Thom

Well, correct me if I'm wrong but none of the factory 4V cams seem to bleed any pressure at all (at least using the figures quoted @1mm of lift) - according to Rob Edwards' chart here, the overlap values I get are :
S3 : -16°
S4 : -13°
GT : -8°
GTS : -25°
By advancing S3 inlet cams by one sprocket (about 19°), this would bring their LSA from 114° to 104.5° and the overlap to +3°.
19° is a huge amount of advance on the inlet though, and would the valves still clear the pistons? Has anyone tried this (stupid?) idea? The positive overlap might bleed "enough" of the increased dynamic compression (gained by advancing the inlet cams) to avoid detonation?

UKKid35

GT cams are worth nearly 20% on a dyno?

SwayBar

I cannot figure out what this means.

Speedtoys

Ill ask again.

What specific HP gains do you expect from a .5 compression ration increase?

SwayBar

Bleeding cylinder pressure has nothing to do with overlap.

Bleeding cylinder pressure is hanging the intake valve open longer as the piston rises in the bore, effectively 'bleeding' pressure, or reducing the 'dynamic compression ratio'.

This is why with a bigger cam change, the mechanical compression ratio is also increased by using higher-compression pistons to compensate for the lost dynamic compression ratio.

hb4

A previous owner (Pappy) installed GT cams in my ''89 S4 with automatic trans. Runs great!

SwayBar

You've been asked twice now about your cryptic remark, and choose not to answer - that's okay, don't worry about it.

Thom

Wrong - the volume of air that flows straight from the intake into the exhaust during a positive overlap phase is a volume that would contribute to increase cylinder pressure if overlap was null or negative.

I agree with the rest of your post though, and in my reply to Ken I tried to say, perhaps not very well, that (positive) overlap and the inlet closing event should balance each other "somehow".

Speedtoys

I cant figure out how to add more words, to a simple question.

Im -not- worried about it. It was your question to begin with.

PorKen

But...you've already established that no 928 cams have overlap

Which is different than this:

Thom

Yes, and?

What happens when advancing cam timing on S3 cams by 8° to get the same IVC as on GT cams?

GregBBRD

Just interested....can you find a performance camshaft for an NA engine, anywhere, that has 104.5 degree LSA?

One of the things that a MAF injection system does not tolerate is any intake "blowback" through the intake system that will reach the MAF...which is why the opening events are always timed ADTC and the base circle diameter continues so far up the ramps.
The problem, for Porsche, was creating a camshaft that was more aggressive, yet still did not have intake blowback to the MAF. On the GT cams, they moved the opening event as close to TDC as they could, without incurring this.

And from there, Porsche did not have to think very hard or long....

When I was a youngster, I once had a famous cam grinder (not for Porsche cams) make things pretty clear, for me....and I've always carried this statement with me, ever since:
"For performance oriented naturally aspirated gasoline engine, you better have a pretty damn good reason to wander very far from 110 +/-2 degree lobe centers."

And GT cams have what lobe centers?????

In reality, the lobes on a GT application pretty much design themselves....
Start with an LSA of 110 degrees. "Crowd" the opening event as close to TDC as you can, without the backflow of the 4 cycle event reaching the MAF. Figure out how much lift you desire. Now connect the "dots" with a lifter acceleration curve that doesn't require a huge seat/opening pressure valve spring that also allows the use of a hydraulic lifter. Back then, camshaft lobes were almost universally symmetrical on the opening and closing events.....so that was a given. Stick the "exhaust event" in the appropriate location in relationship to the intake event.

Done. GT cam.

Thom

Thanks Greg, great post.

The 104.5° figure is just the result of calculating where LCA would be by advancing the inlet cams by one sprocket. I have no personal interpretation there and this is why I meant to ask if this adjustment had at least been tested, even if various rules of thumbs suggest it would be a stupid idea I just brain-farted.

I know of one performance-oriented naturally aspirated engine with 2 LCA values relatively far from 110+/-2°. The Variocam on the Porsche 968 sets the LCA on the inlet cams either to 104.75° or 119.75°. Wouldn't the 110+/-2° value be a rule of thumb meant for cross plane crank V8 engines only?

PorKen

Tire smoking torque off the line but peak HP would take a big hit as the exhaust valves close too early.



One cam tooth is 7.5° (crank). There is roughly a two tooth range in the cam gear slots (±1). IIRC, folks have skipped three cam teeth without valve damage.

GregBBRD

Yeah, I was talking about "fixed" camshafts....
A street engine with variable cam timing is going to shift the cam timing to the extremes....also a pretty predicable amount, right?

A set of 928 camshafts with a "modern" high performance "flat ramp" on the opening side of the intake, in combination with a non symmetrical lobe shape (increase the duration on the closing side), would really wake one of these engines up.
......And not require a crazy valve spring, since the closing ramp would be very friendly. All one would need to do is have a valve spring that can control the lifter as it opens to full lift....
This would allow one to "move" the "significant" open events closer to TDC, have more significant open time, while still having reasonable lobe centers.

Hmmm. I bet there is someone working on this....