9.8:1 to 10.5:1 comp. Can I use CIS?
 

I am rebuilding a 78 euro motor tht has 9.8:1 compression and the big port intake CIS. Question is, can I keep using the CIS system if I go to 10.5 compression pistons, and, will this do much for a horsepower gain?

Thanks,

Jack.

The 78 euros had 8.5:1 just like the US cars. Although CIS will work fine, you won't be able to use 10.5:1 pistons unless you're going to twin plug it. 9.8:1 is as high as you can go with single plug. And you'll also want to upgrade the fan to a later SC fan or a Carrera fan. I have a 78 SC and had considerable detonation after rebuilding using 9.8:1 P/Cs, even in cool weather with a Carrera cooler in the fender and premium fuel. Once the new fan went in, the detonation stopped. The extra airflow keeps the heads that much cooler. Thanks to Grady Clay for that recommendation; I didn't even know the fans were different before he told me.

Oh, and the motor makes 200hp at the wheels, so yes you will have a hp gain with more compression. I also have SSIs and 964 cams.

I think the most important question is what pistons are you using? A CIS or Motronic piston with stock crown profile should work fine at 10.5:1. the 3.2 Carrera was 10-.3:1.

If you are going to another piston like JE or non-CIS Mahle this CR would be too high for single plug. The reason is that the piston shape will divide the combustion chamber preventing flame front travel to the non-plug side of the head. This is where detonation can occur.

I think you will notice a jump in torque from the stock 8.5:1 to the new level. If the motor was rebuilt previously to 9.8:1 I don't think you would notice a significant difference to 10.5:1. I would personally, spend the money on new cams instead of a half point CR increase.

Jack: rpiper said it! Around 10:1 CR is max for single plug.
If you find KS Alusil 95mm P/C's in your engine, consider going with 98mm Mahle P/C's for a real gain. They just bolt on without machining.
With 964-cams, SSI's and bigger P/C's, you'll be getting about 230 horses even with CIS.

I think that a true 9.8:1 is max for single plug and 94 octane (better than we have here now) gas. Realistically, I think limiting your compression to 9.5:1 would be a smarter move in light of todays fuels.

The factory stated compression ratios are not useful indicators of what will work. CC a few 911 motors and you''ll find that the compression is less than the factory figures.For example a 964 motor has a stated compression ratio of 11.3:1. If you actually measure (CC) one you get ~ 10.5:1.

-Chris

Don´t you have a lot of ethanol in the US?
Here in Sweden we are really getting a lot of ethanol pumps.
Ethanol has a lot higher octane than petrol, wouldn´t you be able to use higher CR than 10:1?

Thankyou all for your advice. I thought I had 9.8 comp pistons because it is a euro motor, not the early 8.5, which makes me confused but nonetheless they need to be replaced anyway.

I like the idea of going to 98mm p/c's, where would I find these, or what piston set would you reccomend if i'm going to keep the 95mm cylinders?

The 78/9 3.0 engines all had 8.5:1. This includes, US, CA, and RoW motors. It wasn't until the 80s that the RoW motors had higher compression than the US. This was based on emissions.

Our host has a variety of 95 and 98mm Piston and cylider options. Most engine builders can get these parts as well.

One thing that was neglected in my answer was the question about retaining the CIS. The CIS system meters fuel via the air sensor plate. the more air going into the motor, the more is pushes on the sensor plate, the more fuel gets delivered through the injectors.

the CIS system will be fine if you bump the compression since this just gives more bang for your buck, so to speak. Remember, the engine is a complete system. If you increase compression, usually there is a cam upgrade as well. Depending on the cam upgrade this might require an intake upgrade.

The CIS system is not very tolerant when it comes to changing cams. Usually, high compression engines use a very agressive cam, w.r.t. lift, duration and overlap. This helps to allow the more air into the cylinders to take advantage of the CR and high rpm power. Now, cams that have a high lift, long duration and suitable overlap will not work with the CIS. these types of cams have a lot of reversion at low rpms. This reversion will usually cause confusion at the sensor plate and poor running at low rpms. Usually, this means a 964 profile is about as agressive as one can get and still retain drivability with the CIS.

I would suggest reading Wayne's Engine book for more information. Once you have the basics, there are lots of threads that can be uncovered using the search button in the upper right of this page.