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

[rvanderpyl]

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.

[Porschekid962]

which particular were you planning on using?

[rvanderpyl]

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.

[kenikh]

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

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

[rvanderpyl]

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

[rvanderpyl]

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

[kenikh]

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

[rvanderpyl]

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

[kenikh]

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.

[rvanderpyl]

I used .040

And thanks for that calculator link, very useful.

[Henry Schmidt]

Originally posted by rvanderpyl
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..............

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.

[rvanderpyl]

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