Rear Gear Question
 

When it comes to rear gearing, I am not the brightest candle on the cake so, I need some help understanding the application of rear gear ratios and what the performance results of different ratios.

For example we are running a 4:56 gear and several people that I have talked to have said we are a bit over geared for our application. The people I have spoken to have said that we should be running a 4:30 gear.
My question is if we go to a 4:30 ratio gear as recommended, what performance gains should I expect? Quicker ET? Faster MPH? Both?
Also, if I changed to a 4:30 gear, does your shift RPM change? How about my 60 foot times?

Re: Rear Gear Question
 

What kind of car and class?

Re: Rear Gear Question
 

K/SA 71 Chevelle Wagon 402 big block. 4260 lbs w/driver

Re: Rear Gear Question
 

I would say, if the converter is right..That is, it flashes right around peak torque, then you might see the same 60' but more mph, then maybe gain a little ET down track.
As many here know ..there's only one way to find out for sure.

Re: Rear Gear Question
 

Powerglide or a 3 spd ? I run approx the same setup 65 Impala wagon 396-325, 400 trans 4.56 gear. If your running a powerglide I suspect you would be better off with more gear 5.57 perhaps ---Trevor

Re: Rear Gear Question
 

I run the same 402/315 but in a Camaro, it all depends on the cam, lobe sep.etc. I run a 4.56 with a tight cam,2.75 low, 4800 flash.I think you would lose 60' and ET with a heavy car like yours by changing the SLR.You can get away with a little more rpm on that motor because of more exhaust lift (.430). If you run the big hoosiers 94" as opposed to the 92.5" it will give you around 200 rpm. gsa612

Re: Rear Gear Question
 

I am surprised your only running a 4:56 and not a 4:88 . Even though it's a different combo, I have a 71 Lemans wagon I run in J/SA. To get it moving I have a 2.75 low gear and a 4:56 . Trap rpm is about 6500, I would think your 402 would turn at least that high. What rpm do you run through the lights ?

Re: Rear Gear Question
 

On mine at 3635 (G weight) in good air with the 94' tires I go through at 6700 with .56's.I can get away with .88's at altitude. You would have to check your dyno sheets and see where the motor falls off (cam type). At that weight you might be able to use 4.88's, if you have ceramic lifters make sure you have plenty of spring press, if not they might rattle at the top end with the extra rpm.

Re: Rear Gear Question
 

I agree with a 4,260 pound car.

Re: Rear Gear Question
 

I hadn't read your description yet when posting earlier.
I agree with most..4.30 is not gonna work.

Re: Rear Gear Question
 

My advise it: "Listen to Mark" and save some money down the road....For that matter, if anyone can make a 2 ton car run.......442Olds..knows

Re: Rear Gear Question
 

Transmission is a TH 350 with a 4800 stall converter. Engine RPM through the lights is 6200

Re: Rear Gear Question
 

Is it 2.75 low, how fast have you gone 60', ET, mph?, shift points?. I would try 4.88, your 600 lbs. heavier than me.The difference between .56's and .88's is about 300 rpm on top end.You have to move that weight off the line in order for it to ET well.

Re: Rear Gear Question
 

First gear is 2.75. Fastest the car has ever gone was at Martin last August. 60' was 1.62 ET was 12.25 @ 108 MPH. We have played around with shift points from 6150 to 6250. Didn't really make a difference. We were at the track last Friday and the best I went was 12.39 @ 107. The air was good and it was a nice cool day, should have gone better than what we did.

I have to admit, most of the time when I go through the lights, the shift light does not come on in 3rd gear, so I am not reaching the set engine RPM.

My brother and I feel the car has more in it but are struggling to find the extra ET and speed.

Re: Rear Gear Question
 

Something's not right, your trap rpm seems low with 4.56's, I don't know what cam you got. What's the spring pressure? I'm going through at 6700 with.56's with the 94'tire. Mine goes 11 teens on a average day with 1.45 60' in G.(with no ice or lw oil).I found on these motors because of the cyl. heads (105cc) and the lower piston dome require a different set up than a 325-396 in terms of total timing, jetting, cam position, usable rpm. I would try .88's on your car if the springs check out ok. On a side note NHRA's Danny Gracia ran a wagon like yours a while back(80's). I was going through teardown 20+ years ago and he gave me some good advice/tips on the 402 combo lol..

Re: Rear Gear Question
 

GSA612 you have a PM

Re: Rear Gear Question
 

Would you happen to have run the engine on a dyno, and know what the basic HP and Torque curves are ? When you say you have a 4800 stall converter, is that what it flashes to off the starting line ?

