I have a ’71 LeMans Sport that had a 350/2-bbl engine. We replaced it with a ’68 350 H.O./4-bbl code-YP engine that was rebuilt to mainly stock specs. I like the 350 H.O. motor, and Pontiac only made them for two years. It was bored 0.030 over, and had a valve job and an 068 cam installed, which replaced the stock 067 cam.
It seems like the stock converter is giving me trouble. When I put the car in gear, it wants to fumble around and then go. I have to give it a little gas. My friend says I need more stall speed and I suppose more vacuum. He is very good with carbs, timing, and such. I have 1,600 miles on the new motor and it seems to be loosening up well.
Nobody knows much about Pontiacs around here, so if I need a converter, could you advise me on which brand and stall speed I should choose?
I enjoy your magazine. Thanks so much.
Dave Shaver - Via Internet
Rocky Rotella responds:
Dave, the 350 H.O. was a potent small-cube package. We rebuilt a ’69 example a few years ago, and it generated 330 hp and 375 lb-ft of torque in relatively stock trim using an 068 camshaft. It was quite impressive!
I believe there are a few factors at work that may be creating the driveability issues with your LeMans. The 068 camshaft is fairly radical for a typical 350, but when considering that Pontiac used it in certain ’69 350 H.O. applications, we know the combination isn’t a gross mismatch. I think your rebuilt 350 would operate similarly to a ’69 350 H.O., assuming it retains its No. 18 cylinder heads and has flat-top pistons producing a compression ratio of more than 10:1. With that said, I don’t believe what you’re seeing is the result of insufficient torque-converter stall speed.
If your LeMans is still equipped with a Turbo 350 transmission, the stock torque converter will stall to about 1,600 to 1,800 rpm. As opposed to stumbling or bucking, I think at worst a properly functioning stock-stall converter would more likely make your LeMans accelerate lazily at takeoff; simply giving it more throttle should improve the situation greatly. If your torque converter worked well before the engine swap and you didn’t change it at the same time, then I feel we can safely rule out a faulty converter. But I also don’t believe that a good running 350 with an 068 camshaft should “fumble” with a stock-stall converter.
I suspect your driveability issues are more closely related to the engine’s current tune. You didn’t note the carburetor you’re using, so I can only assume it’s a Quadrajet like that originally installed on the replacement 350. If that’s the case, it’s very likely the internal fuel metering is not calibrated for your current combination. Richening the idle circuits and adding bypass air will likely improve idle quality and stability, as well as off-idle driveability. We outlined that very process as well as the required resources in the Feb. ’07 issue of HPP (http://www.highperformance pontiac.com/tech/hppp_0702_ rochester_quadrajet_carburetor_tuning/viewall.html).
I also suggest verifying that your engine’s static base timing is between 10 and 14 degrees, and the distributor provides 20-24 degrees of centrifugal advance by 3,500 rpm and 10-12 degrees of vacuum advance. With your carburetor and distributor tuned properly, your engine should be pulling at least 10 inches of manifold vacuum. If not, then you may need to check the valve lash and reset it accordingly, and possibly verify that the camshaft was installed correctly and degreed to the factory specification.
In the end, I don’t know that your combination requires more stall speed than what’s available from a stock unit (1,600-1,800 rpm or so).
If after your tune-up you still feel as if you want more stall speed, increasing it to 2,200-2,400 will certainly make the car feel better at takeoff. Selecting a “tight” unit will reward you with stock-type driveability at light-throttle takeoff and lower speeds and excellent acceleration during spirited takeoffs. The added slippage will, however, negatively impact fuel economy if that’s a concern.
I have found that Continental Torque Converters (www.ctconverters.com) and Precision Torque Converters (www.gopnh.com) offer good quality units that perform as advertised.
I think at worst a properly functioning stock-stall converter would more likely make your LeMans accelerate lazily at takeoff; simply giving it more throttle should improve the situation greatly
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