HP and torque relationship in a light car
 

An engine build can be tailored to have a lot of torque or a lot of HP. There are always trade offs. Typically high HP would be made at higher RPMs and less driveability.

What do the engine builders have to say on the subject?

In a light weight Cobra, would you aim for HP and torque numbers to be similar or would you push for torque to be much higher than HP.

For example in a street driven Cobra would you prefer;
520 hp at 6000rpm and 540 torque at 4500rpm or
450 hp at 5200rpm and 570 torque at 4500rpm.

What is your logic behind your opinion?

John

IMHO, peak horsepower matters most at top speed (e.g., trap time in the quarter). Consequently, it's not a factor in most street driving .... all if you obey traffic laws in the USA. Peak torque at the RPM you plan to be at when you want the most out of your engine is more important. In street driving a car as squirrelly as a Cobra, I'd say peak torque should be a big flat curve between 2500 and 5500 RPM. .... One man's opinion.

P.S.
There are two more times when peak HP matters. One is when you put it in your signature block on the forum, and the other is when you're trying to one-up your buddies at the cruise in.

John,

All engines produce the same numbers at 5252 rpm, because it is a constant in the HP/torque equation.

HP=Torque x RPM / 5252

So in your examples above, the second of the two has a torque curve falling flat on it's face, only 700 rpm past it's peak.

570 ft/lbs at 4500, falling over to 454.5 ft/lbs at 5200.

Out of your 2 examples, the small difference in the 2 torque peaks is offset by the 1st example making 455.1 ft/lbs at 6000 rpm.

It all depends on what the owner wants.

My Cobra essentially has a race engine. Solid roller, big cam, power really comes on from 4000-7000 RPM. HP peaks at 637, torque peaks at 569. If you keep it below 4000 RPM, it is very tractable and drivable around town. If you want it to come on strong, drive it above 4000. I like it that way and so far it has kept me out of trouble with a bit less low end torque.

Btw, mathematically, HP = TQ * RPM / 5252. That is true for all engines, and why torque and HP are always equal at 5252 RPM. It is also why, for example, truck engines have a lot of torque but not a lot of HP because they need to produce torque at low RPM for towing but don't rev high. Race cars run flat out at high RPM so torque at low RPM is less important. It all depends what you are solving for, but the constraints are the same. In a Cobra, to me torque is somewhat less important because you just don't need that much for a light car to feel lively.

I have a street Cobra

I have 350 rwtq at 2500 rpms
max of 416 at 4400 rpms
and 350 at 6000 rpms

great for the street!

I can cruise at 1500 rpms and once (only once) I saw it at 7000:eek: still pulling like missile

I called my dyno guy and asked what he set the rev limiter at? He said 7500.
I run a SCT chip which overrides the stock ECU setting of 6250.

I had asked him to set it at 6500. I changed dyno guys.

Dwight

I agree with Lippy
he and I were posting at the same time.


I can blow the tires off my Cobra with easy.
I made modifications the my throttle system to give me more gas pedal, better control.
I run the best tires I can.

Half throttle in first
3/4 in second
and when I get into third a little I can go wide open


I build a 347 for my GT 40 last year and modeled it after my Cobra motor. I called Comp Cams and gave them the serial number of my Cobra cam and told them I wanted to move the tq and hp up a thousand rpms. I think I can give up torque on the low end because of the low gears in the Porsche trannie. The 40 and Cobra weight about the same.

Time will tell if I chose correctly

Maybe this summer!!!! It's been a four year rebuild!

Dwight

Smart guy, that Gaz.

I'm a bit fan of torque motors in a cobra. I find it quite useful when towing my pickup to the mud pulls or when my tractor breaks and I need it to plow the fields with it.:rolleyes:

Still trying to remember the last time I heard a racer (in any autosport) saying: "Sure wish I had a lower reving motor". Then there was the cobra guy saying, "I need to get rid of this 600hp motor so that I can get more torque down low".

Some of us absolutely love the sounds and characteristics of a race motor and don't consider them a negative, however, there is one place that big cams and light flywheels in a high revving motor will really hurt, and that is parking lot speeds. The higher your idle, the faster you have to go in a parking lot, and with a really hot motor you may find yourself slipping the clutch too much just to go a safe speed in a parking lot.

That's a good one, my very peaky 3600 lb car daily driver with 230 HP @ 6500 RPM has no problem idling through parking lots at 1000 RPM and parking in 1st gear.
Joe

Not talking about a peaky motor, talking about motors with really big cams and minimal rotating weight. Makes idle speeds very high and results in high speeds in the pits or a parking lot. Has nothing to do with powerband.

I have a big cam and my car idles at 1200 RPM to assure proper oiling for the solid lifters and valve train. I have a double disk clutch and a steel flywheel (I think it's a relatively light one). Parking lot and traffic driving are no problem at all.

I had the answer in mind when I posted this. Just curious what others thought.

