ptuomov

Here are some completely random turbo cam thoughts. I've been thinking about the cam sets that one could use in a turbo 928 engine. Here are some ideas for a set:




Set 1 2 3 4 5
Intake Stock GTS Stock S3 Stock S3 Elgin/Morton S3+ Elgin/Morton S3+
Exhaust Stock S3 Stock S3 Elgin/Morton S3+ Elgin/Morton S3+ 928MS 32vR3 profile on GTS
LSA 117.3 114.0 114.0 114.0 115.3
Intake LCA 127.0 120.5 120.5 120.5 120.5
Intake advertised duration ? ? ? 260 260
Intake duration @ 1mm 228 219 219 222 222
Intake duration @ 0.05" 223 214 214 217 217
Intake lift mm 9.5 10.0 10.0 10.4 10.4
Exhaust LCA 107.5 107.5 107.5 107.5 110.0
Exhaust advertised duration 236 236 246 246 ?
Exhaust duration @ 1mm 205 205 213 213 221
Exhaust duration @ 0.05" 200 200 208 208 216
Exhaust lift mm 9.0 9.0 9.4 9.4 10.1


The first set ought to work with any FI engine, however mismatched the turbo. The LSA is so wide and the exhaust duration conservative. The subsequent sets are for a well matched but rev-limited turbo engine, and the last sets are for a well-matched turbo engine that can spin at 7000rpm and above.

Comparison to other engines:

4v Modular Ford

See these cams that they sell for the 4-valve modular Ford which is almost a copy of the 928 S4 engine with a bit longer stroke and bit smaller bore:

http://www.compcams.com/Cam_Specs/Ca...?csid=962&sb=2

The _mildest_ turbo cam sold for 4-valve modular Fords by Comp Cams is 106260 / XE258BH-116. It has intake duration of 222 and exhaust duration of 224 degrees @0.05". The lifts are 0.475" and 0.450" for intake and exhaust. It has notes "DOHC-Street performance with excellent torque. Works well in Supercharged or Nitrous engines."

They have two real turbo performance cams too, which have more duration: 230 in / 232 ex for "serious street effort for Supercharged or Nitrous engines." 238 in / 240 ex for "max strip and street cams for Supercharged or Nitrous engines."

Even the biggest set above is milder than the mildest aftermarket 4v mod Ford turbo cam set offered!

Subaru EJ25

Here's another engine that is very similar to the turbocharged 928 S4: Subaru EJ25. The bore and stroke are identical to that of 928 S4. The stock valves are 36mm intake and 32mm exhaust. The Cosworth performance head for the engine has 37mm intake and 33mm exhaust valves -- the same as S4 stock. Of course, the engine is turbocharged.

What kind of cams do they use? The stock cams in the US and Japanese versions are

Cams, Adv. Dur., Dur @ .050, Lift (in), Lift (mm)
USDM EJ257 (’05-up STi) stock, 256°/256°, 204°/208°, .378"/.385", 9.60/9.78
JDM EJ254 stock, 256°/256°, 211°/209°, .374"/.371", 9.50/9.42

Notice the split seat timing for both cams. The two models have different AVCS (variable valve timing) operating ranges, so the cams shape is different.

Now, what aftermarket performance cams are available for this engine? These are more relevant because they are basically designed to be run with the AVCS off. Crower offers two cam sets, the milder one first:

http://www.bryancrower.com/makes/subaru/ej257.shtml

Street/Strip Specification. Plug and play application. Short duration for nice street manners, slight lope at idle. Excellent all purpose spec. OEM spring OK.
Adv. Dur., Dur @ .050, Lift (in), Lift (mm)
272°/272°, 222°/222°, .400"/.400", 10.16/10.16

Race Specification. Longer duration profile, added rpm potential. Intended for fully built engine. Tuning and ECU mods recommended. Requires spring kit BC0600.
Adv. Dur., Dur @ .050, Lift (in), Lift (mm)
280°/280°, 234°/234°, .425"/.425", 10.80/10.80

Again the turbo cam sets listed above are milder than the mildest aftermarket Subaru EJ25 turbo cam. Especially the exhaust timing is much, much shorter.

tveltman

My (extremely limited) understanding of cams is that forced induction engines favor timing with very little valve overlap, which is what the 928 engine has, which is what makes it particularly suitable for boost. I think you could get away with stock cams and not worry about it.

danglerb

I suspect the 928 block will fail long before we get close to the same hp/L as people are getting with the Mod Ford or Subaru motors.

GRTWHT

120* LSA is realy wide.
You could use 110* as a rule for a blown engine.

Glenn
'81 928
AU

entropy_engineering

When making your calculations/comparisons don't forget the rod to stroke ratio. A shorter one will generate a vacuum signal sooner coming off TDC and will start pushing exhaust out sooner from BDC. Shorter R.S. ratio motors will tolerate more overlap and idle well. Also, supercharged engines have virtually no backpressure in the exhaust. I've built over a dozen high performance Mitsubishi engines. Everyone seriously in to those retards the exhaust cam up to 3 teeth and advances the exhaust cam a tooth for serious top end. Of course they are a little "rumpety" at idle.
The theory that you can't use much overlap with boost is a hoax too. Big overlap long duration cams are for high rpm/power and should have turbo(s) sized accordingly. Every turbo car I've seen with much cam made a LOT more power with less boost. Remember the video I posted a while back with the huge Schrick cam in the BMW? It was a 3.4L 2 valve and made over 500 crank H.P. on I believe only 7 psi.
Run as much intake duration as possible because the positive manifold pressure will start filling the cylinders pretty much as soon as the valves open and will continue to do so until the valve shuts or the piston is far enough up the bore to overcome intake pressure.

