The Stovebolt Site Forums The Shop Area The HiPo Shop dual plane manifold for 236/261

Has anyone heard of or attempted to use a slitting saw in a mill, to cut a dual carb Clifford manifold in half, horizontally. The idea would to be to insert a plate to divide the manifold into upper & lower halves. This would be for dual 32/36 progressive Webers and an extension tube would run from the primary side adaptor plate to the lower half and the secondary would dump into the upper half. The desired benefits would be to increase the velocity from the primaries for better torque and to have the secondary flow come off the plate for a larger, more sweeping flow to the valves. The primary would have a longer flow than the shorter flow secondary and the intake could be tuned by varying the distance the plate extended into the head, but it won't be a true tuned intake. Any thoughts or comments?
Dave

How about a single 4 bbl. manifold with a Quadrajet, they have small primaries to address the low end, and then open large secondaries?

Ed

Lipstick on a pig- - - -unless you also increase the airflow past the valves (impossible due to port design) no matter what you do with the manifold will be time and effort wasted. That's why dual carburetors are only for looks, also.
Jerry

Jerry - maybe a male pig likes lipstick???

Dave - I personally don't think the 235/261 develops sufficient RPM to make this modification feasible; but perhaps "panic" will see the thread. He is the manifold guru.

Jon.

It sounds like just a smaller runner intake would do the same thing with out all the cutting and welding. I can see where the smaller runners would increase the velocity, but on this "low" rpm engine, there would be no need for the secondary runners.

Joe

The "sweeping flow to the valves" part of this plan comes to an abrupt end at the back wall of the port. As velocity increases the charge becomes even more disorganized bouncing off that wall. See here.

The flow of air on a stovebolt: horizontal flow into side of air cleaner, 90* turn to go down to oil surface, 90* turn to cross oil, another 90* turn to get back up to carb throat, 90* turn to flow down through carb, 90* turn to enter intake runner, 90* turn to enter port, 90* turn to pass across underside of valve head, and finally a 90* turn to fill cylinder. Now THAT engine is a prime candidate for hot rodding.

Nothing a forced induction system, and port fuel induction couldn't fix. Rod/main bearings be danged.
Carl

OH BOY!! A blown 230 cubic inch inline six. Now we're talking!

Just direct-inject Nitrous Oxide into the combustion chamber, and wrap the oil pan with a ballistic nylon blanket!
Jerry

You know, I rebuilt my first motor, a 235, 48 years ago and as I drove that 54 Chevy, I have no illusions about an old Chevy 6 being a strong hotrod. The objective of the post, other than to complete my minimum number, was to see what comments might be submitted. There seem to be all lot of folks here interested in the old 6's, but there really hasn't been any new development work in the last 60 years. Perhaps some one might think about addressing that back wall or some of the other problems. In my younger days, I helped with the in-law's hogs and I can appreciate the idea of putting lipstick on a pig. Besides, I've got the mill, slitting saw.....
Dave

Lost causes can be fun, but they can also be frustrating and time consuming. You'll accomplish a lot more if you find a 261 engine to start with and build it up close to 300 cubic inches with the stroker crankshaft, then gear it to take advantage of the extra torque. Check this tech tips for "Cadillac 261". It's interesting reading.

http://www.angelfire.com/nv/conv/stovebolt.html

Jerry

Dave, it's your stuff and your tools and time. If it pleases you, I say go for it. Be sure to post the results of your toils so we can all marvel at your work. After all it's all about the fun of making old stuff cool again.

I was curious enough to collect several heads, cut one apart for study, make a flow bench adapter plate, perform various alterations using input from all potential sources (including HRL/Jerry), and finally beat the bushes to find someone with a flow bench. The guy happened to have extensive race engine porting experience so I was pretty excited. He became quite frustrated (me too) as improving flowrate was his area of expertise and the old head is stubborn. He used clay/putty? to make temporary wedges at various positions including the back wall for flow diversion (which would have had it's own challenges to mount in there) but it never really did much good. We concluded the answer is to make a crossflow aluminum head from 3 layers of plate so if your itching to make something meaningful go straight there. That's all I got...the baton of development is yours sir.

I have a flow bench, but never worked on a 235 head. I just finished up a Pontiac set of heads and did my final test. One port was just a little low so out came the grinder again. I made what looked like the right cut. Turned out not to be and it took hours of work just to get it back to where I started. Air flow is a tricky monster to tame. I have a 250 head setting on the shelf with all new parts and about 200 miles on it. I ported it and installed bigger exhaust valves. Air flow was 15% or better with minimal work. On the truck it ran great at WOT and at cruise speed. Climbing hills with your foot into, it would shake so bad it broke a motor mount and shock the air cleaners off. After a lot of trouble shooting and reading flow numbers, dad and I figured out the exhaust was out flowing the intake and during cam overlap, the exhaust was pulling the fuel mixture out of the cylinder creating a super lean condition. With a stock head and everything else the same, the engine runs flawlessly.

Joe

My main objective is to drive at highway speed and to be able to run an air conditioner compressor and I'm running a T5 behind the motor. I'd hate to go stoking a good crank, but I've got a crank in a 235, that's spun a rod bearing. The crank isn't too bad, but I don't know if you can get the 4.125" stoke out of it, that the article describes. I've got a 1954 235 that I'm going to save for a lighter project and use a 261 in the truck. I don't want to be over boring for the sake of only getting a few more inches, but the stroker (if I can find a good piston option) can add a few inches. The lighter 292 rods and the lighter Caddy pistons, would help the bottom end too.

It would seem to me, the sharp bend in the floor of the intake ports is the biggest problem, as the fuel flow will end up with turbulence, as it comes off the port floor. The back wall probably isn't much of a problem as the piston will try to draw the mixture from the port entrance into the cylinder. The valve slamming shut several times a second is going to disrupt flow more, especially with turbulence already occurring above the valve.
Dave

Your reasoning sort of reminds me of the politicians who say "Socialism has only failed so far because nobody's done it right yet!"

Maybe you'll be the guy who succeeds after 80 years of failures by the best head shops in the business. Good luck!
Jerry

That Cadillac 261 setup would be a sight!

I've got a late model 261 stashed away in the shop that will get that conversion eventually, behind about half a dozen other projects! I'm considering making up a modified intake manifold to accept a Rochester 2 barrel carburetor which will probably work better than the eye candy twin carb setups do.
Jerry