In my opinion -
Heavy car or truck;
-as much torque as you can get down low
-heavy flywheel
-HP not even a question

Light car;
-HP in a range that is useful
-Lighter flywheel
- enough torque to get the car moving

Back to my question;
"For example in a street driven Cobra would you prefer;
520 hp at 6000rpm and 540 torque at 4500rpm or
450 hp at 5200rpm and 570 torque at 4500rpm."

I would say the 520hp at 6000 and 540 torqe at 4500 is the best for a Cobra. A 30 pound flywheel in an FE should be in the ball park.

John

How fast is your car going at 1,200 rpm in first?

Lot's of factors play into it. First gear ratio, final drive, etc. I intentionally went with a pretty short first for this reason. Forum consensus was it would be to low and useless, but didn't factor in the need to do parking lot speed on a street car.

Not sure about the RPMs in first. This will be my third Cobra and will have a 3.54 rear, TKO600 and 295-50x15 rear tires.

Since I am building this car I get to choose my HP and torque vs getting what it comes with.

John

Cars (engines) with peaky or very poor low speed torque are unpleasant to drive. Cars with robust flat torque curves are fun to drive. You want a car that is responsive to the driver and gas pedal at all speeds and to the extent possible all gears. That requires a broad, flat and high torque curve.

When you have to be reaching for a different gear and trying to get into the engine's power curve (torque peak) to negotiate something in your outing, your car will not be as satisfying to drive as one that has robust torque everywhere and in all gears.


Ed

Too much gearing
 

Grubby The 1st question is how streetable do you want the car? 2nd question, how much maintainance do you which to do?
In todays world with owning a cobra we have gone to the lower rearend gearing of 3.31 or less, 5 or 6 spd trannies, and dropping the operating rpm range.
I believe in torque motors over hp motors for my driving style and endurance. Can you turn a torque motor to 6,500 rpms, Yes with the right parts, But why? I just keep pulling gears. Some times 5, or times 6.
I have an FI motor at 482 cid with 622pound of torque and 560hp at the crank. My car is 2705 pounds. I ran pilot 335 in the rearend and now run Nitto 05 315's. Control of rolling on and off the throttle is the trick.
I have a hydro roller cam with stage 2 heads for Shelby and a little more work done to them. All port matched. 10.5 compression. Exhaust is the big power killer. Have gotten the back pressure down to between 2-3 psi posi pressure under WOT. Problem is loudness. Most track now have a Noise limit in the 98-105 range. With street exhaust, no problem with race pipes, no good. Street driving need ear plugs in left ear.
My last motor will be a 498 stroker and a torque monster in the 650+ range with a TWM 58MM setup. heads are worked to the max. Camshaft will be in the .650" range. I have found beehive spring to cover this and have safe rpms to 6,500 rpms. Will still be a torque motor. Will have to slow down the throttle speed of this setup.
IMO here's the thing, small blocks like to rev, rearend ratio's in the 3.54 to 4.09. Big block make a ton of power down low and due not need to rev. Reason for lower gears 3.07-3.31. Some guys run a 2.88 jag rearend but the power limit on this gearing is limited to about 450/450. With 30 spline stub shafts, support cover, chrome moly axles, you can push the jag rearend to the 600/600 club but, need to do more maintainance. No 5,000 rpm hole shots and banging gears.
John if first gear in your trans is higher than 2.80's you will never use it with a 3.54. It's good for pulling out stumps at a tractor pull or parade drive. I have a 3.05 first gear and 3.31 rearend and never use this gear. Have to shift within 15 ft. Will be dropping my rearend ratio to a 3.07.
My car needs more attension now with high power. At my age, my driving skills are slipping and the car has become faster than my reactions are when auto cross or roadracing. This is another reason for running more torque and lowwer rpms, longer life. 7years on 452 motor and now 8 years on 482 with same block. I really think you will be happy with a stroker. Rick L.

Rick,

I am planning a car out.

A good deal on a FE engine came along. It is rated at 482hp at 5400 and 542 ft/lb at about 4400. I discussed a cam change with Brent at B2 to push the HP up about 40 hp and 800 rpms. So about 520 hp at 6200 rpm with minimal torque change.

It is a lot easier to change the cam now instead of waiting until the engine is in the car.

The trans will be a TKO 600 with 2.87 first and at 3.54 diff. The same set up in my last big block Cobra that I liked.

John

Good deals make me nervious
 

Grubby How big is this motor?? Who's rockers and shafts are on it?
That's a nice torque motor, guessing it's stroked. Valve springs and coil bind will need to be checked also with a bigger camshaft. Piston clearance to valve also If this cam is more that .650". Are the pistoned dished , flat topped or domed will also effect the camshaft size. Hope the machine shop was a good one. Good luck. Rick L.

Rick,

The engine is a 468 SR FE Roush. I know valve spring and piston clearance need checked. I would also degree the cam.

My guess is flat or dished pistons because the Roush compression is set for street use on premium.

Again, just considering options to get the best drivability within the package I have. A cam is easy to change on an engine stand but a pain to change in the car.

John