Re: Rear Gear Question
 

I have dyno sheets. Yes the conver flashes @ 4800 at the starting line.

Re: Rear Gear Question
 

Just curious what rpm peak torque and peak HP is at. Converter seems pretty tight.

Re: Rear Gear Question
 

I'll look at the dyno sheet and let you know.

Re: Rear Gear Question
 

This is a great thread for me as it answers basically my same question. I am building a 1965 Impala with the 396/325 BBC combo for I/SA at 4100 lbs. I will be using the same motor from my 68 Impala I ran but that was a Stickshift car (Jerico) I will be running a Turbo 350 w the 2.75 first. I will be using the rear from the 68 which has 4:88's in it. I do have a set of 4:56 which I thought might be better with the Turbo 350, but i think I will stick with the 4:88's at first. I have heard the same thing going as high as 4:30's, but i feel with the weight of the car the 88's will be better.

I think my best 60' time was a 1.60 with the 68' , but I feel I never got the clutch set-up right (Mcleod Rev-Lok) I did MPH pretty good (114-117 MPH ) that was at 6600 RPM through the traps, so I am hoping with the automatic I can see the full potential of this combination, seeing that the index is 2 tenths quicker than when i last ran! (I ran 11.70's with the 68)

Any input this way would also be appreciated.
Will Lamprecht
Div 1, 65 imp big ole chevy

l

Re: Rear Gear Question
 

Yeah this is a great thread.
My 1969 Impala still under re-construction has 5.13's.
Have a Budd Rowe built 350/255 in storage with an early cam.
The Dyno shows it is all done by 6000rpm.

Building it to run Stock. For now will run Pure Stock until a modern cam is installed.
Still have the 3800 TCI Converter and TurboAction 350. This will definitely need some upgrade.

Will's 396/325 would be a 427/335 for my car and may consider the larger engine.
Both have the same cam and the shift points should be lower.
Think it is a better combination than the 427/390.

Would need the T350 with a 2.75 gear for either car.
Get the feeling the 5.13's need to be 5.38's for the 350.
But the 5.13's maybe be too much for a 427.

Dan

Re: Rear Gear Question
 

njk 4.88 or 5.13 converter is close; try stock gears in trans.More power always helps. test,test

Re: Rear Gear Question
 

I agree with the testing but Milan Dragway is an hour and fifteen minutes away. Wish the old Detroit Dragway was still there that was a 1/2 hour drive.

Re: Rear Gear Question
 

Just put the 5.86 in like mine. Lmao

Re: Rear Gear Question
 

Re: Rear Gear Question
 

Bobby, I figured you for a 6.50 rear gear! ;)-

Wade

Re: Rear Gear Question
 

Nelson, I started reading this and realized this my old car. I would like to save you some money, don't try 513 gear, it lost ET. I assume that you are still running the motors that I had built from Parson/Meyers, I would start with valve springs and more valve spring pressure. Valves spring pressure is the key to horsepower for stockers and as you know I spent a lot of money on that car learning. Hope this helps

Re: Rear Gear Question
 

Dave, I am not going to go to a 5:13 gear. We still have the 4:56 gear that you had in it. However, my brother talked to Bruce and he said the car should be going through the lights at 6400 rpm and it isn't. We were thinking on going to a 4:88 to get the car off center faster and increase the trap rpm as Bruce suggested..

Re: Rear Gear Question
 

Nelson: Just my opinion and you've been given plenty, but converter seems too tight, maybe 200-300 looser. I'm not sure what circumference tires you have, but they can be used to "tweak" the RPM's a bit, particularly when you're using a 2.75 trans. Don't forget that the wider gearing of the trans. makes the RPM's fall down more on the gear change. If you're running a 94" tire, a 92" tire will help get the RPM's up. A looser converter will help as well on the gear change.

Re: Rear Gear Question
 

the 4.88 is cheaper then the 92" slicks & IF it dose not help pretty easy to sell ;) After testing with the 4.88's you might want to call a converter company with all your info you might be a bit to tight on the converter. ( more gear will tighten converter flash)

Re: Rear Gear Question
 

Jim K we are running a 94" diameter tire. Although, we have about 30 runs on the current slicks and could go to a 92". Who is providing the good converters nowadays?

Re: Rear Gear Question
 

PM sent...Inbox full

Re: Rear Gear Question
 

Mark Y, re-send PM I cleaned out my messages. Thanks