L8 APEKS

Hey guys,

I don't have any manuals in front of me, and I'm not sure if this is in them anyway! I was wondering this...when having a head rebuilt (in my case, an 88 S), and resurfaced...how much material is equivalent to how much compression?

The only car I know off the top of my head (16V 1.8L VW), I think 10 thousandths = 0.25 CR bump.

What would the numbers be for my S? Anyone know? Stock CR is 10.9 :1. I am interested in raising this slightly during the rebuild. Thanks in advance!

L8 APEKS

PS: I just became a Rennlist member a few seconds ago via PayPal. So I want this info to be my reward.

mbonner

Good for you , welcome to the good side,
Mike

Dave

Ya know, I have no idea, but I wish I did! My head was shaved .019" and it looked like it had been shaved before. I'll just call it .020-.025! Between this and my euro (or maybe just US '88) pistons, I just make sure nothing but 93 octane goes in the tank! I'll be watching this thread since I'm still trying to figure out what's under my hood.
Good call on the membership!

L8 APEKS

Thanks guys!

So hey...what's the word here? Any Porsche techs around?

L8 APEKS

Well, here's the info I am going with for my car, in case anyone was wondering.

I knew the bore, stroke, and combustion chamber volumes and gasket thickness. What I didn't know was deck height, so I made guesses here until the calculation for CR matched the factory 10.9 to 1.

From there, I changed deck height to simulate cutting down the head. It looks as if 1/10 of a mm (0.1mm) results in a change of apprx. 0.13 compression points.

In other words...
Shave 0.1mm = 11.03 : 1 CR
Shave 0.2mm = 11.15 : 1 CR
Shave 0.3mm = 11.29 : 1 CR

This may not be 100% accurate, but it should give you an idea. At any rate, I will be aiming for ~11.3 or so.

If anyone has exact numbers for deck height or anything else, please let me know. Thanks!

Danno

There's no set rule like very X-thousands = Y increase in compression. That's because the shape of the combustion chamber isn't a fixed size. So the initial cut gives the most increase in compression because it takes out the most volume from the chamber. On a 4-valve S head, you'd probably want to take out 0.5mm and even then the increase in compression won't really give you that much more. Perhaps +5hp or so. To really take advantage of the higher-compression, you need a new cam-profile with matching head-porting and larger valves a fully-contoured radiused valve-seats. Then an optimised ignition map for this configuration to round it out and now we're looking at a +20-25hp increase (with some loss in the low-end).

Peckster

And you'd lose ability to shave your head later on if and when you have to mill it flat. Not worth it imo. S heads are rare already, for reasons we all know! I'd leave it as stock as possible

Bhj0887

Isn't it dangerous to shave the head, since we have "interference" engines. Wouldn't this result in valves hitting pistons?

L8 APEKS

Heck, 5hp on an N/A is worth it's weight in gold!

It's not like I'm going for 13:1 or anything...but since the head is off, and getting resurfaced anyway...I'm gonna have it cleaned up plus a little bit more, that's all.

Port work would be nice, but worthless to me unless I could do it right (portwork, O/S valves, cams). I would guesstimate that head job to cost around $4,500 on a 16V. No thanks! For what...~20 or 25hp? I could supercharge this thing for less than that and gain 100hp if I was willing to go through the hassle!

But in relation, the ~5hp from decking will be essentially free, since it's being resurfaced for the rebuild anyway.

I just figure, I do enough little things (1 or 2hp here and there) with the rebuild, and by the time I'm done, I may be pleasantly surprised!

iloveporsches

I'd be VERY interested to hear people's opinions on this. Me and a few other people (including ForumlaOne10 from RL) are rebuilding two bad Honda CBR 600 motors. One of the heads is warped out of spec (warped .01", servicible limit .004"). We were concerned that shaving it down that far will result in some piston-valve interference.

Geo

Pretty much. Every little bit on an NA adds up.

Ah the joys of paying more because of the crest on the hood. Might be worth looking into other head builders.

On my SE-R engine I had the head milled, the chambers welded and reshaped, ported (including the intake manifold), new valves, springs, guides, and cams for $2,800. Granted the S cams are double what they were for the SE-R (despite the SE-R being twin cam and the cams also ground from billets). But then again, you're not talking about welding the chambers and reshaping them.