I can't get this thing out of 3rd at full throttle. What am I supposed to do at the strip, back off to half throttle and just sorta cruise the last few hundred feet? Doesn't seem to be the best way to get a good et. I know B&M makes a kickdown valve, but it makes it so the thing will kick down past 3/4 throttle, so that still won't save me at the strip. What about that corvette thingy? Did I just make that up or does it really exist? And can I get into 4th at full throttle with it? Anybody have a part number? And how do I install it? I'm sure B&M would send me a 10 page installation booklet if I ordered their part, but if I get the vette part I have no instructions. So where's it go? In the valve body, pressure regulator, or somewhere else?

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91 Z28
Come see The Vicious...

 

Skip the B&M part and get the GM part. It's cheaper, works the same...

It sounds like you overgeared as you should run though the traps in third at redline.

 

Yup I hate this damn kick down bull ****. I hate racing people and only being able to go 1/4 throttle. If I could just floor it I'd probably beat people alot faseter.

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Chris
91 RS 305 TBI
Eternal Rev

 

Yeah, I cross the finish line at about 6 grand in third. But that's with the tired old stock 305. My 355 will be running soon and I'm gonna need that 4th gear. Anyone have the GM part number? And where's it go in the tranny?

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91 Z28
Come see The Vicious...

 

I agree. OD in a 1/4 mile run is a no-no. Here is a rundown of why you don't want to shift into an overdriven gear in an acceleration application.

You never want to upshift into an overdriven gear in any acceleration situation because it eats up all of the torque/power your engine is making through the increased effort the engine has to endure just to turn the gears in the transmission. You should run through the traps in third gear in a 1/4 mile run as it gives you the highest speed losing the least amount of power through the drivetrain. This is most easily understood if you have a gear set out of a transmission that you can turn the input shaft in each gear range by hand. You can really feel the difference in the effort needed to turn the input shaft this way.

If you have a gear ratio of 3.06:1 (such as in the TH700R4s first gear) it is very easy to turn the input shaft and thus get the car moving down the road quickly but the top speed attainable is obviously very low - thus the need for more gears. When you go down in the ratio (up in the gears) to second (2.54 - I think - in a TH700R4) it is harder to turn the input shaft and still harder when you get up to third gear which is 1:1 in nearly every automatic transmission. Now when you get into fourth gear, which is called overdriven because the output shaft is turning faster than the input shaft, it is VERY hard to turn the input shaft which equates to lost power in the turning of the transmission itself which means bad news in the acceleration department. It is much easier to understand if you can actually turn a gearbox yourself but hopefully my explanation is decent enough.

So I agree that you may have too steep of a rear axle ratio in your car right now if your engine does not pull your car through the traps in third gear. A smaller rear axle gear ratio might be needed.......

The main issue that decides what rear axle ratio to run is what your powerband is like. If your car is all done making power at 4500rpm, you won't want to run much over a 3.42 gear - even if it is a track-only car as your engine won't be pulling the car down the track at the top end since the engine is just winding out without making any power. On the flipside of that, if your engine makes power to 7000rpm, a 4.10 gear is more to your engines likeing as that will put your engine at it's peak power output right at the end of the 1/4 mile, which is exactly what you want. You want to go through the traps, on a 1/4 mile run, with your engine making slightly past it's peak power, in a 1:1 gear ratio (third gear in virtually every auto trans) of the transmission. This relationship, with respect to your powerband, is what dictates which rear axle gear ratio is ideal for your application for 1/4 mile performance. The trick is to balance this with everyday manners for a mostly street driven car - which is made much easier with a TH700R4 since it offers an extra gear for cruising and everyday driving.

Hope that all helps some......

Laterzzzzzzz
Good Luck

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1987 GTA L98 MD8
355, TFS Heads, LT4 Hot Cam
My GTA

The Minnesota F-body Club

 

Grrrrreat. So what you're saying is that if you're planning on running anything faster than 14 seconds, a 3.73 is too steep unless your engine revs to 8000 rpm. Looks like I better get to putting those land-speed-record 2.73s back in.
Or would I do better to just cruise the last few hundred feet with the engine screaming redline? And not increasing my mph.

What was that part number again?

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91 Z28
Come see The Vicious...

