SB/BB swap/ automatic trans issues
Thread Starter
Heel & Toe
Joined: Apr 2006
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SB/BB swap/ automatic trans issues
I have swap my small block for a 396/375HP 1969 Chevelle engine, solid lift cam [stock]. My 69 vette is a automatic with the stock torque converter. My problem is getting it to idle in gear, I have tried both manifold vacuum and Port vacuum and still the idle is not reliable. I assume I may need a higher stall speed torque converter. This is a street car, so any input would be appreciated . The trans in my 69 is stock. thanks
Thread Starter
Heel & Toe
Joined: Apr 2006
Posts: 20
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In time it will die at idle. Stock converter is for my 350, not a 396 with a lot of cam. I have heard that a higher stall converter may be required because of the cam, just trying to find out if anyone has change to a different one that help the idle problem.
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Higher stall will help..but I think you can get there with tune.
What carb is on it? How much initial timing/total timing? What vacuum can on dist.? How much vacuum is it holding?
JIM
What carb is on it? How much initial timing/total timing? What vacuum can on dist.? How much vacuum is it holding?
JIM
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Heel & Toe
Joined: Apr 2006
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Le Mans Master
Joined: Dec 2003
Posts: 5,727
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From: Fremont CA
375HP 396 had loose torque convertor from the factory. Your convertor is probably too tight for this application. You need a 11" convertor at least to allow proper stall for this big cammed motor. The THM400 had a 6 bolt high performance convertor and special high RPM shift points programmed into transmission with special centifugal weights/springs in the govenor and different vacuum modulator settings. This trans was used in LS-6/L-88/ZL-1/427 435HP Vettes and 350 LT-1 Z28 Camaros.
If this motor is an original 375HP 396, you should try to sell it on Ebay with correct castings/parts this motor is worth some serious $$$ even in this market.
If this motor is an original 375HP 396, you should try to sell it on Ebay with correct castings/parts this motor is worth some serious $$$ even in this market.
Last edited by Solid LT1; Aug 15, 2010 at 02:05 PM.
Le Mans Master
Joined: Dec 2006
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From: Oquirrh Mountains
St. Jude Donor '09
I assume 22 is your total with your initial? If so, you are only getting 14* total advance. Set your initial timing to 14 deg. w/the vacuum canister disconnected. That will give you 28 total; but you need 36*. Most of these engines run best with 12-16* initial timing. Make sure your vacuum advance is working too. Lars has a good paper on setting up distributors if you are interested you should look it/him up.
PS - IF, like lt1 states, this is an L78 396; I sold the one I had on Feebay for $4500 five years ago.
PS - IF, like lt1 states, this is an L78 396; I sold the one I had on Feebay for $4500 five years ago.
Last edited by Ben Lurkin; Aug 15, 2010 at 02:29 PM.
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Definitely add some serious initial timing. I'd have it at 18*- 20* at least with the total restricted to 36-38* or so (all without vacuum). Then add in some vacuum advance to help a little more.
The L78 cam wasn't that radical. It's basically an LT-1 type intake lobe and a wide LSA...so no biggee.
While I do agree it would definitely get the little motor with big heads moving a lot better with a looser converter.
As mentioned...if it's got all the right hard pieces they are worth $$ especially if you find the guy that needs his original motor back. Does it still have a VIN stamped on the pad?
Years ago I bought a '73 Lemans GT with a factory bench seat and 4 speed just because the guy had blown the original Poncho 350 and had stuck an L-78 in it. I pulled the motor and dropped it in a Pro Street (actually X- record holding race car) '68 SS396 Nova with an L-88 cam and a Tunnel ram. Sucker ran mid 10's!
JIM
The L78 cam wasn't that radical. It's basically an LT-1 type intake lobe and a wide LSA...so no biggee.
While I do agree it would definitely get the little motor with big heads moving a lot better with a looser converter.
As mentioned...if it's got all the right hard pieces they are worth $$ especially if you find the guy that needs his original motor back. Does it still have a VIN stamped on the pad?
Years ago I bought a '73 Lemans GT with a factory bench seat and 4 speed just because the guy had blown the original Poncho 350 and had stuck an L-78 in it. I pulled the motor and dropped it in a Pro Street (actually X- record holding race car) '68 SS396 Nova with an L-88 cam and a Tunnel ram. Sucker ran mid 10's!
JIM
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From: Taylor Michigan
If i`m not mistaken the L78 camshaft is the same cam as the L72 cam. You need a higher stall converter to help get a steady idle with some more initial lead. And these are just the basics..with out a converter you will constantly giving it gas and holding the brake at the same time.
Thread Starter
Heel & Toe
Joined: Apr 2006
Posts: 20
Likes: 1
Thanks for the advice, I do not know why I did not think to sell the motor, just sold off a '69 DZ 302 block. Your on target with the issues with the torque converter, i have tried every setting I could to get idle in gear. thanks.
