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Dan Jones
Dan Jones
St. Louis, Missouri
(280 posts)

Registered:
07/21/2008 03:32PM

Main British Car:
1980 Triumph TR8 3.5L Rover V8

Offenhauser Dual Port Intakes
Posted by: Dan Jones
Date: July 04, 2021 10:18PM

> Some discussion here [www.speed-talk.com]

Since I wrote those comments, I helped a friend build a fuel economy type Ford 351C-2V using an Offenhauser Dual Port for his Pantera. When it comes to Ford 351 Cleveland engines the "V" in 2V and 4V does not mean the number of valves per cylinder. Instead, it means the number of venturis in the carburetor the engine was originally fitted with so a 351C-2V had a 2 barrel carb and a 351C-4V had a 4 barrel carb. The ports in the 2V head are smaller than those in the 4V heads and there were open and closed (quench) style combustion chambers offered in both port sizes. Open and closed chamber 2V heads came with 2.04"/1.65" diameter valves while closed chamber 4V heads had 2.19"/1.71" diameter valves. Open chamber 4V heads could be had with either sets of valve, depending upon the year.

Originally, it was going to be a maximum MPG build but it morphed into a "performance with economy" type build. The goal was 20 MPG with excellent throttle response. Chuck had some earlier higher horsepower engines professionally built that had issues or didn't meet expectations so he decided to build this one himself with my input. Part of the economy equation was using as many parts as he had on hand. For instance, I wanted to use a spread bore Dual Port with a Carter ThermoQuad but we settled on a square bore intake with Holley 4180 carburetor. Chuck also had a set of closed chamber iron 2V heads on hand that formed the basis of the build. He did some home porting to them and had them measured on a flow bench. I used that data to design the camshaft assuming a compression ratio of 10.25:1 but Chuck damaged a piston while cleaning the carbon off and replaced them with lower compression (9:1) pistons on the advice of his machinist. His machinist was afraid the dynamic compression ratio was going to be too high but he wasn't familiar with the Cleveland's quench chamber which is quite detonation resistant. That was bad advice from a performance standpoint but allows Chuck to run 87 octane fuel which is in keeping with the economy theme.

We flowed Chuck's intake at 0.300" and 0.600" lift on his bench bolted up to one of my open chamber 2V heads (unported with a clayed radius). The back cylinders 3, 4, 7 and 8 flow better than the front four 1, 2, 5 and 6. Using a carburetor body to check the signal at the booster, the secondary (top) runners produce slightly more signal than the front or (bottom) runners. About 3/4" more vacuum. Using runner #4 and flowing the planes individually, the top runner flows 102 CFM at 104 ft/sec velocity and the bottom flows 103.7 CFM at 95 ft/sec for a total of approximately 206 CFM, about what an unported 351C-2V head flows. Velocity is higher in the top runner despite flowing about the same. Chuck's home ported heads flow another 40 CFM at peak valve lift than the unported head we tested the intake on so I was hoping to do some work done to the intake. In prior discussions, an Offenhauser tech rep said that 300 to 400 rpm can be picked up by milling back the port divider 3/4" (keeping it square) at the cylinder head flange. He also mentioned opening up the the two secondary holes into to a big oval will increase performance with no loss of low end. Blending the runner entries to reduce the cylinder-to-cylinder variation should also help. Ultimately, Chuck decided to run the intake in stock form but both of those modifications were done to my 351C-4V Dual Port.

With my mild hydraulic roller camshaft, the engine ended making:

383.5 HP @ 5600 RPM
404.8 ft-lbs @ 3300 RPM

With the original compression ration and Offenhauser's recommended modifications, we might have exceeded 400 HP but probably could have used a bit more carb CFM. Interestingly, Chuck installed the intake with a thick wooden spacer he had on his previous engine (keeps the fuel cool to help with hot restarts). The spacer is a completely open style that would seem to negate the Dual Port small primary effect but Chuck is pleased with the results and very impressed by the power delivery. Compared to his previous engines, the Dual Port provides a very linear and smooth power curve. According to Chuck "it is a sweetheart to drive and it accelerates very well at WOT. The flat torque curve means the power is always there."

