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Old 01-17-2012, 06:55 AM   #1
DejanM31
 
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Default Top speed problem

I'm opening new thread because this looks like a new problem i have.

I have Mustang 65 with 289 V8 stock engine. This car had originaly 200cui V6 engine and 3 speed manual gearbox. Someone swapped the engine with 289 V8. I don't know if 3 speed manual is the same as it was (is V6 original 3 speed manual gearbox the same as in V8?).

The car's top speed is around 55 mph and it is already screaming at that speed. It starts to shake aswell. is that "nornal" or is something wrong?

How can i check axle ratio? Is there anything else i can check? I thought first to solve poor handling, but if i can't go more then 50mph normaly, i won't bother.
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Old 01-17-2012, 08:25 AM   #2
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jack up the back of the car, spin the tire by hand, count how many rotations of tire vs rotation of driveshaft. do you have a tachometer installed? do you know your RPMs at 55?

also mustangs used I6 engines not V6s
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Old 01-17-2012, 08:42 AM   #3
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i don't have tachometer. Sorry.
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Old 01-17-2012, 09:02 AM   #4
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Clutch slipping?
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Old 01-17-2012, 10:18 AM   #5
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How do you know that cluth is slipping? I don't have such a feeling, acceleration is ok, just revs at 50mph are too high i think.
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Old 01-17-2012, 11:01 AM   #6
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The Clutch slipping remark was a question.

But it is an "educated question", meaning that stock, a rear end ratio was never put in a Mustang that would cause an otherwise healthy 289 to "top out" at 55 mph. Additionally, assuming that the stock 6 cyl housing is still in the car, I am sure there were NEVER any gears made for this housing that would cause the top speed of a Mustang to be limited to 55 mph.

Even if the the housing were changed to an 8", I am about sure no gears were made that would impose such a limit.

9" rear? maybe, but I doubt it, even if your rear had 4.55s, I imagine the car would still be capable of near 100 mph. (I think some "truck" 9" rears were equipped with final gearing between 6 to1 and 7 to 1)

IF the engine is truly screaming at 55mph, chances are something is slipping, since manual transmissions don't really slip, I was thinking maybe the clutch is slipping.

IF your car will only drive to 55 mph, the more likely situation is that something is wrong with the engine/fuel system/ignition system that is limiting the power/RPM capability of your engine.

Practically, there are two possible scenarios....

1. the clutch is slipping causing your engine to overspeed....

2. your engine is not in an overspeed condition, you just THINK it is, and if indeed the car won't run past 55, you have an engine or system issue is fooling you.

Keep in mind, your car is not "overdrive equipped", and engine RPM above 3,000 rpm at cruising speed is not unusual, or indicative of imminent destruction for your older model car.

A healthy 289 is capable of running at engine speeds between 4,000 rpm and 5,000 rpm all day long.

It is my understanding that MOST I-6 Mustangs were equipped with 3.08 final gearing....
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Old 01-17-2012, 11:30 AM   #7
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if the gears were switched to 4.55, he would be at 5k+ at 55 mph, otherwise i would check timing and make sure the carb is tuned correctly.
i had the same thing with my car, it would fall on its face at 70ish, havent driven it since the rebuild but im guess it was my ignition and timing that caused it.

as JMD said maybe it seems like its screaming to you? with stock gears my 289 paired to a c4 is at ~3k rpm at 60. at that point its loud enough so that i have to raise my voice to talk with passengers.

i would buy a tach and go from there, i bought mine from autozone for 30 bucks and it works great.
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Old 01-17-2012, 11:58 AM   #8
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Quote:
Originally Posted by MustangFTW View Post
if the gears were switched to 4.55, he would be at 5k+ at 55 mph, otherwise i would check timing and make sure the carb is tuned correctly.
I ran the numbers through an online calculator,

http://www.rearendparts.com/pages.php?pageid=16

All at 55 mph with a 22" tall tire....

4.55 = 3650

3.55 = 2850

3.08 = 2475

a 4.55 gear at 100 mph, 22" tire, 6650
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Old 01-17-2012, 11:58 AM
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