Ratio of restrictor plate to HP

[cstreit]

I've got to drop somwhere between 5-15 ponies out of the new racecar in order to make my class. (Depends on the transmission loss)

I'm thinking of going the restrictor plate route to do this. THe question is, where do I start size-wise? The intake is 3.75" in diameter.... Any guru's out there?

...a throttle stop would probably be easiest, but my thought is a restrictor plate might have the benefit of adding a little torque?

THanks!

[Flieger]

Smaller ports (intake and exhaust matched) add torque down low but restrict top end torque because the air goes faster at lower revs and then hits the transonic realm (stall) and the engine falls on its face. The faster the engine turns for a given torque, the more power is made.

A restrictor just restricts power. Before the air going through the restrictor hits the transonic realm, the engine is just as good as unrestricted. This is assuming the restrictor really is a choke point- it creates all sorts of turbulence and pumping losses.

[Evan Fullerton]

If you are only trying to drop less than 15HP, I would imagine that would be pretty easy to do with a different ECU map that is more conservative on high RPM ignition timing.

[cstreit]

I think I've decided on a throttle stop. Thanks for the info guys.

[Eagledriver]

I would definitly try to do mods that will increase torque at the expense of HP. Things like smaller headers, smaller venturies, advanced cam timing, longer intake runners, longer headers, more restrictive mufflers. With a combination of these things you can dial in the HP limit and have a higher torque curve. Is it a power to weight ratio? You could fine tune with weight.

-Andy

[Evan Fullerton]

Now that is a nice simple solution. Just hook the throttle stop bolt up to a dummy Tilton brake bias adjuster and you might really be on to something.

[Flieger]

It is more efficient to run at wide open throttle until the horsepower limit is reached. I liked the suggestion to put a rev limit on it so that it does not get to the peak power- if the power curve is going up at the current redline.

Cam timing would be even better.

[cstreit]

Well the issue is this has to be ready in a few weeks. This is for a GT3 and so the rev limit would require the unobtainable porsche computer. Max HP is actually well below redline IIRC at 7250 RPM with a rev limit of 8k.

At some point I might go with an ECU remap that might, in fact, improve things overall, but I don't have time or resources for that right now...

A throttle stop isn't the best solution because it would lower the ENTIRE horsepower curve if I'm thinking straight. The key would be only to limit the PEAK power which could be done by some of the above mentioned suggestions. Perhaps with some development time and head-scratching over the winter. I could also just add 55 lbs to the car.... ...or maybe a little of each.

[Jeff Alton]

Yes, you want to get the ECU into WOT mode so I would think if you adjust the TPS to think it is WOT with the throttle stop in place you should be ok. But then I need to check on that motor to see if that is even possible.,.....

Cheers

[cstreit]

Oi, this really isn't that simple is it. Still wondering if a small restrictor might be the ticket now.... ...or maybe weight (yuck!)

[cstreit]

Well some "good news". Finally got the car on the corner scales in full trim. Weighs 2790. So depending on the trans loss percentage (have to get to the dyno next week) I may get away with adding just 25-30# of weight.....

Time to start eating more biscuits and gravy.

Jeff, any idea how sensitive the WOT is to position? a few mm or more than 3/4 of travel. Would be useful to know!

Thanks!

[Flieger]

Can you adjust cam timing to shift the torque down lower?

What about thicker oil?

[cstreit]

I might be able to, but cam timing in that engine is tricky and requires some special tools... Not time for that yet.... I think and ECU remap later on is in my future...

[Matt Monson]

Something else that occurs to me is that I am pretty sure there's a more restrictive exhaust out there for the 93bd tracks like Laguna Seca. While it would cost more than a restrictor plate, you might be able to achieve your goals with a header and muffler change. Though it does seem that you've resigned yourself to weight.

[dad911]

Originally Posted by cstreit

..... I may get away with adding just 25-30# of weight.....

Just set the parking brake if scrutineering makes you dyno the car.....

25# is just a few gallons of fuel. Are you weighed wet? did you add in driving suit/cool suit water/helmet etc?

[Flieger]

That exhaust system will add weight overhung behind the rear axle.

[cstreit]

Dad, Actually no I didn't add in those items, but I did have 25# of ballast sitting on the floor to accomodate miscellaneous.

I'm thinking of ditching the lightweight battery, which with that and the cool suit tank should take up most of it. I'm down to +/- about 8 HP depending on what the dyno shows so untill I do that, I'm just guestimating what I need to do...

I'll post results when I get it done.