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Differences in e.t.s between bias ply and radial tires?

Terry, you're right about the modern cars being heavier than the old muscle cars, which is one more reason by the gap in stock to stock performance shouldn't be as great as it is. Several years ago I was at a car show in Aptos, just south of Santa Cruz, and a couple was there with their two Challengers. They were both painted that purple MOPAR color (IIRC it was called Goofy Grape). His was a '70 model and hers was a current model. I had always thought the new ones looked like the old ones, but the '70 was a lot lower, a lot wider and just generally better looking. The new one looked like a Detroit sedan parked next to it. Still, I would bet that the big mama would clobber the '70 head to head. Beyond tires there is also the issue of computerized ignitions and fuel injection.
 
Challengers are one of my all time favorite Muscle Cars...I would love to have a 340 six pac, 383 or 440 six pac. 340 would be fun to build nowadays since I now have an affinity for screaming small blocks as opposed to my younger days in the 70s when it was all big block FEs, 383s or 440s. You can make those 340s scream!
 
I couldn't give up on my original question about radials and bias ply tires so I sent an email to Hemmings Muscle Machines magazine. The editor responded with the following letter. It's a bit long but an interesting read nonetheless and worth the time:

HI Ken,
I’m going to jump in and offer a response here even though your letter was addressed to Jim McGowan, just because I have some experience with the subject. Your main question seems to be, how much faster would the cars of the late ‘60s and early ‘70s have been if they had radials. If we’re talking about regular street radials, the kind that a newer car might be delivered with, I think the answer is that they wouldn’t be much faster, if at all. If we’re talking about a Radial T/A type of tire on a classic muscle car, then the car in question probably would actually E/T slower. Here’s why:

I used to think that radials were better than the old bias-plies in every way, mostly because they handle and ride so much better—that part I have experience with. My first car was a ’72 Chevelle SS that had stock 15 x 7 rally wheels, to which I was able to mount a set of factory-spec Firestone Wide Oval 60s that I got from a tire shop that had removed them from a Boss 302 in favor of radials. This was in 1984. The Firestones looked awesome—big and fat and square, with cool lettering, but they drove horribly. Narrower bias-plies that I drove with on other cars didn’t “tram” so much but cornering was always awful. Even cheap radials always seemed superior.

So I assumed that they were also better for the drags, but then I got closer to FAST and Pure Stock racing, and the guys there showed me what should have been obvious: there is more rubber on the ground with those “performance” bias-plies than there is with most muscle-car sized radials, especially Radial T/As. The flat, wide tread and the very narrow and simple siping (the tread pattern) of the old tires meant that the actual contact patch covered more area. Also, when you air down a bias-ply, it usually doesn’t cup the tread, lifting the center portion up, the way radials tend to, so you get even more contact, plus the carcass can roll a little on launch—just like a racing slick. The FAST guys do this to the extreme, and we’ve published photos that show white-letter Firestones and Goodyears with wrinkled sidewalls on launch at those events.

If you look at the sidewall markings on your Radial T/As, you’ll find what I did years ago that surprised me: They are classified as a mud & snow tire. So were a lot of the other white-letter radials that were aimed at muscle cars, most of which have gone out of production in the past few years. That means widely-spaced tread blocks that don’t put as much rubber on the ground—not ideal for drag launches. If you were to take a set of more modern performance radial tires with a tight tread pattern and soft compounds, you would get some advantages, but they are only really offered in 17-inch and larger diameters. A set of 17s can look good on an old muscle car, but the shorter sidewall that most of those tires have makes the carcass stiff; they were also mostly engineered to be stiff to enhance cornering. But there again, the design—with that rigid carcass—isn’t great for drag racing. One of the 17-inch radial sizes that actually looks good on muscle cars is the stock size for the 1994-’96 Impala SS, but it’s only available in 2 or 3 different models now. The exception is drag radials, which work phenomenally well, but they are engineered for the drags, and now have lots of racer R&D in their designs. Plus, they’re mostly street legal.

