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Author Topic: 340 Engine... Top Speed "120 mph?"  (Read 2485 times)

Offline Fleet 500

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340 Engine... Top Speed "120 mph?"
« on: June 02, 2009 - 07:41:43 PM »
I've noticed in old road tests that the top speed of compact and pony car Mopars with the 340 engine were usually in the 120-125 mph range. Usually at about 5200-5400 rpm. I have always wondered why actual top speed isn't more like 130-135 at 5500-5800 rpm?

Here are some examples from Car Life magazine:
(All have 340s and 3.23 axle ratio.)

Car---------------    Curb weight-- 1/4 mile--------- Top Speed
'60 Dart Swinger----- 3,310 lbs----- 14.8@96-------- 121@5400 rpm
'68 Barracuda 340-S- 3,470 -------- 14.97@95.4----- 127@5500
'68 Dart GTS-------- 3,305--------- 14.68@96.2----- 122@5400
'70 Duster 340------- 3,105-------- 14.72@94.24---- 120@5200

Any reason why the testers couldn't wind out the 340s up to 5800-6000 rpm and 130+ mph?

Cuda-Challenger.com

340 Engine... Top Speed "120 mph?"
« on: June 02, 2009 - 07:41:43 PM »



Offline HemiOrange70

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Re: 340 Engine... Top Speed "120 mph?"
« Reply #1 on: June 02, 2009 - 11:39:47 PM »
gears

Alaskan_TA

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Re: 340 Engine... Top Speed "120 mph?"
« Reply #2 on: June 02, 2009 - 11:53:58 PM »
&.....

You are not going to hit the actual highest possible "top speed" in a 1/4 mile test.

Offline Fleet 500

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Re: 340 Engine... Top Speed "120 mph?"
« Reply #3 on: June 03, 2009 - 12:12:01 AM »
&.....

You are not going to hit the actual highest possible "top speed" in a 1/4 mile test.
I think their top speed runs were separate from 1/4 mile tests.

Alaskan_TA

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Re: 340 Engine... Top Speed "120 mph?"
« Reply #4 on: June 03, 2009 - 12:26:54 AM »
I see that know, I was looking at your chart wrong.

There is a formula you can use that combines horsepower, tire size, rear gears & so on to arrive at the actual top speed mathematically possible. I do not have the formula avaliable, but I bet one of our racer members does?

Offline Fleet 500

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Re: 340 Engine... Top Speed "120 mph?"
« Reply #5 on: June 03, 2009 - 12:54:55 AM »
I remember a road test of a '68 340 Dart in a magazine based in England. The tester got it up to 140 mph but it probably had less emissions equipment on it compared to the 340 engines sold in the U.S. Axle ratio was not mentioned.

Offline cuda

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Re: 340 Engine... Top Speed "120 mph?"
« Reply #6 on: June 03, 2009 - 07:29:09 AM »
Yup gears and open road are the answer. I have an O.D. hooked up to my 340 and i have gone 130 with under 5000rpm. That is were i get scared and i stopped. But the car was still pulling well and i bet 150 was very possible.  :burnout:

Offline kissalien

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Re: 340 Engine... Top Speed "120 mph?"
« Reply #7 on: June 03, 2009 - 01:16:16 PM »
Our old cars were horribly inefficient aerodynamically. Tops speeds were much lower than you'd imagine because of it.

Offline Fleet 500

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Re: 340 Engine... Top Speed "120 mph?"
« Reply #8 on: June 08, 2009 - 09:36:10 PM »
Yup gears and open road are the answer. I have an O.D. hooked up to my 340 and i have gone 130 with under 5000rpm. That is were i get scared and i stopped. But the car was still pulling well and i bet 150 was very possible.  :burnout:
Yeah, I believe it; if it was still pulling at 130 mph.

Offline LAA66

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Re: 340 Engine... Top Speed "120 mph?"
« Reply #9 on: June 09, 2009 - 12:05:39 AM »
 When young and bold we once clocked 120mph on bias ply delta 60 tires. My buddy was watching the speedometer as I had to keep an eye on the road. It was around 2am on an empty hi-way. Drifting the 70mph (design) curve was thrilling to say the least. :misbehaving:

 Gears were 2.76's in the rear and a 3.23 speedo pinion. 130-something actual is my guess.

