JE Pistons

Thanks in advance for any responses !!

GM

[Steve@Rennsport]

Well sir,......there is no way to raise the CR 1 point without new pistons.

You have only so much room to deal with deck height and you will increase chamber volumes by twin-plugging (necessary for anything over 9.8:1 on pump gas.

[rvanderpyl]

How much could you pick up by machining heads and getting the piston end of the rod bearings replaced and bored off centre?

[fast0911]

Steve,

Thank you for the response... I was afraid of this but wanted to confirm it before contemplate the idea of twin plugging the motor..... The cost of new pistons was not part of my budget. If I move ahead I wonder if there is a market for lightly used JE Pistons?

Thanks again,

GM

[kenikh]

You could get a longer stroke crank, like a 74.4mm from a Carrera...

[fast0911]

Kenik,

That would get me higher CR but I would need a new crank which is probably more expensive than Cylinders..... I have a 70.4mm crank so I think the extra 4mm would probably make my CR way higher than 10.5....Also, I want to stay with the shorter stroke.

Based on Steve's comment it looks like new Cylinders are the way to go....

By the way I saw your posting pictures on the heads you have in your car..... A work of art !
How does it feel revving to 8K?

GM

[kenikh]

Originally Posted by fast0911

Kenik,

That would get me higher CR but I would need a new crank which is probably more expensive than Cylinders..... I have a 70.4mm crank so I think the extra 4mm would probably make my CR way higher than 10.5....Also, I want to stay with the shorter stroke.

Based on Steve's comment it looks like new Cylinders are the way to go....

By the way I saw your posting pictures on the heads you have in your car..... A work of art !
How does it feel revving to 8K?

GM

Steve did them! Motor isn't assembled yet, though. Still waiting for my custom Moly rods from LN Engineering.

[911pcars]

"Is it possible to remove about .5mm off the heads to reduce the Volume in the combustion chamber to increase CR (This will of course bring the cams closer to the crank centerline). But what if one was to add .5mm of gaskets between the Engine Case and the Cylinders to still maintain the same centerline distance between the cams & crank as before..."

Doing both will net you with zero change in compression ratio. However, removing .5mm from the head-to-cylinder sealing surface will bump the CR slightly. As Steve suggests, make sure you maintain a minimum deck height as well as adequate piston-to-valve clearance.

Bringing the camshaft closer to the crank via machining will cause the cam to sit off-center in the cam box opening. Removing a like amount from the cam box-to-crankcase mounting surface will re-center the cam. The above machining will also create more slack in the timing chains which will position the idler arms too close to the chain box wall. To maintain the correct geometry, install oversize idler sprockets (made by Steve as well as other Porsche machine shops).

Sherwood

[fast0911]

Sherwood,

I do see it now.... If I remove .5mm from one end but add gaskets on the other than the net effect is zero.... It is actually very obvious now that I think about it. I think I was lacking some sleep yesterday when I thought it could be done.

Thanks for your input.....

It looks like I will have to look into selling my 9.5:1, 98mm JE Pistons and getting some new ones...

GM

[ChrisBennet]

I take it you haven't CC'd your setup yet? I would do that and you may find out that your "9.5:1CR" pistons are actually 10.5:1. The last 2 sets of "9.5:1" JE's I got (for a 3.2SS I was building) CC'd to 10.5:1. This is why you should never assume anything and measure everything.
-Chris

[lucittm]

Fast0911,
Ways to increase CR without replacing pistons:

1. Cut a new top ring groove at the very top of the piston.
2. Weld (buildup) on top of the pistons and remachine.
3. Weld (buildup) inside heads and remachine dome.
4. Shave .5mm from heads and install a .5mm shim between heads and cam towers.
5. Install a turbocharger or supercharger.
6. Install longer rods.
7. Install Carrera crank (already discussed).

Can't think of any more and I would not attempt any of them on a nice engine like the one you have. Since new pistons are outside the budget, I would imagine all of the above are too.

Mark

[fast0911]

Thanks to all with the advice.....

I am going with the obvious choice and get new Pistons.....

If anyone is interested in a set of slightly used JE 98mm 9.5:1CR let me know......

George

[ChrisBennet]

Originally Posted by fast0911

Thanks to all with the advice.....

I am going with the obvious choice and get new Pistons.....

If anyone is interested in a set of slightly used JE 98mm 9.5:1CR let me know......

George

Please read my previous post.
-Chris

[lucittm]

Can you post some pictures of the pistons and maybe include the part numbers???

Thanks,
Mark

[fast0911]

Chris,

You make a very good point... I will have to check to see what my Pistons CC at exactly.
However, I think if I am off a bit I will not be above 10:1CR since I am currently single plugged and would have experienced detonation in the last 8yrs of running my engine. I will check to see what I have so I can advertise the cylinders for sale with an accurate CR....

Thanks,

George

[Stein]

You could "stretch" the rods by offset drilling pin bushing. Discribed in Wayne R. Dempsey's book.
Just make sure all are centered and have same offset.