Just want to say great website, very informative.

My question is regarding what tire size I should be aiming for with new gears.

My car specs are:

1984 Trans Am,
355 SBC
(estimated 400 - 500 HP without nitrous)
3000 RPM 9" stall (maybe 3,500....)
TH350
4.56 rear end
Stock Height Suspension (50/50 in rear, 90/10 in front)

My old gears were 3.73 (or 3.70 whichever was gm) and I was running 29x9x15 hoosier slicks.

Without being tuned very well or nitrous the car ran 12.6 in the 1/4.

To say the least, it launched like a minivan.....

Anyways, I am going to buy some MT ET Street Radials, just wondering what you guys think on tire size.

Any help would be apreciated. Let me know if you require additional information to make your best guess.

Thanks Guys.

BTW here is my car: http://www.cardomain.com/ride/2372309

 

No wants to help? =P

 

You need to give us at least 24 hours to read the messages. You're not as bad as some who complain that we're not answering an hour after the initial post.

You were running 29" tall tires with a 3000ish stall converter and 3.73 gears. I can see why it bogged off the line. Did you even get out of second gear before you crossed the finish line?

Going to 4.56 gears, I'd recommend 28" tall tires to get the engine up into it's powerband and make use of the whole rpm range in all the gears.

The last couple of years I was running 32" tall tires with 4.86 gears and this winter, dropped down to 4.57 gears to bring the rpms down some. With my new heads, I may have to put the 4.86 gears back in. The engine was really running out of air above 7200 and the new heads should let it breath to 7800. I'll make some passes with the 4.57 gears before deciding if I need to switch back or not.

You were using 29" tires with 3.73 gears. To use all 3 gears, you probably had to shift at 5500 rpm. Without knowing the cam specs, your engine probably wants to go to the upper 6000 range.

 

28" tires and 4.56 gears put me at about 6800-6900 at the line, not using nitrous.

 

Sounds like you guys have come to an agreement on a 28" tire.

As far as im concerned, EvilCartman and I have the same car.

So, If im at 6800 - 6900 at the line, I guess thats no prob. I have the rev limiter set at (7000 or 7500)

Last time I raced I was shifting at 6500, I do beleive I was around 4500 - 5500 at the line.

I guess that all jives with what you guys are saying.

So as far as buying tires, do you guys find the following tire an acceptable street / strip tire?

Forth one from the top.

http://www.mickeythompsontires.com/s...ETStreetRadial

3754R P275/60R15 28 X 11.50R15 33 R2 7.5-9.5 8 11.0 9.5 28.0 87


I will have to assume this tire will fit in my wheel well. I am not sure at this time the width of my wheels.

When I get home I will post my cam specs


Thanks guys, I greatly appreciate the advice!

 

Rev limiter should be set just before where the cam card says the valves will float. Shift point should be at the top of the HP curve just before it starts to drop.

There is no good "street/strip" tire. Driving around on the street with a tire designed for the race track wear it out quickly. Street driving has a lot of hot/cool cycles which dries out the tire. There could be a lot of rubber left but it's still considered worn out if it can't get sticky and hook. You should have 2 sets of tires. One set for the street and another for the track.

I've had many gear ratios and tire sizes over the years. I was always adjusting to keep the shift point in the high 6000 or low 7000 range.

 

Yeah that tire will fit no problem with the right wheels. mw66nova has a set, not sure of the size tire he's running though. Here's a thread showing some pics of his car and new wheels click here They're a good tire but as Stephen said, they'll wear out quicker if driven on the street.

 

Ahh, thanks for the link, the tires look good.

Here are my cam specs:

Part
Number Grind
Number Value
Lift Advertised
Duration Cam
Lift Duration
@.050" Lobe
Center1025EP-25.507"
.507"280°
280°.338"
.338"244°
244°108°

Its an Engle Cam.

Under the cam specs its says:

Street & strip, brackets, moderate built. 10 to 10.5:1 compression. Powerband 2500-6500


So with that new information, are we still talking the same stuff? lol

Any opinions are greatly appreciated!! Thanks Guys.

 

I know what floating means, but im not sure how to interpret the rpm from the cam card at which the valves will start floating. Any advice?

Just from what that website my cam is from, my PB is up to 6500, so I should shifting right around 6500....correct?

Thanks guys, you have great advice.

