Have a chance to buy a complete 3.55 for a decent price. Current ratio is 3.08 with a TH350 auto. Would this make any signicant difference in my 454 BB? I have not driven the car with the new motor..so I have no referenence point. Want to use it for street / strip /stop light action. No Interstate. If it was your money, would you pick something else?

 

For the few tenths of a second you gain in the quarter mile I don't feel it is worth it. You have a bigblock and it doesn't like high rpm on the open road and if you want to take it to the strip it usually involves open road running.
Your rpm will be too high without overdrive to enjoy the car.
I have had 308, 355, 370,411's , 456 in various cars and I am back to 308's in the vet and the 456 in the mustang are going to be replace with 355 and I already have overdrives in both.
At most you will gain a few tenths in the quarter. Are you that despirate for a few tenths???

 

Well Norval, I just don't know. I keep buying all this stuff and have never really ever driven the car (still on stands). I want the car to pin me in the seat with a big $%# eatin grin when it's floored. Not concerned about ever driving down the freeway...just around down. The 500 hp BB may have the oomp to pull the current 3.08s..I don't know...but that's why I'm asking. I run my motorcycle at the strip and 1/10's are an eternity at the strip...

The 3.55 rear unit I'm looking at is complete for 150.00. So if I can get a couple of 1/10s for that it's cheaper than expensive engine work - right?

 

For that price, I say go for it. Try 3.55's for a while and see how you like it. Either way, you nail it and nothing but tire smoke...

 

Tire smoke is good....

 

hows it going...i'm in a similliar situation....i have a 454 in my 77' with a 700 trans but the gears r still 3:08....i was thinking of changing up but backed down.....she still is bad enough.....but it's still up 2 u....and that is a great price....but like norv... said it's not really that big of a diff. n believe me u will luv the cruise wich u cant do with high rpm......

 

My sb turns 3400 rpm at 70 mph with 3:55 gears. Nt much fun to drive above 50 miles per hour for long periods. It is just OK around town. I have just installed a 5 speed to get me, both good acceleration off the line and to cruise at 70 at 2200 rpm. If and when I ever need to change diffs I will go with a 3:08 or 3:336. With your bb creating even more heat the 3:55 would make it run a little hotter, even around town unless you went to an overdrive tranny.
Bernie

 

I agree with Norval on this one. I was running 456's in my 1979 350-375hp with a TH350 auto, shift kit & 2400 stall. The 456's were very quick from a standing start, but to run 60mph my engine was running about 4200 rpm. My engine at this rpm was drinking fuel, and cruising around town with 456's it swallowed fuel.
After a talk with Norval & Doug (carguy4sure) at a local cruise night, they suggested I switch to a lower gear (308). I installed 355's, 4 weeks ago. I road tested it to make sure everything was set correct for the summer ahead. It still has good standing start take off, but my rpm is lower at normal speeds, which in turn I hope my engine will not consume fuel faster than my wallet can supply the money to feed it. I'll run 355's for the summer ahead, and if I still cusume large amounts of fuel, next fall it will be 308's or an overdive transmission.

I also have a 1971 BB 454 - 4 speed and I'd need a bank loan to feed it fuel, if it had 456's. Gearing is cheaper than engine work in the short term, but the extra fuel you'll burn will in no time pay for the motor work.

 

i have 3.55 in my car now, with my 383 and had a car before with the 3.08. and i like the 3.08 much better. now i am only running a th 350 trans but even stomping it around town the gears just seem to be too low. i really wind the hell out of the engine, even up the high way if you just romp on it for a few seconds the car really does not like to get of 90 mph. not that i ever drive that fast but for 150 bucks cant beat it

 

$150 bucks to try 3.55? Go for it. If you don't like it, sell it for $150. I ordered my car with 3.55s, which were too low for all of the hiway driving I was doing. Once I put the 700r4 in with .7 OD, 3.55 seems perfect. Are you gonna run overdrive?
Bee Jay

 

You might want to do some investigating, Bernie. That rpm at that mph and the factory tire height says more like a 3.90 gear. Either the tach is wrong, the speedo is wrong, you got tiny tires on the thing, or you don't have a 3.55.