 

8000rpm? Well, no. You can most certainly run a 3.73 gear in a car that is faster than a 14, but the engine will have to have a powerband that supports that kind of rear axle ratio. It's all a package and it all works together... If you are telling us that you are having to shift into fourth gear at the end of a 1/4 mile pass because your engine is just revving to beat hell without making any power anymore, then you have too much rear gear and should put something a little lower (numerically) in there.

I wouldn't go that far. I am going to assume you are kidding with that comment as I am sure you know that a 3.42:1 or some other gear in between a 3.73 and a 2.73 would get you where you need to be. If you find your car is only ending up a few hundred feet 'short' in 1/4 mile as far as it pulling in third gear goes, than you are likely not that far off with your current gear. I would say something like a 3.55 or 3.42 gear would work well with your setup.

Well, no, that is not the answer either. Obviously the proper rear axle gear ratio is the answer to what you need for your car to run it's best. But I would bet that running the car through the traps screaming past redline would yield better results than upshifting into an overdriven gear ratio.


GM part# 8673948 is the kit for the 'no downshift into third from fourth under 3/4 throttle'. This valve is also supposed to allow WOT upshifts into fourth gear. The B&M valve for the 'unwanted downshift condition' supposedly makes the 'no WOT upshift into fourth gear' worse. Apparently there is an issue with the GM one limiting the downshift into third to the point that the car must be floored completely to get the shift to happen at lower speeds (for passing) but that doesn't sound like it would be much of a hinderence to me..... I guess there just is no completely perfect valvebody out there for EVERY situation........ but the GM valve should take care of your situation.

Laterzzzzzzzz
Good Luck with all-a-dat........

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1987 GTA L98 MD8
355, TFS Heads, LT4 Hot Cam
My GTA

The Minnesota F-body Club

 

Ok matty, one more question:
Where does it go?

Better make that two: when I get my T56 will I still have this problem? I mean, is 1:1 the same in all transmissions, and if so, why would anyone want to run a 4.11 (unless they had huge tires)?

[This message has been edited by ViciousZ (edited October 11, 2001).]

 

Because some peoples cars don't start getting into their powerband until 3000+ RPM's and redline over 7,000RPM. Then you just don't want 4.10+ gears... they are pretty much required, as for a high stall converter to keep the car from being a dog out of the hole.

A high stall converter and deep gears behind a motor that is pretty much done at 4500rpm make no sense and will slow the car down.

 

Matty huh? Alrighty.... ........ Well, Michelley.... ...... The sleeve is for the 3-4 throttle valve as the 3-4 TV valve is what controls 3-4 upshifts (and downshifts). It is located in the 'front' of the valvebody, right in front of the 3-4 shift valve. I would imagine that the kit includes instructions...... but I have not actually installed one of these kits personally. BTW, I was looking through my Hydramatic Product Update manual and I spotted another update that covers this issue but only on 1989 B-body (Caprice) cars with 5.7L engines. It addresses your condition precisely but it should already be fixed on your transmission since it is newer than 1989...... Did your B&M shift improvememnt kit have you modifying the 3-4 throttle valve in any way?

If your engine still has the same powerband, and you still have that rear axle ratio that is evidently too steep for your application, then yes, you will.....sortof. Manual transmissions will also have a 1:1 gear ratio but it varies which gear it is depending on how many gears you have. 5th gear in a T56 is the 1:1 gear so you will want to blast through the traps in 5th gear. If you had a T5, 4th gear would be 1:1. As far as the mechanical relationships go, a manual is no different than an auto. You still don't want to go through the traps in an overdriven gear, you want to be in a 1:1 ratio while you pass through the traps. So your problem will pretty much be the same but the actual gears will be different.

I think the above answer should explain why people run really steep rear gears. He hit the nail on the head. The people that run these gears successfully virtually require a gear that steep and an extremely high stall converter because their engine's powerband is so high that it needs to get it's rpms up really high really fast just to perform. Just remember that it is all a package and no one rear axle ratio is THE ratio to run in every situation.

Hope that all helps ya.......
Laterzzzzzzzzzz

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1987 GTA L98 MD8
355, TFS Heads, LT4 Hot Cam
My GTA

The Minnesota F-body Club

 

I know this is a really old post but I need one thing explained to me:

Shouldn't the effective gear ratio - the ratio at the wheels - and not the gearbox ratio be the thing to consider? I mean, so what if the flywheel is turning faster than the rear of the transmission. Wouldn't a lower rear gear compensate for this condition?

Thanks for your help!
Kyle