Okay I want to have everyones imput on these 3 different intakes: Offenhauser Dual Plane intake, Offenhauser Single Plane intake (Also knwon as C series), and the Clifford Single plane intake. I want to know pros and cons of these different intakes please.

 

In general, single plan intakes are typically for mid to high range operation (typically 3500 and up to 7000+) power bands where dual plane intakes actually have the intake runners split horizontally, creating almost a dual power band ie. typical operating range is 1500 to 5500 rpm...which is best for street vehicles.

However and I will emphasis however.....I had a friend who put a dual high rise intake on a chev truck with a 350 V8 and a 3/4 race type cam grind (around 290 duration and 500 lift) with two 450 4 barrel carbs. These intakes are for 4000 rpm + operation and his engine would just be a dog in daily driving. He was out of money, didn't have any other intake available & fowling plugs and I had an idea, to remove the 4 barrel carbs, sell em and buy 2- 350 cfm holley 2 barrel carbs with the 50cc squirters. long story short, he sold his 4V carbs installed the 2-V carbs with that high rise intake and that truck (even by my own standards) absolutely hauled AsXXXXXXX!!!!!!! Sometimes the most incorrect combination of parts when adjusted just right will really perform, just takes time and patient.

 

I run the offy dual port on my 300 I6 and love it. It will lug down to 500 rpm and pull all the way to 6,000 rpm. I had the ignition tuned to my set up and it works really well.

 

The Offy DP is a Dual Port, not a dual plane and utilizes a 4v carb but not a 2v and probably works best for low to mid range torque. It is designed for the carb to set 90 degrees to the engine with the primaries on the engine side which requires a little adapting and improvising (not difficult) for linkage.
The open or C type may be better for mid to higher range operation if that is what you are looking for, requires minimal linkage adapting and has the option of using a 2v or 4v with the appropriate carb/manifold adapter.
I opted for the Offy DP with a Holley 8002/450cfm Economaster Spread Bore which is a replacement for the QuadraJet carb. While the secondaries are only about a 1/16" larger than the primaries it still requires something like the TransDapt #2034 or Edelbrock #2696 square bore/spread bore adapter (Offy has one but it's listed for $73!) mounted inverted on a square bore adapter to the square bore manifold to get clearance for the linkage.
One of the Clifford manifolds (don't know which one at the moment) is designed to use heater hose connections to heat/cool the manifold. The Offy's are designed to bolt on to the stock 300 log exhaust manifold as a source for intake heat. Since I am using EFI exhaust manifolds I had to make an adapter plate with heater hose fittings to bolt to the bottom of the Intake for heat/cool. (Simple adapt and improvise project.) As far as pro v con, it depends on what you want to accomplish. Once you determine the performance level you want (And I include driveability and reasonable but sometimes elusive mpg as part of performance) Read, read, read this Forum, the FSP Forum, Bronco Forums,...even the Slant 6 Forum (gulp), take notes, bookmark web pages, ad infinitum, and start making a list of the mods you think you will need to accomplish what you want...and go for it!

 

One thing you can do to improve performance is to install a single plane and split it with a plate under the carb into two manifolds front and back which will act like a dual plane manifold for a V8. The carb would have to sit sideways just like it does on the Offy dual port manifold. The plate does not have to seal perfectly either, 90% is probably OK.

 

I just had to smile when I read your post, you must either be an 'ol timer or were raised with someone who used to build their own parts!

 

"Tinkering" is all part of the fun of working on these I6's!

 

Well Currently i have the offy dual port. With a 390 cfm 4-bbl holley. So what kinda gas millage do you get with yours Harte? I am going to put headers on it though so i will have to buy the heater spacer thing you were talking about. Or i could build a heating shroud.

 

I run the C series, and mine definitely won't do that. On my engine, it doesn't really like higher rpms, but lugs great down to about 500 also. Personally, the single port is the way to go since you can run a 2bbl on it.

 

Yeah mine will pretty much do the same except not all the way 6,000 RPM more like to 3,500-4,000 RPM. but you have allot more engine work then me.

 

I won't have enough running time on my new engine to give you any mpg figures yet. Gotta get ready to go OTR and won't be back until about 1st of April. I was hoping it would be done in time to take a shake down run to Seattle and back but I am running out of time for that. Darn!

 

We used to go hunting im pomeroy WA allot I think thats south of spokane.

 

I AM an old timer and have built many parts for my own vehicles and a few for others. You don't suppose those NASCAR guys buy all of their parts either...
A manifold plenum divider is easy, one plate to split the manifold and one plate to sandwich in under the carb adapter plate to hold it brazed/welded together. I have built whole manifolds. With the splitter three cylinders draw off each side of the carb which greatly improves the carb vacuum signal and also fuel distribution.