Which motor would you want in your car, (for auto cross) Options

[ottox914]

and why...

Lets assume a 2000lb car, and no particular class. Suspension is well sorted, tires correctly sized (big enough but not uber huge).

Motor #1: is pretty pathetic below 4k rpm or so. Between 4K and 7K it will top out at 200hp. Somewhere in there it will top out at 175tq. Valve train and internals will support 8K rpm, so there is a little room above the max hp rpm.

Motor #2: has some power below 3k, definately pulls harder starting there. Starting at 3K it makes 150tq, tops out at 200tq at 6k rpm. Somewhere in there it tops out at 175hp. Engine will rev to 7k rpm w/out blowing parts all over, so you can go beyond the power band if needed.

Lets further assume both cars run LSD, and can run any ratios for the 5 foreward speeds, so the box can be optomised for each motor in this 2000lb car. Both motors drop off rapidly after 6k and 7k rpm.

Which motor would you want in your car and why?

[messix]

use the torque young jedi use the torque!

[Andyrew]

motor 2

TQ is guuud.

and yes, luke is in my avitar script... lol

[Allan]

Short AX course = Torque.

[ewdysar]

Like the other answers, Slow (under 80 mph) wants torque, fast (80 and up) needs HP. Or AX=Torque and Track=HP.

Eric

[Demick]

Torque is what accelerates your car. Horsepower is just a measurement of combined torque and speed - and is really not a very useful measurement (unless you are in marketing). Look at the torque curves and you will find your answer.

Demick

[Brando]

I choose #2...

More Torque = better acceleration, which is what you need coming out of those tight turns.

[J P Stein]

If you were running against me, I would agree with all these guys (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

[jhadler]

Mid range torque is what you want, not peak power numbers. The fatter the torque curve, the better for autox.

-Josh2

[Dave_Darling]

If we assume that we can change all of the gearing, I would be very tempted to go with the more powerful engine. Remember--the transmission is a torque multiplier, so making torque at higher RPMs with the motor lets you put down more torque at the wheels--assuming you have the correct gearing to use it!!

Power is not just a marketing number. Power is what is used to change the energy of something--in this case, that something's velocity. So more power, assuming you have the gearing to use it, means a greater change of velocity.

Now peak numbers, those really are pure marketing. As Josh2 said, it's all the shape of the curve. Though I mean power curve, not specifically torque curve.

--DD

[Dr. Roger]

BROADER torque band...

then match the best gears to keep it in the torque. BAM! (IMG:http://www.914world.com/bbs2/html/emoticons/cool.gif)

[rick 918-S]

QUOTE (Dave_Darling @ Dec 1 2005, 05:40 PM)

If we assume that we can change all of the gearing, I would be very tempted to go with the more powerful engine. Remember--the transmission is a torque multiplier, so making torque at higher RPMs with the motor lets you put down more torque at the wheels--assuming you have the correct gearing to use it!! Power is not just a marketing number. Power is what is used to change the energy of something--in this case, that something's velocity. So more power, assuming you have the gearing to use it, means a greater change of velocity. Now peak numbers, those really are pure marketing. As Josh2 said, it's all the shape of the curve. Though I mean power curve, not specifically torque curve. --DD

PHAT (IMG:http://www.914world.com/bbs2/html/emoticons/pray.gif)

[J P Stein]

Ya'll must be talkin' about a real hi-torque rig ....like an S2000 Honda. A real AX dog, eh?

We all have our problems to solve.
My motor is a tweener....I can safely rev to 7300....maybe 7500, but I'd hate to find I was wrong about that last bit (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

I run out of gear at 38 mph in 1st and 58 in 2nd. Since my motor is done I *have* to change gearing. With an 8k motor, I wouldn't. Changing gearing in a 901 is not cheep, but if money is no object, have at it.

Shifting costs time and complicates your AX life. The less, the better. There's the reason for the Honda reference.

[Andyrew]

an S2k has like 3.5k of usable rpm range... Thats pretty darn good to me...

Now if it was a turbo honda v tech with 1k usable rpm range... That would suck... (one frequents the scca solo 2)

[ottox914]

I posted this same question to another forum, most of the replies were: Go for the TQ!!!. This one was interesting. Waddya all think of these numbers:

"I made up a couple torque curves based mostly on Dave's criteria. I'm not exactly sure what units he was using for torque, but I assumed lb-ft. 200 lb-ft at 6000 RPM would be quite a bit more than 175 HP- 228, in fact, so I just used 200 lb-ft as the peak torque number and 175 HP as peak power.