Note that, per the rules, Pure Stock racers can run radials (but not drag radials), so long as they are within the range that corresponds to the stock tire size. Yet very few do, because they need all the traction they can muster, and that comes from bias-plies. I’m told that he compounds of reproduction bias-ply tires seems better than the originals, both in terms of ride and handling and traction, this coming from guys who were around to experience the originals, but there’s no quantifiable proof, and NOS original tires will still suffer from age.

So why are some Pure Stock cars so much faster than the original magazine tests? Because the guys racing them today take advantage of maximizing every aspect that is allowed. They are very good at accurate tuning and knowing what timing curve works best, while many average muscle car guys set the initial timing and leave it at that. Carburetor tuning is also very important, particularly tuning for air quality conditions at the track—serious racers adjust as the day goes on and the air changes. That was rarely ever touched by road testers back in the day. The PSD racers have to run stock air cleaners, but they can run modern high-flow filters. Most of them run electronic ignition conversions in stock distributors, both of which offer very slight advantages on their own, but when stacked together with other small enhancements, it really starts to add up. Then they can raise compression 1.5 “points,” and they can mess around a little with the cam profiles, though they must maintain, I think, 16 inches of idle vacuum. Really serious guys will rebuild their engines with low-tension, file-fit rings and make some alterations to the oiling system to control the oil more, keeping it off the rotating parts. They have to run un-ported, stock exhaust manifolds, but they can put a mandrel-bent 2-1/2-inch exhaust on, with performance mufflers, so long as they are oval, or stock-shape (I think). All of these little things, paired with good driving, can make a surprising difference. Lots of average muscle car guys put aftermarket upgrade parts on their cars before they ever maximized what they had to begin with—there is a lot left on the table in the small stuff.

When I worked at Muscle Mustangs and Fast Fords magazine in the ‘90s, we tested new Mustangs regularly, and the staff often got the best ETs of any magazine. It wasn’t because we had special cars or were cheating, it was because we had a good track (Englishtown) at a decently low altitude and we would test mostly in the spring and fall, when the air was good. Then we would pay attention to ignition timing, air cleaner obstructions, tire pressures, and some other things. Finally, we had good test drivers who would get most out of a car. We’d sometimes do lots of fiddling and tuning at the track, but it would show what the car was capable of.

Your Mustang sounds like a good example of a car with lots of untapped potential. The combination you outlined should make that car go faster, in my opinion—you’ve got a good gathering of components there. The trap speed is showing that the ET is pretty much in line with the power, if I speculate on the car’s weight (3,400-ish?). You could probably get it down to a 14.1-14.0 or so if you were determined, but I think there’s more power in the engine that you’re not getting right now. Carb tuning and ignition tuning would probably yield significant results. Our contributor, Ray Bohacz, is a master tuner. He once found something like 60 horsepower from a 440-powered Charger on the dyno, when the owner was already happy with the way the car ran. Old V-8 cars with carbs and distributors will run with the tune way out of whack, and can feel fine going down the road, but there is very often untapped potential.

That was probably a lot more then you were asking for, but once I started, it just kept rolling. Hope it answers your question, and offers some insight. I’m sure there are others who might have differing opinions on some aspects of my response, but this info comes from my own experiences over a number of years with magazines (about 23 now) and more in shops, at race tracks, etc. I’m no racing or tuning expert, but I’ve tried to pay attention over the years.
 
good info.
All the bias ply tires I ever had were hard as a rock. you could put a 100K miles on them and no tread wear! But I always had cheap brands. Maybe those Goodyear and Firestones are better.
 
I had a pair of Blue Streaks briefly on the back of my '64 Falcon Sprint back in the day. The only way I could afford them was to buy them from a local tire recapper (do they do that anymore for automobile tires?). Unfortunately the previous owner had used them as drag tires and had secured them to the wheels with sheet metal screws to prevent them spinning on launch. The day after I got them I went out to the driveway to find two nearly flat rear tires. I guess I could have put inner tubes in them, but I just took them back to the recapper and got a pair of cheater slicks for the Sprint.
 
I just remember that, when the guy aired up the tire and he put it in the water tank, it looked like some sort of bubble show.
 
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