0 - 60 mph :) - Pontiac GTO Forum
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post #1 of 29 (permalink) Old 09-17-2019, 03:29 PM Thread Starter
 
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0 - 60 mph :)

Well I just used dragy and did my first 0-60 run. Needless to say, the launch was less than ideal, but the car is clearly underpowered. I will post the data shortly, but 0-60 mph was 7.6 seconds. I was hoping for 5.x....I have a bit of work to do.....

1965 GTO Convertible 3 speed tri-power drum brakes originally

Current setup:
400 cu in with tri-power
4 speed Muncie
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post #2 of 29 (permalink) Old 09-17-2019, 04:58 PM
 
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I love your post! So total newb question, when I floor my goat, and after I finally get traction, it seems like it downshifts, but I watch the RPM go over 4,000 and I'm worried it will not shift into the next gear, likely resulting in an explosion of what was once my beautiful engine, so I let off on the gas. Is there a trick here, should i be letting up on the gas to let it shift? Ok, maybe I made 4,500 once.
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post #3 of 29 (permalink) Old 09-17-2019, 05:36 PM Thread Starter
 
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Originally Posted by Ebartone View Post
I love your post! So total newb question, when I floor my goat, and after I finally get traction, it seems like it downshifts, but I watch the RPM go over 4,000 and I'm worried it will not shift into the next gear, likely resulting in an explosion of what was once my beautiful engine, so I let off on the gas. Is there a trick here, should i be letting up on the gas to let it shift? Ok, maybe I made 4,500 once.
Well my car is a 4 speed manual and I am shifting at ~5000 - 5500 rpm. Your automatic should go above 4500 before shifting if you are going WOT.

1965 GTO Convertible 3 speed tri-power drum brakes originally

Current setup:
400 cu in with tri-power
4 speed Muncie
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post #4 of 29 (permalink) Old 09-17-2019, 07:03 PM
 
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Originally Posted by cij911 View Post
Well my car is a 4 speed manual and I am shifting at ~5000 - 5500 rpm. Your automatic should go above 4500 before shifting if you are going WOT.
Ok, maybe I just need to be a bit more brave and wait it out!
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post #5 of 29 (permalink) Old 09-17-2019, 08:45 PM
 
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The governor of your auto trans can be modified to shift higher or lower. My trans guy rigged his last bracket car to shift about 5200, if I remember correctly. He just launched in drive, & let the trans shift itself.

https://www.hotrod.com/articles/hppp...ing-turbo-400/

https://www.jegs.com/i/JEGS/555/62070/10002/-1

A healthy 400 should be plenty safe to 5500. But, most smaller cams don't make good power to 5500.

Anyhow, if you're afraid you'll over-rev & blow your engine up, you can install a rev limiter, & set it at 4800, 5000, or where ever you want it. I used a rev limiter on all my 455 bracket cars. They saved ALL of those 455's, some several times, when they'd break the trans or rear end.

https://www.ebay.com/itm/MSD-MSD-872...32862842fdc94d

https://www.amazon.com/PerTronix-600...ustomerReviews

You can buy from Advance Auto & use an online 25% off code. That makes it the cheapest price for lots of parts. I've saved quite a few bucks with the 25% off deal.

https://shop.advanceautoparts.com/p/...B&gclsrc=aw.ds

Pertronix also makes a points replacement & HEI module with built in rev limiter.

https://www.summitracing.com/parts/pnx-d72000/
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Last edited by bigD; 09-17-2019 at 09:20 PM.
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post #6 of 29 (permalink) Old 09-17-2019, 10:34 PM Thread Starter
 
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BigD - What about gearing ? My 4 speed + 3.55 + tire combo requires that I shift into 3rd gear to hit 60+ mph. Given all the low end grunt, would it not be better to have a taller rear and only have to do a 1-2 shift to hit 60mph and be easier to launch? (Plus added benefit of lower RPM on the freeway.)

1965 GTO Convertible 3 speed tri-power drum brakes originally

Current setup:
400 cu in with tri-power
4 speed Muncie
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post #7 of 29 (permalink) Old 09-18-2019, 07:35 AM
 
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Originally Posted by cij911 View Post
BigD - What about gearing ? My 4 speed + 3.55 + tire combo requires that I shift into 3rd gear to hit 60+ mph. Given all the low end grunt, would it not be better to have a taller rear and only have to do a 1-2 shift to hit 60mph and be easier to launch? (Plus added benefit of lower RPM on the freeway.)
As I see it, higher numerical gear ratios have only one main advantage. Straight line acceleration.

