ptuomov

We don't know for sure. Probably not before 6000 rpm. The cams, exhaust manifolds, and compression ratio have been changed so we don't know before we try it. The main symptom will be the victim cylinder knocking.

Thom

I'm curious about how low you went on the static CR, as NA-style cams with "lots" of overlap may require "lots" of boost to compensate for a possibly "excessively low" effective CR, and this may turn out counter-productive with the upper rpm flow limitations of the stock intake... Looking forward to the results anyway, and would be interesting to know some data (static CR and cam specs) if you don't mind sharing even before testing.

Lizard928

That was the old version of thinking. It has since been shown that a turbocharged car responds well to overlap as does a NA car.

Thom

Well, how long is a piece of string?

Go "too wild" on the overlap with "too low a CR" and the engine will not respond well regardless of IVC, or more precisely, will be fine off boost with IVC "early enough" but knock right away with even "little" boost, or will be a dog off boost with IVC "late enough" and require "too much" boost to run only half decently.

ptuomov

There's nothing wild in those cams. They are 218 intake and 208 exhaust duration at 0.05" lift with 114 LSA. The static compression ratio is 8.6:1. This is with 2x37mm intake valves feeding a 620cc cylinder. ( More details here: https://rennlist.com/forums/928-forum/560928-twin-turbo-928-fixed-and-back-out-there-terrorizing-the-streets-82.html#post13294122 )

By my guesstimates, the cams and other improvements about make up for the loss of efficiency from lower expansion ratio in normally aspirated mode. It should all play very well together, with the main unknown being the rpm at which the cylinder #3 blowdown pulse will start screwing up the overlap of the cylinder #1. The exhaust manifold shape impacts that in addition to the normal 1D pipe parameters, which makes it too hard for me to simulate reliably.

ptuomov

Bypass circuit being fabricated. These pipes will have a good priority and recirculate inside the main filter.

DKWalser

Beautiful work! I like how, to the extent possible, everything is symmetrical.

Thom

Thanks for the cam specs - this thread has so many pages I missed them somehow.
On my 944 turbo engine I have not been able to find quite a satisfying compromise between mid range torque, high rpm power and engine overall silk-smooth running, even with different intake runner length to vary ram effect rpm, which 87+ 928 intake already simulates with the flappy. OK my engine was a satisfying compromise, but you know how it goes, we always want more...
I believe that variable cam timing would help a lot, and I expect you to test 968 heads in the medium to long term.

The symettrical engine bay layout does look ace. Keep up the great work.

Strosek Ultra

968 heads will only fit on the right hand side cylinder bank of the V8 engine. The left hand cylinder bank is sitting 25mm to the rear making the timing gears not to line up. It would be possible by welding and machining to modify a 968 head to fit on the left side, a fairly extensive work though.
Åke

ptuomov

It's always a compromise. In our case, we have the advantage of 5L displacement, four valve heads, and active resonance intake that in normally aspirated stock form puts out about 320hp and a very flat torque curve. It's relatively easy to reach the traction limit of street tires in low gears over a broad power band once that package is turbocharged.


But more is never enough and too much is just right, so now here we are with a lower static compression, slightly bigger cams, bigger intercoolers, and 3 inch plus compressor inlet pipes.

Thom

Thanks for the information. It sounds like welding on 928 heads a 3rd mounting point on the 3rd spare plug well to fit a Variocam tensioner would be a lot easier. I also forgot that the 968 head combustion chamber is shaped for a 104mm bore, which is not desirable on a stock 928 block. Custom cams made with 18 sprockets for the cam chain instead of 19 would also be needed. The 968 head cover is a straight replacement with 928 4V units, but holes would have to be added for the breather system.

ptuomov

In addition, in my opinion, variable valve timing is not something you really need in a turbo engine that isn't tightly emissions controlled. Porsche not only didn't use variable valve timing for turbo engines back then, they also stuck with two-valve heads pretty late in the game! There are in my opinion higher marginal returns to effort and dollars in other areas. For example, one could cast new exhaust manifolds and twin scroll turbine housings, and alleviate both the 180-degree and 90-degree exhaust interference problems. But we're hoping to show that even that is unnecessary when components match well.

Thom

I was more thinking as cam timing as a solution to reduce overlap at higher rpm so that it doesn't put to evidence too much the flow restriction of the intake manifold at higher engine speeds and keep the engine to run smoothly. I imagine that a 700rwhp engine like yours will build revs in the blink of an eye if the rear wheels slip (which they will) and keeping it to run as smooth as possible on all the rpm range can only help with reliability.

ptuomov

For these cross plane V8 engines, we have bigger problems to deal with...

With single scroll turbos, the 928 engine with its 122mm bore spacing is limited to making its peak power at about 6000-6500rpm and the best we can hope for after that is extending the power curve at that level while efficiency suffers. Simultaneously, at low rpms we have the 180-degree interference problem that the four bangers also have. So there's a natural power band for a twin turbo v8 that depends the physical distance between the 90-degree interference cylinders (shorter the better, extending the top end of power band) and the physical distance between the 180-degree interference cylinders (longer the better, extenting the bottom end of the power band). It's more important to have different cam timing for different cylinders than having the whole cam shift in synch with rpms. Porsche Cayenne Turbo has half the exhaust lobes different size and Toyota has a patent on shifting the center lines between cylinders of a V8 engine.

ptuomov

(The new iPhone mobile interface of this site is terrible for posting photos. Took me ten minutes to give up editing this post.)

Including the compulsory shots from Thira (Santorini).

Air-oil separator bracket design. The idea is to make a bracket that holds the air oil separator. The bracket will likely be welded to the boost pipes. The goals are visual symmetry and some adjustability of the mounting angle. The separators are identical and thus not mirror images of each other, which means that mirror image brackets don't work. The V8 BMW engine has two smaller mirror image separators, so one could use those for an engine that flows less air. For this project we opted to use 2x the largest BMW separator from a six banger.












New fuel line arrangement, with the new bracket:



The first version vs the final version of the fuel line bracket:




Oil cooler hookup with the rubber isolators:





Is there a more sure-fire way to get people to take photos than this kind of sign?