Has anyone tried a RPM Air Gap on an otherwise stock HP500
#12
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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.
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.
#13
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.
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.
#14
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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!
Cam was reccommended by Teague. Crane 134561. 236-246, .553-.571, 114*
But it sure sounded sweet!
#16
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.
#17
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Thread Starter
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.
#18
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Thread Starter
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?
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?
#19