Don Terrill’s Speed-Talk

I've got a customer manifold on the bench right now to open up the plenum for a 4500 carb. I've got a few options I've been considering, but I'm really curious as to when you would use the cloverleaf pattern for this and when you'd use the simple open pattern.



It's for a big block Oldsmobile and I've never heard of one *not* picking up power with carb spacers, so I think more plenum volume is good, but I'm sure there are considerations for carb signal or something like that?

I'm not a porter! Just a guy with some machine tools.

The problem with the spacer in your photo is the fact that airflow directly beneath the throttle plates if going to be severely and adversely affected. Probably to the extent that the engine will run better with the 4150 style carburetor than with the 4500 unit. If you were to enlarge the plenum with a vertical mill, you would disrupt the design on the ports where they intersect with the plenum floor and walls, which will "probably" also result in a negative gain. It is always best to use an intake manifold that was designed for the carburetor, rather than trying to adapt carb to manifold.

Bill I'm joking here...I've heard the opposite of your advice, I know that a dual 4bbl adapter to a single 4 intake can work very well. We outfitted grandpa's station wagon with a dual 4 adapter kit and he was able to pull the camper trailer up the mountain all the way in 2nd gear instead of dropping down into 1st like the last time.

To cloverleaf or not depends basically on how far the runner entries come into the plenum , wether you want the increase the plenum volume by the ammount the open design does and wether you want/need the carb low end signal benefit the clover leaf can provide.
What does that plenum look lime without your adaptor on top?

2X. The cloverleaf approach maintains the length of four of the dividers and even 1/2" of spacer would add as much plenum volume as would opening it it up to a square. Also, the sensitivity to plenum volume will be lessened with a big flow 4500.

is there material on the intake to work the cloverleafs back into the runner entry area? Seems like the air and fuel would just flow down and hit a ramp pushing it towards the center of the plenum, deflecting it away from the runner openings.

Yes, you'd likely have to weld up the top of the runners on the outside, then grind a ton out below the four corners on either side of the dividers to allow smooth flow into the top of the runners. They'd end up radiused back to the plenum corners at their top edges, looking like 'elephant ear'-style dividers.
So, no 4500 pattern manifolds available?

Thanks for the feedback - there is no adapter on there, just a print of the CAD for the cloverleaf on paper and an open style gasket on top of that, laid on the intake itself.

I have about 1.25" from the carb pad to the runner entries vertically, and because of how the runners are paired it would be possible to transition them into the cloverleaf. Oldsmobiles never lose power with more plenum volume that I've ever seen so I think we're going to open it up to the square shape and then he'll probably even toss a 4-hole to square 1.5-2" spacer on top of that.

It is a little odd, too... the rear and front runners are not even remotely centered on the plenum. The way I'd prefer to do this is mill and inch and a half off the manifold, weld on an inch and a half spacer, and then let the porter worry about shaping things correctly. Making the spacer and welding is much faster than dicking around moving that hole pattern.

It sounds like the customer just wants it opened up and the holes moved, though, so I guess this discussion is just academic. I'll post up some pictures.

My memory is not what it use to be but I think the Olds factory dual plane intake was offset like that and most aftermarket manifolds copied that so the w30 w31 cold air induction air filter location matched the hood. Dan

if you turn the carb just a small amount 2 opposite corner holes will line up, plug the other 2 holes with aluminum threaded bolt and jb weld ,drill and tap 2 new holes and the carb will probably line up better with the runners,. cloverleaf and blend into the runners
Len C

This is what I ended up with. I just hand drilled and chamfered the old holes, made some slip fit plugs and welded them in place, then let the VMC chew on her for a bit. I think with the 1.000" depth the porter has enough meat to contour the runner dividers and entrances.

Bet there's a couple of dozen guys whose fingers are twitching to plant a cutter on that inviting expanse of aluminum...

I'm trolling Chad Speier - just look at that smooth, smooth finish...

Here's a picture the customer sent me this morning after the porting:

Rick Finsta wrote: ↑Wed Mar 21, 2018 10:56 am Here's a picture the customer sent me this morning after the porting:

That looks real nice.