C4 verses 4 Speed - Vintage Mustang Forums
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post #1 of 18 (permalink) Old 06-19-2019, 08:43 AM Thread Starter
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C4 verses 4 Speed

I'm still kicking around a way to lower my rpms on the highway for my 67 coupe ! I know, I already got alot of flack here for wanting to do this ! I'm an old guy (73) living on social security so I can't spend a ton of money on a new top loader or an overdrive automatic. I have a friend with a complete 4 spd set-up I can get for little or nothing. My question is, would this lower my rpms at all ? Pretty sure I have 3.50 gears in the rear !
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post #2 of 18 (permalink) Old 06-19-2019, 08:48 AM
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I doubt it. Because 3rd gear on the C4 & 4th gear on the 4 speed are both 1:1 ratio.
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post #3 of 18 (permalink) Old 06-19-2019, 08:59 AM
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The only ways to lower RPM are to swap in a lower numerical gear, swap in a 5 speed or drive slower. Seeing as you have stated your financial constraints, option three might be your best bet.

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Patrick
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post #4 of 18 (permalink) Old 06-19-2019, 09:09 AM
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Quit worrying about it and just drive it with the engine turning 3200 rpm. It will thank you for letting it "air itself out".
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post #5 of 18 (permalink) Old 06-19-2019, 09:11 AM Thread Starter
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OK...I get it ! Thanks guys for your input
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post #6 of 18 (permalink) Old 06-19-2019, 09:27 AM
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Swap in some 2.80 gears...easier and cheaper than changing out the trans.
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post #7 of 18 (permalink) Old 06-19-2019, 09:30 AM
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It will lower the rpm some but I'm not sure how much. If what I have read is correct the stall rating on the stock converter was supposed to be 1800 rpm but in my experience it was a bit less. If you are running over the stall rating the slip amount drops. Maybe you might get a couple hundred rpm improvement from the 4 speed but I am just somewhat guessing. Not sure that is worth the trouble to do the swap.


It must be worth something though because all modern automatic transmissions that I am aware of have a lock up converter which removes the slip. When this was a relatively new thing I had an IROC with that lock up in it and it was annoying. I often wished I had a switch to disable it or enable it when I wanted it. They are so smooth now that you can't really tell when the converter locks up or unlocks.


It might be easier to just run a taller tire when it comes time to get tires.


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1979 Ford F150 custom, 302, C4, AC, tilt wheel, main transportation
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post #8 of 18 (permalink) Old 06-19-2019, 10:44 AM
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No flak from , I love having overdrive. But.
Technically you get slightly better mileage from a four speed just because in high gear it is "locked". A solid no-slip connection between engine and transmission. Where OEM's got the idea for a "lock up" in newer automatics. Old school automatics have converters that always slip a bit, thus worse gas mileage. The reality is that the MPG difference is very small in a car that's only driven lightly.

Overdrive is great, I am addicted to it. But it doesn't make actual financial sense. The gain in mileage is really only found at highway cruising speeds. THe math has been done, somewhere, but essentially you'd have to drive on the interstate regularly for years or see gas go up to ten bucks a gallon before you'd even break even on what you'd spend on swapping to an overdrive transmission.

Me, I'm not sensible like that. I just swap in overdrive transmissions for fun.
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post #9 of 18 (permalink) Old 06-19-2019, 02:44 PM Thread Starter
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Thanks guys.....Like I said, I'm old so driving the 67 with flowmaster exhaust & turning 3500+ RPMs on the freeway is very annoying driving to long distance car shows & roll-ins. I'm restoring a 70 fastback right now also. It has a 351 Cleveland withy an FMX but it's going to be awhile before its on the road so I don't know what the difference is between an FMX & a C4. I have a spare FMX laying in my garage. Guess I will skip the 4 speed swap for sure or maybe put it in the fastback !
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post #10 of 18 (permalink) Old 06-19-2019, 02:52 PM
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Not much difference between the 2 autos. C4 is lighter, and has better parts available. Fmx is getting hard to find parts to rebuild them, and fewer options to hop them up. Run a C4 with a transgo shift kit, a good servo, and put it behind that C. You will be plenty happy. But if you are looking for lower highway rpms, then you need overdrive or higher gears like a 3.0, or 2.8.


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post #11 of 18 (permalink) Old 06-19-2019, 03:50 PM
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Quote:
Originally Posted by GOT-my*1967 View Post
driving the 67 with flowmaster exhaust
I think I found what is annoying you while driving at highway speeds.



I've started a blog about my car and adventures: http://65mustangfun.blogspot.com/

1965 Fastback, 289, Toploader 4 Speed, owned by me since June 1980. Originally a C-code with a C-4. 5R09C16****
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post #12 of 18 (permalink) Old 06-19-2019, 04:12 PM Thread Starter
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thanks coupster
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post #13 of 18 (permalink) Old 06-19-2019, 05:12 PM
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Lee nailed it...

Regards,
Patrick
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post #14 of 18 (permalink) Old 06-19-2019, 06:43 PM
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Where is this 4 speed you speak of?

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AKA "Dr. Wolf"

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post #15 of 18 (permalink) Old 06-19-2019, 06:45 PM
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I think I found what is annoying you while driving at highway speeds.

Ding ding ding. Thats exactly what I was thinking. Put in a good set of mufflers. I put in a set of Hedman headers turbo style mufflers as they had the right inlet and outlet location specd my exhaust system. Made in USA, very nice quality and no drone.

To lower rpm, taller tires or taller gears. Im very happy with my 2.80 gears.

Tom

One thing great about getting older. A life in prison sentence is less of a deterrence

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