jptsetme

My '75 4-speed's transmission was the weak spot in the car when I bought it a few years ago. 2nd gear synchronizer was shot and the clutch was mostly shot. A couple of years of driving, and one day it gave it up. I've been told it may be the throwout bearing, but the symptoms are difficulty getting the clutch to disengage and slipping in all 4 gears (driving home that day at 55mph on the highway, if I pushed hard on the gas, it would slip immediately and the tach would spike).

My goal since I bought the car was to do a 5-speed swap, so it didn't make sense to pour money into repairing this transmission. Instead I saved money (with the car not running for a while) and am now ready to buy a kit. But I've run into a surprise and could use some opinions/advice.

One of the main reasons I wanted a 5 speed was because my RPMs were so high on the highway -- 70 mph = 4000rpm. This seemed to match with what people reported for 4.11s, so even though I knew that was a rare rear end ratio, it made sense. But, now that I'm ready to buy a kit, I went to verify the rear end ratio, and it's a 3.36 (verified by turning the rear wheel once while counting driveshaft rotations). This leaves me scratching my head about why my RPMs are so high. I verified that the transmission is a Borg Warner Super T-10 (it has casting number 1304065903 on it). My first guess is that the engine is weak? I know it's a V-8 with a Holley 650 Double Pumper carb (List 4777-4), I know that the block is not original, and I know that the intake manifold is original (stamped 346249). Beyond that, I don't really know what's under there (see my old thread on this for pictures and some more details if they could help: http://forums.corvetteforum.com/c3-t...rcheology.html )

I had hoped to someday replace the engine anyway, but it's definitely not in the budget right now. I'd like to move forward with the transmission swap so I can drive it again and then consider the engine at some point down the line.

To be honest, I'm a novice at all of this. My questions are:

1) Since the transmission seems to be the original, I believe I can still go ahead and put in a 5-speed that's designed to replace an original 4-speed (leaning towards the Keisler RS400). Is there any reason to worry that this would not work (based on the uncertainty for why my RPMs are so high)?

2) Any guesses as to the cause of the high RPMs? I don't think the tachometer is wrong -- the engine is roaring pretty good at 4000 RPMs. If it's a weak engine, what might that mean? Smaller than a 350 -- what are the other likely candidates? 327, 302, something else? What's the least intrusive way for me to figure out what I have?

Thanks for any help you all can provide!

qwank

Maybe you counted the number of turns wrong? you can get a little more accurate by turning the wheel 10 times and counting the driveshaft rotations then divide that number by ten.

You can still do a 5 speed

jptsetme

I'm very confidant that I counted the rotations right. I did it at least 5 times to be sure, b/c I was completely expecting 4.11s. The last time I tried it, I rotated the tire three complete revolutions and I got 10 revolutions of the driveshaft.

RobbSalzmann

Based on this I would SWAG that the rear is a 3.42. With 4.11s, using your procedure you would see a bit more than 12 turns of the driveshaft.

81_vette!

My 1977 4-speed specs elude me as well. 86k actual.

I have confirmed that my RPM gauge reads TWICE the actual RPM's for the most part. I Idle at 1500 according to the dash gauge, but my RPM tester/meter returns an actual reading of 750.

Driving 65 on the highway would yield an RPM reading of 5500+. My needle would be on the yellow/red line.

My four speed is very tight and responsive - but it's like driving a tractor. You can start off in 4th gear without so much as a ping. I somehow thought the PO had put in short track gears... or original? I dunno.

Also, my speedometer is 10 mph off as well when you get above 40 mph. I am running a different set of wheels/tires... but it was 15 mph off before they were installed.


I am beginning to feel some sporadic "funkyness" in first gear... so I suspect a transmission/clutch might be in my future as well.

I just wish I could get all the details from the PO, especially the top end kit.

TheSkunkWorks

Either get an accurate count of how many times you have to rotate the driveshaft to turn the wheels over once or turn the wheels over 10 times, counting how many times the driveshaft rotates, and divide by 10. FWIW, engine torque/power doesn't affect gearing; rather it's gearing that affects how much available torque/power is multiplied. As for inaccurate tach readings, that's another issue to solve, but doesn't affect actuall RPM or gearing.

So, you really need to get an accurate read of your rear ratio before you commit to the transmission, then look for transmissions that will yield somewhere in the ballpark near 10:1 multiplication in 1st for use with common C3 tire diameters. There is some wiggle room, but you don't want to stray any further away from the ~10:1 rule of thumb than necessary, particularly in the direction of less multiplication with a weak mill. Conversely, anything deeper than ~12.5:1 combined will feel like granny-low in a dump truck whatever engine is ahead of it.