> You want signal at the carb, and this doesn't do that for you.

Somewhere around here I have a copy of David Vizard's big intake test on a 350 SBC. In that test, the Dual Port was the only intake that increased fuel economy (decreased brake specific fuel consumption) over the factory dual plane intake manifold so it does apparently work to some degree.

> The divider actually hurts top end flow, which is what you want a single plane intake for.

Agreed the divider hurts top end flow but it's not a conventional single plane and wasn't designed for top end flow. Back in the 1970s, following the fuel crisis, there were a number of intake manifolds designed for fuel economy. These were typically some variation of a small runner, small plenum, single plane and usually had provisions for EGR. Holley had the Zora Arkus Duntov designed "Z" Street Dominator intake with a split plenum, a cross-over passage between runners 4 and 8 and trapezoidal plenums with dual bolt patterns so either a small CFM square bore carb or a larger CFM spread bore carb could be directly mounted. Edelbrock had the Streetmaster and also the SP2P, the latter being a small runner dual plane. None of these were designed for maximum horsepower but I've done a fair bit of dyno testing with a 351C-2V Holley Street Dominator small port, small plenum, EGR single plane and, on the right combination, it outperformed all the other street dual and single planes, including an Edelbrock Performer high rise RPM. Despite having the smallest ports (considerablly smaller than the heads), the Street Dominator made better power than the Performer RPM across the entire RPM range (3000 to 6200 RPM) tested and peaked at 450 HP so wasn't exactly weak.

The Dual Port approach does seem to be handicapped by the low carb height and, with its teeny tiny ports, the Buick/Rover is probably the least likely candidate to benefit from the Dual Port design. On the other hand, my 351C-4V sits at the opposite end of the cross-sectional area spectrum:
Rover_351C_Dual_port_01_90.jpg
Rover_351C_Dual_port_02_90.jpg
Notice the difference in port size. It's interesting to note the Offenhauser tech said to make the secondary oval on the 351C-4V Dual Port but the Offenhauser/JWR Dual Port is oval on the primary side.

> Might be OK if you cut out the dividers. That's what I did when I turned it into the base for a blower intake

You did that on an a JWR Dual Port? Did you cut out the top/bottom dividers down the full length of each runner or just hog out the plenum leaving the runner dividers in place? Seems like it would be a bunch easier to have started with an Offenhauser EquaFlow 360 which is externally the same shape as a Dual Port but is essentially a big plenum with a side-to-side divider. That's the intake used for Roots type superchargers on 351Cs. You just mill the center divider out.

Dan Jones


joe_padavano
Joseph Padavano
Northern Virginia
(156 posts)

Registered:
02/15/2010 03:49PM

Main British Car:
1962 F-85 Deluxe wagon 215 Olds

Re: Offenhauser Dual Port Intakes
Posted by: joe_padavano
Date: July 05, 2021 08:45AM

The performance of one specific intake on one specific engine with one specific set of build parameters isn't exactly a statistically valid sample, and how a Ford intake performs has precious little bearing on how an BOPR intake with different runner shapes, different cross section, and different flow requirements will perform. Knock yourself out with this intake.


BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(6468 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

authors avatar
Re: Offenhauser Dual Port Intakes
Posted by: BlownMGB-V8
Date: July 05, 2021 12:06PM

Dan, that was over 2 decades ago but as best I recall I removed the dividers entirely, milled out the top of the plenum chamber to a large rectangle and welded on a large rectangular flange to mount the intercooler box to. I had the intake on hand and wasn't going to use it for anything else because I felt the stock Buick 4 bbl intake was as good or better.

I think you rode in the car with that engine, an experimental intercooler using heat pipe tech bolted to the flange with an Eaton M90 above that. It put out 16psi of boost and I drove it until an unfortunate mod that didn't include a flame arrester in the crankcase vent line resulted in a crankcase explosion, at which point I migrated to the iron block motors to cure a troublesome issue with head bolts constantly loosening up.

But in any case, originally the JWR intake went on the HiPo Buick 215 replacing the stock intake without other significant changes and gave no improvement. However that was long ago, certainly over 30 years now, so I could be mistaken about the other changes. It seemed a cam and distributor change occurred during that period as well but I don't recall the exact sequence. Also I was winding that particular engine to 7k regularly which is really beyond the design specs of the JWR.