 

One thing - check out that $150 differential. If it's from someone you know ok, but if it's on ebay,in the paper, junk yard- buyer beware!

I looked at a few used differentials, not one of which I would use before rebuilding it. Granted, since I rebuild these I may be more critical but what are you saving if there's a problem? FYI the last BB diff I did was a new 336 gear with M-20. Works pretty good.

 

One thing to keep in mind with an automatic is that the torque converter acts as a torque multiplier when it "slips", unlike a clutch in a manual trans car where slipping simply amounts to your engine torque going up in smoke with no multiplication.

You will typically find that manual trans cars will have higher numerical axle ratios than an otherwise similarly equipped auto trans car. The reason for this is so that the bulk of torque multiplication occurs through the rear axle of the manual trans and limits the amount of slip required of the clutch. The automatic turns the slippage into additional torque to overcome the torque multiplication disadvantage of a smaller rear axle ratio. The net effect is to achieve similar rear axle torques for either the manual or automatic equipped drive trains. The automatic offers the advantage that at highway speeds where little torque converter slipage occurs, the engine turns at slower speeds due to the smaller rear axle than the manual trans car with the larger axle ratio.

For your automatic, going to the higher rear axle ratio will reduce the torque multiplication effect due to slippage of the converter, and trade it off for less convertor slippage and more torque multiplication at the rear axle. This is especially true for a light car like a Corvette which accelerates relatively quickly any way.

So what does this all mean? You may or may not feel significantly greater acceleration by going to the 3.55's. If you had a manual trans, I'd say you'd definitely feel it.

For $150, I'd say that's an excellent price for a Corvette rear end in good shape. What have you got to lose except for the time involved in installing it....or possibly the time involved in removing it to reinstall your 3.08's if you don't like it?

 

I think the gear change would be like adding about 100 hp. Pretty cool!! I run about 2800 rpm @ 70mph with my 3.08, so with 3.55 you should run about 3200 @ 70. Your choice. For the price, I would jump on it. If not, where are you? Can I buy it?

 

I am going thru the same problem and the only answer I could come up with was going to a 500 TKO transmission. Leaving my 3:08 rear end in place. In my case, I will have the equivalent of a 3:88 in first and a .64 OD in fifth.

 

My 79 was running the same as Bernie's and it is definitely 3.55's It sucked cruising the highways

Chris

 

I'm almost convinced......

 

Doug/carguy4sure had the same problem. He had 3.55 and hated buzzing the motor so high running with Brian and myself.

A tremec with it's low 3.27 first gear and .68 overdrive is the answer. It gives you a great 1st gear off the line and a great top end cruisng gear.
You can even run 4.11 rear gears and have a final drive ratio better then a stock set of 3.08's

Gearing and non overdrive transmission don't make for nice cruising.

 

I changed from 3.08 to 3.55 with a TH400. I didn't start to enjoy the change until after I put the 200-4R in. I did drive it on the E-way for my trips to NCM Labor Day weekends. Before the OD I was lucky to get 11mpg and after I get closer to 17mpg. The OD took 1000rpms off the engine at 70mph.

 

I understand what you guys are saying about the overdrive. BUT, my intentions for the car right now (still have not driven it) is only to drive it once in a while....you know that Saturday night cruise around town thing...no long Sunday drives in the country. I don't care about gas mileage (right now) but want the car to pull really hard from the "tree" or stoplight (I know Norval is cringing about the stoplight thing ) If you think the 3.08 is more than enough for a BB...I'll leave it at that for a while and channel efforts and $$ elsewhere, but if there is some serious acceleration to be gained with the bigger gears, I want to head in that direction -- I just don't know how far to go (3.70 -4.56), that's why I'm asking for advice.

I do apprecaite everyones advice! My car has turned out pretty good so far, mostly built on the advice of Forum members over the past 2 years

Mike