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jptsetme

Thanks for everyone's input on this.

Following TheSkunkWorks' advice, I crawled back under the car and turned the wheel 10 times. I got 33 and a shade more than 1/2 rotations of the drive shaft, so it's definitely 3.36. From what I've read, that was the standard ratio in 75, with a 3.08 economy option (plus all the special order ratios), so it makes a lot of sense (aside from the high RPMs which I still don't understand).

The transmission I was most seriously considering is a Keisler RS400/RS500, and it has a first gear ratio of 3.37. If I understand correctly, that would give me an 11.3:1 final drive first gear ratio. A Tremec TKO-500 with a 3.27 first gear ratio is 11:1. (Assuming I just multiple the two ratios -- tires are 245/60R15). From what you've said, these ratios both seem high -- I don't want first gear to be so short it's unusable.

I'm thinking that, since 3.36 was a really common differential, there must be a lot of folks who have installed these two transmissions. Can anyone comment on the first gear final drive ratio? Happy, changed gears, otherwise?

Also, I had hoped to try to do this myself but I just don't have the time. (I also don't have the skills, but I was hoping to learn.) At this point, I just want to be able to drive the thing again. Can anyone recommend a good installer in the Raleigh/Durham (or nearby) area? Thanks again for everyone's help.

BKbroiler

Since you know that your clutch slips badly, that could be the cause of higher than expected rpms at 70 in 4th gear.

scottyp99

Your tach might be off. Beg, borrow, or steal an aftermarket tack, hook it up, and see what rpm you are really spinning at. The stock tachs are notoriously inaccurate.


Keep the shiny side up!
Scott

BKbroiler

Electric tachs? What makes you say that?

TheSkunkWorks

FWIW, there are more gearing options, and here are a few others that might be worth your considering...

The Richmond SS 5-speed w/OD is available with 3.06:1 1st and 0.77 5th gears, yielding 10.28:1 and 2.59:1 respectively with your 3.36 rear.

The LTG SS700 Wide ratio boxes have 2.97:1 1st, for 9.98:1. Top gear options are 0.68 or 0.80, for 2.28 or 2.69 in 5th.

IIRC, you can option a TKO with 2.87:1 1st and yield 9.64:1, with 0.64 and 0.82 top gears available for 2.15 or 2.76 in 5th.

That said, 11:1 isn't too awfully deep as to be monotonous. I had a 12.21:1 combination in my '78 for a while, and IMHO that was getting near the borderline behind the built 427 in at the time, but acceptable. Ran a 14.14:1 combo in it for a short time, and that setup drove me nuts during normal driving, tho it pulled like a beast. FWIW, my former ZR1 with 4.56 rear and M22 box came out to 10.03:1, and that was pretty much a factory race car.

In any case, while focusing on 1st gear, bear in mind that it is possible to go too tall in OD and cross the point of dimenishing returns, so I'd suggest sticking with >2.5:1 final.

Hope something I've shared helps, but don't overthink it on my part. YMMV

mrvette

YES, my electric tach is inaccurate, it's 400 rpm high coming in at 3000 rpm and stays that way, but at idle it's fine up to ~2500 rpm....

I know this because of my tektronics scope on the tach wire.....

I also know the speedometer WAS totally inaccurate, as I had chosen the driven gear for the tranny by some tables off the net, and they all came up with the same gear teeth count....RONG.......

the speedo was ~12% over indicating....maybe more....

SO I bought a Garmin GPS some years ago, corrected the speedometer to be dead nutz on till 70 mph, after that it says 80 and I"m going 78, when it says 100, im going only 94......I think it's done deliberately by the factory....

Jedi926

heck, my tach doesn't even work!

BKbroiler

I was asking about tach accuracy because mine is very accurate, at least in high gear comparing to calculated rpms at a given speed.

qwank

go with a Richmond 6 speed with a 3.06 first

MotorHead

A TKO 600 with 2.87 first gear is good, that's what I have with 3.08 rear end. Gear ratios and rear end ratios are just bandages for low power motors, don't get me wrong here even the new Corvettes have gears but like I said they are just torque multipliers.

A Top Fuel car has one gear and it can't even start with that gear totally engaged it has a set of clutches that come on at different speeds until the end of run they are all locked up at 330mph or more so you can see if you have the power you can go below the 10:1 gear ratio in first just fine.

I agree with the slipping clutch being the problem of high rpms on the highway.

I had a '68 Chevy pick-up with a granny gear for 1st, never used it, started in second gear