Rear end efficiency

[Milwaukee Midget]

I'm looking for the most efficient rear end I can find.  The class I'm building for permits a rear end change, but no quick-change rear ends.  I know Ford 9" rears are popular with drag cars because of their durability, but I also know that they are not always the most efficient. 

I'm building for the I/GT class.  The car will probably weigh in at about 1500 lbs, and horsepower looks like 120 @ 7000-7500, so I need to be able to spool it up pretty tight, but it doesn't have to be as strong as most LSR cars.  The stock MG Spridget rear sees a lot of use in vintage and SCCA racing, but do any of you have any thoughts on what might be a better choice?

Pinto, Vega, Datsun, Toyota, Mitsubishi, Dana, Chevette, Morris, Opal?  It needs to comply with section 2.E of the rules.  Maybe something I'm unaware of?  I'm not averse to thinking outside of the box on this one, but I would need to be able to buy a set of gears off of the shelf in the 4:11 to 4:30 range.

Any ideas from you Brit, Aussie or Kiwi racers?

OR -

Any ideas on making what I've got less of a power-suck?

Thanks.

[RichFox]

I do know that Gary Williams bought a 350 Monza which seemed to have the same rearend as a Vega, and installed a 454 LS7 and four speed. Thing would do some fantastic burnouts with the 13 inch rubber bands it came with. never broke it and had a good gear selection at Pick your Parts. Went around 186 mph

[SPARKY]

EARLY ford bevel ---
GM 7.5" will have the greatest selecton of gear ratios

[Dr Goggles]

I'm looking for the most efficient rear end I can find.  The class I'm building for permits a rear end change, but no quick-change rear ends.  I know Ford 9" rears are popular with drag cars because of their durability, but I also know that they are not always the most efficient. 

 but I would need to be able to buy a set of gears off of the shelf in the 4:11 to 4:30 range.
Thanks.

......could be worth having a sniff around Toyota or Mazda van rear-ends I have an inkling that some of them are in that range , built to be durable at the power range you are talking about........and not as hp thirsty as the bigger diffs.

here for example is a post I found on a messageboard in Oz.....

I know this is not the usual vehicle that is modified on toymods, but can anyone help with this. I have just completed an engine and gearbox mod on my YH71 HiAce ,the engine is a 4.1 ltr falcon with 3 spd auto. The problem is the toyota diff ratio is to low at 4.3:1, that converts to 28kph per 1000rpm. .....

there ya go

[Harold Bettes]

When was the last time that you heard /observed someone A) breaking a drive axle B) Killing the ring and pinion on the salt?

Not meant as a wise Acura question. Simply adding to data base from lots of runs of others.

Thanks in advance.

Regards,
HB2

[SPARKY]

Rule of thumb---the closer the pinion shaft is to the center line of the ring gear --the less drag it will have.

[Milwaukee Midget]

Rule of thumb---the closer the pinion shaft is to the center line of the ring gear --the less drag it will have.

This makes sense.  If I'm following Sparky's logic correctly, if the pinion is closer to the centerline of the ring, the gears would have less of a helical cut, less surface area, less side loading, and less friction.

Essentially the same reason straight cut gears in a tranny are less parasitic.

[Dr Goggles]

sheesh , I just noticed where you live ......that sounds like heaven...

[Stovebolt]

Most Jap and Pommy rear ends have ratios in the 4's, so they;d be no good for you

No Q/C - bummer.

Maybe a Ford 7.5"in a 2.9 ratio.

touble is you cannot gear it too high in the smaller classes, as you won't have enough neddies to pull over it

[Milwaukee Midget]

sheesh , I just noticed where you live ......that sounds like heaven...

Don't let the nickname fool you, pal.  Milwaukee may be a lot of things, but heaven ain't one of 'em.

Although we are proud to claim that Gary Gabelich's "Blue Flame" was built here.

[SPARKY]

gms are considered the most efficient, then dana,  botton of list, ford 9"---the new fords are really old gms---now made by American axle---also require  C clip eliminators

[JimL]

Depending on the width you need (between the backing plates), I know of several Toyota options.  Early Corolla 1600 Station Wagon had very narrow axle with 6.7" diff (but not the gear ratio you want).  1976-79 Corolla Liftbacks with manual tran will have some 4.11 ratio 6.7".  You'll also find 6.7" in Corolla GTS (mid-80s) that will plug right into the very narrow early Corolla axle housing....some had 4.30 gears.  The old Corolla axle is only about 44" between the backing plates.  These 6.7" diffs are very easy to turn and will handle 200-250 HP.

