Bump compression 2.4cis?

bgyglfr · 09-09-2014, 04:47 AM

I am getting my engine back from the machine ship this week. My plan is to build the stock 2.4cis motor for my 73 up but add some extra HP where possible while keeping the original fuel injection.

I was thinking 2.7s cams since I have some lying around. Simple enough upgrade.

Would it help much to bump the compression a bit too? I have a set of NOS stock Mahle P&Cs for this car. I was considering running a set of 2.2E or S pistons in these cylinders. Is it worth the little bit of extra HP or should I just keep it simple and run the stock P&Cs?

I know the original pistons have the swirl top but I've heard it doesn't make a lot of difference as long as the cams have limited overlap for the CIS.

If I did go more compression, I might consider grinding the cams to SC profile as well.

E Sully · E Sully's Garage

From what I've read, the 2.7 S cams are as far as you can use with the 2.4 cis. No SC profile.
2.7S CIS, Left 911 105 143 00, Right 911 105 144 00
The original engine was 8.0-1, but the 2.7S was 8.5-1, so I don't see a problem with a small increase in compression using the later S cams. Others with more experience would give better information though.

BURN-BROS · 09-09-2014, 07:19 AM

Running 2.2 pistons will result in an increase over their original compression.

A 2.2S piston is 9.8/1....If you run it in a 2.4 it is 10.3/1, far too high for your build.

bgyglfr · 08:49 AM

I agree. I was hoping to get to around 9-9.5:1. I guess what I'm getting at is the stock pistons are 8:1 and I already have a NOS set. Is it even worth trying to bump it to 9:1 or better? My understanding is that higher compression will allow a more aggressive cams (by cis standards) like and SC grind. If the power is only going to be slightly better than I might as well stay with stock.

Stock build 140 hp
Add 2.7S cams (easy no brainer) HP increase somewhere between 140 and 175 that the 2.7 makes. 150 maybe?
Add higher compression. Any HP gain from above?
Add SC grind to higher compression. Would love to see something in the 170-180 range. Possible?

73T
68L

Tom F2 · 09-11-2014, 06:22 AM

Best bump in power for a 2.4 CIS motor, IMHO, is to have the case modified for 2.7 pistons and cylinders, then go with the 2.7 CIS pistons and cylinders. Used p/c sets pop up from time to time, so you might save a few bucks that way. This motor will not have a great top end, the 2.4 ports are just too small, but it will have lots of torque down low, so it's really fun to drive. I doubt that you'll feel a difference with the bump in compression ratio that you are talking about. And you are giving up the CIS piston crown, which is actually a good design for its purpose.

bgyglfr · 09-13-2014, 10:04 AM

Tom,
Thanks for the input. I actually have a set of 2.7Cis P&Cs but I don't want to alter the case. It's matching numbers to the car. I just picked up the case, crank and heads from the machine shop yesterday. Ready to assemble. I think I will go ahead with the stock P&Cs and run the 2.7 cam. Looking forward to getting it together.