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Offenhauser Dual Port intake manifolds feature completely separate runner systems for the primaries and secondaries for more power across the rpm range. At low-load, the primaries feed the air/fuel charge through the smaller bottom passages at near sonic speed, improving power and efficiency. When the secondaries open, their charge goes through the bigger, cooler upper passages. Then, in turn, it is rammed into the cylinders when it encounters the high-velocity mix from the primaries. The results are better fuel economy and a 15-30 percent power increase across the entire rpm range. These manifolds are available in a variety of styles and options for many year/make and engine applications.
Offenhauser Dual Port intake manifolds feature completely separate runner systems for the primaries and secondaries for more power across the rpm range. At low-load, the primaries feed the air/fuel charge through the smaller bottom passages at near sonic speed, improving power and efficiency. When the secondaries open, their charge goes through the bigger, cooler upper passages. Then, in turn, it is rammed into the cylinders when it encounters the high-velocity mix from the primaries. The results are better fuel economy and a 15-30 percent power increase across the entire rpm range. These manifolds are available in a variety of styles and options for many year/make and engine applications.
should be better then a stock gmc/toronado intake right? And not much taller? I have 1-2 inches clearance at the moment.
I had one on my Chevelle for a while a loooooong time ago. It worked ok with the Q-jet but I swapped it out for a single plane when I stepped up the engine quite a bit.
You might want to ask Holley how it would work with the Sniper. Not sure how they separate the mixtures from primary to secondary etc. It might not like it.
The Offy Dual Port manifold is poor choice for matching up with the Holley Sniper EFI, unless you modify both the manifold (to fix a flow restriction) and the Holley Sniper (to change it to progressive opening).
There was an Old GM tech paper floating around that talked about all the flow testing GM did for the Toro/Motorhome low profile intakes with a claim that they would outflow the stock Olds big block intakes of the time. In a motorhome the additional high rpm flow you might get out of the bigger runners in the Edelbrock Performer dual plane intake may not be useful to you anyway. The dual plane Performer may also be too tall to fit in a GMC motorhome. The Holley Sniper comes in a spread bore version, so you could just go with the stock cast iron low-rise intake with a spread bore Holley Sniper. Somebody is reproducing the low rise toro/motorhome intake in aluminum but it is expensive.
The Offy Dual Port manifolds were a crude implementation of what later became known as dual-runner intakes. Do a search and some reading and you will find there are low rpm benefits with the Offy Dual Ports, just like there is with modern dual-runner intakes. You will also find mostly negative opinions about the Offy Dual Ports and the reason is that the casting of the carb pad necessary to match it to a progressive 4bbl carb absolutely chokes the secondary runner feed to the front 4 cylinders. The primary feed is not similarly restricted, so you will get the dual runner benefits at low rpm and cruise but once your secondaries are opening, secondary flow area to 1 & 3 and 2 & 4 are a fraction of what feeds 5 & 7 and 6 & 8 secondaries. Because of this restriction, the Dual Port manifolds as-cast will hurt mid to high rpm performance. The spread bore version has smaller primaries obstructing the flow path so they are less restrictive than the square bore version of the manifold shown in your photo.
The Dual Port manifold only makes sense when topped with a progressive 4bbl carb (primaries open first transitioning to secondaries as you get into the throttle), or the progressive equivalent with a throttle body EFI. I believe the Holley Sniper opens all 4 barrels simultaneously, so it would not be a good match for a Dual Port manifold. There is a Holley linkage kit to make the Holley Sniper progressive, but as I noted above that square-bore Dual Port is not a good choice if you need to get in the mid to high rpm range.
I do run a highly modified Offy Dual Port intake to get better street manners out of 468 Olds, but I modified the hell out of the intake to get rid of the restriction to the front secondaries because I didn't want to sacrifice mid to high rpm performance. I called the old retired Offy engineer (since passed) who designed it and talked with him about the restricted secondaries and how it might be fixed. He said they had plans to make a "race version" that changed the way they routed the front secondary runners but things didn't work out.
Here are a few shots of how I modified an Offy Dual Port to get rid of the restrictive secondaries. I put way too much time and money into it and wouldn't recommend it unless you like to make aluminum chips. Never got it to a dyno but seems to do the job. Tamed down the idle and cruise on a big cam and improved low end torque versus the Performer on it before. No noticeable loss in high RPM performance up to my TH400s 5100 rpm shift point.
Spread bore Offy Dual Port before mods
Restricted secondary runner to #1 and #3 on the spread bore, restriction is worse on the square bore version.
