Performer Rpm Vs Rpm Air Gap Opinions?

[horseballz]

Hey All,
Both of these manifolds appear nearly identical, other than slightly smaller outlet ports (1.04"x1.85" on the Air Gap vs 1.05"x1.86 on the RPM) and the air space on the RPM Air Gap. My curiosity is whether or not the Air Gap unit might help alleviate some of the carb/fuel boiling issues with our current crappy fuel blends when combined with summer temps and air conditioning. Do both of these manifolds perform the same, other than the benefits of the air space? Is it worth spending the $$$ on an upgrade from an RPM to the Air Gap? All thoughts/comments are welcome.
Thanx,
Gene

 

[The_Dude]

Everything I've read suggests that the Air Gap is superior. I have one on my Mach 1. The drawback, at least on a 351w, is the stock distributor wouldn't fit anymore. You also need to check your clearance with the hood. I have a drop base air cleaner and have about 1/8" of clearance.

 

[horseballz]

I already have a Performer RPM and according to Edelbrock's specs, they both have the same manifold height. On your 351W, did you upgrade from another Edelbrock product or from an OEM manifold?
Thanx For The Reply,
Gene

 

[The_Dude]

OEM 4v.

 

[Edster]

Are your heat risers blocked off? If not that may be a cheaper alternative than buying a new intake. I tried that and it seems to work. I'm not runnin A/C yet. Thats another project!

 

[horseballz]

My Performer RPM (purchased about 1999-2000) does not have an exhaust heat passage. It has shallow recesses cast in the place where the passage would be, but they go nowhere. Edelbrock must have changed their castings over time. I see where you live and I'm betting that once you add A/C, percolation/fuel/carburetor boiling may become one of your new pet peeves! Other folks, on this site, are running non air gap manifolds, also with blocked or non existent heat risers and from your neck of the woods are experiencing issues. It would not have been a problem, years back, when fuel was better, but with our crappy fuel blends that are more susceptible to boiling, it has become a HUGE issue. Our fuel won't likely be getting any better anytime soon!
Gene

 

[BuckeyeDemon]

there is a lot of difference between an RPM and Air Gap for a 351W.

there isn't really any difference between an RPM and Air Gap for a 302 except for the opening.

have you considered a phenolic spacer? could help slightly, although the bolts still provide some heat path.

have you considered running a return fuel line/bypass regulator to keep your fuel cool?

 

[horseballz]

Already have a 1 inch phenolic spacer, haven't tried a return line/bypass regulator, but I don't see how that would help eliminate the boiling/percolation that happens when you turn the engine off and go into the grocery store for 5-20 minutes. Just wondering if it's worth the $150 to do the upgrade. I realize the RPM Air Gap is only $50 more than the Performer RPM, but I can likely only get $100-$125 for the used RPM, plus gaskets, sooo.........
Thanx,
Gene

 

[BuckeyeDemon]

Already have a 1 inch phenolic spacer, haven't tried a return line/bypass regulator, but I don't see how that would help eliminate the boiling/percolation that happens when you turn the engine off and go into the grocery store for 5-20 minutes. Just wondering if it's worth the $150 to do the upgrade. I realize the RPM Air Gap is only $50 more than the Performer RPM, but I can likely only get $100-$125 for the used RPM, plus gaskets, sooo.........
Thanx,
Gene

are you seeing it poor out of the primary and secondary or is it just one bowl? if your fuel is boiling at turn off, i wonder what it's doing when the engine is running producing heat?

if your fuel is cooler to start with then it might take longer to heat up to boiling after shut off.

