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Like Chip mentioned, I bet the guys that go faster with radials either have a rim that's too narrow or the sidewalls are dead......

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the above statement applies to 90% of the improvmenets that guys claim from a new tire size or brand change. More so the dead sidewalls. We tend to pay much more attention to wear on circumferance than sidewalls. I have seen improvments equal to what claims are made here regarding switch to radials that came fom putting tubes in tires that had dead walls.

The radial, of comparable size, provided you can hook it, will be faster than the bias tire, every time. The radial tire construction, is simply better. Yes the sidewall is stiffer, but the tire as a whole is stiffer. The tire stays more "round" during the run, which is quicker and faster. We run both slicks and radials. With identical tune ups and identical 60fts, the 330 and 660 numbers, are quicker with radials, EVERY time.

Monte

im going to a 325/50r15 on 12 rim

My experience falls in line with most here. The radials either hook or spin hard. My Dart was quicker and faster with the radials than it was with new slicks, 275/60 MT ET Streets and new 28 X 9 MT slicks.

I haven't tried slicks with the 408, it has gone 1.38 60' on pump 93 with no power adders on a 325/50 MT ET Street Radial. Over the winter I plan on buying a extra set of rear wheels and a new pair of slicks. There is only one track here that it will consistently hook at with the radials. I bracket race so it will run on slicks next year.

This is hard to articulate, but I'll try.

I always consider that most all things your car does takes energy to make that thing happen. This energy is supplied in the form of torque from the motor. Anything that takes energy without effectively moving the car forward steals that much energy / power from your ultimate goal of getting to the stripe. Now, A bias tire with softer sidewalls flexes much more at the sidewall (and I believe in the treadpatch) than a radial tire. Flexing the tire takes energy. This energy that is used to flex the tire is energy that dies in the tire, and is not transferred to the track. The stiffer radial tire flexes less, and thus is more effective at transferring all the energy its recieving into rolling force.

The flex is also what makes them hook, so its a double edged sword. But, as said, as long as the radial tire hooks, you're now using more effective energy to push you forward, as less is spent flexing the tire. I think this same principal can be applied to rear suspension (but thats another discussion).

Also, as far as growth. My philosopy here is that growth will only help if you're at the end of your motor's power range, and it needs more gear. If you're still in your effective power range at the top of the track, then you don't want growth, the lower effective gear ratio offered by the non-growing tire will offer acceleration at a faster rate than the higher effective gear.

These are merely my thoughts as a reasonably intelligent person, offered up for discussion. Your thoughts may vary.

Are we discussing treaded radial race tyres or radial slicks? I've found that a radial slick will et and 60ft better but are not as forgiving on less well prepped tracks.
I've no experience with the treaded radial race tyres but have listened to many tales of mixed results with them, seems they will work well on a good track, similar to the above.

The OP was refering to ET Street Radials, so they do have a tread design.
Personally I have seen a few guys at our local track running just radial slicks. They have pretty well the same problems as the street radials.Both them and myself usually just hope the track stays together for the final rounds.