Cut top down and mill out the primaries and remove secondary plenum divider wall between the primaries to open up flow area.
Restriction to front secondaries greatly reduced by creating open plenum for secondaries, no longer split left to right.
Used thin wall tubing for full inside diameter primary downlegs. Cast primaries were 1/4" thick before. Cut new top plate to match.
You can see you will need an accurate estimate of the room you have for a taller intake. Even a stock Olds intake is 1 1/4" taller than what you currently have.
The Offy 360 is so old that I'm not finding height information on it. But as you've been ably advised, it's not a good choice for your GMC anyway.
The goofy Offy dual port intakes are gimmiky crap. There's a reason why in the last half a century, not one other manifold manufacturer has released a similar design. It's two flat single plane intakes, one on top of the other. There is no thought to runner shaping for flow, and in fact each of these single plane intakes (one for the primaries, one for the secondaries) is just a big flat plenum with short runners to each port. Try not to think about what the flow does when that small primary runner empties into the larger port in the head. Since they are aluminum, they don't even make a good doorstop. Run away.
The Offy Dual Port manifold is poor choice for matching up with the Holley Sniper EFI, unless you modify both the manifold (to fix a flow restriction) and the Holley Sniper (to change it to progressive opening).
There was an Old GM tech paper floating around that talked about all the flow testing GM did for the Toro/Motorhome low profile intakes with a claim that they would outflow the stock Olds big block intakes of the time. In a motorhome the additional high rpm flow you might get out of the bigger runners in the Edelbrock Performer dual plane intake may not be useful to you anyway. The dual plane Performer may also be too tall to fit in a GMC motorhome. The Holley Sniper comes in a spread bore version, so you could just go with the stock cast iron low-rise intake with a spread bore Holley Sniper. Somebody is reproducing the low rise toro/motorhome intake in aluminum but it is expensive.
The Offy Dual Port manifolds were a crude implementation of what later became known as dual-runner intakes. Do a search and some reading and you will find there are low rpm benefits with the Offy Dual Ports, just like there is with modern dual-runner intakes. You will also find mostly negative opinions about the Offy Dual Ports and the reason is that the casting of the carb pad necessary to match it to a progressive 4bbl carb absolutely chokes the secondary runner feed to the front 4 cylinders. The primary feed is not similarly restricted, so you will get the dual runner benefits at low rpm and cruise but once your secondaries are opening, secondary flow area to 1 & 3 and 2 & 4 are a fraction of what feeds 5 & 7 and 6 & 8 secondaries. Because of this restriction, the Dual Port manifolds as-cast will hurt mid to high rpm performance. The spread bore version has smaller primaries obstructing the flow path so they are less restrictive than the square bore version of the manifold shown in your photo.
The Dual Port manifold only makes sense when topped with a progressive 4bbl carb (primaries open first transitioning to secondaries as you get into the throttle), or the progressive equivalent with a throttle body EFI. I believe the Holley Sniper opens all 4 barrels simultaneously, so it would not be a good match for a Dual Port manifold. There is a Holley linkage kit to make the Holley Sniper progressive, but as I noted above that square-bore Dual Port is not a good choice if you need to get in the mid to high rpm range.
I do run a highly modified Offy Dual Port intake to get better street manners out of 468 Olds, but I modified the hell out of the intake to get rid of the restriction to the front secondaries because I didn't want to sacrifice mid to high rpm performance. I called the old retired Offy engineer (since passed) who designed it and talked with him about the restricted secondaries and how it might be fixed. He said they had plans to make a "race version" that changed the way they routed the front secondary runners but things didn't work out.
Here are a few shots of how I modified an Offy Dual Port to get rid of the restrictive secondaries. I put way too much time and money into it and wouldn't recommend it unless you like to make aluminum chips. Never got it to a dyno but seems to do the job. Tamed down the idle and cruise on a big cam and improved low end torque versus the Performer on it before. No noticeable loss in high RPM performance up to my TH400s 5100 rpm shift point.
Spread bore Offy Dual Port before mods
Restricted secondary runner to #1 and #3 on the spread bore, restriction is worse on the square bore version.
Cut top down and mill out the primaries and remove secondary plenum divider wall between the primaries to open up flow area.
Restriction to front secondaries greatly reduced by creating open plenum for secondaries, no longer split left to right.
Used thin wall tubing for full inside diameter primary downlegs. Cast primaries were 1/4" thick before. Cut new top plate to match.
great information, I see what your talking about on the front runners being restricted by the primary tubes.
Seems like the quadrajet version is the way to go with this if it were to be used. (Less restriction, progressive linkage)
thanks
