John H

Mark,

If my old white 993 wasn't faster after the cat bypasses, it sure sounded like it!

Bull

Mark, I would go with the bypass. It will definitely shift the hp and tq curves, but given the gears you have to deal with, I believe you will be better off in the 4500 - redline range. I have seen dyno curves before and after for 3.6 motors and all seem to look better in that range. It is what will really make the discussion about 2nd gear at T4 at VIR make sense (in whatever thread that is).

Any chance you can get some dyno time?

1AS

Torque and horsepower are related by rpm, and are always the same at 5250rpm. Torque is really just the amount of bang you get on each explosion. HP relates that to how many expolsions you get. Therefore, if you lose some power on each impulse at low revs, but regain it thru gaining revs faster or having a higher redline, then the higher hp is faster. This assumes you have proper gearing to take advantage of the hp curve, and are willing to use it.
Torque is what makes street cars feel fast, since high torque engines really push the car when the engine is loafing. However, hp is what really counts in racing. Drive a Honda S2000 for the ideal example of low torque (feels like crap at usual street speeds), but high hp (comes to life when you beat the crap out of it). I wouldn't drive one of those to work, but would galdly take the engine in a race car. AS

Nader Fotouhi

Mark,

Just Curious as what is the reason for running cat by pass on the 993. Can you not go with a straight pipes and a open muffler to reduce noise?

Mark in Baltimore

Wow, thanks for the great responses. As I am beginning ot understand it, "It's the area under the curve in the gears you use that matters, not peak torque or peak hp" as Matt writes. 993's with the stock, widely-spaced gearing are not ideal; it would be so much better to have a close-ratio gearbox but, well, that's not going to happen anytime for the next ten years with this car, for I just do not have a GT budget.

I'm not sure I understand your question at all, Nader. The cat bypasses are "straight pipes". The Fabspeed Supercups are open mufflers. I would most definitely not be installing straight pipes and open mufflers to reduce noise.

Jean

Mark

Leaving the HP vs. torque debate aside, I doubt you will have a (meaningful) difference between setups if you do not remap to maximize the benefits.

The debate between torque and HP is also a bit irrelevant here, since, first of all we are talking about perceptions of the owner without any datalogged proof, and second of all is because I would not expect you to be running below 4.5-5K RPMs too often on the track, which leaves the torque equation out, even if the gains are real.

Here is a datalogged difference of the same car and same road and conditions before and after changing exhaust, a remap and better fuel if I recall correctly.



The result is after a WOT run of 12 seconds the modified car would have been ahead by 2 car lengths....which could be seen as important.
Keep in mind a few things...This car had a remap done to optimize the changes, and timing was adjusted as a consequence, also this run starts at <3K Rpms, therefore the engine goes through a wide range of torque delivery and utilizes the benefits of the cat changes.

The only way you can determine whether you will be better off is through a datalog of the track you run on and superimpose the two curves, anything else is speculation IMO.. Especially without a proper tune post modifications.

Personally, I would save the money of a dyno run which will not bring any real value added in terms of on-the-ground lap time projections, buy a DBox datalogger or other, do a few runs and then spend time analyzing, both the differences between pre and post modifications, as well as what are the areas where you can improve your driving, this is where you will really get those extra tenths!

986

Mark,

Attached link to an interesting article about the relationship between hp, torque, acceleration from standing start, and acceleration at speed. (At the bottom of the page, there are a couple more links re: how to compare power curves, etc.)

http://craig.backfire.ca/pages/autos/horsepower

Cheers,
Toby

Nader Fotouhi

I think I understand what you are doing now. I had never realized that the straight pipe that replaced a removed cat is called a cat bypass.

38D

He can't remap and stay in his current race class. The gain of the bypass is small, but every little bit helps when racing!

Bill Verburg

the only thing that gives you a push to accelerate is torque. Hp is just a quantity derived from torque and time.

That said and hopefully understood, it is best to have torque at high rpm because then you can then use gearing to multiply the torque(and transform it from rotary motion to linear aceleration.

Again the thing that cause acceleration is the forward thrust(measured in pounds in the US) - air & mechanical resistance. Linear thrust can be calculated for a known torque curve(measured in ft-lb) and the gear ratios and the drive wheel loaded radius.

Here are 2 thrust curves, identical engines and tires for both, but w/ different gearing. the red is a g50/20 the blue a g50/30. The y axis is in pounds of thrust(push) the higher the more acceleration. Of corse aero resistance increasingly becomes a factor as speed goes up. That is not considered here.

chris walrod

This, in addition to, potential weight savings at the heavy end of the car.