MrLexse

Could modifications to the 32v engine be made to allow for the use of the 944 turbo cams ? If so, how would they adapt to a s/c'd engine (as opposed to turbocharged)?

ptuomov

If I go with stock cams, I will have to start cranking up the boost after 5400 rpm. The VE stays nice and falt with stock S4 cams up to 5400, but then starts falling off sharply. See the dyno graph of John Kuhn's all-stock-internals S4:



Now, I have two options beyond 5400rpm. First, run more boost. Second, run bigger cams. (Thrid, both. ) The problem with running much more boost after 5400rpm is additional pumping losses and hotter charge air, which in a vicious spiral requires more boost. So I am thinking of using the cam route as well.

An additional plus with the cam route is that the S4 cams are so insanely conservative (short duration and low lift) that bigger cams don't cost anything at the low end! The S4 cams are just plain bad cams.

ptuomov

The sims take into account rod to stroke etc.

You means retards the intake and advances exhaust or what?

I am with you there. If the lower static compression the engine runs well without boost with bigger cams, it'll run great with those bigger cams and properly sized turbo.

I basically agree, with the following caveats:

- If the exhaust gas is not evacuated well, there's going to be high pressure in the cylinder near TDC after exhaust stroke. I am thinking that it doesn't pay to open the intake before the cylinder pressure is lower than the intake manifold pressure. If one does that, then there will be reverse flow and that flow will have inertia so it'll want to continue in that wrong direction. Therefore, opening the intake early will only work if the exhaust gas is escorted out on time. Escorting the exhaust gas out requires large enough turbine, well flowing and fast enough port, early enough exhaust cam opening, etc.

- Closing the intake valve late will work great if the static compression is high, if the intake port is flowing fast in the right direction, and if the (static?) pressure in manifold is higher than the pressure in the cylinder. If the intake port is slow, static compression low, and the boost is not yet generated, the engine will bog down.

But as I said, I agree with you, more duration WITHIN REASON is the way to go.

ptuomov

Subaru block will fail logn before we get to the same hp/L as people are getting with Subaru motors! ;-) The stock open-deck Subaru block sometimes reacts poorly to 800hp out of the 2.5Ls.

ptuomov

I agree about 120 degrees, but most of the turbo cams that I see and like have LSAs in the 114-116 degree range. Including the mod Ford turbo cams in the original post.

ptuomov

The 944 turbos are all 2v per cylinder. I think the 16V engines might be able to use some components from 944 turbos with a great deal of trouble, but the 32V can't use anything.

Dennis K

What this points out is the need for billet cams.

Right now we are forced to regrind used 928 cams, which means our options are limited. It's great and works extremely well (my motor is a testament to that) but there is so much more that could be achieved if we had the freedom to grind any lobe on a cam blank.

The market is so small and everyone has their own agenda - people are building 5.0L, 6.5L, 7.0L (!), supercharged, turbocharged, whatever. Also each engine builder has their own philosophy of what they want to see in a cam. Some want long duration, some want big lift, some want narrow lobe separation, others want wide. If we had a source for cam blanks, everyone could select their own grinds to suit their various needs & goals.

2-valve 944 guys seem to be able to get OEM German chilled cast iron blanks. It'd be great to see something like that available for 4-valve 928's. Or, if we could make it worth his while, get another run of steel billet cams from Mike Simard. Look at what he's achieved on his own motor. It's the next step.

At some point I want to see cams that look like this on a 928:

123quattro

I think a lot of this is a moot point unless you are trying to get the last 10% out of any given engine. You could spend thousands of dollars to get custom cams cut, or run one more psi of boost, which is free, for the same effect. Once you start putting positive pressure into an engine it becomes much less sensitive to plenum volumes, throttle sizes, cam timing, and bend radii vs. a NA engine. Retiming the cams is worth doing, but I don't see the real benefit to getting new ones cut. There is much lower hanging fruit.

ptuomov

I agree with this, but it's still the case that boost means heat which means inefficiency which means the need for more boost. I much rather have a great breathing engine and boost that moderately than overboost a constipated engine.

From the modular Ford dyno test database, one can see that cams are more ciritcal for an NA engine than boosted engine, so I agree with that. The horsepower gain at no boost is about the same as at moderate boost, which makes it a smaller percentage and thus not a multiplicative effect.

I also agree that for the first couple hundred horses from stock, boosting is much more cost effective. However, once you've got the boost in teens and your transmission can't take more torque, the intake manifold, heads, and cams are the next logical place to modify.

ptuomov

Billet cams are a part of the answer. However, those billet cams need to come with a rev kit. Furthermore, they have to come with a rev kit that extends the effective lifter diameter. For example, the indexed 997 race lifters with the runway on them in your photo would do. But that's going to be expensive, real expensive.

In my opinion, what is needed even more for NA applicaitons than a billet cam is a new intake manifold. For NA I'd much rather take a new, straight-shot short-runner intake manifold and reground/welded cams than the stock intake manifold and the best profile on billet cams.