Machining cylinders/heads for higher compression?

[Patronus]

Bruce Anderson wrote that you can machine away the cylinder head's sealing surface up to 1mm in order to improve compression. It sounds tempting to me. I know that the chain housings will also have to be machined, etc., which is no problem.

Before I ask my three questions, here's my background: My car is a 2.7S with CIS, we have 93 octane fuel in the interior of South Africa (about 1km above sea level) and 97 octane at the coast.

1. Has anybody on this board machined his CIS cylinder heads to increase the compression? If so, please tell me whether it was worth the effort. Please tell me everything, good and bad.

2. Why machine the heads and not the cylinders? It's so much easier. I'm sure there must be a reason?

3. How much hp can be gained by doing it?

(If there's a post/article discussing this point, please point me to it. I did a quick search, without success.)

Thanks

[jluetjen]

For the most part, HP comes from the cams - not from the CR. You'll most likely get an increase in torque from machining the heads, but it won't be huge. I believe that you'll get about 1/2 of a point increase in CR by taking 1mm off of the heads. So your CR will go from 8.5:1 to ~9.0:1 or about a 6% increase. As a result, your torque will increase across the rev range. Your HP may go up by about 2% (3.5 HP) as a result of the increase in torque at the peak HP engine speed of 5700 RPM. This is assuming nothing else is changed.

While you have your heads off, you could also change to 964 cams for example and the new package will most likely make about 185-190 HP. I'm guessing you will also feel a "seat of the pants" change in the torque.

BTW - I'm basing my estimated HP numbers on calculations that I've done on the BMEP for different engine combinations and the affect of cam's on where the HP will peak. Basically, 964 cams seem to generate about the same BMEP as the stock cams, but at a higher engine speed of 6100 RPM rather then 5700 RPM. This will result in the HP increase. Increasing the CR should fill in the bottom a little bit to make up for the "peakier" engine.

Those are my guesses.

[911pcars]

Patronus,
Go for as much compression ratio you can get as it's relatively cheap power although the proper pistons are the way to go for substantial changes. The 9.5:1 range is what most builders here recommend for our pump gas (91 octane). A more radical cam could tolerate a higher C.R. due to the effectively lower compression pressure. CIS can tolerate more lift but not as much overlap.

I would also suggest equalizing all the cylinder heights to make sure the clamping force is the same across the entire cylinder head/cam housing plane. A little bit of C.R. can be gained here as well from removing from the cyl. head surface although this is more a case of making sure everything is the same.

Also suggest you double check valve-to-piston clearance to make sure you have a minimum clearance, especially if you're thinking about changing to higher-lift cams. Due to the removal of cylinder/cyl. head material and depending on your engine pieces, the timing chains will have more slack. Excessive slack will bring the tensioner idler arm (driver side esp.) very close to the cam box housing. Oversize idler arms can be made to compensate. This is briefly described in B. Anderson's book. You do have it don't you?

Hope this helps,
Sherwood Lee
http://members.rennlist.org/911pcars
www.seinesystems.com