Author Topic: Diff Ratios (Image Heavy) (Read 15731 times)

Barney · « **on:** October 14, 2015, 06:01:55 pm »

Hi Guys
Been scratching my head over which diff ratio to fit to my 65 fastback. I upgraded my gearbox from a toploader to a new Modern Driveline T5, now the existing 3.25 diff is way too tall.

The engine is a grumpy high comp 302 with solid mechanical and vic jnr heads/manifold so it will need some revs to cruise nice, I wanted to target 2000rpm @ 100kph, does this sound reasonable?

The new T5 has a 0.63 Over Drive so its a decent drop in revs from 4th to 5th.

From my calculations a 3.9:1 diff is required to cruise at 2000ish RPM @ 100kph, but my brain is telling me this is a drag racing ratio and I have go my figures wrong!

Drive line details are:
302W
0.63 OD 5th
3.9 Diff
Tyre is a 226/60/15 (635mm/25" Diameter)

Whats your thoughts?

Thanks

Ash

boofhead · « **Reply #1 on:** October 14, 2015, 06:11:55 pm »

302's love to rev and any old school guy knows to get them to perform then let them rev. In summary, Do not be afraid of the revs. So from your description of your engine you really should have at least a 4.11:1 ratio. You will find in over drive it will be around 2k on the highway.

GLENN 70 · « **Reply #2 on:** October 14, 2015, 06:20:17 pm »

You have a littler rever and with a big intake manifold and a large solid cam so low diff gears like 3.9 will be ok and it will go like hell yet with the 5 th gear will cruise on the hwy at low revs . But with the solid cam it might not like top gear cruising at 2000 rpm and be happier at around 2500 rpm . Go for a drive now and see what revs it's doing in 4 th gear and see how the engine is running at those revs ( not camming around ) that's the difference between having a big cam in a manual and having a auto with a big hi stall .

Barney · « **Reply #3 on:** October 14, 2015, 06:48:18 pm »

Thanks for the feedback guys. Good call on the test drive will see how it goes in 4th at 2k

Ash

GLENN 70 · « **Reply #4 on:** October 14, 2015, 07:25:02 pm »

In 4th gear now at 2000 rpm will be just under 50 mph/80 kph . Just see how it likes it cruising at 2000 rpm and as I said it might like 2500 better .?

PET-289 · « **Reply #5 on:** October 14, 2015, 09:01:42 pm »

I'd go 4:11's with that set up

GLENN 70 · « **Reply #6 on:** October 14, 2015, 09:20:26 pm »

4:11s

GT350H · « **Reply #7 on:** October 15, 2015, 08:09:06 am »

Agree, 4:11's.
I run 4:11's with my T5 and 17's and its 100kph at 2000rpm

GLENN 70 · « **Reply #8 on:** October 15, 2015, 08:51:08 am »

Barney what diff is in the car now 8 inch or a 9 ? Open , LSD , etc ? . The 3.25 is a very good ratio and very saleable as gears or a complete center . The only problem with 4:11's is they can be noisy and each gear will be short .

boofhead · « **Reply #9 on:** October 15, 2015, 08:56:23 am »

A lot of the after market gear sets tend to be a little noisy. I do agree you will have no problem selling the 3.25:1 as that is a great gear for a stock or very mild build with no over drive. The 4.11 is available for either 9 or 8 inch rear.

BTW: The later performance mustangs with the T5 from the factory had a 3.73:1 rear. The closest ratio for the 8 inch rear would be the 3.80:1 while the 4.11:1 will give you a small kick up from there without being to much shorter than the factory fox mustang of the T5 era.

GEOFF289 · « **Reply #10 on:** October 15, 2015, 11:14:00 am »

You want to give some thought to the other end of the equation too.

If your T5 has the standard 3.35 first gear it's going to be just about redundant with 4.11's at the back and for all normal driving purposes you'll be in 2nd in about 3 feet. A 2.95 first gear is better.

Have a look at this calculator http://www.allfordmustangs.com/Detailed/608.shtml.

Barney · « **Reply #11 on:** October 15, 2015, 01:56:36 pm »

Thanks Guys

It's a 9" diff, with LSD.

Only has small wheels 225/60/15 (25" Diameter)

I should be able to take it for a drive early next week to see how it goes at 2000rpm in 4th, I just hope its not camming around.

GEOFF289 your link is not working

Ash

Barney · « **Reply #12 on:** October 15, 2015, 01:57:28 pm »

Quote from: GT350H on October 15, 2015, 08:09:06 am

Agree, 4:11's.
I run 4:11's with my T5 and 17's and its 100kph at 2000rpm

What t5 is it, do you know the over drive ratio?
Also, what diameter are your tyres?

Thanks
Ash

Azz67 · « **Reply #13 on:** October 15, 2015, 02:08:52 pm »

I have recently done the T5 swap which has a 3.35 first gear and I have a 3.5 8 inch diff in mine.
While this combo could be viewed as a little on the short side it is very much driveable, and most definitely a sh*t load of fun.

cheers,
C.

