ptuomov

Well well well... I can help by stimulating scope creep on that project! ;-)

If the pistons are out, dishing the pistons is a part of a cost effective solution to lower compression. Rebco is a specialist on piston lightening and dishing, and they are very reasonably priced. I think they charged me about the same for the work as the postage was... They may want a chamber mold for best results, there are instructions how to make one on the their (poorly maintained) website at http://www.rebco-machine.com/ . That said, the chamber geometry is so simple that you can probably just take a photo of it and write in some measurements.

I'd drill the piston for oil drain holes, if they don't have those already.

I don't know whether you're planning to change the rod bearings, but if you are then you should consider this solution from a reputable source: https://rennlist.com/forums/924-931-...aring-fix.html. Then again if the original rod bearings look fine and haven't lost their crush, one might just want to put them back in -- hold your nose if necessary! ;-)

Overinvesting in head work is not cost effective if you keep stock intake manifold. I think the stock intake manifold works pretty well with turbos. If you really want to do something to the heads while you're at it, I think something like Ake's budget intake porting recipe with stock valves would work well. The description of the budget intake porting recipe is here is here: https://rennlist.com/forums/928-foru...l#post11970385 I'd keep the exhaust seats wide, 2mm wouldn't be overkill. The original stock valves for 928 four valve heads appear very high quality and have taken turbocharging heat loads very well. In my opinion, the same can't be said about the replacement valves being sold by Porsche. The original valvez rulez, keep them if they are in good shape!

What cams are you currently running? If they are stock GTS cams, they should work fine with their negative 25 degree (!!!) overlap at 1mm. Pretty much any "typical" factory or performance cams that are used in 928s will work well with John's kit. So whatever cams you have now should work with high probability. My question about cams turns out to be irrelevant.

Maybe you want to baffle the oil pan a little bit with some horizontal sheets, but everyone's got an opinion about that and there are enough threads talking about those kits as it is.

I think John's kits solves everything else perfectly adequately for a fast street car. The stock '87 engine that has taken a lot punishment in my car without batting an eyelash has everything else factory stock except the powder coating on the valve covers and intake.

RKD in OKC

5000 miles ago the drivers cam broke off at the gear. Did a standard valve job, nothing fancy. All 16 valves on the drivers side bent, replaced. Assume they were just porsche parts. Replaced GTS cams with GT cams and stock valve springs. Crank bearings looked good, did not pull pistons. Since that rebuild the oil usage has been steadily increasing to the 1 qt every 250 miles it is at now.

Mechanic not sure if it is rings themselves (pistons not drilled) or ovaled cylinders but compression and leakdown says its block and not heads.

Does the replaced valves kill it?

ptuomov

Interesting.

Since you don't have to rev a turbo engine too much, the replacement valves are probably fine. The problems that I've heard about relate to high rpm operation. They are probably fine, especially if the valve seats are as wide as stock (very wide) and if the valve guides are not too worn and transfer heat as intended.

Do you know where the oil goes? Does it go past the rings into the combustion chamber? Or does it go out the breather system into the intake?

I have one theory (don't I always! ) that might be worth ruling out.

The stock GTS intake cam closes 61 degrees ABDC. The stock GT intake cam closes 42 degrees ABDC, assuming it's installed the same way as in a GT. Those are at 1mm not at seat, so let's throw in +10 degrees more to both to approximate seat timing. For the stock GTS cams, you get an effective compression ratio of about 7.9. For the GT cams, you'll get an effective compression ratio of about 9.0.

Two implications.

First, I bet your car runs like a raped ape!

Second, it may be knocking more than the stock GTS because the effective compression ratio is increased. I've been convinced by others that increased knocks immediately lead to increased blow by, and I know that increased blow by leads to increased amount of oil pushed into the intake from the breather system (which in turn leads to more knocks and the usual GTS death spiral is completed...) This might explain the increased oil consumption.

