Thread: any insite on the 700R4??

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Well I still think about a 3.70 rear gear with the 700R4 trans. I am farming out the change of my rear gear after Christmas from 2.79 to ? Henry Rifle comments that with the 3.06 low gear in the 700R4 a 3.50 or 3.55 gear should be plenty low enough for acceleration. Then I looked at the site on acceleration:

http://www.alienplanet3.com/hotrodmath/quarter.html

There if I put in 2500 pounds and 260 H.P. at a mere 4200 rpm the program predicts a 1/4 mile time aound 12 seconds and recommends a 3.27 rear gear. Although I know that the laws of Physics can be used in an ideal way to assume perfect traction and predict these times, I do not know the assumptions of the transmission gears in that calculation. Assuming there is only one shift in the 1/4 mile run I would guess my trans would use low (3.06) and second (1.64) gears from a standing start but I have no idea what gear ratios the acceleration calculator uses, so what does the 3.27 rear gear recommendation mean? Interestingly, if the weight of the car goes up to more like that of a Corvette around 3300 pounds the predicted rear gear gets lower. Soooo, the question is, maybe a 3.55 rear gear is too low for a light roadster? Apparently there are a few places which still offer a 3.25 rear gear so that is possible, but then I will not be able to get into OD gear until a higher mph, so maybe after all the 3.55 rear gear is an all around compromise considering that I am using tall tires of 735/75/15? Just wondering while I consider what to tell the guy who will rebuild my 8" rear with a new gear.

Don Shillady
Retired Scientist/teen rodder

Don, I tried inputting some scenarios into their calculators and came up with some real suspect results. I would not trust them.

Pat

Don,

A couple of thoughts:

First, what do you tell a guy that wants to build a hot rod with decent gas mileage?

Most of those horsepower/ET/speed calculators are based on empirical data, not Newton's laws, so they don't make assumptions about gearing and traction. They are what they are, and your setup may or may not get the performance predicted. The best predictor, though, seems to be trap speed.

The difference between 3.27 and 3.55 gears is only 8.5%. Considering all of the variables, is that really significant?

In a light roadster, 3.73's may be overkill, considering your desire for gas mileage. But, I don't think you can go wrong with either 3.27 or 3.55.

Of course, I'm putting 3.73's and 440 HP in my lowboy, so what do I know?

Oh, and I'm with Pat. That calculator looks a bit hinky.

Originally posted by Streets
Dan ... The "Granny Tranny" talked about here is NOT what you'd use fer Draggin material.. 1st gear would getcha 6,000 RPMs at a speed of 10 MPH with the standard 4:10 rear gears usually found in trucks with this tranny..(And the engine would blow up before the 60 foot mark) It weighs in the neighborhood of 300 pounds and is a top shifted tranny, MOST everyone that has a big heavy truck or a heavy load to pull uses these trannys if they like sticks.. When just cruisin' around town you start out in second gear.. saves on the killer engine rpm's!! It also has a PTO drive on the passenger's side AND a bolt-on u-joint assembly!!

that sound's like the one i just put in my 79.

I knew that a sm420 was not suited for drag racing and, I just wanted to see if anyone would catch on.

i catched on all right
no truck tranny will drag race unless the truck has had a trans put in or it was a highporformance truck when it came out that is for the 4speeds.

i dont know much about the three speeds besides i DON'T LIKE THEM!!!!

Henry and Stu, Thanks much for your comments. I went back and played with the acceleration calculator some more. I have spent most of my career fitting polynomials to data and I am familiar with the case that a certain equation may fit the data in a narrow range but not elsewhere. I tried their first set of mph-rpm equations and got the same numbers as on another site so I think that part is right. Then I tried the acceleration formula with:

2500 pounds wt.
250 H.P. at 4500 rpm ( I have a low rpm cam with 1.6 rockers)
28.82" tires (735/75/15)

their formula predicted:

11.99 sec 1/4 mile elapsed time (?????)
108.7 mph trap speed

Suggested ideal rear ratio: 3.55

The 12 sec elapsed time seems pretty optimistic, but the input seems realistic and makes me feel better about the 3.55 rear gear. From my perpective I recall that the AMBR No.1 roadster did not break 15 sec elapsed time with a built flathead, so even 13 sec seems pretty quick to me. I guess a 3.55 rear gear will be a good compromise for me with a relatively mild SBC. Maybe Proz or someone else with actual speeds and times could check the formula in this site, but I guess it looks more and more like the 3.55 rear for me and hopefully I can crack the 20 mpg barrier at least on the Interstate. Thanks for your comments.

Don Shillady
Retired Scientist/teen rodder

Ah yes . . . I did some power spectral density function applications in my earlier years. Determining what lab generated inputs to hydraulic actuators would create realistic loadings on structural members used in earthmoving equipment.

....I had an '84 Camaro with a 700R4 that was built by Art Carr {years ago before Art sold the business} with a converter {don't remember the stall} the whole set up was around $2500 or so. But anyway, it wasn't a cheap unit. It was behind a 415 small block that dyno'ed at 499 Ft/Lbs @4000 RPM & 438 HP @ 5250 RPM {flywheel dyno}. I had 4:56's with Posi. in it & cheater slicks. The car was stripped big time, 2800 pounds the last time I weighted it and I took some stuff off after that. Anyway, the trans blew up so I personally wouldn't use a 700R4 unless it was for a non-performance application. They said the 700R4 would handle my HP, but it didn't. I took it back and got a GREAT deal on a 400 turbo so that was ok but I'd start out with a stronger trans if you plan to hot rod around .... Bill.....

Here we go boys and girls read the info on this site and u will have a better understandin of the matter at hand.http://www.transmissionhead.com/ just a detailed site for these trannies and it covers just about everything in those trannies!((((((c-mon get ur head into it !!))))))lol

Bill,

Two questions:

1. How did you cool the 700/R4?
2. Did it have an 4th gear lockup?