Offline Bullitt-

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Re: 340 Engine... Top Speed "120 mph?"
« Reply #10 on: June 09, 2009 - 12:51:21 AM »
IF the speedo can be believed I was in a 71 Charger 340 4speed doing about 160.. was this Jaguar V12 that needed a lesson back in '77.
Personally had my '73 340 to 125, that's 6K+rpm with 3:55 gears
Wade  73 Rallye 340...13 R/T Classic   Huntsville, AL
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Offline Ross

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Re: 340 Engine... Top Speed "120 mph?"
« Reply #11 on: June 09, 2009 - 01:25:29 PM »
I've had my 340 up to 135mph  (auto with 3.23 gears)
The tach was broken then, so i cant tell you rpms

Pretty scarry at that speed, but she would have gone more...
'70 340 Challenger

A whole lot of rust and very little time to work on it...

Offline Mr. 440SixPack

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Re: 340 Engine... Top Speed "120 mph?"
« Reply #12 on: June 09, 2009 - 01:54:31 PM »
Well i have to say that this is a great topic. I am always dissapointed when looking at these old road tests... top speed of 120 mph for a 300+ hp car is kind of a joke, dont you think so? Ofcourse these cars weren't designed for breaking top speed records because of their bad aerodynamics and tires but I am sure that every stock mopar would get to 130 mph and more without problems.
My dream: 1970 Plymouth 'Cuda 440+6 Pack/4 speed Pistol Grip/Dana 60 rearend/PS, PB/Lime Light/Shaker hood/Rallye dash...


Offline NoMope Greg

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Re: 340 Engine... Top Speed "120 mph?"
« Reply #13 on: June 09, 2009 - 02:42:56 PM »
Well i have to say that this is a great topic. I am always dissapointed when looking at these old road tests... top speed of 120 mph for a 300+ hp car is kind of a joke, dont you think so? Ofcourse these cars weren't designed for breaking top speed records because of their bad aerodynamics and tires but I am sure that every stock mopar would get to 130 mph and more without problems.


With that kind of power, aerodynamics wasn't the limiting factor; it was definitely the gearing.  Remember, the transmissions are all direct drive in high gear and the rear ends are geared more towards acceleration.  Add in the relatively small tire diameters of the time and that limits speed.  If you could get it to hook, it would get to 130 quickly and stay there, but any faster would induce valve float and cut power.  Modern cars accelerate as fast as the old cars with higher top speeds through a combination of overdrive transmissions with more gears, taller rear ends gearing and taller (stock) tires. 

Using some of the data from one of the road tests Dutch posted, the stock tire from the factory for a 'Cuda or Challenger R/T was an E60-15, which is a 25.5 inch tall tire with an 8.70 inch (221 mm) section width .  Using this calculator (http://www.kabamus.com/garage/gears.html) , the speed at 6000 rpm is 128 mph.  Change to a 3.23 gear and the speed at 6000 rpm changes to 141 mph.  Go with the 2.76 gear found in a bunch of 318 grocery getters and the theoretical top speed with the stock size tire is 165 mph!!

Compare that to a 2008 Challenger SRT8.  The stock tire is a 255/45R-20, which is 29.0 inches tall - 3.5 inches taller.  The rear gear was a 3.08 and top gear in the transmission was a 0.83 overdrive.  The theortical speed at 6000 rpm?  204 mph!!  To limit it to the same 127 mph top speed as the original requires going to a 4.90 rear gear.  Or, to compare another way - keep the 1:1 transmission gearing and 3.55 gearing from 1970 and only change the tire - top speed goes from 127 mph to 146 mph just by going to the larger tire.
Greg
2003 Ford Escape XLS
Currently Mopar-less :(

Offline HP2

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Re: 340 Engine... Top Speed "120 mph?"
« Reply #14 on: June 09, 2009 - 02:48:36 PM »
I would have to ask what constitutes Car Life's, or any magazines for that matter, top speed test. Is it 1/2 mile? 1 mile? 3 miles?  I seriously doubt any of them loaded up and headed out to El Mirage or Bonneville to do that test, where there is much more run out space for a top speed run. Was it something like Maxton where they did a standing start, 1 mile run? I could see that keeping speeds somewhat reserved, yet still being within a distance they could achieve on some desolate highway, which is probably how they conducted those tests.

Of course once you do get unlimited run out, then you get to deal with the aero issues. Sure, you can get one of these old cars up to 130 reasonably easily, and touch 150 if you push it, but the nose gets light and steering gets vauge. Much beyond that is not going to be safely possible in a stock type car. Once you drop the height, slow down the steering ratio, add some weight, and redirect some air, them maybe 165-170 could happen if you got enough oats in your engine. Sustained speeds beyond that aren't going to happen without major work, hence the reason for the winged cars.

Cuda-Challenger.com

Re: 340 Engine... Top Speed "120 mph?"
« Reply #14 on: June 09, 2009 - 02:48:36 PM »