I have the Offy manifold. Yes it has great low end. No it does not flow well at high RPM, but I have to ask what you consider high RPM. It matches the very well with my cam which drops off at 4,500RPM. It would not do much good to have an intake that was good up 6000 if your cam is only good for 5000. I have a 78 Bronco with a 400, so the low end - get the heavy truck up and moving - cam works great with the Offy.

I had the Weiand for a couple of years before coming across the Offy. When I swapped I rebuilt the E-brock carb at the same time. So it is hard to say how much of the improvement I got from the intake and how much was due to the retuned carb. I gave away my weiand to a friend and have never regretted it.

IF you are looking to turn 5000+ RPM then the Offy is not for you.

 

Interesting. Summit lists two different models of the Offy dual-plane for the 351M and the 400. Wonder what the differences might be?

OFY-6033-DP for the 400
OFY-6141-DP for the 351M.

http://store.summitracing.com/egnsea...5&autoview=sku

 

Good question. I got my used and it does not have any numbers cast in it other than the cast date on the bottom. Mine was cast in 1980. They only thing I could think of that might be different would be flow. They might open the 400 up a little, but that would be just a guess. I bet they are the same, just a different model number.

 

What would you guys think of using a 351w dual-plane manifold and a Quadrajet with adaptors? I am thinking of using this because I plan on swithcing it over to efi down that road and I thought that using a factory style manifold would be the easiest.

Stephan

 

I just looked at an Offenhauser site and it appears to me that the 400 is for 71 up and the 351M is 75 up. I really doubt there is much difference at all, I suspect it has to do with the EGR hookup. Since both 351M/400 had same heads, intake ports should be the same. In my opinion, I would go for the Cheapest!!

Both are the "DP" which is dual port. The entire manifold is divided to each intake port vertically, which means carb primaries feed down to the lower portion (plenum) all the way to the ports and the secondaries feed the upper portion. Each series is available for square or spread bore carbs (NOT for Fords spread bore design).

 

Sorry for the double, but one of my 10 thumbs hit the wrong key

"Dual Port" intake is not the same as the "Dual Plane" intakes

 

I think the difference in dates is due to the fact the 400 was in production in 71 and the 351M came in production in 75. I would bet there is no difference.

 

One of those Offy manifold is for a Spread Bore carb, like a Quadrajet. The other is for a standard Square Bore Holley, or Edelbrock carb.

The Spread Bore carb will give you great throttle response, due to it's small primary Venturi, and good power at WOT, since it has a very large secondary Venturi. The fuel mileage should be improved with a Qjet as well.

 

That offy intake seems even more restrincting than a stock ones, does it have smaler ports?

 

I had a 6061DP (spreadbore) on a 460 motor once upon a time--. I doubt they are any more restrictive than anything else for the range they are supposed to operate in. The carb primaries are isolated from the secondaries completely, and feed the lower half plenum to the head ports. Many moons ago I seen to recall a test article (if you can believe any of those) comparing them to others and it seems that even though the principal is good, the dont perform much better that any of the dual planes in either area.

I wasnt impressed with the area in which the secondaries had available just below the carb flange to feed the front 2 intake ports on each side. Looking from the secondary flange openings forward where the primary tubes went through to the bottom plenum, sure wasnt much room for a lot of air voluum to pass around the primary tubes (that was on a spreadbore which = smaller primaries). Thats the area needed for higher rpm flow---even on a torque engine.

My favorite for injector bungs, other than a single plane is the Offy "360 equal-flow". Its a single plane design with a separator plate beneath the carb opening that divides the single plenum into 2, a right and left side. Supposed to have advantages of both designs. Unfortunately, they only show it for the 302/351W and a 351C 4 bbl.

I think DL is on about the quadrajets--I learned a lot about them on inboard, I/O engines (trying to get better economy). For example, just a soon as that secondary air butterfly moves off the seat, even a little, it pulls fuel. That tension spring is not nearly enough to delay the opening where it needs to be, particularly on a marine engine where there "aint" no vacuum!!

 

hmm i think thats the diea ofa a quadrajet (besides giving the exact cfm need), operating as any other carb in matter of performance but giving you a slightly better improvance in gas milliage....

 

I think the QJ works pretty well for both. Except on a marine engine (and to a degree on heavy vehicles) where the flow demand on a loaded engine will open the secondaries a little, when the primaries are perfectly capable of supporting the load. The metering rods are able to keep the mix right for the full range of the primaries (or change them!). Without a wideband (on a marine engine?) I dont know how you would determine that, but one thing for sure is that when those secondaries (air butterflies, not bore butterflies) crack just a little, the economy goes to he---ck. On two marine engines (SB 350 chebbys) I found that you could ty-rap the air valves shut and run within 200-300 rpm of top engine speed wide open, on primaries alone. I put a double spring on the air valves and upped their off-seat operation another 1000 RPM. There is ~ 100 rpm range in which the secondaries will open, and if your cruising speed is even 100 rpm above that, a noticeable hit in economy.

Not claiming to be a carb expert, but I have played with them---.

 

Sorry to hijack, but what do you guys think of the performer 400 edelbrock intake? I noticed they are cheap and plentiful on ebay

 

just read, i guess...

 

The Edelbrock Performer 400 is a good manifold. It comes in two versions. An EGR and Non-EGR version.
The Non-EGR version has a small carb mounting surface and is prone to cause vacuum leaks, but that is easily fixed.
The EGR version doesn't have that problem.