toofast928

Has anyone tried taking the 4.5L, L-injection to 10:1 CR? No need to advance the cams and should make +20 HP/+ 20 TQ. Having the heads milled isn't to expensive. Whatyouthink?

SharkSkin

Just remember, any material that you take off of there will result in the right cam being off by triple that amount. Let's say you remove 1mm. So the cam belt will effectively be 1mm longer than stock at the first sprocket. Add another mm as the belt goes past that head/seam again, and another mm on its way to the other head. You might want to follow PorKen's threads on adjustable cam timing if you plan to make any drastic changes to the head.

2V4V

Don't forget that anytime you mill the heads, as Dave pointed out, relationships change.

Not only the relationship of the cam drive, but also the things that connect to the heads (i.e., the thermo housing).

Also, you will be moving the valves closer to the pistons - don't forget to check clearance if you mill beyond the factory limits.

Most all things are do-able, just a question of ROI.

Greg

toofast928

T-Belt timing should not be a problem because the manual states the head deck can be cut to a certain point. Just take the slack up with the adjuster.
gbyron your right the thermostadt housing would have to be milled to refit the heads but this is not unique to the 928. If take 0.020 off the heads that's about bumping the CR by one point. Of course I'ld have to measure the cc of the piston and head to be exact.

GlenL

I was wondering how you figured the CR. How about these numbers:

4474 CC displacement or 559cc per cylinder

If 9:1 is right, that'd be 62.1cc unswept and you want 10:1 or 55.9cc or a decrease by 6.2cc.

With a piston diameter of 4.75cm that's 70.9 cm^2, you'd need to take 0.087cm or about 0.9mm off the block to get 10:1.

From the heads I've seen, the chamber does not match the piston circle (There's a "quench deck" IIRC!) so there'd be more to cut off the head.

I'll _guess_ you'd need to cut 1.5mm off the head or about 0.06 inches.

At that point, would the pistons need relief cutting?

2V4V

Tony,

True, true. The manual says that you can deck the heads up to about .40 mm.

They also recommend that you use the thicker 1.4mm head gasket (vs. the stock ~1.1 mm) to offset any shave deeper than that.

And it would appear you're looking at a way deeper shave than .4mm.

If you get a chance to look at some unassembled 2V heads, you'll see what Glenn is talking about vis-a-vis the combustion chamber shape.

Once again, if you want to do it, I'm not trying to rain on your idea - pretty much anything is possible. Just not sure if it'll be worth the time and money to do it. Do the math and see how it works out. I can't say I have, so I can't say you're wasting your time. Just never a direction I persued.

Greg

toofast928

hmm, Ok, have to look at this deeper. Let's go back to the thread of advancing the cams using the 32v cam gears.......

Sueden

Another way to go are the 4.5 euro pistons which will also bring it up to 10.1 although this may be the most $.

-------------
Dennis
82 weissach

Joe Ricard

Well that's a new one on me I did not know that the 77 thru 79 Euro 4.5L 240 HP engines had 10:1 compression. However the 80 and up Euros were 4.7L and did make 10:1. and 300 HP.

LT Texan

toofast928, some of the Tech Spec books give the max matierial that can be removed (not much, and I don't have your year's book). Looks like even 1mm is beyond specs. I bet I'd do it if I was stuck with the 43mm valve heads. But I bet you'd spend less on fitting 45mm heads.

btw if you're "toofast" already, why mess with it?

toofast928

The manual states can shave .04mm to stay in tolerance. If more has to be cut off the head, than a 1.5mm thick head gasket can be used. Makes sense when a head is rebuilt the machine shop removes about .002 to make the deck flat again. (the thicker head gasket would be used to keep the CR stock 9:1) So their's no mechanical fitting issue shaving the heads down to 10:1 CR. The question is can the thermostadt housing be machined to match the heads if they are "milled"?

Sueden is correct, Euro 4.5L did come 10:1 CR. I think if I dive into the short block I would start with a 5.0L first.

toofast928

On second thought the pistions would need to be relief cutting............

toofast928

www.bgsoflex.com/crchange.html

2V4V

Tony,

You running MegaSquirt? I've got a couple built and benched, but have yet to find the time to do the install.

Greg

SharkSkin

Read carefully, or measure carefully before assembly. Or both. The thicker head gasket may be recommended either to resolve a clearance issue, to resolve the cam timing issue, or both. Maybe compression ratio was a consideration. Maybe the thinner stock gasket would yield unacceptable combustion chamber clearances if the head is shaved too much.

Nothing wrong with going outside factory specs, as long as you do the homework to understand what all of the implications are. If all the tolerances should add up against you you might be in trouble. Just do your homework and measure everything up before you cut anything. If you don't have your own surface plate and measuring tools, find someone who does.