Here: are some of the advantages of lower numerical gear ratios:

(1) Lower highway & cruz rpm.

(2) Better gas mileage.

(3) Should give engine a longer life.

Some disadvantages:

(1) Less acceleration.

(2) Might cause more clutch wear, because of more slippage required for getting the car moving.

SO, for street driven cars, the best gear ratio is a compromise. And, it depends on the driving habits & goals of the driver.

(1) If you do LOTS of high speed freeway driving, then ratios between 2.41 & 3.08 might suit you best.

(2) If you prefer lots of acceleration, & don't really care about fuel mileage, then ratios between 3.55 & 3.90 might be better for you.

(3) So, the compromise gears might be 3.23 to 3.42. I think these were the only 2 options for the '78 W72 4-speed Firebirds.

All this is assuming a 1:1 high gear ratio. An overdrive trans changes things.

Taller or shorter rear tires can also change things, slightly.

4-speed manual trans are available with numerically higher 1st gear ratios, which will work better with numerically lower rear gears. My '69 GTO had a 2.20 1st gear ratio, with 3.90 rear gears. I loved it. But gas was lots cheaper back then. I think the most common 4-speed 1st gear ratios were in the 2.50's & 2.60's. I think 2.88, 2.98 & maybe even 3.08 are available.

https://midwestmuncie.com/product/au...ear-end-gears/
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post #8 of 29 (permalink) Old 09-18-2019, 07:46 AM Thread Starter
 
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Interesting.....I would have thought the car (with all the low end torque) would benefit from a 3.08 or even lower (taller) as the tires would not just spin and would save you a gear shift (time) from 2-3 in order to get to 60 mph.

1965 GTO Convertible 3 speed tri-power drum brakes originally

Current setup:
400 cu in with tri-power
4 speed Muncie
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post #9 of 29 (permalink) Old 09-18-2019, 08:54 AM
 
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Interesting.....I would have thought the car (with all the low end torque) would benefit from a 3.08 or even lower (taller) as the tires would not just spin and would save you a gear shift (time) from 2-3 in order to get to 60 mph.
If you have lots of tire spin, your 0-60 time is not gonna be quick, regardless of what rear gear you have. Back when I raced my '69 GTO with hard street legal tires, I popped the clutch at about 3000 rpm, but i couldn't go to full WOT until just before the tires quit spinning. If I went to WOT any sooner, the tires would just keep spinning for quite a while longer, resulting in a much slower ET.

With a numerically low rear gear, if the tires don't spin much, but rather hook up at low rpm, it might actually "bog" the engine down, til the rpm get up into near the peak torque range. The bog will be worse, if the cam does not provide much low rpm torque. So, the trick is to get just the right amount of tire spin, but not too much, & going to full WOT at exactly the right time.

With the engine in it's peak torque range, and the tires hooked up, acceleration will obviously be better with a numerically higher rear ratio. And, as for the extra shift, with a little practice, the shift only takes a small fraction of second. So, the numerically higher rear gear ratio will more than make up for anything lost to the extra shift
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Last edited by bigD; 09-18-2019 at 09:11 AM.
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post #10 of 29 (permalink) Old 09-18-2019, 10:25 AM
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Average times 0-60 in the muscle car era for the GTO ranged from 4.9-7.5 seconds or so, depending on all of the variables covered by Big D. A 4 speed, 3.90 geared tripower '65 was seconds faster than a 2 speed automatic, air conditioned, 3.08 geared 4 barrel GTO of the same year. My '65 came with tripower, 4 speed, gauges, and a 3.55 posi. No power steering, no power brakes, no seat belts, no outside rearview mirror. I didn't mind the 3.55 gears when I got the car almost 40 years ago when the national speed limit was 55 mph. Where I lived, everybody cruised 60.....65 was a guaranteed ticket. When the law got repealed in the '90's, I changed to 3.36 gears and 29" tall rear tires, which allows a comfortable 70 mph cruise just under 3000 rpm. My '67 automatic car had a 3.36 open rear end.....I changed that to a 2.56 posi about 10-12 years ago. 80 MPH cruise is 2450 rpm getting 20-21 mpg.....90 mph is right at 3000 rpm. The engine runs cool as well. I use this car for long trips over open country. 1000 mile weekends are not uncommon. With my lazy gears, I'll bet my 0-60 is about 8 seconds.....but I don't care. My car will hold second gear until 105 mph....which is more than the top speed of many shorter geared cars.
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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, LeMans & GTO Engine Tuning and High Performance

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