Expert help needed 2.5

[911quest]

I have all of the parts to assemble a 2.5 the pistons are 12.1 compression this is way to high for street I know, but is there any way to lower the compression ratio of this pistons cly. base gaskets or is it possible to rebush the rods to make the wrist pin lower then stock this engine is going to be a street/DE engine it will be in a 914/6 any help??

[dtw]

Yes, both are common options for lowering the c/r.

[Steve@Rennsport]

Hi:

Depending on the design of your pistons, its might be very difficult to get the CR down into pump gas range using single-ignition or not: thats a big difference.

I would NOT reduce CR by stacking base gaskets for reduced deck height as that really makes these engines more detonation prone. Maintain .040 deck and see how much you can machine the piston domes.

Offset boring and rebushing the rod's small ends will help.

[kenikh]

Let's take a step back. What kind of pistons are they? Did you get the 12:1 by measuring them or is this number off of the box? If off the box, I would highly recommend measuring them before assuming your CR is too high. If it really is 12:1, you should be able to take some material off of the top of the piston to reduce CR.

[911quest]

Thanks for the info I wanted to stay away from the base gasket idea, the Pistons are JE's I haven't personally measured them but the parts came with a 914 I bought from a guy here in town that works on only Porsche, and builds alot of engines the 914/6 was his race car and he raced it for several years so I have no reason to doubt that they a 12.1

[911quest]

[kenikh]

They look it. You should be able to cut some CR out of them, but how much depends upon crown thickness.

[911quest]

When they machine the crown do they just remove material from between the valve pockets?

[Steve@Rennsport]

JE has several forging designs and the dome thicknesses do vary.

I would kindly suggest that you have someone who is intimately familiar with these JE pistons or call Jerry @ JE to find out precisely how much can be safely removed from the domes.

Minimum running thicknesses will vary according to your final CR and RPM range.

[911quest]

Thanks Steve your help is greatly appreciated

[Steve@Rennsport]

Originally posted by 911quest
Thanks Steve your help is greatly appreciated

My pleasure,...

[ChrisBennet]

Originally posted by 911quest
Thanks for the info I wanted to stay away from the base gasket idea, the Pistons are JE's I haven't personally measured them but the parts came with a 914 I bought from a guy here in town that works on only Porsche, and builds alot of engines the 914/6 was his race car and he raced it for several years so I have no reason to doubt that they a 12.1

When it comes to engine measurements my motto is "Trust no one, measure everything yourself." That goes double for JE pistons.
-Chris

[911quest]

Chris your right and I agree with you 100% I just wanted to know my best options before the situation arises I think my best option is to find some 90mm RS pistons cause I have a 70.4 crank and all the parts to make a 2.7