ET gain with gear change - Chevelle Tech
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post #1 of 23 (permalink) Old Feb 21st, 20, 9:34 PM Thread Starter
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Eric
 
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ET gain with gear change

I am thinking of changing rear gears to pick up some ET. My 71 Chevelle currently has a pump gas 540, AFR 305 heads with a Straub hydraulic roller, 3500 stall and 3.50 gears. Car runs in the 10.50 @ 129 range. I thought about picking up another center section with a 4.30 gear. Will I have a noticeable gain or is it a waste of time and money
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post #2 of 23 (permalink) Old Feb 22nd, 20, 5:45 AM
von
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Re: ET gain with gear change

In theory it should provide a noticeable ET reduction, BUT it could cause a (further?) loss of traction or further expose needed suspension mods (also reducing traction) for drag racing. Either or both of which may not reduce ET. Hook it up though and you should be smiling.
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post #3 of 23 (permalink) Old Feb 22nd, 20, 8:10 AM
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Re: ET gain with gear change

Do you have enough rpm?

Jim

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post #4 of 23 (permalink) Old Feb 22nd, 20, 8:59 AM
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Gerry
 
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Re: ET gain with gear change

What size rear tire are you running?
My 4.11s with a 28 tire cross line @ 6300 rpm unless I spin on the line.

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post #5 of 23 (permalink) Old Feb 22nd, 20, 9:29 AM
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Rick
 
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Re: ET gain with gear change

What is the RPM for the engine and what are you currently crossing the line at?
Lower gear will make more engine torque to the tires at launch. That is a big move you mentioned. Like said above, it could cause loss of starting traction and suspension mods.
To me any move lower than 411 would not be the best move.
Is your torque converter where it should be?


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post #6 of 23 (permalink) Old Feb 22nd, 20, 10:06 AM
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Claude
 
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Re: ET gain with gear change

Quote:
Originally Posted by dell72 View Post
I am thinking of changing rear gears to pick up some ET. My 71 Chevelle currently has a pump gas 540, AFR 305 heads with a Straub hydraulic roller, 3500 stall and 3.50 gears. Car runs in the 10.50 @ 129 range. I thought about picking up another center section with a 4.30 gear. Will I have a noticeable gain or is it a waste of time and money
I think it might be worth a try If it doesn't work as expected, you can always go back to the 3:50's

Claude.

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post #7 of 23 (permalink) Old Feb 22nd, 20, 10:43 AM
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Re: ET gain with gear change

Id rather have a dead-hooked 10.50 than a car that might run 10 teens if it hooks.

I dont like being manipulated!
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post #8 of 23 (permalink) Old Feb 22nd, 20, 10:50 AM
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Re: ET gain with gear change

I will give you 2 examples of gear change results

Car one was with 3.73 gear to 4.56 gear with a 427 BBC 1 tenth and 1 mph

Car two was with 4.10 gear to a 4.88 gear SBC no change in ET or MPH slight better 60ft time.

With car 1 went to different converter car picked up 2 tenths an 1 mph with 3.73's put 4.56's in car slowed down
as the 427 was not able to pull the top end as well.

Seems to me that the high winding SBC love lots of gear but most BBC's seem to like the torque to pull that car down the track. I bet a converter change would make more of a difference. Waste of money, hard to say but that's why you test stuff to see what works best.
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post #9 of 23 (permalink) Old Feb 22nd, 20, 11:33 AM Thread Starter
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Eric
 
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Re: ET gain with gear change

The car currently has a 28x10.5 slick and crosses the line around 5500. The cam is good to around 6200-6400. I would ideally like to try a 3.89 or 4.10 gear, but I found a 4.30 geared center section for sale.
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post #10 of 23 (permalink) Old Feb 22nd, 20, 11:59 AM
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Chris
 
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Re: ET gain with gear change

Quote:
Originally Posted by dell72 View Post
The car currently has a 28x10.5 slick and crosses the line around 5500. The cam is good to around 6200-6400. I would ideally like to try a 3.89 or 4.10 gear, but I found a 4.30 geared center section for sale.
The cam is good for 6200-6400 based on what? Has it been dynoed so you know where it makes peak power? What is your normal shift points?

Pick the gear ratio that gets your trap RPM to be at your shiftpoint/redline based on your tire size. That's the easy part. How much it picks up in ET, if any, that's the unknown and only one way to find out.
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post #11 of 23 (permalink) Old Feb 22nd, 20, 12:34 PM
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Re: ET gain with gear change

BIG motors do not always respond to lower gears, you may well find that changing from your present 3.50 to 4.30's slows you down, assuming you hook well now, might even have a tendency to slow your 60 foot down, I'm assuming this car is not street driven much, as 4.30's will take most of the fun out of that. Might want to do a little more research into what some of the other forum members with similar or quicker set-ups are running.
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post #12 of 23 (permalink) Old Feb 22nd, 20, 1:07 PM
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What's the time slip look like? What's the converter flash and drop back to?

I have a 4.30 gear behind a pump gas 555 but does not see long trips...
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post #13 of 23 (permalink) Old Feb 22nd, 20, 1:52 PM
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Re: ET gain with gear change

I think I'd stay in the series range. Traction is usually the issue with lower gears. There is a point where you have to spend more money for an anti roll bar, good control arms and pricey d.a. shocks to make your car stick. But if you are willing to spend the money you can do it. I think I'd not go above a 4.11 gear though even with other changes.

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post #14 of 23 (permalink) Old Feb 22nd, 20, 3:18 PM
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Re: ET gain with gear change

I am guessing it has a 9".
If so, pick up the center section and see how it works.
Otherwise call one of the convertor guys first. See what they suggest.
You could use shorter tires. Going down 1" is very noticeable.
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post #15 of 23 (permalink) Old Feb 22nd, 20, 4:08 PM
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Re: ET gain with gear change

I would stay with what you have. We had a 69 Camaro with a 505 inch big block Chevy that ran on street tires through the exhaust 1060s 130 miles an hour. We went from 373 down to 308 with almost no difference in E.T
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