Ok where to start....i'm looking for advice on a combination that someone is using that works with 2.56:1 diff gears. My 69fb just doesnt move ike it should. No neck snapper off the line (2.56:1 see's to that) but from 50mph on , she flys. Perfect for our 110kph (about 65-70 mph speed limit)
I currently have a 400, TH350 and peg leg 10 bolt (2.56:1).The 400 has mild work done to it (big valve #16 heads, Comp XE268H 51-232-3 cam (old grind) and valve train, Edelbrock RPM, Hooker long brach headers 650dp Holley and HEI. The tranny is a new stock TH350 with stock converter.
I realise i could change my diff ratio to work better with my cam but the 'long legs' really work for me. ( i travel about 70 miles on the hiway daily ).... ( no im not buying a Hyundai as the wife keeps begging me to do)
Should i change cams (would rather not) , buy a stall converter ( ??? stall speed) or search for a diff with maybe 3.08's
Id appreciate any advice and welcome input from anyone running a similar combination or doing big miles frequently.
Sounds like you need a low gear set transmission. Its a lower first and 2nd gear but still 1:1 in drive. Much cheaper option then changing the rear end ratio and/or putting in a OD trans.
a mild stall would help to but you would have to be really careful on your choice as you don't want your curise speed rpm under your stall speed. GM had a factory stall that may be an option. May also want to try calling Continental for their thoughts too.
I think a 3.08 will be the way to go (now i have to find one around here) Hadnt thought of the lower gear set , i will be making calls to my workshop and may go with both.
I run a 3.08 (and all final drives are 1:1, except w/OD) and with 235/60R14 tires, I'm spinning about 3500 RPM's at 70...keep some extra money in the wallet for the additional gas you'll be burning up.
I have 2:56 in my tempest and its just a joy to drive on the interstate. I hope to change out the 326 for a 400 but I will keep the rear gears right where there at. Just love driving the car even though it doesn't have enough power to even spin the tires.. My 68 bird has 3:36 and short 25.5 tires. It just sucks on the interstate and I tend to look for back roads to use. 55 mph isn't too bad. My plan is to go with a 8.5 with 3:08 gears for a happy compromise some point in the future..
My point is be careful on what you decide to change. An overdrive setup would be the ideal compromise as they usually have lower first gears but they wont work with your current gear. So it would be some $$$ to change both the trans and the rear.
Changing the cam for more grunt can be risky. May wipe a lobe, may run into pinging. If you have a closing event on the cam that allows more cylinder pressure at lower RPM you may run into pinging issues and the cam will most likely run out of breather well before 5500 rpm.
If you keep your 2.56 gears, then I think the Continental converter is good advice. According to what I've read, they provide less slip at cruise speed, but "flash" to stall speed when you nail it, better than any other brand out there. So, you should be able to get at least 3000 actual flash stall, but still be OK at cruise speed, with a Continental. The only negative I've read about the Continental converter is it's price. They ain't cheap.
As mentioned, a 2.75 ratio low gear set will also improve launch power. Many of the Stock class racers use this mod. But, that deal ain't cheap either. They're about $500 plus labor to install.
Id like to keep the 2.56:1 gears and didnt think 3.08:1 would change that much as it sits somewhere between my 2.56:1 and the 3.55s i had.( which sucked on the hiway) Im going to contact Summit today for a Continental price.... hopefully thats the answer.
Does anybody else run 2.56:1 gears and if so, what combination (cam, stall) are you using ?
It has been discussed that Continental no longer has any 13" converters since the cores they use have dried up. That would be the one you would want. I would still contact them and verify. The are not cheap converters at all but you get what you pay for.
"It has been discussed that Continental no longer has any 13" converters since the cores they use have dried up. That would be the one you would want..."
A "tight" 10" is what I'd recommend. I think they can build 'em to "flash" anywhere between about 2500-3500rpm, or so. The 10" will also be lighter, which should help slightly, with both acceleration and fuel mileage.
You don't want any stall converter with 2.56 gears.
Been on several Pontiac sites for about 5 years or so. Never heard anything like that before.
So, please explain why you think a stall converter should not be used with 2.56 gears. ???
Maybe he means you should have a manual transmission with that rear end?
1968 400 Coupe, verdoro green, black vinyl top 1968 400 Convertible, verdoro green, black top 1971 Trans Am, cameo white, auto 1970 Buick Skylark Custom Convertible 350-4(driver)
I mentioned you would need to be careful with the converter selection.
This is quote I saved from the PY site.
"When I redid the TH400 in my '67 GTO last March, I installed a GM6-HS converter in place of the stock GM6 converter. The GM6-HS is spec'd for an LS6 or BB 'vette application, according to the info I could find. I am no trans expert, but the stall speed is listed as about 1800-1900 rpm....a little higher than the stock GM6. It works better than the original TC with my 2.56 gears out of the hole, and it doesn't slip at highway speeds (2200-2400 rpm /70-75 mph. I could actually feel a difference, seat of the pants."
This is not a big change as far as stall speed but IMO would be perfect.
Well i spoke to the guys at Butler and they recommended a small stall 1800 - 2000 which will put me under my cruise revs of 2200. The GMS-HS would suit if it suits the Pontiac with TH350
If you can afford one, I'd still check out Continental and see what they have. They have the rep of providing the least amount of highway speed slip, with the max amount of flash stall. I'll refer to it as the best flash stall to cruise stall ratio, for low rpm cruisers.
For example: they might be able to build a converter for you with which you can cruise at 2200rpm, but which will flash to well above 3000rpm.
Might be worth checkin out. Let us know what you find out.
I am on the hunt for a safe-t-track (posi) rear with 3.08 which should work better with my cam but maintain hiway mileage. I'll set this up, then email Continental for converter/advice. Including postage to Aussie it could get very expensive.
I spent $670 on my Conti tight 10" this summer. They don't make the 13" any more do to the lack of usable cores. It will be interesting to see how they reply.
Well Continental replied and as 'roht' suggested, they recommend a 13 inch converter. Due to not having any stock we may look at sending mine over to be modified....$500 for the mods and about the same for postage.
Well Continental replied and as 'roht' suggested, they recommend a 13 inch converter. Due to not having any stock we may look at sending mine over to be modified....$500 for the mods and about the same for postage.
IMO that is not a bad price for a Continental converter. But like I said before it may be over kill for what your doing.
