850HP on 315 30 18 tires?

[69Boss572]

Is anyone here running 700+ HP with low profile tires, or has experience they can offer?

I bought 14"/13" big brakes for my 69 Mustang, so it requires 18" wheels to clear and I can do a max of 315 30 18 tire. (Rear end has a trackloc diff)

My engine was just dynod at 850HP/800TQ. Since my 69 mustang is probably 1000lbs lighter than a Dodge Demon and 50 more all-motor horsepower, I'm rethinking my brake, wheel size and tire choice.


Should I downsize my brakes so I can run 16-17's on the street, maybe even a 15-16" on the rear if I hit a dragstrip?
I could always put bigger wheels on if I wanted to, but you can't go down smaller than calipers and rotors will allow.

Wildwood offers a 12" rear rotor setup that works with some 15" wheels.

I did find this photo of 315 30 18 Hoosiers, though you wouldn't run this tire on the road I don't think.

[srode]

There's quite a few brands that make a 315/30/18, curious why you are worried about that size? If a 315 is all you can fit in your wheel wells, it's not going to get any better with a smaller wheel diameter.

[69Boss572]

The general take is that taller sidewalls give more grip in a straight line, but not around corners. So it's a trade off - But I have no idea how much, or if I should be concerned about it.
I do know with a ton of power, you're only using the full power of the engine in a straight line.

[68Formula]

The Demon's tires are 315/40R18s which have about 5" of sidewall.

[69Boss572]

Yeah,
315 40 18 has a 5" sidewall (Demon)
315 30 18 has a 3.7" sidewall (My planned tire)
There's 35% more sidewall with the 40 series tire. I reached out to the place who pushed me onto the larger brakes and they insist there won't be much difference in traction between a 30, 35 and 40 series tire. I'm not so sure.

[Vimes]

With that kind of power I think I'd stick with the larger brakes. Stop is more important than go.

If concerned about traction, contact the tire manufacturer you want to use and see what they recommend based on your setup.

[Lonnies Performance]

I did something similar with large brakes & my smallest possible wheel is an 18".

Unfortunately, a lot of new cars use a similar size wheel/tire but most are much heavier cars, which greatly helps traction.
Guys on the CTSVowners.com forum have a lot of tire info there.

I have an S10 which is also a light vehicle with a relatively light rear weight.
At the time I was under 600HP but I'm similarly concerned since I just added a supercharger.
Even with a set of 28 x 10.50 slicks measuring over 29" tall on 18's I needed more traction... it is nowhere as good as a 15" slick or soft street tire for that matter.

Also, with a N/A engine, I'm assuming it is an upper rpm setup so walking it out from an idle is not likely a good option.

Weld has some 15" wheels that work with 12" Wilwood brakes.
I recently used a set of these 15x11 Bead locks on a friends Silverado that just barely clear.
I had to use the high pad version as it offered more brake clearance.


I would try to stay conservative on the rear brake diameter to allow a smaller dia wheel if necessary.
I'm considering trying to adapt a smaller rotor/caliper to the same mount on my S10 so I can swap wheels/brakes for the track.

[Tynitty]

Nitto nt01s come in 315/30 r18 they are a super grippy tire…just don’t drive in the rain and you’ll be good

[mean buzzen half dozen]

Don't forget, with a taller tire you also have more contact patch.

[icemanrd19]

I’m making 750 rwhp on a 315/30/18 Bfg rival. Big factor playing into is your gear ratio as well. Stick with a 18” and just run a taller tire on the drag strip to help you in gearing and traction like a 315/35/18

[protour_chevelle]

Its all in the suspension. My car made a little over 1000rwhp. It got down at the track on a 15" drag radial(1.26 60' times). When I put the 315/30/18 the car was still a monster from a roll but not from a dig. The mustangs have a very forgiving suspension. Much like Vipers. You can put a 18/20" drag radial on a viper and knock off 8 second passes with ease.

Not much replaces a 15" drag setup though. All in what you really want. Max effort? change the brakes to fit a 15.

[Zachalanche]

how heavy is your car? my 70 mustang weighs in at around 3100. It was probably in the 3400-3500 range with the old iron block, heads, and all the original interior stuff. Either way, it's a lighter car than the newer cars that have massive brakes these days. Additionally, most years of mustangs have commonly had very small rear brakes. would bigger be an improvement? yes, but do they need to be 13s probably not.

How is the car being used? Do you need to dissipate tons of heat? or do you just need to generate a lot of stopping force once in a while? Smaller brakes can still stop the car adequately. And, even though you can accelerate quickly, and have the power to probably go insanely fast (on paper) remember you are working with 60's Aerodynamics, and while that car probably has a better Cd than many other cars from that era, it does let a lot of air underneath itself which is not good for super high speed performance. I'm guessing you might hit 150 in the quarter, and if you were on a road course with a lengthy straightaway (say .5 mile) you could be north of 160. That would be the scenario where I would want some big brakes with cooling, and probably some aero mods that most would be unwilling to do to a vintage mustang.

I'm running 13s front and 12s in the rear, (tires are 295/3018 / 315/30/18 R888r) and my car stops hard enough that I have bent the rod ends on the upper a arms. I think I get good traction, but I'm only working with around 600 or maybe just shy of 600 FWHP. Even at that, from a dig the car doesn't hook great, but that's not what the car is built for.



In summary IMHO so much of the braking force is on the front brakes that you could easily size down the rears to fit smaller wheels even 15s if you really want.