The Stovebolt Site Forums The Shop Area Engine Shop Changing 4.11 rear to 3..55 on 1951 Chevy p/u

Hello,

I have enjoyed many of the posts over the years….great advice on a great subject…old trucks. So here goes my first post on the boards. I have a 1951 chevy pickup, series 3100, with the original 216 engine, 6 volt system and rear end. The tranny was changed out from a 3 speed column to a 4 speed floor prior to my purchase. I have done work to it over the 12 years I have owned it, so it runs and drives pretty good. I still use it to carry items such as hay bales, camping gear, canoes and other items. I take it on the highway, which brings me to my question. It runs at a high rpm at 50 mph because of the 4.11:1 rear end. I am comfortable at that speed given the trucks braking and handling capabilities, but would love to bring the rpm’s down a bit to lessen the strain on the engine. I was looking into the 3.55:1 ring and pinion installation and wanted to know if anyone has done this with an original 216 engine and how much it dropped the rpm. I realize the parts and installation kit cost close to $800 alone, but it would be worth it if it brings the revs down.

Thanks

blaise

It should drop your engine speed from 2381 rpm at 50 mph to 2057 rpm with 29" tires. Use an online rpm calculator, there are many out there.
You will lose some low end torque as well.

Hi Blaise,

Welcome to the 'Bolt.

I did just that. The first thing I did to mine was do a disk brake conversion and the second was the 3:55 R&P with a complete rebuild while it was down. At that time my truck had an VERY good running 216 and the SM420 like yours.

But there was a little hitch... I had no second gear synchro and I was concerned with the taller R&P I might get into an embarrasing situation if I had to downshift into second. Would she buck or would I have to come to a full stop to downshift.

When the rear end was completed I went for a drive and guess what... yea, nightmare time... middle of town mid-day... traffic is down to a very comfortable 2nd gear speed... and me unable to downshift.

I came to an almost stop and that 216 pulled 3rd gear like a mule... smooth as silk in 3rd gear. Very impressive torque... you're going to love it, especially with a 3-speed full synchro trans.

BTW, Cruising at 50 is a breeze now.

Blaise,

Do a search for user name CMAYNA

If you look at his posts from back in 2003, 04 and 05, he did the rearend swap.

He has lots of pics if you follow the links. I seem to recall the process was documented step by step. Not sure why that L i n k pops up..guess that is new since the new layout.

I have the kit in the garage and hope to get the 3:55 project going in the next few weeks. I want mine done so I am ready to go when the snow melts and the streets are clear of salt.

I've got a '46 Chevy with a '54 235 in it. I did the rear 4:11 to 3:55 back in 1997. I can't say much about the 216, but the 235 has plenty or power to pull the 3:55. I run 16 x 7.00 tires and run at about 62 comfortably. I'm running the heavy duty non-synchro 4 speed trans.

If you're planning on running faster, be sure your brakes are in top shape and give a look at your front end for wear. I run bias ply tires and they can be a handful at highway speed, even with a tight front end and steering box.

David B

My 4 speed (47 1/2 ton with 216)is worn and the synchros (if it had any)aren't there but I can double clutch and get the gear box into second without grinding or coming to a complete stop.

As hard as I tried, I just could not get the thing to downshift... and I've done it many times over the course of my 62 years. I know it doesn't make sense but it is what it is.

thanks to all of you for the advice and guidance. I did go to the rpm site as ghostrider suggested and it was a big help figuring put the rpm’s I was running with…..and knowing that czechman was successful in his change out to the 3.55 was the little unknown fact I was looking for. So I will proceed on the change out to the 3.55. I will still cruise at 50 mph since it is a comfortable speed for me….except I can do it more quietly. I will post a photo or two once I figure that out…..for now, thanks again for taking the time to respond !!!!

blaise

https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Board=1&Number=66872&Searchpage=1&Main=10318&Words=%2B3.55+%2Brear+SWEET&topic=0&Search=true#Post66872