Best size tire

[Ratbird]

I was wondering if anyone has advice on the best tire size for drag racing my Rat T-bird. Or perhaps a link to some info.

I'm guessing it'll all depend on a lot of factors so I'll reiterate what I have. It's a 3650 pound 59 T-bird with a 410 bored .030 over. Edelbrock 750 carb on an Edel RPM Performer intake with Edel RPM Performer heads. Has FPA headers, Lunati's version of the Cobra Jet cam. Pertronix ignitor instead of points. Detroit locker 4.00 rearend with a wide ratio 4 speed toploader. CenterForce dual friction clutch.

Redline is ~6000rpm. No dyno info but it should be about 400-410hp with about 450-460 torque. Timing is all in at about 2800-3000rpm. I don't know where the optimal power band is in the rpm range, but it feels like it pulls the best right around 4000 - 4800 rpm.

The reason I ask is because I currently have 8 inch wide, 28.5 inch diameter old school cheater slicks - they're mostly for show, but they do hookup pretty well with the Detroit locker. If I put on some highway tires on that are 6 inch wide, 26 inch diameter it seems to be jumpier, or at least more responsive to the go pedal. However those things spin way too much. It's a street/occasional strip car. I'm thinking it would be faster in the quarter with a smaller diameter tire.

thx in advance, DAve J   

[cjshaker]

As long as you can get them to hook, and provided you aren't hitting your redline at the end of the quarter mile already, smaller tires has the same effect as lowering your rear ratio. That will definitely increase the felt response. You might try an online calculator to see what tire size will get you closest to redline at the stripe.

[Ratbird]

That makes sense Doug, thanks. I've only been to the track once and my top speed was 94.4 mph. However that was with a fuel starved motor which was cutting out at the end of 2nd and 3rd gear. Plus it had a 3.89 differential then too. Since then I have replaced the fuel pump and lines with 3/8 inch, and put in a 4.00 locker, and upped the heads to the Edels. Soooo, I'm hoping if I can get it to run well I might end up at 96mph range.

The calculator says at 96 mph I should be at 4527 rpm with these 28.5 inch tires. If I were to go to a 27 inch tire it would be 4779 rpm. I think what I'll do is see how it does with the ones I have now (28.5) and then decide what size to buy next.

One other thing if you don't mind. Since going to the Detroit Locker differential I'm experiencing a little axle wrap. About 2 years ago I had the stock leaf springs retentioned  and added an extra leaf which put the car nice and level in the back, and worked well with the old 3rd member too. Can anyone recommend a decent set of traction bars?

thx, Dave J

 

[fastback 427]

I would fab your own. The one size fits all are too short to touch directly under the front spring eyelet. They touch the spring and allow it to still wrap. I just built a set for my fairlane, cost 25 bucks for 1.5 x 3 x 1/8" tubing and angle iron. Plus I moved the mounting tabs so they wouldn't rub the tire on the inside. The generic ones do.

[bn69stang]

Dave look at the shelby under rider traction bars , i went from a 4 leaf spring to 5 leaf mid eye spring and the shelby uner rider traction bars , they bolt to the axle u bolts perch and weld a bracket by the front spring perch , and bolt up ,, no axle wrap , car handles better ,, i like them ..Bud

[Ratbird]

Thx gents, good info to know being that I have zero experience with traction bars.
Bud, are the ones you're talking about like these? http://www.laurelmountainmustang.com/products/1967-70-traction-bars-under-ride-1pr?gclid=CJyIqoifk8ECFe0-Mgod7VcAWw
I like the idea of them helping handling, this old bird is quite shaky. I've got a buddy with a welder that can help me install them. I'm having my local mechanic replace all the front end bushings next week, that should firm it up a bit. I tried replacing them myself but I'd need a press of some kind to get those things apart. I was going to go to a beefier front coil spring, but since "adding lightness" with the aluminum intake, heads, and removing the bumper, I've dropped about 180 pounds off the front. 

Dave J   

[bn69stang]

Yeah Dave thats them , im running them and what a difference . I did them and sub frame connectors .. Bud

[TomP]

They will look silly on the car because of the short height but 8.5 x 26" slicks would be better.

[Ratbird]

Ah shoot, those ones I showed a link to won't fit Bud. The holes are on 3 1/16 inch centers. Mine are 3.5 inch centers. I couldn't figure out how they would bolt onto the u-bolts either. The plate for the u-bolts that has the hole for the shock isn't flat, it's an odd shape.
Does anyone have any pictures of how they look installed? Trying to get ideas for fabricating some. 
I'll post some pictures of my axle and frame soon, it's a little different because I have a uni-body car.

TomP, thx for the info on the slicks, I'll have to see how it does with the current set up and then decide after i record some RPM's at the end of a 1/4 mile.

Dave J

[ScotiaFE]

Have you looked at Caltracs.
They work excellent and no welding required.

http://www.calvertracing.com/index.php

Not really a fan of the under riders. I have seen them tear out the welded section on a hard sticky hook.
Although I did use them on the Stang many years ago, but the Caltrac system is really the way to go.

[Ratbird]

Those Caltracs look great, but I think I'll go with a set of slapper bars for now. From the Caltrac web site it looks like they can make them to order too.

thx, Dave

[fe66comet]

I was considering Cal Tracs but a four link is lighter, cheaper and infinitely tunable. Granted it does not require any modifications but you still are limited to narrow tires and leaf spring technology.