[69redbird]

Hey,

I know this queation has come up a thousand times but, I have a 12 bolt rearend with 28 spline,mini spool, 4.88 gears and 1/2 studs and c-clip eliminators.( Which by the way is for sale). And now in the car is a ford 9"rearend with 35 spline , spool, 3.89 gears and 5/8 studs. My question is does it take more H.P. to turn the 9" then the 12 bolt? I ask that for the reason both were next to each other and I spun both over and the 9" was alot hareder to turn. Is this normal? And if so, how much more H.P. would it take? Thanks

[FBNRacing]

My 9 inch is a bit stiff also. I think the 12 bolt would be a little faster but only by 5 hundreths, mostly cause they are lighter. I would keep the 9 inch as it wont be breaking and sell the 12 bolt to someone trying to put a stret car together.
Rex

[issach428]

the answer is yes. How much i couldnt tell ya. Maybe someone else could give you a guestimate.

[WARPed]

Guaranty it will be well under ten horsepower difference

[KEN CROCIE]

Ford 9" = 2-3 % more drag than 12bolt. This has been publised in several different books over the years.

[69redbird]

Thanks guys, I went to the 9" and 3.89 gears and a housing back brace so I can start spaying a 250 shot. The 4.88 's put the motor a 8100 rpm's with a 28" tire. And hopefully the 3.89's wil put me around the 6800-7100 range.

[Tom Vaught]

When I worked for Truck Operations, I did Parasitic Loss Testing.

The normal rule of thumb is that the Aero number at 55 mph is 60%
the Tires are 30% and the driveline would be 10%. A typical F-150
at the time might have had a 18 hp EPA HP number.

Aero = 10.8 hp
Tires = 5.4 hp
Driveline = 1.8

Naturally the aero goes up as the speed goes up but the tire rolling resistance and the driveline losses usually stayed about the same.

If the trans is taken out of the equation (about 60%) of the driveline
then the rear was about 40% or .72 hp. That would make the GM
rear axle about .69 hp. Not enough to worry about for most cars.

Say the numbers were much higher, like 10 times higher( 7.2 hp vs
6.9 hp, you are still only talking .3 hp difference.

JMO

Tom V.

[69Goat1]

What 12 bolt has 28 spline? - my car 12 bolt is 30 from the factory!

[Jim Robertson]

FWIW. In the late 1980s, a local super stock racer did a back to back test (same day) of the 12 bolt vs the 9". Money was no real obstacle so I believe the equipment was of top quality with all the options. Same gear ratio and approximately same rotating weight, the third generation Camaro was .05 - .06 quicker with the 12 bolt. I know the guy tried very hard to minimze the variables in the test. After the test I do not believe the 9" ever went back in the car...so at least he was convinced the 12 bolt was quicker.

Because of this and other good info I recently laid down $2700+ for a 12 bolt with all the options for my car instead of the 9".

This info is worth only what it cost...$0 :-)
Jim

[RAIV55]

Good information Tom, I figured it was way less than .05 seconds that was previously posted but since i don't know for sure how much didn't want to open my mouth.

It is similar to the alternator losses, for years I ran without one thinking I was saving big power, found a formula for figuring HP to generate 60 amps..... something like 2 1/2 HP. Needless to say now the battery charger stays home.

Chevy style 12 bolts should have 30 spline axles, 10 bolts have 28

[WDCreech]

I run a Chris Alston "Fab 9" because I change ratios frequently for eighth and quarter mile tracks. If you run the same gear all of the time, I say it's six of one half dozen of the other, but there are good 9" parts almost everywhere.

[FBNRacing]

WD did you ever try running the same gears at both the 1/8and 1/4 track?
rex

[FBNRacing]

"FWIW. In the late 1980s, a local super stock racer "

these guys really know how to get everything out of a car.
Rex

[FBNRacing]

"I figured it was way less than .05 seconds that was previously posted but since i don't know for sure how much didn't want to open my mouth"

so you didnt think ole rex knew what he was talking about huh?
Rex

[PONTIAC DUDE]

Just install the 12 bolt conversion carrier/axles in the 9" housing.

Best of both worlds.

[69redbird]

FBNRacing, The reason i ran the car 1/4 mile when it is set up for 1/8 is, the track that I run at is 1/4 unless you are at an event, and my car is no were near ready for that kind of racing. I have had it for about 7 months and still going through test and tune. I know that running the 1/4 mile with those gears put me way over my cam power range, I think my peak H.P. is 5500 to 6500 range and drops off quickly after that. And also the biggest reason is low on money. Thanks

[White Warrior]

I run the Strange 12 bolt drop in section in my 9" Ford housing to reduce parasitic drag. I never did an A-B test,although that may happen some day as my backup rear is a regular 9". There is a price to pay. After 2 seasons on my 5:13 Pro gears,there were cracks on each tooth of the pinion gear.
I have a 4 speed car and it hits the driveline pretty hard on launch. The maintenance cost will be a little higher but you will see a benefit.

[FBNRacing]

RedBird,
what im wondering is if you gear the car for the 1/4 mile and use those gears in the 1/8 how bad does it, if any slow you down in the 1/8th.

When my car was in pretty much stock form running 7.20s in the 1/8 i had a 29" tall tire with 4:88 then when i broke them i went to 4:10 and it seemed maybe 5 hundreths faster and it would carry the front end a little longer.

Im just wondering what your car liked.

thanks Rex

[69redbird]

FBNRacing,
My car ran 7.30 with the 4.88's, I have not had a chance to run the 3.89's yet. I am going through the supension right now. and i need to get some new slicks, I believe the slicks I had were about 5 years old, not worn down just old. I think I can get better times when i get better traction. Last December the track was cold and slick, I had a car in front of me put some fluid on the track and the had to put some track bite down. When I left the line my wheels went up about a foot and carried them about 20 ft. but when the front came down the car went straight to the ceter of the track and fished tailed the rest of the way. But with the 3.89's I plan on going with a new supension set up and spraying 250 off the line, so who know what will happen.

[ponjohn]

My thought is the 3% loss of efficiecy is worth the piece of mind. At 1000 hp you're talking 30 hp.