Technical > Jon's Wheel House

Tires for 7.0

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GlennLever:
Working on tires for a FED

Current set up is for 8.000 index,  top end RPM is 6,700 at 165 MPH on 31 X 12 X 15

New engine and desired index is 7.00 around 7,800 rpm engine is good for 7,500 to 8,000, would like to see 7,500

Rear gear is 3.73

Looks like a 33 inch tall tire would be around 193 MPH 6 -8 % slippage in the torque converter.

How much will the tire grow? Is this calculation correct with tire growth?

JrFuel Hayden:
6% converter slippage seam s a bit high and of course 8% looks even looser. i assume you are running  a 8" converter, maybe even a 9". We run along with all the other JrFuelers run 7" converters , and I get 5% running 7.0's at 188-190 injected alky with a 1435 lb car with driver. I would guess you are much heavier with your blown Pont, and the extra weight I see in your car.
Most JF cars run the Goodyear 31x12x15, it weighs 22.6 lb, we work to reduce the rotating weight. One pound of rotating weight is = to 9-10 lbs of static weight, so theory goes if you can cut 100 lbs off your car you will pick-up a tenth, there fore if you cut 10lbs of rotating weight you could get a tenth.
I do see a number of 7.0 Pro rs running 33x12x15 GY on their blown alky cars. There is only a couple of JF teams running M&H 33x12x15 tires, but their combo has the engines out 55-60"o they need more bite because they have less static weight on the rear-end. Also one of the teams just won the Heritage JF at the the 2018 March Meet winning with a 6.87 , 193 mph, with a 387 ci SBC.
I know it seems silly to compare your car to the light weight 10,000 rpm [ no blower] JF cars, but you are asking about 7.0's @ 193 mph.
The reason most teams don't run the 33" tires is the tire is too big and it pulles the engine down out of it's power range, also tire shake shows up from dead hooking the heavier tire.
Now i know you may need a 33" tire because of you should have more torque with your blower, plus your Pont combo will put out more torque than a high winding SBC.
When we won our last Heritage JF championsip it was with our Gene Adams built early Hemi, again injected on alky, but we ran some 7.0's with my 7" converter, but tighter than the SBC converters, like flashing at 8300 rpm vs the SBC 87-8800 rpm. The Hemi makes more torque and I need a tighter converter. The Hemi ran 870 hp at a peak of 8800 rpm, with an average HP of 863 running between 8000 to 9000.
i don't know if this will help you and your combo , but it's what has worked for me. 
Call me , 805-444-4489, if you are interested in the light GY 31x12 or the GY 33x12 slicks, i can give you a discount.
As far as your question of tire growth I have found to get the RacePak to compare/meet the ET slip I only have to add 1/2" to the tire Diam to make everything match, ie RPM to trap speed, the reason is the converter slipage, so I have found the formulars are real close to my racePak info by just pluging in the tire size 31 or 33, even tho the tires will grow 3-4" . Some tires will grow different than other brands. It seems M/T grow the least, but Hoosiers grow more but not as much as my GY 31" tires. at the March Meet where we were runner-up our trap speed was 186, with the engine running at 9800 rpm with my 4.88 gear.
Feel free to call me,
Jon

GlennLever:
Thanks for the info, when I get closer I will call.

Went shopping for double bead-lock wheels, and found an interesting thing. Both Jegs and Summit list available ship date on all 13 X 15 X 3 inch back space 5on5 as May!

31's will not work for me as I need to keep RPM down in the 8,000 range

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