Mark in Baltimore

In this thread from last year, the question of horsepower versus torque was brought up. I, however, want to ask a question that is specific to my car and situation, if that makes any difference.

I am considering getting a cat bypass for my 993 race car and running them with Fabspeed Supercup open pipes. The bypass builder, a fellow Rennlister, said that, with the exact same set up, he noticed a loss of low-end torque but that upper range power seemed better. For a race car, is it better for lap times to free up horsepower at the expense of bottom end grunt? I don't want to get these pipes and then get slower as a result of them. Keep in mind that my car is fairly porky at a bit over 3,000 lbs.

How many people are running cat bypasses in their stock cars? I know different motors and different designs affect the efficacy of components such as cat bypasses (and muffler bypasses and headers, etc.). Are most people running the cat bypasses for the sound and decreased weight? Or is there a real performance benefit, even if there is a loss in torque?

If anyone can point me to any old threads or if anyone can shed more light on this subject beyond the one thread that I found, I would appreciate it.

forklift

Here is why I bought a cat bypass for my 964. 14 rwhp on the dyno via Tom W.

https://rennlist.com/forums/showthre...at+bypass+dyno

I thought I could feel the difference in 2nd gear between T5 and 6 at Summit, but it was probably just a bit cooler that day.

Mark in Baltimore

Thanks, Jim. Unfortunately, his test/thread did not address torque, so I'm not sure if his torque figures went up or down and, if they did go down, how detrimental that would be to lap times.

FlyingDog

If your 993 has adjustable cams, try advancing and retarding them to see how the torque/hp trade off feels.

Veloce Raptor

IMO, it depends on the mix of tracks you intend to race. HP is great for qualifying, but IMOP torque wins races, since it allows you to get out of the corners faster with less rowing of the gears. Imagine that each gear change--up or down--requires a total of 0.5 seconds off the gas. If you can save 4 of these per lap by having to do 2 fewer downshifts, that's a bunch of time savings over ther course of a race!

Ray S

Mark,

It has always been my understanding that in racing HP (up high) is more important than torque (down low).

To illustrate this just look at a F1 engine. The 2006 2.4 litre Toyota RVX-06 V8 engine produces 740 bhp at 19,000 rpm and outputs 274 N·m (202 ft·lbf) of torque. That's about the same torque rating as my Boxsters 2.7, but waaaay more HP.

Mark in Baltimore

Aaaah, the debate continues . . . thanks for the response thus far, guys.

Matt, I'm not acutally sure if the 993's cam can be adjusted but will check into it.

FlyingDog

If you want to read endless pages of banter on hp vs torque vs gearing, search Mark Kibort's posts in the 928 forum.

Mark in Baltimore

Thanks, Matt, I might do that, but I'm hoping someone can separate the wheat from the chaff. Do you have the Cliff's Notes on the thread?

jrgordonsenior

Remember if you go to the bipasses and straight pipes you should check your air/fuel ratios as you will likely lean out your cylinders which can burn up pistons quickly. A aftermarketchip/flash can address that issue. Tom W. mentioned that in his article too....

DaveM993

Interesting. Then why are racing gears so close together? When I look at the various trannies for the 993, the Cup ones and RS gears are alot closer together then the stock North American trannies (G50/20). That tells me I am going to be rowing through the gears much more with the racing gears than the stock ones.

stownsen914

My simple understanding of the torque/horsepower situation from reading over the years boils to the following:
- Never under-estimate the value of torque. People put way too much emphasis on horsepower.
- Yes, F1 cars have a lot more hp than torque ... but remember how many usable track gears those guys have. I'm not even sure myself -- is it 6, 7? Most of our cars (even the 6 speeds) really have 2nd thru 4th or maybe 5th to work with. F1 cars keep their engines in the narrow usable power band with all those close ratio gears.
- Torque is what makes a car accelerate. Horsepower helps the car maintain its speed, and becomes more valuable at high RPM. For a relatively heavy car with stock gears, you will generally be happier and faster with a lot of torque than with a lot of hp.
- With all that said, peoply often confuse low-end grunt with torque. Low-end grunt (let's say < 3000 RPM) is great on the street but pretty much useless on the track. If the bypass kills torque at < 3000 RPM, then I would say don't worry. If it kills torque at 4000 RPM or higher, then it might make the car less drivable on the track.

It sounds like you're on the right track to look for a torque/hp curve comparison. Then you can evaluate whether it makes sense for your application. Good luck!

FlyingDog

My layman's interpretation: Ideally, you will shift gears when (decreasing) HP in the gear you are in equals (increasing) HP in the gear you will be in after shifting. You want a broad torque curve that covers the rpms you use in each gear, but outside of that it's worthless (unless you do standing starts). It's the area under the curve in the gears you use that matters, not peak torque or peak hp. To really make the choice of whether you want to trade off hp vs torque, you need to look at a dyno chart and what rpms/gears you will use on each track. If you can change gearsets, the equation gets far more complex.

Don't forget that TQ x RPM / 5252 = HP

Veloce Raptor

Because maybe the Cups and RS's engines were tuned for higher RPM running, whereas Mark is using an essentially stock motor, which tends to run out of breath at the top of its rev range, perhaps?

38D

Long answer, you want the most area under the curve in rev range you use the most. For you car, that should be ~4.5k to redline. Torque and horsepower below 4.5k are irrelevant (which is what I would call "low end").

Short answer, get the cat bypass for sure. Every C1 car runs one, and it is basically the same engine as yours.