Gents,
Ok I know the 18 spline trannys tougher,but how much more? I read somewhere that internally there the same,gear,syncro's etc,etc. The big diff is the input shaft. How much power can a 23 spline take, are there any of you hitting them with over 600 hp? And can they hold up? Thanks-Gary/68 blown roadrunner (with 23 spline 833) yet to be smacked on the strip!!!

no , the gear pitch is different in a hemi box , and the difference is in the stronger direction , but they are a little noisier because of it.
also the gears ride on oilite bushings , the 23 splines are gear on the mainshaft directly , lastly starting in 68 the hemi box got a larger set of main bearings , though the 70 up B/E 23 spline trans to go behind a big block also got the larger main bearings .

unless you have a +700hp engine and you are dumping the clutch at 5000 rpm and doing full power full throttle shifts all the time a 23 spline will be fine on a street car .

And remember the 23 spline is stronger than anything available from Ferd or GM in the muscle era.
I find the 23 spline a bit faster to shift but it's not super noticeable.
You can tear up about six 23 splines for the cost of an 18 spline too.

Sheldon

I've had a 23 spline in my GTX for the last twenty years. I couldn't find a clutch disc for the 18 spline back then so I used the tranny from a 69 383 Bee. It has held up fine. I finally put the 18 spline in last month.

I twisted a 23 spline input shaft after many years of abuse. I just replaced the input shaft, and the front bearing. The trans is still in the car and still alive, with 23 splines. I hit it hard though. 3600 lbs, 500hp and slicks. I leave at 5500- 6000.

Thanks very much to you and all the other legends who responded. I like the part about it being stronger than any GM or Ford trans from the era!-Gary

Here is a third gear that was behind a mild 440 with bias ply 14"tires

I have a 23 spline behind a built 440 in my 68 Charger and regularly abuse it. It absolutely will not break. I've heard the rear end will blow (8 3/4) before a 23 spline breaks. If you're running street tires, you can't break it.

Same deal here, only manual trans broke was a
3 speed behind a 6 banger.
23 spline been fine for street duty.

I know 2 friends that blew up 23 splines in the last year behind a 440 and a 500.

Do not beat on it with sticky tires.