2V4V

Lag,

Sounds like you're on the right track. Not 100%, but IIRC, there have been a coupla SAE papers on Miller-Cycle theory. I imagine they came up in your search, but if they didn't make it to the top of the 10K matches your search(es) produced, you might find them informative.

(This of course presumes my memory about the origin of those papers I read years ago was correct...)

Thanks for the compliment, but I simply feel it's my patriotic duty to pitch-in for a fellow 928er / (and I presume) single-malt drinker.

Greg

Warren928

Lag,

I was reading you post on teeth for the timing belt. So what your saying is that on an 86' 32v engine that if you jump the intake cam one tooth that you've moved it 7 degrees or 20 degrees? How can you make it more adjustable in finer degrees? Could the cam cogs be slot machined so adjust timing on the intake valve or are they already?

I havent taken apart the engine yet so Im just trying to get a mental picture. What would the ideal compression ratio be for an 86' 32v engine running 12-15psi without a serious intercooler system, butmaybe more a light duty intercooler?

Lagavulin

Sorry 'bout the confusion!

On any of the 32v 928 engines, on the end of the exhaust cam is a big sprocket where the timing belt goes. This sprocket has 48 teeth, so if you take 360 degrees and divide it by 48 teeth, then, each 'tooth' is worth 7.5 degrees.

Underneath the valve cover, both camshafts have a 18 tooth sprocket which connects the two cams by a chain. It is here where the camshafts stay in sync with one another. So again take 360 degrees and divide it by 18 'teeth', then each tooth is worth 20 degrees.

So, to advance or retard the both cams at once, it can be done using the external timing belt sprocket. Additional finer granularity is available via the slots machined into the sprocket, but again, it affects both cams simultaneously.

However, as stated before, timing the cams independent of one another is only possible in 20 degree chunks due to the 18 teeth internally on the cam sprockets.

I'm not quite sure what you're asking since I do not know what you mean by 'ideal' compression ratio. My answer will assume that you meant to say 'what would be the ideal static compression ratio'...

That is really hard to say since running that much boost would require an intercooler no less than what I'm running right now as a bare minimum. If you would read any of my previous posts on this topic, I always exhort people to spend the extra money and get an intercooler, including posts regarding Andy's kits.

So, a 10:1 engine running an 85% efficient intercooler at 15 psi would require a static compression ratio of 9.40 to keep the combustion chamber temp under Corky's 1075 with a value of 1074.84.

Warren928

Thanks Lag for the calculation and information. Now I understand cam adjustments alot better.

I think a 10% safety margin in corky's max temps would be ideal in figuring in static compression ratios. As you said, 9.1:1 would keep it just under the max temp. Adding alittle insurance in the event something is running not quite right would be very wise. That would mean 967 degrees F @ 10% margin, but maybe thats not a realistic option. What would the static timing need to be to put a decent safety margin into the equation?

Some safety margin would be best, just in case. That would make for 928 SC buyers who could breathe alittle easier every time they step on the gas hard!

Ive heard that the 928's cylender head shape is good for SCing, due to heat dissipation and shape. Is that safety margin already calculated in, or is corky bells math temp theory unmodified?

An 85% efficiency ratio intercooler is probably a very heavy duty, large unit, correct? Im just guessing here.

PorKen

How much adjustment is there with the pulley itself?

The slots look fairly long.

Normy

Warren...one quick little point-

-The relationship between crankshaft speed and camshaft speed: The crank spins at twice cam speed. Which means that the cam moves twice as many degrees for a given amount of crank travel if you adjust it. Thus...if you move the cam one tooth...you will NOT be advancing or retarding it by 7.5 degrees, but by 15 degrees!



N!
'85 S2 5 Speed

Warren928

Thanks Normy-

So advancing the intake timing only can be done by 15 degree increments. What would 15 degrees give for a static compression ratio? Would that much change in cam timing adversly effect the combustion process?

Lagavulin

Thank you, and you presume correctly!

This is statement is correct.

This statement is wrong.

You are forgetting that the crankshaft remains stationary while changing the cam’s timing. The end result is a change in the relationship position-wise between the stationary crankshaft position and the modified camshaft position. Because the crankshaft remains stationary while manipulating the camshaft, the 2:1 ratio does NOT apply. Thus, the change in valve timing degrees is determined solely by the amount of change in camshaft rotation degrees.

Let’s make this easy to see by fabricating an example.