I do know I wasn't sufficiently impressed with the JWR to think twice about cutting it up, compared to the other intakes I had to work with so that should say something.

JIm


roverman
Art Gertz
Winchester, CA.
(3188 posts)

Registered:
04/24/2009 11:02AM

Main British Car:
74' Jensen Healy, 79 Huff. GT 1, 74 MGB Lotus 907,2L

Re: Offenhauser Dual Port Intakes or not.
Posted by: roverman
Date: July 05, 2021 02:30PM

IF I don't run out of time, I like a really good Rover single plane, with a plenum divider that telescopes, up from the floor.
Huffaker said about every .25" off the floor, raises power band about 500 rpm. Art.


Dan Jones
Dan Jones
St. Louis, Missouri
(280 posts)

Registered:
07/21/2008 03:32PM

Main British Car:
1980 Triumph TR8 3.5L Rover V8

Re: Offenhauser Dual Port Intakes
Posted by: Dan Jones
Date: July 05, 2021 05:13PM

> The performance of one specific intake on one specific engine with one specific set of build parameters isn't exactly a statistically valid sample

I agree and did not mean to imply otherwise. Because it works well on one application, doesn't mean it will work well on another. Even within the same engine family, it might work well on one combination and poorly on anther. The only reason I'm interested in it is because I want to run a Carter ThermoQuad on a 351C-4V and it's the only intake that lets you do that.

> I removed the dividers entirely, milled out the top of the plenum chamber

Impressive. I was eyeballing mine and that plenum is funky. It wasn't clear what it would take to remove it all. I wanted to mill a Buick 300 intake from a 4 hole plenum opening to dual oval openings but I'm told that won't work as it'll break into voids, though I've seen it done on a Buick 215 dual plane.

> Huffaker said about every .25" off the floor, raises power band about 500 rpm

I'm not sure I follow. Do you mean lowering or raising the divider increases power band RPM? I've done a lot of intake testing on 351C Ford V8s with modified and unmodified intakes and, while the power produced can vary dramatically, the peak power RPM generally only moves around by +/- 100 RPM or so. We've found runner extensions into the plenum like these can improve some single planes (widen the torque curve).
351C_2V_Weiand_Xcelerator_plenum_mods_1_80.jpg


BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(6468 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

authors avatar
Re: Offenhauser Dual Port Intakes
Posted by: BlownMGB-V8
Date: July 06, 2021 09:51AM

I ended up with a large rectangular cavity all the way to the bottom of the intake IIRC. I'll try to look around and see if I have a photo somewhere that might help but I can't promise anything, that's really going back a ways. Before this website and before MGE.

Jim


88v8
Ivor Duarte
Gloucestershire UK
(1041 posts)

Registered:
02/11/2010 04:29AM

Main British Car:
1974 Land Rover Lightweight V8

Re: Offenhauser Dual Port Intakes
Posted by: 88v8
Date: July 12, 2021 05:39AM

Slightly mangled note from the Mez site Michael Perkins says: "The JWR/Offenhauser Dual Port 360 manifold was to be an improvement over the dual plane Offenhauser that was made back in the 1960ies. It has divided runner, one for the primaries and one for the secondaries that until the fuel leaves the intake and enters the cylinder head they mix. It was a slight improvement but did tend to run rough and rich. BL Motorsports used this intake with a Repco twin DCOE Weber adaptor until the "Swan Neck" quad Weber intake was developed."

[www.mez.co.uk] about half way down the page.

Ivor



roverman
Art Gertz
Winchester, CA.
(3188 posts)

Registered:
04/24/2009 11:02AM

Main British Car:
74' Jensen Healy, 79 Huff. GT 1, 74 MGB Lotus 907,2L

Re: Offenhauser Dual Port Intakes
Posted by: roverman
Date: July 12, 2021 05:36PM

"Huffaker said raising plenum divider .25" off of floor, raises power band about 500 rpm". In other words, their single plane,
instead of divider touching floor of plenum, shorten by .25",( clearance). .5" about 1,000 rpm/etc. Art.


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