Top Speed 71' Roadrunner

Technical Question and Answer - On topic to 71-74 Plymouth B-bodies only.

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pats71
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Top Speed 71' Roadrunner

Post by pats71 » Wed May 12, 2010 6:43 pm

Anyone know the top speed of a 71' Roadrunner with a HP 383? Did the factory ever try top speed tests on these cars? I know mine has the 150mph speedo but was wondering if it could reach 200mph if it wanted.
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Re: Top Speed 71' Roadrunner

Post by landon1 » Wed May 12, 2010 8:44 pm

depends what gears...
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Re: Top Speed 71' Roadrunner

Post by 471Magnum » Wed May 12, 2010 9:32 pm

Is this a serious question!?!? 200 mph!?!? Get Real.
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Re: Top Speed 71' Roadrunner

Post by CtownRunner » Wed May 12, 2010 9:49 pm

200 is Salt Flats race territory.
Good question though.
Just what is the top end speed on a 71 383?
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Re: Top Speed 71' Roadrunner

Post by pats71 » Wed May 12, 2010 10:56 pm

It's not like this is an unlikely question. The aerodynamics are definately there to pull it off just like the superbird could, although were not sporting the spoiler and noise.

I have 3.23 sure grip with a 4 speed. It seems like everything is there to make it possible. I was just curious and thought I would ask cause everytime I look at the car it looks like the aerodynamics could rival the superbird since it is not the squared body of the 70' roadrunner.

This is just a hypathetical question. I know this is salt flats ideally, but if someone were to get a wild hair on the highway...you never know lol.
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Re: Top Speed 71' Roadrunner

Post by 471Magnum » Wed May 12, 2010 11:18 pm

:roll:
Okay, here's the deal...

With 2.76 gears and 28" tires, 5000 rpm is 150 mph, assuming no drive line (converter) slippage.

Not sure where peak power is on the curve, but that is a BIG assumption that a stock big block is going to have the horsepower to pull 5000 rpm through 150 mph air.

Furthermore, at 150 mph, with the slightest cross wind, YOU'RE GONNA DIE!

Run a stock big block over 5000 rpm for an extended period inching up on that speed, and it will probably scatter, oiling the tires in the process, AND YOU'RE GONNA DIE.

3.23 gears will get you 129 mph at 5000 rpm. Probably a much more realistic figure.

The slipperiest car of that Nascar era was probably the third generation Charger, and they used gobs of Hemi horsepower to hit 200 mph.
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Re: Top Speed 71' Roadrunner

Post by 471Magnum » Wed May 12, 2010 11:45 pm

Here's another little piece of math that will cut through the bench racing BS.

A four speed 3.54 geared hemi car is going to be wound out at 6000 rpm... doing 141 mph.

An automatic 3.23 geared hemi car and it will be all done at 155 mph.
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Re: Top Speed 71' Roadrunner

Post by pats71 » Wed May 12, 2010 11:50 pm

Ok. Like I mentioned, just a simple hypathetical and if it is possible.

I'm sure the effects of the gull wing/chin spoilers would definately come into play in this type of scenario.

It would make more sense for a 383/400 to be able to pull off this type of situation simply because they are running a smaller stroke compared to that of the 440/hemi. That's why the "B" engines are known as high winding motors that run at the higher rpm's much easier and make more power in the higher rpm range. If it were a "RB" motor, I would be a little more skeptical.

My only arguement to this being possible is whether the stock carb could actually keep up with the demand of the motor. I've got a Barry Grant 750 that will be going on so that should definately be able to supply enough fuel in this hypathetical scenario.

If these cars were able to hit 150mph from the factory (I'm basing this on the speedos they put in these cars), then it should only take minor mods to reach close if not just under 200mph.

Based on your math you just gave, a 4 speed 3.23 geared car should be able to at least reach 165-170mph. If you add in an aftermarket intake and headers, that should free up the motor to go that much faster.

Please do not take this post as any sort of serious. It was just a thought that came to me while I was driving for 3 hours straight. I WILL NOT attempt this once my car is done. I just thought of it as a funny post that would start a new conversation.
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Re: Top Speed 71' Roadrunner

Post by CtownRunner » Thu May 13, 2010 6:48 am

I like the way you think!
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Re: Top Speed 71' Roadrunner

Post by bruce » Thu May 13, 2010 8:40 am

FWIW, in Gary Beineke's quest to see over 200 mph in a '71 b-body he's using a 975 hp fuel injected hemi AND the nose and wing of a Daytona. A stock 383 Road Runner seeing 200? Not gonna happen.
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Re: Top Speed 71' Roadrunner

Post by 471Magnum » Thu May 13, 2010 12:33 pm

bruce wrote:FWIW, in Gary Beineke's quest to see over 200 mph in a '71 b-body he's using a 975 hp fuel injected hemi AND the nose and wing of a Daytona.
To be fair, he's trying to do it over the course of a mile. 650 hp might get you there given a long enough run at it.

But.... a 383 ain't gonna get it done, not even going down a hill being chased by a tornado.

A stock big block is done making horsepower at 5500 rpm. That's all the heads are good for. Warm it up a bit, and you might pull to 6000, but not in 150 mph air, much less at speeds above that.
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Re: Top Speed 71' Roadrunner

Post by bruce » Thu May 13, 2010 1:00 pm

471Magnum wrote:To be fair, he's trying to do it over the course of a mile. 650 hp might get you there given a long enough run at it.

Good point. :D
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Re: Top Speed 71' Roadrunner

Post by 471Magnum » Thu May 13, 2010 6:03 pm

pats71' wrote:Based on your math you just gave, a 4 speed 3.23 geared car should be able to at least reach 165-170mph. If you add in an aftermarket intake and headers, that should free up the motor to go that much faster.
It's not my math. It's everybody's math.

A 383 is not going to turn over 6000 rpm for very long, and it won't be making any kind of power. 140 tops with 3.23 gears.

Your factory tach and speedo are pretty much worthless at that end of the range, so if you are dumb enough to try it, use a GPS.
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Re: Top Speed 71' Roadrunner

Post by 71GTX » Fri May 14, 2010 12:54 am

dont know if this has been seen yet by anyone on here..... http://www.youtube.com/watch_popup?v=p1cLfiVSyrw
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Re: Top Speed 71' Roadrunner

Post by CtownRunner » Fri May 14, 2010 6:40 am

WOW.
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