I'm having some trouble figuring out my car. Here's the story! While cruisin on the highway at 60 mph's, my car is reving at 3100 rpm(tire size is 255-60R-15). About 26 inches in height.

I've been racing my car a bit and although my L48 automatic has been running 14.0's. My 60 foot times are crap (2.13). And my mph is only 97-98.
I'm crossing the finishing line at almost 5500 rpm.

With these numbers, I was thinking that I must have had too much gear in the car. Because I'm almost revved out at the finish line. Like could they be 4:11's . But then with 4:11's wouldn't I have a better 60 foot time?

Tonight I was under the car, looking for a rear end code. No sign of any code. So I did the wheel turn and count the revs of the driveshaft. Looks like 3:55's back there.

Now, do I have (a) a trans problem, (b) inaccurate tach, (c) bad torque convertor, or what...........?

I'm running street tires at the track, great burn outs, no spinning on launch (19 psi) in rear. 45 psi front.

Can someone advise me so I can dip the 60 foot times and push my car into the 13's.

Thanks for any advice you can pass along!

indynut :banghead:


[Modified by Indynut, 4:09 PM 8/31/2003]

 

You didn't say what year your car is, or any other details other than it being an L48. So I'm assuming it's all stock?

First of all, the 255/60R15 tires are 27" tall. And yes, your speedometer could very easily be off. It all depends on what the previous owner(s) had done. Sometimes a transmission is changed and they never bother with the speedo gears.

However, 14 seconds in the quarter could probably be expected from a 70's era stock Corvette. Remember, this was when Detroit was trying to learn how to make a V8 run clean. They initially though the answer was to drop compression to ridiculous levels and stifle the exhaust flow.

If you're still running stock, there is a whole bunch you can do without even considering a new engine. First thing I'd do is pull the heads and raise compression. Rather than paying a machinist to mill the heads for smaller combustion chambers, I'd get S/R Torquers, or the new Vortech heads which are even better. Big power gains for little money. The Vortechs need a Vortech intake. No big deal.

Then consider modernizing that cam about 3 decades.
If you have an automatic, put in some better stall. And a 3.55 gear is not bad, but is probably a little tall to be a decent performance gear. A 3.70 is better, or you could bolt on shorter drag radials back there, such as a 275/50R-15 and make the steeper gear ratio cheaper than a rear end change, and get better traction while you're at it.

Other things you can do is to upgrade your ignition, they've come a long way since the 70's too. If you're going to do all this, check your local laws, and if they allow it, go back to a true dual exhaust. You can put high flow cats on it to keep it clean.

I'll bet with all this, you can shave off a second or so and get solidly in the 13's. Then if you want more, try a 125 hp shot of nitrous. No problem getting 12's then. Plus you don't lose your driveability.

 

I almost forgot. To get to some excellent gear ratio calculators, try http://www.smokemup.com

You'll have to register like any forum, but they've got some great stuff there.

 

Indicated speed to RPM is not at all affected to tire size, it's only dependent on having the proper speedometer gear for the corresponding rear end ratio. 3100rpm at 60mph sounds like 3.70 gears to me.

 

2700 @ 60 mph = 3.70
This is with a 4 speed.

I don't think the torque converter loss would bring you to 3100.
3.55 would most likely be the stock combo for your car.

Crossing the finishing line at almost 5500 rpm seems high.
The tach could be reading a bit high, however I've never seen this.

 

Holy poopie if your stock 79 l48 auto is running 14s then my almost fully modified 4 speed car will easily run 13s. But those numbers do look a little weird.

 

Most '79's were 3:55's You had a stock Y pipe.. Ugh.. Can be easily changed, your crossmember will support 2 pipes. Long tube headers will also fit. I agree on the heads and Cam as well. easy swap outs.

 

I believe you are incorrect. If you look at any of the calculators they want you to input tire size (height) along with the other numbers. A shorter tire will give you the effect of a lower (higher numerically) rear axle ratio and conversely a taller tire gives the opposite effect.

Changing tire sizes and not changing anything else will result in an inaccurate speedometer.

 

I agree that changing tire size will result in speedometer error. That's not the point that I was making.

I did not say "actual speed", I said "indicated speed to RPM". You could drive the car on the rims, or with 31" mud bogger off road tires. In each case you'll still be turning the same RPM at the same indicated speed. The only way you'll change the ratio of indicated speed to RPM is by changng the rear end ratio, final gear ratio of the tranny, or swapping out the speedo gear. Your actual speed will change with the change in tire size, but the indicated speed at the same RPM will not.

 

[/QUOTE]
I did not say "actual speed", I said "indicated speed to RPM". You could drive the car on the rims, or with 31" mud bogger off road tires. In each case you'll still be turning the same RPM at the same indicated speed. The only way you'll change the ratio of indicated speed to RPM is by changng the rear end ratio, final gear ratio of the tranny, or swapping out the speedo gear. Your actual speed will change with the change in tire size, but the indicated speed at the same RPM will not.[/QUOTE]

I agree, but the original post said he was doing 60 mph and had a certain tach reading. Unless he knows for certain what his speed is, he cannot tell what the rear axle ratio is based upon rpm. As mentioned, you need to input tire diameter into the calculator formulas.

 

OK, here's more info on my car. Since I posted I've been under the car and changed the rear diff fluid and check out the numbers. I've spun the tires and counted the revs of the driveshaft and also decoded the rear end. My car has 3:55's in it. It is a 1979, L-48 with an automatic. I have added headers, and an Edelbrock Performer Cam and Intake. I've recurved the distributor. Timing is set at 36 degrees total, like Lars advised with the recurve. Is the stock convertor the problem or is my tach not accurate. A review of the details goes like this.

60 mph's is 3100 rpm
60 foot times 2.13 is the best
almost 5500 rpm at the end of the 1/4, @ 98 mph
car weighs 3640 with me in it
rear end is 3:55

trans is a TH350 and is my next thing to tweak. I'm planning on a trouble rebuild and also a shift kit and was planning on a new convertor (B&M Holeshot 2400) to help my 60 foots and also my ETs.

Could my tach be messin me up a bit here or what?

With those gears should I have a better mph than I have?

Thanks as always