A Question about Compression Ratio

[126coupe]

I realize their are several ways to Adj. CR. My Question is:
If my copper base gaskets are .25 MM and I want to increase my CR by reducing the thickness of these gaskets are their copper gaskets available thinner than .25MM?
I want to avoid taking material off the heads or removing material from the base of the barrel themselves.

[Carrera3.5L]

I've only seen base gaskets in .25mm, .5mm or 1.0mm thickness.

Ralph

[126coupe]

Thanks Ralph,

Removing and tearing down the 2,5L this weekend on Saturday. Come on by, I would like to pick your brain.
41 Lexington Way. Cold Beer or a Margarita awaits you.
I will put your name on the list!

[126coupe]

So My easiest option would be to have the heads fly-cut the same time I have the heads twin-plugged?

[Henry Schmidt]

On a 66 x 90 2.5 engine it would take 1cc of chamber size to change the compression .2
66 stroke
90mm bore
69 ccc chamber
1.0mm deck
compression ratio 10.26:1

change only the chamber 1cc c r= 10.46:1

As you machine the heads you change the overall dimensions of the engine. Chains get loose, chain boxes need machining, fan shrouds start to fit strangely.

The right way is to buy the compression ratio pistons you need.

[911pcars]

Changing the relationship between cylinder head and piston also changes the deck height. Thus, you should check the deck height after machining and make the appropriate +/– adjustments.

Henry is right. I don't know the history of your crankcase, cylinders and cylinder heads. However, the total amount removed from that combination must also be deleted from the chain box in order for the cam to remain centered in the chain box opening. Or work backwards to determine how much to trim the chain box.

At a certain threshold, the chain idler sprocket will end up too close to the chain box wall as a result of taking up excessive chain slack. There are solutions, but you may not have to go that far.

Sherwood

[126coupe]

Thanks! My Mahle P and C's that are coming out of my engine are low hours (40 hours) since new. I want to re-use them as long as they measure out. I was going to just re-ring them.
The Mahles are the racing 10.5:1. Should I leave the CR at 10.5:1? I want to twin plug, is it necessary to twin plug at this CR? Will I see a performance gain? I am increasing the cam from DC60 to DC80. From my readings going over 10.1:1 twin plugging helps. Thoughts? Opinions?

[911pcars]

Yep. High compression is the answer for increased torque and power (as well as fuel mileage), but only if the amount doesn't exceed the limits of fuel octane or combustion chamber design. Race fuel fixes the former. Twin plugs mitigate the latter.

With a race cam, the effective compression ratio will be somewhat lower, bringing it further down into more of a comfort zone. Maybe too low for your purpose-built engine.

Mahle pistons seem to claim a higher-than-actual CR. You should CC the combustion chamber/cylinder/piston assy. to verify the CR of your engine.

Sherwood

[126coupe]

Originally posted by 911pcars
Yep. High compression is the answer for increased torque and power (as well as fuel mileage), but only if the amount doesn't exceed the limits of fuel octane or combustion chamber design. Race fuel fixes the former. Twin plugs mitigate the latter.

With a race cam, the effective compression ratio will be somewhat lower, bringing it further down into more of a comfort zone. Maybe too low for your purpose-built engine.

Mahle pistons seem to claim a higher-than-actual CR. You should CC the combustion chamber/cylinder/piston assy. to verify the CR of your engine.

Sherwood

When I built this engine back in 8/04 I did all the CR measurements as per the Porsche Performance book. cc'ing ect. I was 9.8:1. I cant believe these pistons have this low of compression. Its quite possible that I measured dome volume incorretly. I believe you when you say Mahle exaggerates their CR. I will remeasure eveything on assembly and aim for 10.5:1-11:1