Ok it is time to spend a bit of money, the car is a 2004 C5 coupe 6m with 30K, fresh tires and brakes, K&N air filter, magnaflow exhaust cat back cross over. One more thing, I’m in CA and need to stay CARB Legal. I’ve been eyeing the head/cam swap for street/strip which tunes to about an additional 60hp for CARB Legal application. However when I talked to tech he has me now thinking the better way to go is to change out the 3.42 differential to a 3.90 to gain about 70 ft lb of torque. He is indicating that the car will be quicker, pull harder but will cruse comfortably on the freeway at 1750 RPM at 70 mph in 6th. Cost of the differential swap is half the cost of heads and cam.

One, can anyone confirm these numbers? Two, what would you do? Three, why?

Yes I had to ask the question, “What about top end?” to which he answered when was the last time I ran the car up to 190 mph.

 

Torque = Off the Line, from a roll, or out of corners.

HP sells cars and mods Torque wins races.

changing to a 3.90 would be a bit faster off the line.

 

Torque is what pulls you into the seat when you hit the gas. I would rather have more torque than hp.

 

One being a function of the other, I don't believe you can trade.

 

Probably 99+% of my driving time is below 5k RPM where torgue is the dominant force your dealing with.

I'll take torque - everytime.

My 403 build


New 427 build-656/615 engine dyno:

 

Torque is good! More Torque is better!

 

Isn't there a direct correlation between the two?

 

Torque X RPM / 5250 = Horsepower

It is possible to sacrifice a broad torque curve and peak torque number while persuing a peak HP number. High, broad torque motors are much more fun to drive.

 

YES they are

 

There is no such thing as torque vs HP.

HP = torque X RPM / 5250.

Nothing more than a simple mathematical relationship. The rpm's can be engine rpm or wheel rpm.

With gears you change the torque multiplication between the engine output shaft and the wheels.

Power is a measure of the amount of work done during a period of time. The work being done is the acceleration of the car. With gears, you change the operation (well, you hope to) so that the engine on average is operating at a higher rpm where it produces more power. More average power means you will accelerate the car to a higher speed in the same period of time. The engine isn't producing more power, you are just making better use of it.

Gears is the best bang for your buck. That is the reason why picking the right rear-end gears ise a key part in any racing endevour.

On the other hand, the engine HP power increase will also come with a torque increase. The torque increase will roughly translate directly through the drivetrain gear multiplication factors to the wheels. However, the HP increase will be the highest at the upper rpm's of the engine operation which is still only used as you reach the top of each gear.

Another thing to consider is that with a manual you are not putting any power to the ground when the clutch is pressed while you are changing gears. If the rear gear ratio is too steep then you will lose any gains during your gear changes.

Peter

 

Since you are in CA you might consider a set of milled AFR's and gears. The AFR's have a CARB EO# and have produced up to 40 hp with a stock cam. Gears will give you more acceleration as well. It would also still be CA legal in all regards.

 

Then you really should put 2.73 gears in.

They will help keep your engine in that desired high torque RPM range for a longer duration, so certainly the car would be quicker, because you'll spend more time at the top of your tq curve...

 

Peter

 

My post was a bit, ok, a LOT facetious, and I think Kevin realizes that.

 

since you want to stay "smog legal" i assume you won't be doing long tube headers.imo,if you do heads/cam and not headers you are leaving alot on the table.
depending on funds,you might look into a turbo.
otherwise,i would suggest the gears first.

 

If I were to do heads and cam I'd at least do shortie headers. They are CARB ok.

 

Horsepower sells cars, torque wins races. That said I'm very greedy and have to say whynot both, it only costs money and when you die you cant take it with you.

Good luck with your heads, cam, headers and gears.

 

3.90's will put the biggest smile on your face for the fewest dollars and fewest headaches. Gears are a very easy mod with the biggest WOW factor associated with them.

I agree with most everyone else, give me a nice broad flat torque curve any day over peak HP anyday.

 

GM has an emissions compliant LS3 crate motor that would guarantee a smile for every mile.

 

the engine makes the torque not the gears.. The gears will get you up to the peak power zone faster and keep it the through the gear changes.. Your torque and horsepower will be virtually the same no matter what gears you have. I'm not sure where the extra 70 ft pounds is coming from by changing gears..