The Stovebolt Site Forums The Shop Area Driveline Another 4.10 RP swap question

I was chatting with my machine shop today, and mentioned that I was going to swap out the OE RP to a 4.10 on my 3600 to get the RPM's down on the 235 (want to keep the SM420). He opined that these engines, even after a rebuild couldn't last with a lot of miles at 60-65 MPH. I mentioned that some of you guys were achieving +/- 2000 RPM's at 65 mph with the 4.10 swap (in hindsight I may have made that up)...and he questioned at 1:1 in 4th, a 4.10 would get you down that low. So I found a rpm calculator on the web, and sure enough at 65 mph, 1:1 trans ratio, and a 29" tire (I guessed 17" + 12" of rubber - check me on this guys) and I get +/- 3000 rpm's. I want to be closer to 2000. My calculator indicates I will need a trans ratio of .7 to get to ~2200 rpm's. I know John did the swap, but don't know if you had a tach John. Question - have any of you guys been able to get rpm's down to 2-2300 with tires/RP swaps?

I'm no expert on six cylinders, but 2000 rpm sounds very low to me...you'll be way below the engine power curve. I think 3000 rpm sounds better. I think most of the guys swapping rears to go faster are still running the engine at close to max rpm to achieve those higher speeds.

Mike B

Just look up your tire size on the Tire Rack, click on specs and find the revs/mile. Multiply that figure by the rearend ratio and you have your RPM at 60 mph. I believe with my 4.10 and 750/16 tires, I am around 2800 RPM at 60 mph.

A Motor repair manual I've got handy lists a 1955 235 passenger car engine as getting its peak torque at 2200 RPM. That's the general area you should shoot for to get the best throttle response at cruising speed. Max HP happens at considerably higher speed, like 3800-4200, depending on which model I look at, but that's only for pulling heavy loads at wide open throttle in the lower gears.
Jerry

Do the gear swap and don't look back.

Don't get excited about also finding an OD transmission, probably not enough power in the old Stovebolt to go that fast. Or if thinking not to go fast but achieve slower engine then as Jerry points out you need some RPM to get down the road, not lug the engine.

Start with the gear swap in any case, easy, pretty cheap, and worthwhile.

Allen, I agree with Grigg...do the swap and don't look back. You will not regret it.

Here are a few measurements from when I did mine that you might find helpful. Not scientific, but the measurements were done comparing my speedo at 50 MPH to my GPS. Before any swapping, my speedo was dead accurate at 50 MPH according to my GPS.

When I changed the 4.56 gears over to 4.10's, I picked up 10% which is consistent with the math. When the speedo read 50, the GPS showed I was going 55.

Then I switched out the factory 17.5" rims with STA bias ply tires to a set of factory 19.5" rims with Bridgestone 8R19.5 radials. This gave me another increase of 10% over the 4.10 results. The compounded increase was 21%. After the two modifications, the truck was moving at 60.5 MPH with the speedo reading 50.

You can use this info to get a feel for what the swap will do.

Matt

Thanks Fellas great advice. I'll settle for 2800 or so and keep it at 60-65. As long as I'm not causing a bottleneck, will give more folks a chance to look at my truck as they pass by me!

Thats a good looking tire Matt, is it on an 8 lug 3600?

Allen, yes.

I did the center swap on my HO52 to 4.10/4.11 gears with a T5 and 225/75 R16 radials and it cruises very smoothly and quietly at 58 mph on relatively flat ground. Mine has a .78 5th gear I believe.

Here in Eastern Ohio we have some steep grades and 4th gear can only hold 40-45 with a run for it. But I love the T5 and would never want to give it up.

Where are you guys finding gear sets for a conversion? I've searched everywhere and havent came up with anything!!

I did the 4.10 swap in my '48 3/4-T with the tires in my signature. Love it.

I don't like higher than 45-50 MPH because I've felt drum brakes fading to nothing (notta, nichts, etc) when a brake tromp was needed at higher speeds.