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The RPM Air Gap has the divider milled down out of the box and that is why it works well with the Sniper. Problem with the Sniper is that it gets its MAP signal from only one venturi.....and on a dual plane, the two sides are different on exactly what requirement they need.......on a carb this is fine because a booster pulls what the booster can pull....but on EFI, both sides are dependent on a signal from one side.
The other thing is that a dual plane intake was ONLY designed to signal a carb and for NO other reason. There is zero benefit and even a loss running a dual plane with an EFI because one sides runners do not flow as well as the other side........the Air Gap RPM style intakes do the best job of flowing air but by design are flawed on one side compared to the other.
EFI does not need a signal as mentioned and works best with an open plenum.....
Try E-Bay for a nice used unit......hundreds of old intakes in perfect condition for dirt cheap......
Jebby
I've not done the measurements, but is it true that the RPM manifolds have unequal length runners? By extending the plenum height I would assume they have some leeway to equalize runner length/volume/flow. I can see why this would be compromised as you go shorter.
For what it's worth, I am running MSD Atomic EFI on an RPM Air-Gap. I've had this setup on a ZZ4 manifold (low rise dual plane), a Weiand 7525 (low rise single plane) and now this. The RPM Air-Gap makes better torque production and high RPM power than either intake.
Holley says the Sniper height is 3.28”. I assume that’s from base/bottom to air cleaner mounting surface.
Does anyone know what is total overall height from base to the nub by the air cleaner bolt hole?
I don't have that measurement for you. However, a little more insight on manifolds.
I tune fuel injected Harley's for a living, so I was a bit baffled in the beginning myself. You see the Harley's run sequential port injection. The system is incredibly similar to these aftermarket throttle body set-ups for our older cars. But the sequential port part is the big difference.
I have read more than once about the MAP sensor being on one side as being the trouble. And milling out the divider or putting in a spacer will help solve this issue. Perhaps.
However I have studied and learned a fair bit in the last 2 years with one of these systems on my car.
In sequential port. the injector for the one and only cylinder it feeds opens just as the valve starts to open. Stays open just long enough to supply the required amount of fuel for the amount of air being pulled in and is totally closed well before the valve is closed. then stays closed until the next cycle.
On the throttle body set-up. the injectors do turn on and off. but in relation to the amount of fuel needed by the entire engine. So in essence, they are a form of constant flow injectors. When the intake valve is closed. Fuel is "Stored" in the intake manifold waiting for the next intake cycle.
With a duel plane manifold. 4 runners are fairly short and near vertical. And 4 runners are long and near horizontal.
Guess which ones hold more fuel?
Now to compound the issue. the system is running a single O2 sensor. reading an "average" of 4 cylinders.
2 of those cylinders have short / near vertical intake runners and 2 have long / near horizontal runners.
While cruising along at steady speed. the engine runs great. no problems.
Take your foot off the throttle and decell. the throttle plates go shut and the injectors fire almost no fuel. the suction in the intake pulls the fuel out of the intake runners. the short ones dry out much quicker. just then you re-stab the throttle. if it was a short decell, like coming into a curve. the short runners could be dry and the long runners still have some fuel left. you punch it and 2 cylinders are lean and 2 rich and your O2 sensor thinks everything is about right. But you know it's just not right. After a couple seconds everything evens out and it's running fine.
However, when I re-stab my throttle after a short decell. I want it perfect! On a really twisty road this is noticeable!
On a single plane manifold, all 8 runners are fairly vertical. and very close to the same length. So the wetting affect is really close cylinder to cylinder.
As per the power (RPM) ranges listed for single plane manifolds.
That is for a carburetor sitting on top that needs vacuum to pull fuel through jets.
I don't have that measurement for you. However, a little more insight on manifolds.
I tune fuel injected Harley's for a living, so I was a bit baffled in the beginning myself. You see the Harley's run sequential port injection. The system is incredibly similar to these aftermarket throttle body set-ups for our older cars. But the sequential port part is the big difference.
I have read more than once about the MAP sensor being on one side as being the trouble. And milling out the divider or putting in a spacer will help solve this issue. Perhaps.
However I have studied and learned a fair bit in the last 2 years with one of these systems on my car.
In sequential port. the injector for the one and only cylinder it feeds opens just as the valve starts to open. Stays open just long enough to supply the required amount of fuel for the amount of air being pulled in and is totally closed well before the valve is closed. then stays closed until the next cycle.