With the power gains of the cat bypass, wonder what advancing the cams -some- would do in terms of the overall benefit? Its believed the bypass nets an 8hp gain. Advancing the cams to bump the lower end of the power curve might get the best of both worlds. In other words, install the bypasses, advance the cam and net the same factory original 280hp with a healthier torque curve all with a weight savings.

Jean

I apologize, I have just reworked the graphs (same runs)

I noticed that one of the tested runs was not on full throttle at 36mph, therefore it was giving an unfair advantage to the "modified" run from the beginning, as the gains made at 36mph were being carried forward throughout the run.

To give a more accurate picture, I have recut the graphs and extrapolated performance between 66mph and 110mph approximately, where both cars were already WOT and starting at about 4500 RPMs, mor ein line with a track situation as well.

As you can see, the difference in gains is now minimal, and again, this involved a remap, which as Colin says it is forbidden by your class regulations.



Colin, I agree with you regarding any time gained in a race regardless of how small it is of course. I do think however that good use of a datalogger will yield better improvements than a cat bypass modification with respect to lap times, assuming Mark has not used one still, which of course I could be wrong.

Getting both would be the best option!

SundayDriver

Let's get to YOUR question, not F1 engine design or what works better when you can rev to 10,000 rpm. You have a car with an engine. You are not asking about major changes. The weight will remain about the same, the usable RPM will be the same and the gear ratios remain the same. So what is the impact of losing torque (and therefore HP) at lower RPM in order to gain at higher RPM?

You do not want to give up power in the normal range you run in. If your gearing is such that your lowest RPM on track is 4500, then losing torque/HP at 4700 is probably a bad trade off. It will hurt you a lot coming off corners.

But what if the trade off region is bleow your normal RPM range? Let's say you lose at 4000 to gain above 5500. In an error free lap, you will be faster. But here is where the trade off is - when you make a mistake and the revs fall to 3500, you will pay a much larger penalty for that small mistake than had you not modified the torque curve.

I have seen this a lot in DSR. A newer driver opts for a built engine, making more HP but it is peakier. The result is slower laps because of the mistakes.

So, you need to understand the RPM range you normally use and then decide if you are willing to pay a bigger price for mistakes on track or want a more forgiving car with better torque at lower RPM's.

Hope this help cut through the noise on this subject.

Steve Weiner-Rennsport Systems

Well said,...!!

That is precisely what Mark & I talked about on the phone this afternoon,....

mark kibort

Ill give you the cliff notes of the endless posts relating to the comparison of torque and HP.

Basically, its is ALL about power.

Acceleration =power/(mass x velocity)

What this means is that at any same vehicle speed (when comparing two same vehicles with different engines) acceleration is directly proportional to power, and inversely proportioal to speed.

What happens, as with Veloce's post below, people get confused wtih engine torque and rear wheel torque, or thrust force. (total force acting on the car to acceleratate it). This thrust forces (or rear wheel torque, if we are talking about same diameter tires) will be identical at any same speed, if the two cars are utilizing the same power at that speed. coming out of corners, as long as you are in the correct gear, it wont matter. As was said, it is really about area under the HP curve used. the greater HP-seconds available, the faster the car will accelerate.

When folks talk about wanting a flat torque curve because of its percieved advantages, that is incorrect as well. What they are really talking about is a flat HP curve (or flatter) this allows for a broader set of gears to be used for more average rear wheel torque or thrust force. Closer ratio gear boxes allow for an engine with a peaky hp curve, compete with a car with an engine with a broad HP curve. (Even if the peak HP of both are the same)

An extreme example is the new R10 Audi and the new Pugeot Diesel race cars.
they only have 5 speeds. Even though they have a tremedous engine torque value, the rear wheel torque values (or accelerative force) would be identical at any speed to their gasoline engine, high reving, competitors, as long as the power available was the same. close ratio gear box 6 speeds of their competitors allow this to happen.

in the example of 4500rpm to redline (for 75% gear spacing), says that we can maximize acceleration based on the shape of the hp curve in this range of rpms. In order for us to get greater acceleration, we will need more engine torque (or HP) in the rpms from 4500 upward. However, if you do a mod that increases torque in this range and sacrafices Hp up top, since you spend more time at the higher rpms as you accelerate, this could have a detrimental effect on the total amout of HP utilized (HP-seconds).

I have a set of HP and torque curves that show two 440rwhp race cars. One Viper and one GT3RS porsche. the viper has almost 2X the torque of the porsche , yet the porsche would accelerate as fast as the viper at any speed , off any corner, because, the HP is the same. And, the HP curves are near the same shape. any differences are made up for by the porsche and its peakier HP curve, by having a closer ratio gear box. (assuming same weight cars and ignoring aero, etc )

Hope that helps.

Mk