993 RSR cams - adv or retard for more low end torque

JoeMag · 01-06-2019, 04:38 AM

Something for the experts... I have 993 rsr cams in my motor (3.8 rsr p/c, 1 3/4" straight pipes, 993 plastic intake (non-vario ram), 964 heads 51.5 intake, 43.5 exh).

Motor torque below 4.5k rpm drops off like crazy (~225 ft-lbs at 4.5k, and 175 ft-lbs at 4.0k). Was wondering if there would be any benefit to advancing or retarding the cam to improve low end torque? On the higher rpm side I'm limited by intake.

Frankly I spend very minimal time below 5k as I have a close ratio 6spd box, but if I do happen to end up there motor is really dead.

Steve@Rennsport · 01-06-2019, 08:03 AM

Joe,

Those are race cams and as such, they make power between 4500 RPM and 7800 RPM. Frankly, thats an excellent powerband for a competition camshaft.

You can advance the cams a little bit (depending on your piston-to-valve clearances) for a little bit more low-end torque, however this isn't going to net you 20 lbs-ft where you want it.

If you spend minimal time below 5K, this shouldn't matter.

Keep shifting.

JoeMag · 01-07-2019, 08:56 AM

Thanks Steve... Just to clarify, by advancing the cam, I would take the pin out from pic and rotate cam slightly "clockwise" and put pin back in next avail hole. I think pin would go into hole one hole counter-clockwise to where it is now on sprocket gear.

Also, by checking piston to valve clearance, would I be correct in assuming if I set lash on #1 (intake and exh) with all other rockers backed off, then turn rocker screw on #1 intake and exh 1 turn add'l clock wise, and slowly rotate engine and if I do not get any interference I'd be fine?

faapgar · 01-07-2019, 09:02 AM

I would not use this method.Not using a dial indicator might make you retarded and definitely not advanced.

Mahler9th · 01-07-2019, 09:36 AM

I cannot answer the question about cam timing but I had a similar 3.8 with 993 RSR cam profiles and a 993 intake. And 1 3/34 OD SCARGO headers with aftermarket EFI.

I used (and still do) the factory intake flapper... closing it between about 3400 and 5400 rpm at high throttle openings with an RPM switch.

My application was racing, and my configuration fell short of my competitors because I did not have ITB's. At the time I converted my engine to these specs, I could not justify buying a set of a set of TWM/Borla ITBs sized properly for my engine.

I did want as much driveability and power as I could achieve within reasonable cost constraints. So my friend and engine builder suggested that I try a larger, aftermarket throttle body, and a change to the geometry of my throttle linkage.

I bought and implemented an 80 mm BBK throttle body. Quite a bit larger than the stock 993 TB. Did not measure the power and torque after this change... we just tuned in more fuel on track based on the lambda numbers. This was a worthwhile change in terms of power. That modified BBK TB is listed as for sale in the classifieds as I can no longer use it based on current class rules. Bolts right up to 993 intake.

To help with light throttle response, I lengthened the arm of the throttle bell crank on the transmission by about an inch. This helped quite a bit, and was an easy to implement modification.

JoeMag · 01-07-2019, 02:23 PM

Mike -- Thanks for the info on TB. My application is totally race too... Interesting on the flapper valve on intake. when we tuned my car, we found almost zero difference in low end torque with valve closed and open, so we just left open all the time. I'll need to take a look at what size TB I have on my car...

Mahler9th · 01-08-2019, 09:50 AM

The modified BBK TB I have for sale has an 80 mm plate. I cannot remember the stock 964/993 size(s), but they are smaller. The difference in potential air flow and therefore power "depends" on the implementation.

My friend/engine builder does on track tuning-- not sure if he has ever dynoed my configuration to see what difference, if any, the flapper valve makes. Or to optimize its use. With my current cams, I am sure it is important. With my old rsrs cams, much less so, if at all.

I started heading toward ITB's after a few years with my rsr cam/modified 993 intake/big throttle body was implemented. I even bought a set of used TWM/Borla ITBs. They turned out to be from a friend, purchased through an intermediary (long story). Then I decided on a different racing class and could not use the ITBs. So I sold them.

If I were to consider ITBs again, no doubt I would choose the AT Power parts. They make power.

I think they (AT Power) may be working on some single throttle body designs for Porsches. My guess is they'd flow more and make power as well.