Q: Rebirth of OHC-6
I have a 1968 Firebird with a 250 O/H cam 1bbl carb. And am having some serious running problems, so am considering dropping the 400 pontiac engine I have (from a newer Firebird) into it. Will a 400 bolt up to 250’s stock tranny which I believe to be Pontiac’s 2-speed (automatic, no a/c) or maybe powerglide.(Did Pontiac put Powerglide’s in, in 68?
A: Tom Can I ask why you want to change the motor if the OHC-6 is repairable? You have alot of other things to consider when doing this swap such as,motor mounts,tranny cooler,larger radiator,rewire the engine section of harness, cooling baffles around the radiator. An a/c equipted car is easier to change as it comes with these baffles regardless of the orginal power train. Not enough of the OHC-6s left… This motor basicly revolutionized the industry as it is the basis of most OHC-6s(4 and 6cyl) today.
A: Fixing of Problem
I would suggest first looking at the carb. The carb setup sometimes causes a vacuum leak towards the cam cover, due to the stud being difficult to reach. The idea of a steel gasket would solve this problem. The second area that I would look at would be the vacuum advance on the distributor. If it gets gummed up, it won’t release correctly. Just some ideas.
I don’t have the carb numbers with me, but I will try and get them for you. There were I believe four different carbs used: automatic, automatic with air, manual, and manual with air. The air condition carbs used a solenoid to adjust the idle while the air was on. If you need a carb, myself and several others on this list have a bunch laying around.
History of Engine
The OHC-6 engine is often misunderstood. The belt was the part that worried most new car buyers between 1966 and 1969. Unfortunately, the belt holds up very well, it is the oiling system that needs work. Oil starvation of the top end (i.e. camshaft) causes many camshafts to be wiped out. The engine came in two versions, a 1-bbl base engine with a 9.5:1 compression ratio and a 4-bbl “Sprint” version. The Sprint version included a 10.5:1 compression ratio (determined by the cylinder head), dual valve springs, a 4-bbl Quadrajet (the first production use of this carb), a high-lift camshaft, and dual exhaust manifolds. The Sprint version was rated at 215-230 HP depending on the year. The engine was 3.8 litres (230 CID) in 66-67, and 4.1 litres (250 CID) in 68-69. The best engines to build up are the 1968 blocks.
Most OHC-6’s that are still around are 1-bbl automatics. I believe this is due to the fact that these cars were not beat on as much as the 4-bbl/manual transmission versions. With an advertised redline of 6500 RPM, the manual versions tended to be abused and have the oil problem noted above. The ideal engine to build with today’s gas is a 1968 block with a 9.5:1 cylinder head. A Sprint intake and exhaust complete the package. Finding the correct carb can be a “challenge”. It seems that most of them have disappeared, with the exception of a few expensive NOS pieces. A standard Pontiac (fuel inlet on the front, not side like Chevy) Q-jet can be modified to work with the OHC-6, I will be doing this soon, and will let you know the details.
If you plan on converting to a manual transmission, the four-speed, the z-bar and the transmission are different then the V-8 version. The z-bar is longer to accommodate the difference in engine width, and the transmission uses a 3.10:1 first gear. This is very important in getting good performance from a standing start with a Sprint. The automatic that is used with the OHC-6 is a Super Turbine 300, which is closer in relation to a THM350 then a PowerGlide (from what I have been told). There are two versions of this transmission, an air-cooled used on the OHC-6, and a water-cooled used with the V-8’s. The transmission was also used in various Buicks and Oldsmobiles, and it is getting a little difficult to find anyone knowledgeable about these transmissions, or to get parts.
There are a few companies that offer speed parts for the OHC-6. The most notable is Clifford that produces camshafts, headers, and Weber carb kits. Although the engine will rev to 6500-7000 RPM, the usable power of the stock setup is closer to 3000-4500 RPM, as it runs out of breath any higher (although neat noises will happen !!!).
I hope this information has been helpful. I don’t want to get a reputation for ranting (although this one is a little better organized!), but a healthy OHC-6 is a beautiful thing. The folks on this list will have tons of information about this engine, as well as anything else you will need for your car.
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Content last modified: January 16, 2014 at 3:09 pm