GLENN 70 · « **Reply #14 on:** October 15, 2015, 02:49:48 pm »

Its the big cam causing the problem . Stick with the 3.25 and stay in 4 th gear at 100 kph . 2700 rpm at 100 isn't that bad , then at 110 kph you have 5 th gear . To pull your diff apart then the center then buy a good set of 4:11 or 3:9 gears and fit them into an old LSD center ,is it worth all the trouble ? .if you want to sell your center , As I said a good 3.25 LSD 9 inch is a good seller ,so maybe a old center with new low gears and a tru trac is the go . With 4:11 or 3:9 ratio the old LSD will be useless . Just throwing a few ideas out there .

boofhead · « **Reply #15 on:** October 15, 2015, 04:45:55 pm »

The advantage of the 9 inch is primarily there many more gearsets to choose from.

You need to gear it so you are not loading the engine up at cruise. Your running large heads and a solid roller cam. Great choices for a high performance engine but it does mean your power would likely start to come on 3000 revs or so and go through to 6500 and you shift at 7k or so. Maybe the Cam is not the size I am assuming it is.

Any way, even 2k would be labouring the car at cruising speed. If your too tall your going to hurt the performance of the engine, burn excessive fuel and not have lot of fun driving the car. I hope you have a world T5Z so you have a stronger box and the 2.95 first gear. In any case, gearsets in the box can also be changed.

Barney · « **Reply #16 on:** October 15, 2015, 05:08:49 pm »

The Cam
http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=830&sb=2

The box is the top one in this list
http://www.safepay.net/cgi-bin/shop/cart.cgi?db=mddata.txt&category=Transmission&merchant=moderndriveline

GLENN 70 · « **Reply #17 on:** October 15, 2015, 05:24:46 pm »

Ok it's a solid roller , lower the diff gears .

boofhead · « **Reply #18 on:** October 15, 2015, 07:20:10 pm »

Cam is where I was expecting and the gearbox is the exact same model as mine (and purchased from the same supplier), e.g., an aftermarket world T5Z. So first gear is 2.95. I am staying with my original suggestion, at least 4.11:1 (though you could run a little more [say up to 4.56:1] if you looking for even better 1/4 mile times).

GEOFF289 · « **Reply #19 on:** October 15, 2015, 07:41:25 pm »

Quote from: Barney on October 15, 2015, 01:56:36 pm

GEOFF289 your link is not working

OK, has in the past. Try this http://www.allfordmustangs.com/Mustang_Tech/Calculators___Decoders/index.shtml, then 3rd from the bottom

GLENN 70 · « **Reply #20 on:** October 15, 2015, 07:53:18 pm »

4:56

gee you missed out 4:33 . so Barney what are you looking for a hwy cruiser ? Not really with that combo .

boofhead · « **Reply #21 on:** October 15, 2015, 08:37:12 pm »

I know 4.56:1 is short though you need to look at my recommendation, e.g., 1/4 times.
I could go through the calcs but you will find it is closer to optimal when the Cam is looking for around 7800 rpm at the time the car crosses the line at 1/4 mile marker. Then if your looking for highway cruising then in 5th gear with a 0.63 overdrive the rear is equal to 2.87:1 . Now as you would know this is longer than (though close to) the factory provided in the very basic models. So is it really a silly option for the highway - ford thought the 2.89:1 was good for standard 289 cars and 3.o0:1 for higher models. Not so silly is it.

Barney · « **Reply #22 on:** October 15, 2015, 09:18:37 pm »

Thanks for the input guys, and good conversation

I have only just built the engine and the tune is still a bit off, but I have to say it doesn't feel too bad down low. Yes its gonna want to rev but I don't think it will be as bad down low as first thought. Correct me if i'm wrong but the compression @ 10.8 will be helping a bit as well.

At the moment 5th is too tall for cruising we know that, but it is also too tall in first gear, so I will defiantly need at least a 3.9 or 4.11.

GLENN I wanted a car that sounded as good as it looked, and I really like the mechanical solid lifter sound. But it would also be nice to pluck 5th and make it half decent on the highway.

Maybe I stuffed up with the build and got a bit carried away, but I don't think i'm the first or will be the last to do that!

Ash

GLENN 70 · « **Reply #23 on:** October 15, 2015, 09:51:16 pm »

Before we work out the gear ratio lets see how it drives at 2000 rpm in 4 th gear . 2000 rpm might be to low for smooth hwy cruising for that cam . 2500 might be better ? To get a gear ratio so it will be 2000 rpm at 100 kph in top gear will have to be low . That cam and engine needs low gears like 4.11 to work and not be hard on the clutch .

Barney · « **Reply #24 on:** October 15, 2015, 10:15:57 pm »

Quote from: GLENN 70 on October 15, 2015, 09:51:16 pm

Before we work out the gear ratio lets see how it drives at 2000 rpm in 4 th gear . 2000 rpm might be to low for smooth hwy cruising for that cam . 2500 might be better ? To get a gear ratio so it will be 2000 rpm at 100 kph in top gear will have to be low . That cam and engine needs low gears like 4.11 to work and not be hard on the clutch .

Good plan.
I am currently having the headers modified as they rub on the towers. BTW they are 1-5/8" into 3" collector then down to 2.5 for the exhaust.
So hopefully by early next week I can give her a tune up and do a RPM test. I will keep you all posted.

Diff Ratios (Image Heavy)

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Author Topic: Diff Ratios (Image Heavy) (Read 15731 times)