Do you have access to very high octane gas, such as unleaded race gas? If you do, one thing you could try is to run a couple thousand miles burning race gas. Maybe logging the knocks with ST2. There's an outside chance that this experiment will show a lot lower oil consumption. I am not saying it's likely, but it might be worth the shot.

While the oil consumption is one issue, the high dynamic compression ratio in turbo use is another thing.

In turbo use, I'd want to bring that compression ratio down if possible. 10.4 CR with 5.4L V8 implies a chamber volume of 71.8 cc. If you want the static compression to something like 8.9 to get to effective compression ratio of 7.7 with GT cams, you'll need a 85.4 cc chamber. You would need to find 13.6 cc from somewhere, and that would probably require using all the tricks in the boook: thicker head gasket, dish machined to the piston, and some grinding of the combustion chamber.

Another alternative is to swap the GTS intake cams back and leave the GT exhaust cams in. That would restore the stock effective compression ratio, move the LSA to a very turbo friendly 113.3 degrees, 1mm overlap to -15 degrees. You'd keep the better flowing exhaust cams. Then, just machining a dish to the piston would probably be enough to get your desired dynamic compression ratio.

RKD in OKC

So GTS cams are better than GT for boost.

My mechanic has been doing a lot of research on the oil usage and thinks maybe the cylinders might be ovaled. Might open the door to bring the compression down with new pistons and rods.

Guess will have to get in to it to see what is realliy going on.

ptuomov

It's more complicated than that. I don't know for sure. But what I think is that if you can either lower the static compression sufficiently or run very high octane fuel, GT cams are better. If you can't lower the static compression sufficiently and have to run low octane fuel, then GTS intake and GT exhaust cams may be better than GT intake and exhaust cams.

BC

You don't need to lower that compression of you want to turbo it. Just use e85.

RKD in OKC

Had a 951 2.8 with o-ringed heads running 16 psi of boost making 408 rwhp and stopped there because did not even like thinking about having to hunt gas stations to get 101 race gas or E85 or even pouring toulene in the gas either. Just not a viable option for something I might actually want to drive. Might as well make it run on unicorn farts.

Strosek Ultra

Now when my all new GTS crank is sold, I have a set of Clevite racing rod bearings left over if anyone is interested. https://rennlist.com/forums/924-931-...aring-fix.html
Åke

BC

Just to clarify. That was many years ago now. There is a massive increase in e85 stations across the country.

Play on.

ptuomov

I am also an "all gas man" and I think race gas, E85, or unicorn farts aren't part of a daily driver recipe. However, race gas can be very useful in testing and trouble shooting, which I consider different from actually running the car day to day on it.

The good news is that the stock S4 engine (and presumably other 4-valve 928 engines as well) doesn't seem to need any head gasket trickery to last with 16 psi at higher rpms. The stock head gasket works well. All one needs to do is to use an rpm-specific boost profile in a controller such as eboost2 and give it the right amount of boost at each rpm to get close to the knock limit. (Tuning relative to the knock limit is a little like ECB meeting its inflation target: ...inflation rates below, but close to, 2%.)

BC

E85 is not to be grouped with the other two.

ptuomov

Perhaps. I know where to order race gas and Cambridge is full of old Subaru Outbacks with unicorn stickers. Haven't seen E85 for sale anywhere here, so perhaps it's even rarer! ;-)

BC

Todd Swears by it. For high boost applications, even more so ones that were not originally meant for it, its perfect and easy to get in most states.

http://www.e85vehicles.com/e85-stati...achusetts.html

Imo000

E85 would need to become as common as diesel otherwise you would be always hunting for it. Up here I haven't seen any gas stations selling E85 yet. Maybe there are a couple but I haven't seen them.

BC

That's a bummer. Around here I get around just fine. I can get pretty far, looking at the e85 maps of the country.

At least where I will be traveling by car (Read, with the 928, or with a truck running ETOH and towing the 928) - it works.