Should I change to a 3.7 or keep the 3.36 ratio?

My diff has been making some noise for a while, and it's a bit worse now since fitting the 383 stroker... so I will be pulling it out and rebuilding it.

Currently running a TKO-600 with the 0.82 5th and a 3.36 diff ratio.
The gearing at present makes it an exceptional tourer.

Out on the highway it is great.... 5th gear sitting on about 2200 at our (Australian) national speed limit of 100kph, and about 2800 at 80mph when chasing club members in their C4's and C5's on cross country club runs.

Off the line it requires a bit of clutch slip to make a smooth transition, or a clutch dump to leave lots of smoke.:o but it's OK.
Traction is pretty good with the BF Goodridge T/A radials as long as I don't dump the clutch. It spins a touch, grabs, then gets going. hard!

Engine builder did suggest 3.7 diff gears to get best performance benefit from the motor...
Got a quote in on a new 3.7 gear set, and diff rebuild, and it is quite significant due to the exchange rate at present. ~ $2100

I'm curious to know how much difference I would notice in changing out the 3.36 diff for a 3.7 ??? Anyone done this?

Is it very noticeable from a performance perspective?

Could it shave 1/2 a second off a 1/4 mile time?
or will it become an uncontrollable tyre smoker and test my reflexes at every mid corner power down?

Obviously rpm will increase on country drives, maybe an extra 250 rpm.

But will the extra cost of new gears be noticed in seat of the pants performance improvement?

Should I change to a 3.7 or keep the 3.36 ratio?

 

A 3.71 rear end would make the first gear in your 5-speed pretty much useless off the line. You already need to feather the clutch to prevent excessive wheel spin. But, if you want to lose that highway cruise economy in exchange for faster take-offs, you can install a 3.71, some bigger drag tires on the rear, and beefier components in the rear suspension. Doesn't sound like a good 'trade off' to me. I think you have the right combination "as is".

 

Tom,
What is the first gear ratio?
If it is 2.87 then your current overall reduction in first is 9.64.
Going to 3.7 the overall first gear reduction becomes 10.61.
IIRC, 10:1 is supposed to be optimal for our cars and you are playing each side of it.
I currently have 3.27 x 3.7 =12.1, which is far too low and I am looking at 3.08s to give 10.07.
I have bags of Torque to play with (will pull away in 5th from 1000rpm cleanly)
I would have thought your 383 should have ample torque to pull 9.64 without slipping the clutch.
Remember, the muncie only had 2.2 or 2.5ish first gears and we lived with that until 5 speeds became popular
Oh, and 3.55 gives 10.18

 

yeah..... I'm undecided.
If there was a reasonable performance gain to be achieved by changing ratios... I'd consider it. But I'm concerned that all I'll achieve with a 3.7 ratio is lots of wheelspin. That's NOT what I want.

Maybe I'll just rebuild it with the 3.36 gears and save $630

 

My car has an auto trans, but when I switched from 3.08 to 3.73 gears I cut about 1/2 second off my ET.

 

383's are stump pullers that make alot of low end torque. Unless you're drag racing the car I would stick with the 3:36 for a nice all around driver that you can take places. I have a 383 in my 61 with a 4 speed and a 3:08 rear. Plenty of power and it's great on the highway.

 

stick with the 3:36, i love that ratio in my 72 coupe

hth,

Sully

 

Stick with what you have. Going to a 3.70 is only a 10% change, but it will be lost since you already have hook up problems.

 

Im on the other side of most that have posted. I think a 3.73 would be ideal. No clutch slip on takeoff. Into your powerband quicker on upshifts and into your powerband better at cruise. Cruising in your cams powerband will probably keep your fuel consumption close to where your at now due to running closer to your most efficent RPM's. Changing out your clutch early due to slipping it to take off will offset your costs on a gear upgrade somewhat. With the 3.73 gears you should be able to take right off with no slipping or dumping the clutch. As far as quarter mile times maybe not a 1/2 second but better times due to staying in the powerband better throughout, especially on a well prepped track. Your engine builder is right, that motor wants to rev a little and the 3.36 rear doesn't allow you the full benefit of your build specs. I had the same issue of either having to slip the clutch or rev and dump with tire smoke when I had a 3.08. Went to a 3.55 and it changed the personality of the car totally. Where you will really notice a difference is WOT running through the gears. It will feel like you added 50 HP throughout your run.

 

I would not think that's the case since you have a 5-speed box. 63Mako...do you have the same transmission? [If so, I would certainly defer to the person who has that set-up.]

 

why not spilt the difference and go 3:55?

 

No I don't, I had a wide ratio Super T-10 4 speed with a similar build level. That is what he has with an OD. Just has been my experience when selecting gearing, the best overall performance comes from keeping the engine in its optimum (cam) powerband for as much of the time you drive as possible. His engine wants rpm, in first gear, when taking off and cruising, to be efficent. The 3.55 would probably work well also but I like closer trans ratio spread, more "seat of the pants" power. I would definitly want to get past the point I had to slip the clutch to take off.
To the OP. How is your timing curve? 36 All in @ 2500, proper vacumn advance. That can make a big difference in your ability to take off - low end power.

 

I have a 3.70 rear in my 5 speed with a 500HP/385ci motor. I would highly recomend the 3.70 rear. The .82 final is a little tough with the 3.70 though. If I were to do it over again I would use a 3.55 ratio. 1st gear is not to short at all. BTW going from a 3.36 to a3.70 is a 10% difference. It is worth the change.

 

I've got 3.70s waiting to go in my '75 behind a BW Super T-10 wide ratio. Cannot freaking wait to get those bad boys in there.

 

The increase in your rpms would be 10%. 3.70/3.36 = 1.101 So, your speedo would be fast by that amount and your rpms would be up by that amount. I don't know if you would get a real, seat of the pants difference from that gear change. I've swapped out rear ends going from a 2.79 to a 3.55 in one case and a 2.73 to a 3.73 in another. Those both were noticible when you stuck the accelerator to the floor in both cars. I would think you would want to go to a taller set of gears, like 4.10s, rather than the 3.73s. The only problem is that the gear swap can spoil the car as a highway cruiser, but you've got a 5 speed, so it might not matter that much.

 

I'll be using a 3.45 first with 4.56 rear in my 80!!!

 

Holy crap you will be in 2nd gear before you hit 60 feet.

That is going to launch very hard if using slicks.

 

It should leave just like this. This is the motor, trans and rear combo thats going into the 80.

http://tinypic.com/player.php?v=2n6hjpf&s=4

 

So your RPM would be 2420 at the speed limit, 3080 at 80 MPH and 160 plus @ 6200 RPM. There will be a noticable difference running through the gears. It will feel stronger. If the RPM's listed above are not a problem that is the way I would go.

 

No problem hooking up there.