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Has anyone tried a RPM Air Gap on an otherwise stock HP500

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Has anyone tried a RPM Air Gap on an otherwise stock HP500

Old 02-08-2014, 01:52 PM
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My Cafe came with RPM Air Gaps on HP500s. It was recommended that I go back to the stock single plane intakes, so I bought a set from an OSO guy on here. I still have one used RPM if you decide you want one (the other was sold).
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Old 02-08-2014, 02:48 PM
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Years ago I had a single engine boat with a 454 mag. GM dual plane and weber carb. from the factory. I did a cam change and at the same time I went with a Victor Jr. single plane and 800 cfm Holley with a 1" spacer. I did this irregardless to what a very knowlegable engine builder in the area said. He told me I would have to "spend over $1000 to get a better intake then the stock GM high rise". After putting this together and running the combo I did have reversion issues partly due to cam choice. I switched back to the GM dual plane and this did help the reversion. It was also a night and day difference in the way the engine ran with the dual plane, 800 Holley, and plane old 1" open spacer. I never went back to the single plane.

If the results would be even close to the same on a HP500 then I feel it may be worth a try. I however do not want to go to the expense to find out I was wrong. Thus my thread question here. I will not that I may have lost a bit of top end going back to the dual plane but it was worth the loss for the gain everywhere else. Kinda the same when I went from a 3 blade prop on that boat to a 4 blade. Lost some speed but gained midrange cruise.
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Old 02-08-2014, 03:16 PM
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Originally Posted by GETTINBYE
Years ago I had a single engine boat with a 454 mag. GM dual plane and weber carb. from the factory. I did a cam change and at the same time I went with a Victor Jr. single plane and 800 cfm Holley with a 1" spacer. I did this irregardless to what a very knowlegable engine builder in the area said. He told me I would have to "spend over $1000 to get a better intake then the stock GM high rise". After putting this together and running the combo I did have reversion issues partly due to cam choice. I switched back to the GM dual plane and this did help the reversion. It was also a night and day difference in the way the engine ran with the dual plane, 800 Holley, and plane old 1" open spacer. I never went back to the single plane.

If the results would be even close to the same on a HP500 then I feel it may be worth a try. I however do not want to go to the expense to find out I was wrong. Thus my thread question here. I will not that I may have lost a bit of top end going back to the dual plane but it was worth the loss for the gain everywhere else. Kinda the same when I went from a 3 blade prop on that boat to a 4 blade. Lost some speed but gained midrange cruise.
Interesting. Do you recall the approximate specs on the cam? Were you running stock Merc manifolds? If your cam specs were close to those for the HP500 and you were running equivalent exhaust you should expect similar results. If you stayed with stock exhaust when you swapped cams and induction, that may have limited what the single plane could do.
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Old 02-08-2014, 03:33 PM
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IMCO Powerflow with silent choice for the exhaust.
Cam was reccommended by Teague. Crane 134561. 236-246, .553-.571, 114*
But it sure sounded sweet!
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Old 02-08-2014, 04:01 PM
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Originally Posted by GETTINBYE
IMCO Powerflow with silent choice for the exhaust.
Cam was reccommended by Teague. Crane 134561. 236-246, .553-.571, 114*
But it sure sounded sweet!
My buddy has those flat tappet cams in his scarab with dry Stelling exhaust. They sound very healthy
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Old 02-08-2014, 04:19 PM
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That cam has about 10* more intake and exhaust duration than the cam I am running. Mine is 226*/230* @ .050, .612/.596 on a 114* LSA with the 4/7 - 2/3 swap (LS firing order). I'm running Lightning headers with built in silent choice collectors. On paper, you would think I wouldn't have anything to worry about reversion wise, but Bob has told me that he cannot guarantee that I would avoid reversion with the diverters closed. Cam specs can be deceiving, especially when comparing a roller to a flat tappet, because the rollers usually have more aggressive profiles and get the valve up off the seat much more quickly. That's one reason I was thinking about the Air Gap with my setup - to stack the odds a little more in my favor. I also like the fact that the AG is already plumbed for 4-corner water lines, which I understand helps prevent hot spots in the rear cylinders. I have to say that the Dart looks faster.

Last edited by Budman II; 02-08-2014 at 04:22 PM.
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Old 02-08-2014, 05:20 PM
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Originally Posted by Budman II
That cam has about 10* more intake and exhaust duration than the cam I am running. Mine is 226*/230* @ .050, .612/.596 on a 114* LSA with the 4/7 - 2/3 swap (LS firing order). I'm running Lightning headers with built in silent choice collectors. On paper, you would think I wouldn't have anything to worry about reversion wise, but Bob has told me that he cannot guarantee that I would avoid reversion with the diverters closed. Cam specs can be deceiving, especially when comparing a roller to a flat tappet, because the rollers usually have more aggressive profiles and get the valve up off the seat much more quickly. That's one reason I was thinking about the Air Gap with my setup - to stack the odds a little more in my favor. I also like the fact that the AG is already plumbed for 4-corner water lines, which I understand helps prevent hot spots in the rear cylinders. I have to say that the Dart looks faster.
You should be at the helm looking forward when going fast. Not looking at the fast looking intakes. That is unless you just happen to be going fast in reverse!
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Old 02-08-2014, 05:25 PM
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Something else to think about. Both Scoggin Dickey and what used to be Gilbert-Sallee did versions of 502's using the Crane 741 or 761 cam making 600 to 630 horsepower. Those used RPM Air Gap intakes.

I also wonder if the Air Gap had been availible when the HP500 first came out if Merc's choice would have been the same?
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Old 02-08-2014, 05:47 PM
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Originally Posted by GETTINBYE
You should be at the helm looking forward when going fast. Not looking at the fast looking intakes. That is unless you just happen to be going fast in reverse!
Might have to paint it blue to make it look faster.
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Old 02-08-2014, 05:52 PM
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Maybe. But that is some high dollar blue paint.
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