2.7 PC on 66mm crank -> 7:0 CR, how about the other way?
 

According to BA if you take 2.7 Cylinders and Pistons (8.5:1) and put them on a 66m Crank your get a 2.5 with just slightly under 7:0 compression

What happens if you go the other way? Add 1.5?

So take a set of 2.7 cylinders, and pistons designed to have 9.5:1 CR on a 66mm crank, now use those with a 70.4mm crank. 11:1 CR?

Any clearance issues? Would twin plug be good enough to run this on 91 gas.

ok but...
 

which particular were you planning on using?

I ordered a set of 9.5:1 CR pistons for a shortstroke 2.5. But now I just bought a 2.4 motor so I will have a 70.4 crank and rods as well. So I am wondering if I could use the pistons on the 70.4 crank to make a 2.7 liter instead, but with a high compression ratio.

I don't beleive the relationship is linear like that. You need to use a calculator like this:

http://www.dunegoon.org/compression.html

I may have found my answer, again from BA, he mentions putting the pistons from a 2.2s into a 2.4 to increase the compression. In this case he says that the longer stroke should increase the CR by .55 above what it was in the 2.2 (9.8:1) to make 10.35:1, but when it is measured it actually is 9.66:1. So it looks like I might be able to do this. I'll have to measure everything up when I get the P/C

Using that calculater it appears I would end up with 10.05:1 which would probably require dual plugs considering our premium gas is only 91 octane around here. But still doable.

Thanks

Make sure to adjust the timing appropriately, because that is pushing it on single plug.

No I think I will go dual plug, I'd rather be on the safe side.

You'll be money with twin plug. What deck height are you running? You can go higher than 10:1 with twin plug, so if you are calculating CR at .050", you could probably get it a little higher with an .040" deck height, which is about as close as you want to get.

I used .040

And thanks for that calculator link, very useful.

Re: 2.7 PC on 66mm crank -> 7:0 CR, how about the other way?
 

I have read and heard this comment for many years and it is just an error. Bruce is very knowledgeable ( especially about which tool kit or radio belongs in what car) so I will assume it is just a typo.
If you put 8.5:1 pistons from a 70.4 mm crank on a 66 mm you will loose about .5 points of compression not 1.5 points.
If you reverse the process and put 8.5:1 pistons from a 66 mm crank on a 70.4 crank you will get an increase of about .5 points of compression.

Do the math and you'll see for yourself.

Thanks Henry, didn't really make any sense to me, especially when he mentions putting 2.2S pistons in a 2.4 and getting .5 point of an increase over on a 66mm crank.

By the way can you PM me what you would charge to rebuild a 2.4T MFI pump to 2.7RS spec's?

Thanks