 

Not sure how you can figure valve float from the cam card either lol Think I've set the rev limiter at 7300 in mine. My cam says it's good from 2500-6500 but it still pulls beyond 6500 a bit. I'd say when you take it to the track, just play around with shift points and see how the car reacts.

 

Will do. Thanks for the advice!

 

Just judging by the cam I had 246/254 duration @ .050, I'd say that cam should be making peak power in the 6400-6600rpm area. You could rev up to 7000rpm before shifts, but there's no gain in turning more than 6500rpm through the traps since you then pass up peak power before the finish line. Counting on everything working like it should, my guess is an upper 11 second pass at 115mph. 4.56's and a 28" tire puts the motor at 6900rpm, which is too high for the fastest that motor/cam could make the car. I say stick with the 29's you have at least to see how you like that gearing, but personally I'd have 30" tires on those gears.

 

Many cam manufactures don't list the valve float rpm. I've only had a couple of camshafts that listed the float rpm.

Float rpm isn't always exact. If your valve springs are too weak, the valves will float at a lower rpm. If you've upgraded to heavier springs, the valves will float at a higher rpm.

The camshaft lobe pushes the valve open. As the lifter goes over the lobe, the valve springs close the valve. When the valve hits the seat, if the springs are not strong enough, the valve will bounce on the seat. This is valve float.

Always install the recommended valve springs for the camshaft you use. Only with a billet cam can you install heavier springs.

To be on the safe side, set your rev limiter a couple of hundred rpm above the powerband. Since your powerband goes to 6500, you'll actually start accelerating less if you push it much past that so a rev limiter set at 6700-6800 will be fine. Over 6500 rpm, you'll still be accelerating but the powercurve will be starting to drop off. It depends how flat the curve is at the top. A flatter curve can make more power over a wider rpm range.

The powerband of my camshaft is 5000-7500 and I have the rev limiter at 7800 but I was shifting at 7400 so I never hit the limiter. I also use a 6000 stall converter and launch at 4500 rpm where the cam starts to make peak torque.

For all out drag racing, you should have a stall speed about 1000 into the powerband. For street use, it should be about 500 rpm into the powerband which is what you have.

You should be shifting around 6500 according to your cam card and if you have the proper gear/tire combination, you should be crossing the finish line at or just past the shift point in high gear. So if you shift both times at 6500, you should cross the finish line in the 6500-6700 rpm range. It's not uncommon to have the shift light come on around the 1000' mark. Shifting at 6500 and crossing the finish at 4500-5500 (which is a huge rpm range) is far too low needing a much deeper gear or a shorter tire.

I always recommend to put the tallest tire possible under the fenders then adjust the gear ratio to best suit the tall tire. A tall tire gives a larger front to rear footprint and will give better traction than a shorter, fatter tire with the same surface area on the ground.

Driving around on the street, you can use just about any combination of tires and gears. Pro Street car can have 33" tall tires and 3.42 gears. They'll be a slug off the line but cruise easily down the highway. 8000 rpm small blocks could run 5.13 gears with 28" tall tires for maximum acceleration off the line and quick ET's.

If you're only using a 7.5" 10 bolt, you're limited to 4.56 gears as your deepest gears which is going to limit how tall a tire you can use. At your rpm range, that would be around 28" tall. To use a taller tire and deeper gear, you're going to need to increase the max rpm of the engine.

 

Man, its like cracking open an encyclopedia with you guys, great info.

Well, I guess the conclusion is, try the 29" slicks I have, if I am crossing the line with a lower rpm then 6500 then I need to go to the next smaller tire.

Really appreciate the help guys.


One more question, is there an accurate way to estimate HP from cam specs?

Given the fact the motor is a 355 SBC, Alumin Brodix heads, Super Comp Hooker Headers, 750 DBL Pumper....etc?

 

From what I've seen about your motor, it is pretty similar to mine a few years ago other than a 355 vs my 383. Mine dynoed 405rwhp, and ran mid 11's with a best of 11.44@ 119.

 

So you would estimate the HP in between 400 - 500, pretty close to my estimate.

Thanks for the replys, you guys have a great website here with even nicer people.

Keep up the good work! BTW, I started her last night for the first time in over a year and a half, started right up with no hesitation, she sounds like a monster with open headers.

Again, thanks guys for all your input!