On the throttle body set-up. the injectors do turn on and off. but in relation to the amount of fuel needed by the entire engine. So in essence, they are a form of constant flow injectors. When the intake valve is closed. Fuel is "Stored" in the intake manifold waiting for the next intake cycle.
With a duel plane manifold. 4 runners are fairly short and near vertical. And 4 runners are long and near horizontal.
Guess which ones hold more fuel?
Now to compound the issue. the system is running a single O2 sensor. reading an "average" of 4 cylinders.
2 of those cylinders have short / near vertical intake runners and 2 have long / near horizontal runners.
While cruising along at steady speed. the engine runs great. no problems.
Take your foot off the throttle and decell. the throttle plates go shut and the injectors fire almost no fuel. the suction in the intake pulls the fuel out of the intake runners. the short ones dry out much quicker. just then you re-stab the throttle. if it was a short decell, like coming into a curve. the short runners could be dry and the long runners still have some fuel left. you punch it and 2 cylinders are lean and 2 rich and your O2 sensor thinks everything is about right. But you know it's just not right. After a couple seconds everything evens out and it's running fine.
However, when I re-stab my throttle after a short decell. I want it perfect! On a really twisty road this is noticeable!
On a single plane manifold, all 8 runners are fairly vertical. and very close to the same length. So the wetting affect is really close cylinder to cylinder.
As per the power (RPM) ranges listed for single plane manifolds.
That is for a carburetor sitting on top that needs vacuum to pull fuel through jets.
And you just described exactly why you need to make manual fine tuning adjustments of the fuel map and acceleration table to fine tune what can’t be learned through close loop operation. Great description!
I've not done the measurements, but is it true that the RPM manifolds have unequal length runners? By extending the plenum height I would assume they have some leeway to equalize runner length/volume/flow. I can see why this would be compromised as you go shorter.
For what it's worth, I am running MSD Atomic EFI on an RPM Air-Gap. I've had this setup on a ZZ4 manifold (low rise dual plane), a Weiand 7525 (low rise single plane) and now this. The RPM Air-Gap makes better torque production and high RPM power than either intake.
Sure it does.....because the Air Gap flows better than either intake and has the divider cut down......
All Dual plane intakes have unequal runners......nature of the beast. You can hold it in your hand and see that they are all different especially side to side.......
ctmccloskey...thanks, I called EFI System Pro. They pretty much insist the Sniper performs well on practically any dual plane manifold, including my 2116 Performer Vortec. And suggest using the EFI sealing plate to prevent the whistling. So right or wrong or whatever... I'm pulling the trigger and buying the whole thing from them and going with their advisory. I'll post progress, - or lack thereof...
On another note, thinking ahead, - what do you guys recommend as a good connection point for 'clean switched 12 volt' power source to power the EFI?
But I need it switched on and off with the ignition key.
You do, and that's what Rescue Rogers meant by a "Switched Relay", the ignition-only 12V signal will turn the relay on and send battery power to the EFI.
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
if you dont have expereince with relays, they are just an elctro magnet that is turned on buy one power source to close another switch that is equal or higher power. I use them on high power headlights, electric fans, fuel pumps and my sniper when I ran it. And my MSD
Make sure you have 12v to ignition when the key is in the START position. Many don’t as the starter solenoid provides 12v bypassing the ballast resistor or wire. On my ‘64, I ran 12v through a fuse box and a set of toggle switches to ensure there was no interruption.
if you dont have expereince with relays, they are just an elctro magnet that is turned on buy one power source to close another switch that is equal or higher power. I use them on high power headlights, electric fans, fuel pumps and my sniper when I ran it. And my MSD
yes the relay would switch the primary battery 12v, got it no problem with that. But what’s a good source to excite the relay when I turn on the key? “+” side of coil? I can figure this out but if somebody’s already got a nice clean method I’ll do that and save my brain cells for something else. A nice clean stock looking installation would not include toggle switches.
Simple, test your ignition switch. If it provides 12v to the ignition ON terminal when in the START position, you are golden. Wire your relay as follows:
Pin 85: ground
Pin 86: ignition switch ON position
Pin 87: Sniper B+
Pin 30: battery + via a fuse
Well I do plan to send a wrap up to this story except I’m waiting on the fuel pump. Holley has a nice unit that fits right into the tank, sending unit and all, except…. backordered. Latest ‘promise’ date is 11/26, but not holding my breath. Hoping it’s not sitting in one of those 100++ container ships off the California coast.
Sooo, all dressed up and no where to go…
Yeah that’s what I’m thinking. It’ll show up and I’ll get everything ready to fire it up just as the first salt hits the highways here in beautiful Southwest Ohio LOL