Let’s assume that with ‘stock’ timing, the intake valve starts to open at 0 degrees TDC. Let’s also assume that we want to advance the camshaft timing so that instead of opening at 0 degrees TDC, it starts opening BEFORE TDC (..or BTDC) because we would like more low and mid-range power.

To accomplish this task, with the crank held stationary at 0 degrees TDC, we take the camshaft’s 48 tooth sprocket at it’s current stock position, and rotate it one tooth clock-wise for an advance of 7.5 degrees (..360/48).

The end result from the engine's perspective is that the intake valve will now start to open at 7.5 degrees BTDC, NOT 15 degrees BTDC as you stated.

Warren928

Lag,

Do you think theres enough safety margin in the 9.4:1 CR at 15psi or would more cam timing cause an adverse efffect of running the engine?

Lagavulin

The numbers say that running 9.4:1 at 15 psi is safe from a detonation standpoint. By the way, I forgot to mention this in regards to your earlier post, but there’s really no need to have a 10% ‘safety factor’ as that’s way too big, and you’d be doing nothing but wasting efficiency/horsepower.

The whole idea behind Corky’s calcs and what’s he trying to communicate is to avoid what you are proposing, leaving horsepower on the table. By the same token however, the calcs are there to reveal the stupid stuff too like running too much boost for a given static compression ratio.

The best case scenario is running a static compression ratio/boost combination which allows the maximum amount of horsepower along with accompanying reliability.

However, now that we’re digging a little deeper into engine operating principles, we see that it’s not so cut-and-dried anymore because of the affect of camshaft timing and it’s resulting actual compression ratio.

When I first started studying supercharging, I was intrigued by Mark at FAST and his successful running of a 10:1 compression ratio, non-intercooled 5 psi of boost, 1985 32v 928. According to the numbers, they say that that combo generates a combustion chamber temp of 1167.38 degrees, well over Corky’s recommended max of 1075. How is it possible that he could run those apparent numbers without busting any rings from temperature-induced detonation?

At the time, I figured it was a result of the aluminum cylinder head and it’s pentroof combustion chamber design (..which by the way was used in the Chevy Cosworth Vega long before Porsche employed it in the 928). Although I’m sure the head does play a part, it’s my thinking now that it’s role is relatively minor compared to the part played by the 85-86 intake cam and it’s 50 degrees ABDC closing point.

To make a long story short, a stock 85-86 engine can run more boost safely (..detonation-wise) than a stock S4 simply because of the difference in cam timing, which in this case is considerable when looking at the differing intake closing points and how it affects the engine's actual compression ratio.

Normy

I beg to differ Lag, but I believe I am correct...only I may not have written it very clearly.

Yes, during an adjustment, the crankshaft will be stationary....

[I recommend performing an adjustment such as this with the engine turned off. Fingers are indeed nice things to own... ]

HOWEVER....it is true that desired adjustment degrees need to be halved in order to achieve what you are trying to achieve. For example, lets say that your engine closes its intake valve at 15 degrees ABDC, and you wish to retard the cam in order to achieve intake valve closure at 45 degrees ABDC. How many teeth do you need to skip? The answer is 2. Since the cam really does work in reference to the position of the crankshaft, and the crank spins at twice camshaft speed, then each movement of the cam degree-wise equates to twice as many degrees of crankshaft movement.

N-
'85 S2 5 Speed

slate blue

Lag I believe the comp ratio for the 86.5 or the early 32 valve engine is 9.3 and not 10.0 this should also be factored in.

Old & New

Might it be practical to move the cam chain one tooth, and the belt one or two teeth in the opposite direction to achieve useful timings?

Lagavulin

Normy, I concede, you are correct! Thank you for being patient with me.

The proper way to view this problem is as N! suggests, which means that instead of doing this calc:

360 degrees / 48 teeth = 7.5 degrees per tooth

..do it this way:

720 degrees / 48 teeth = 15 degrees per tooth

This is because the crankshaft rotates twice for every rotation of the camshaft, thus 360 x 2 = 720 degrees.


Someone wanted to know how big an adjustment the slots in the timing belt sprocket provide; it looks like 3 teeth worth, or 45 degrees of crankshaft rotation.

Since N! has set me straight, I'm thinking one tooth advance on the belt maybe only possible to do on the large GTS cam, but then that would mess up it's exhaust timing by making it close much further below from TDC (..27 degrees BTDC) than it already does.

Again, thanks to N! straightening me out, one tooth on the chain is way too much since that would equate to 40 degrees of crankshaft rotation ..(20 camshaft degrees) x 2.

Normy

Someone's gotta keep you in line....wouldn't want you to disappoint Corky-

N