Look under the hood, or on the door frame, for a silver plate.  It will have a line coded A/TR or something similar with a letter and three numbers.  For a 6.7" you are looking for T followed by 292 (T292 = 4.10) or T followed by 082 (T082 = 4.11) or T followed by 282 (T282 = 4.30).  The GTS Corolla may read T283 if it has LSD with 4.30 (the 3 on the end is 2 pinion LSD).  You'll find some taller 6.7" gears in the old straight axle Cressidas and Coronas from the early 80's.

If you need tall gears (such as for a roadster) with very low drag and a good rear end width (about 47" between backing plates) for '27T body, use the 7.5" Hilux/pickup axle.  the 7.5" has "fuel economy" type gear alignment (for the early days of MPG leaders).  These axles are in 1975-1988 pickups (check the vehicle plate as shown below).  They were also in the cheaper grade early 80's Celica and Supra straight axle models.

7.5" gears run from low of 4.56:1 to high of 2.92 (or 3.07:1...found one in a junkyard - forgot to get the code...sorry). 
The 7.5" axle code is F followed by three numbers.
F462 = 2.92
F392 = 3.15
F012 = 3.30
F382 = 3.41
F372 = 3.58
F302 = 3.73
F312 or F072 = 3.90/3.91
F082 = 4.11
F282 = 4.30

You'll find LSD center carrier in '82-'86 Supra and Cressida which can be plugged into the truck pumpkin.  Use the truck bearings, and whack about 1/8" off the inner end of the axle (I use Dremel cutoff wheels to keep from cooking the end of the axle).  Keep in mind that these axles are only good up to about 350-400 HP.  You can get them to full lock by shimming the springs and plates.

One last thought for those seeking tall gears for small engines.  The Starlet had some VERY tall gears for a lightweight car.  The ring gear is only 6.38" and the cars were designed for 45-50 MPG ratings.  '81-'82 had 3.15 ratio and '83-'84 had 2.92 ratio.  The Starlet Cup racers could run about 150 HP through these for course racing.  It is an easy axle to mount because the 4-link brackets are already in place.  This little axle is so light you can carry the whole assembly by yourself!

Hope this helps.
Regards, JimL

[doug odom]

Does anyone know where and when the tests on rear end efficiency was made? By whom? How can I see the results? I would be interested in what is the percent of loss with each type. Were they preped or stock out of the box?
Doug Odom in big ditch

[Sumner]

If I remember right when I was looking for a rear-end for my lakester I looked at ones they were using in dwarf cars and other mc powered cars.  Seems like someone had about 2.50 gears that had been made for one of the Toyota rears that most of them used.  Do a search and if you can't find it I might still have the link,

Sum

[JimL]

Dwarfs and Legends also use the 6.7" rear end.  I did find an old 2000GT manual that shows about 96% power delivery through the diff.  It actally shows more loss in 1st-3rd and 5th (overdrive) to a maximum of about 6-8%....first time I've ever seen a number in print.

I also remember the story about the fellow in Denver, in the 70's, that built and drove what he called a "Sprank", which stood for Sport Tank.  He got tired of getting hit in his Opel (I think that's what it was) so he made his own body out of diamond plate, shaped like a small armored car.  He did a bunch of work to carry the load, then installed a Corolla 1600 Hemi in it with a pair of 4 speed transmissions.  He split shifted 3rd-4th on the rear trans to get enough ratios to pull the weight.  Got onto local TV with it, and always won when somebody hit him! 

I've always thought a dual trans, properly done, might give enough gears to help a small engine at Bonneville.  We had good runs with the Prius in '04 after I figured out to just push it far enough to get past the MotorGenerator #2 low RPM 3-phase pulses (we'd geared way up and it would hop-spin the tires at near 0 RPM/max torque).  As soon as it was rolling, we let the engine do all the work.  It acts like an infinite ratio transmission...the engine stayed at 6500 RPM for about 2.9 miles! 

It was an eye-opener to me because it only had about 115 HP, and by the time it got to the traps the computers were stealing energy from the crank to feed the battery and transaxle generator.  It finally ran over 134 even though it fades fast at the end.

It sure would be nice to know all the tricks for reducing losses with small output packages.  The little stuff is fun to play with...and cheaper, too!

Oops...got to rambling...off topic.

Regards, JimL