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Premium Member
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Discussion Starter #1
So I am reading this months Hemming Muscle Machines magazine. And in this months edition they feature a 1971 Plymouth Road Runner Hemi (1 of 55 built)

They note that the car ran a 13.73 bone stock out of the factory, equipped with D70-14 (which converts over to like a 200/70-14, based on this chart http://enthusiast.yearone.com/faqs/wheels-tires-brakes/tire-conversion-chart/, if I am reading it right)

I find it absolutely mind boggling that a car with that much power (425hp and 490lb ft) can hook up with tires no wider than a stock SC.

Maybe its smarter to not spend the premium for wider tires, and instead invest the money into your engine?
 

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The Parts Guy
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:uppoint: And what was the 60'? How about the MPH? I wouldn't classify wider or softer compound tires as a gimmick.
 

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Discussion Starter #3
no 60' mph was 104, and 0-60 was 5.7, out of a 2 ton car.
 

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The Parts Guy
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Premium Member
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Discussion Starter #5
you win lol, I just found it amazing that car running skinnies like that could run a high 13's, but then again a 426 hemi has gotta run like a bat out of hell.
 

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Super Moderator
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Only 13.73 with 425 HP at 4000 lbs? A bone stock MN12 would run two seconds slower with 200 HP at 300 lbs lighter. Something doesn't add up...

A given compound can provide a certain amount of acceleration friction grip for a certain unit of vertical pressure per each unit of surface area. To increase that acceleration friction value to get more launch grip, you have to increase vertical load pressure and/or increase contact patch size. Since you can only increase vertical load pressure to a certain degree and to varying degrees of benefit, you have to increase contact patch eventually if you need more grip.
 

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My dad would tell me that's why he raced his 289 mustang from a stop. He almost always had a smaller motor with less power but with a decent set of fat tires and a good launch he could always beat them on the 1/4 mile. (It wasn't stock but it wasn't pushing anywhere near 425hp) If he raced from a roll it was over.
 

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50 years of Mercury Cougar 1967/2017
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I don"t thing a hemi car came from the factory with
D70-14.

The hemis were and still underrated as far as factory
Horsepower is concerned.
 

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Super Moderator
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Polyglass tires were terrible for grip in general, no matter the size. In fact I recall reading the bigger wheel/tire combo was done not for grip but for the additional load capacity do to the RB/Hemi engine weight.

Most pseudo car journalists love to point out the fact that a modern Camry can out accelerate most of the Muscle cars from the era but they never seem to account for the fact that the specs they're referencing were taken on the gawd awful tires they were running in tests back then. Modern radials actually perform equal to or better than vintage slicks even. Considering the Hemi's 425hp rating was actually closer to the RWHP number, that 13.7 run probably involved a lot of spinning that wouldn't have happened with a decent(wide) modern tire.
 

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Voice/Data Guru
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My uncle's 72 Duster 340 came with polyglass tires, I can remember them things hard as a rock:eek: and right about no grip.
 

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WOT Junkie and avid corn burner
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I remember a 66 Fairlane that my dad had. It had a 390 4V with a 3.25 open. I spanked it from a dig when my Tbird was stock....but he quickly started catching up to me until we had to hit the brakes. Even that was only rated at 315 bhp, through a C6 none less! I tell ya what though, the '62 Galaxie XL500 convertible he had was a beast. 406 Tri-power with the 4 speed. That thing hauled it! I was with my dad when he raced a supercharged 3V Mustang on the highway. We walked his a$$ like it was nothing. He barked the tires when downshifting one gear at 70 mph! :eek: Only rated a 405 hp. After a stock rebuild, it put out 468 hp on an engine dyno.
 

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We'll my sho stone stock ran 13.55 @105 mph dead hook as it's awd
 

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On cars 1971 and older they used "gross HP" rating, and after that they used "net HP" rating. So a 375 HP 440 Magnum suddenly became only 275 HP, which more closely matches what they use today. No net rating for the 426 HEMI, since they stopped making them in 1971.

And as already said, with today's tire technology, that same HEMI car would run mid 12's. A lot of HEMI cars came with 14" x 5.5" wheels, WTF! A 6" wide tire is not going to do much.

Actually, there is a whole Muscle car drag racing series where they have to run the 60's & 70's cars on the bias ply tires, and they run in the 11's and 10's with very minor mods, and hook up pretty well. But I am sure the bias ply reproductions of today are better than the originals.

Al
 

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PostSlut
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my 73 bird still had original recall firestones on it when i bought it....bias ply.
Now, its got modern radials, I run 235/75/15's. yes, truck tires!
 

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Super Moderator
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On cars 1971 and older they used "gross HP" rating, and after that they used "net HP" rating. So a 375 HP 440 Magnum suddenly became only 275 HP, which more closely matches what they use today. No net rating for the 426 HEMI, since they stopped making them in 1971.

And as already said, with today's tire technology, that same HEMI car would run mid 12's. A lot of HEMI cars came with 14" x 5.5" wheels, WTF! A 6" wide tire is not going to do much.

Actually, there is a whole Muscle car drag racing series where they have to run the 60's & 70's cars on the bias ply tires, and they run in the 11's and 10's with very minor mods, and hook up pretty well. But I am sure the bias ply reproductions of today are better than the originals.

Al
Oddly enough the 71 Barracuda brochure I have lists both the Gross and Net ratings and they have the Hemi down a 350. That's definitely still underrated by the current standard since those cars consistantly put well over 400 to the wheels in modern tests(although some restorations do tweak a thing or two from pure factory spec).



What's funny too is that they clearly fudged the rating for the 340 4bbl as well, and they practically spilled the beans on that chart. Both the 383 and 340 made 275 gross but then there's a whopping 45 horsepower difference between their net ratings :tongue:
 

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I believe I ran F70-14s on my 73 mustang. Dad didn't think I needed such "huge tires" but did agree they looked good. I'd have given anything for a set of the new radials the newer cars had but just didn't have the money. Didn't have too much problem with traction. But then I only had a 302 2V.
 

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A bone stock Honda Accord would kill that Hemi on a road course. !970's muscle cars were not known for handling. Skinny little bias ply tires were definitely part of the problem.

The 1995 T-Bird I drove in college had 205-70-R15 tires on it, and that car was a bit too prone to fishtailing, especially if the roads were wet. My Current 1996 T-Bird has 235's on all four corners, and handles much better. I can still hang the back end out on a wet road if I get too crazy, but it's much better than the 1995. Of course, it has aftermarket springs and Tokicko shocks on all four corners, and sits a couple of inches lower as well, but I think the tires are the biggest part of the equation.
 

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Back in the day, it was said that the 426 Hemi would make its rated 425 HP running on seven cylinders. It was definitely underrated, in a failed attempt to keep the insurance cost semi-reasonable.

And yes, traction was a BIG problem on the muscle cars. When I took my dad's '68 RoadRunner with 383 automatic to the dragstrip, it had to be eased off the line or the first 330' was all wheelspin.

Later, I had a '71 RoadRunner with 440+6 and a 727 Torqueflite. It would burn rubber all the way through first gear and half the way through second gear on stock size tires. I added Centerline wheels with G60-15 tires and the car launched much, much harder.
 
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