If both engines were geared to top out at, say, 65 MPH in 2nd gear, the torque monster would have a negligible advantage up to around 25 MPH. From 30 MPH to 40 MPH, it would have a fairly sizable advantage. By 45 MPH, the high-HP engine would take the lead and hold a large advantage all the way to redline.

The torquey engine would have the advantage on low-speed courses and coming out of slower corners, but remember that you can't always use all of your available power on corner exit. As soon as the course opened up a bit, the high-power engine would shine and the torquey one would fall on its face.

It's like comparing my old 626 to my Miata. The 626 made 130 lb-ft and 110 HP, while the Miata makes around 110 lb-ft and 130 HP. Driving around town at low RPMs, the 626 felt considerably faster than the Miata, but the second I wanted to accellerate onto a highway, the 626 would get really slow about the same time the Miata would start to make decent power. The torque of the 626 would have been worthless in an autocross, however, becuase it could never get it to the ground coming out of a corner, and by the time it would have been at a point where it could use all that torque, it was gone."

This almost defends DD's position in his reply.

This is interesting. Keep it coming.
__________________

[Dr. Roger]

QUOTE (Dave_Darling @ Dec 1 2005, 06:40 PM)

Now peak numbers, those really are pure marketing.  As Josh2 said, it's all the shape of the curve.  Though I mean power curve, not specifically torque curve. --DD

like i said, broad torque band.

a torque band that covers a broad RPM is way more usable that a peaky torque band.
torque band the shape of mt everest sucks.
torque band the shape of JLo's back side is probably a winning torque band for AX. less shifting. 901 isn't a very close ratio tranny.

you need a specific torque band/shape to fit the application.
as well as gearing
HP does come in to the equation whenit comes to speed.
The Ferrari 3.0 liter Formula One engine is rumored to produce about 860 hp at 18,500 rpm. That would be about 1.33 ft-lbs per cubic inch.

what does the 914 put out per c.i.? pretty close.

This post has been edited by rogergrubb: Dec 2 2005, 12:41 AM

[Trekkor]

I like my rpm's up between 5-7k, all the time.

As far as pulling at a lower rpm band, IMO, you shouldn't be down there except for at the start, which is off the clock.


KT

[jhadler]

The issue is having a -broad- curve. Acceleration is a matter of gearing the drive to match the motor. The important thing here is to remember this:

At higher speeds, your acceleration is less than at lower speeds. A car does not accelerate at the same rate all the way up to 100 mph. It starts out with very high acceleration, and as the speed builds, the acceleration declines. For one thing you start pushing more air. But the point here is is that at higher speeds you spend more time in the high rpm bands. That's why a peaky motor serves well in a road racing environment, where the usable power band is matched to the nature of the car. However, in an autox, you are runnning up and down through the rpms all the time. You are very rarely camping out in the sweet spot of the power band for more than a second or two at best. What you want, is something that will help you maintain your best possible acceleration at low speeds as you can.

And again, it's not the numbers, it's the shape of the curve. The 626 has a tractor motor (flat torque, low rpm power), the miata has a sports car motor (peaky torque and power curves, but -broader-). You're comparing apples and oranges...

How's that?

-Josh2

[MattR]

How about this: you cant tell shit from the motor. You have to look at the ENTIRE package. Look at transmission gearing and car weight among other things. Having 1000 ft lbs of torque in a short geared 914 is kinda pointless... The engine is easy to focus on. Try looking at the entire package to get an idea of "what will be faster" (damn ricer forums...).

[airsix]

QUOTE (trekkor @ Dec 1 2005, 10:52 PM)

I like my rpm's up between 5-7k, all the time. As far as pulling at a lower rpm band, IMO, you shouldn't be down there except for at the start, which is off the clock. KT

I agree. I'm no autocross pro, but I rarely EVER see any rpm lower than 3,500-4,000 from start to finish. If I'm on the gas and the revs are below 4,000 I consider it a royal wrong-gear screw-up.

I always think (IMG:http://www.914world.com/bbs2/html/emoticons/huh.gif) when I hear people here say "It'll have no torque below 3,000rpm". If you're on the course and below 3,000rpm other people are growing old wating for your run to finish! (IMG:http://www.914world.com/bbs2/html/emoticons/laugh.gif)

-Ben M.