OPEN PLENUM 4 BARREL INTAKE GOOD?Bruce?

jambottle

jambottle

Senior Member
i AM CONSIDERING USING THIS INTAKE WITH A 390 CFM CARB.255 MERC ENGINE VERTEX L100 CAM.mY CONCERN IS HOW WELL IT WILL IDLE WITH THE OPEN PLENUM(SP).pLAN IS TO GO WITH 4X2 97`S ONCE I GET IT RUNNING GOOD.Hope Bruce see`s this and chimes in.The 3x2 is also home made.
 

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Don't think the open plenum is what you are looking for. Signal strength at the venturis will be compromised. Not only idle, but low speed performance will not be the best.
 
open plenum manifolds are designed to produce max h/p at high rpms, they usually dont start producing power till around 2500 rpm all the way to 5000 or more rpm, not good for low end torque, if you want to use a 4 barrel manifold get a dual plane one
 
flatjack9 said:
Bubba, what are the specs on that engine?
Click to expand...

Got them wrote down somewhere?
Less than 300 CI, snyder camshaft,offy heads , merc crank etc...bigger valves ..we bought the motor built up.

Our other engine is at the dyno shop now , we built it and are adding Hilborn fuel injection etc. Bigger better motor ( we hope) 200 hp would make us happy i guess........
 
During my stock car days we hung around Bill Barth's shop in Bp CT. He built some mean SBC for the track and drags. The rules for our engines at the Danbury speedway was single carb bolted to a stock intake. By this time most guys were running the 2GC or the 2100 Holley on a bored out stock intake.. After seeing how well the open plenum intakes worked on the SBC I decided to make one for the Flathead out of a 33-4 camel back intake. I completely machined out the center of the intake and inserted small aluminum tubes into the cavity then fiberglass the whole thing. The thing had trouble idling and driving around the pits, but once it got going all hell broke loose and we were tore down for cheating. But they never looked at the intake. I still have it. My grand son has computer designed an intake that is made for Bville, waiting for some dyno tests.
 
That 4 bbl manifold appears to be a standard 180 degree dual plane manifold with the plenum enlarged to accomodate a 4 bbl carb. mount. My opinion is it will work OK and idle fine.
Jim
 
It will not work well for normal street driving. The low speed engine torque will be seriously sacrificed.
 
Jambottle>>>mY CONCERN IS HOW WELL IT WILL IDLE WITH THE OPEN PLENUM>>>

Try it. You might like it. Or not.

Jack E/NJ
 
Hey, it's a hot rod, try it you might like it. If it doesn't work well you could always take it off and I wouldn't think it would be too hard to add a center divider to split the plenum and make it think it was more normal. The NHRA used to have a motto that said "Ingenuity in Action", I'd at least give it a shot!
 
Oh yes; i plan to set up the engine with that manifold and run it until i get the 4x2 carbs done up.Dave got the L100 cam done and sent.just waiting for Bubba to finish converting my vertex.its a canadian 255 so i plan to use an olberg filter of my old drag car.It has been 55 years since i have run a flathead in a drag car.i hope this one works better than that old slug.if not i still have the old full race nailhead that replaced the flatty in 1958!:D
 
You may find that a water heated carb spacer ( '60's Ford) will improve performance on the low end. It will also have a port for a PCV system and help eliminate any icing tendencies. I have 2 Zephyr engines using single plane open plenum 4 barrel manifolds. One engine stock and one modified and they both idle fine with using the smaller Autolite 4100 carburetors.
 

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Open plentum will Idle ok but street driving there not to good. Opens start at 3000 and Dual plennum starts idle to 500-6000 and has smooth transitions and are designed for street. Open is great for racing where you launch at 3-4000 and they work well at high RPM's. I have run both on the street and went to the dual .
 
Dee said:
You may find that a water heated carb spacer ( '60's Ford) will improve performance on the low end. It will also have a port for a PCV system and help eliminate any icing tendencies. I have 2 Zephyr engines using single plane open plenum 4 barrel manifolds. One engine stock and one modified and they both idle fine with using the smaller Autolite 4100 carburetors.
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Am i correct in thinking that the autolite 4100`s are 400 + cfm`s?I think my manifold is still duel plane to some degree.Getting it running and on the street is my main goal.Ideling and driveability are secondary at this stage.I will report back;when i get it going.One of the big plusses of the flat motor is the easy intake change.(I guess that`s why i like the nailheads too).:D
 
Yes, I'm using the smaller 4100 Autolites ( one '57 Ford and one early '60's small block Ford) I believe they are rated at 480 cfm. The larger 4100's are rated at 600cfm and are a little too much for the flathead Zephyr. You do need to be very careful about sealing the base from any potential vacuum leak on these carbs. I've converted both to manual chokes and utilize the stock controls on the dash. I have found that these carbs require more "choking" on cold startups than the old Holley 94 style carbs. I have also used the small 4150 Holley ( '57 Ford) and it too runs very nice.
 
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