 

[horseballz]

Demon,
No "pouring out" of fuel, just the typical heat soak types of issues. When you turn the car off in 117 degree heat, in the sun, after having run the A/C, after 5-15 minutes, the fuel in the float bowl is below the sight glass (QFT carb) and starts as an obviously flooded engine, needing to hold throttle to the floor to get it to start and the associated rich smell that comes after starting a flooded engine. The longer it has been sitting (up to 20-30 minutes) it tries to die as it would with a dry fuel bowl, unless you keep the rpms up to refill the bowls enough to keep it going. All truly typical heat soak issues, especially when you combine it with todays crappy fuel. The phenolic spacer reduced it considerably and making a new steel line from the pump to the carb, keeping it as far away from the block/intake/thermostat housing as possible and wrapping it with crinkled aluminum foil helped a little bit more. When it does start and try to die, it's difficult to tell if it is truly vapor lock or simply empty fuel bowls, as both situations cause the exact same symptoms, except that empty bowls require a little less cranking to overcome than a true vapor lock. I have no problem just dealing with it, but my wife might get stranded because of not knowing which condition, or combination of conditions, she is experiencing. If the Air Gap manifold would give me nearly as much improvement as the phenolic spacer did, it would put me just over the edge into the range of the issue being livable. You see, the newer blends of fuel with 10-15% methanol/ethanol percolate and evaporate at a lower temp and faster rate than a mostly petroleum fuel, but once it gets into the manifold, under the closed throttle plates, the air is generally sealed off so less evaporation, equalling a flooded condition, but due to the venting in the carb, whatever fuel doesn't percolate/bubble out into the manifold continues to evaporate as time goes by. I see this same issue, described in different ways, over and over on this site and others, particularly from folks living in the hottest climates. Just looking for a REAL solution, other than EFI. If I wanted to deal with a computer controlling my engine, I wouldn't be driving a 68 Mustang, 69 F250, 65 Falcon Wagon and a 56 chebbie pickup! Sorry for the long rant.
Thanx Again,
Gene

 

[BuckeyeDemon]

Demon,
No "pouring out" of fuel, just the typical heat soak types of issues. When you turn the car off in 117 degree heat, in the sun, after having run the A/C, after 5-15 minutes, the fuel in the float bowl is below the sight glass (QFT carb) and starts as an obviously flooded engine, needing to hold throttle to the floor to get it to start and the associated rich smell that comes after starting a flooded engine. The longer it has been sitting (up to 20-30 minutes) it tries to die as it would with a dry fuel bowl, unless you keep the rpms up to refill the bowls enough to keep it going. All truly typical heat soak issues, especially when you combine it with todays crappy fuel. The phenolic spacer reduced it considerably and making a new steel line from the pump to the carb, keeping it as far away from the block/intake/thermostat housing as possible and wrapping it with crinkled aluminum foil helped a little bit more. When it does start and try to die, it's difficult to tell if it is truly vapor lock or simply empty fuel bowls, as both situations cause the exact same symptoms, except that empty bowls require a little less cranking to overcome than a true vapor lock. I have no problem just dealing with it, but my wife might get stranded because of not knowing which condition, or combination of conditions, she is experiencing. If the Air Gap manifold would give me nearly as much improvement as the phenolic spacer did, it would put me just over the edge into the range of the issue being livable.
Thanx Again,
Gene

sounds really similar to what i was going thru.

 

[horseballz]

Did the return line/bypass regulator cure your problem? Are you using a mechanical or electric fuel pump?
Thanx,
Gene

 

[BuckeyeDemon]

Did the return line/bypass regulator cure your problem? Are you using a mechanical or electric fuel pump?
Thanx,
Gene

i have a vic jr (open under intake), 1/4" phenolic carb spacer (needed this to get the correct shaker height), Carter 172 mechanical pump, 1/2 fuel line, 950HP carb (with see thru glass).

symptoms were just like yours except for the flooding. i have a fuel pressure sensor on the fuel rail that routes to a gauge in the interior. i would routinely watch the pressure drop from around 8 psi at cold start to around 3 psi when hot. i also noticed i couldn't see fuel thru the sites within a very short period of time after a run. i never saw the fuel entering the carb and i never had an incident where it acted like it was flooded, nor smelled like it was flooded (that's the one difference between what you described and my situation). the other day i had it around 200 and after 30 minutes tried to trestart the car. it didn't want to start and once it did, i had to keep blipping the throttle so the squirters would get some fuel in the engine. the fuel pressure needle was bouncing between 0 and 2 psi.

so i bit the bullet and installed a bypass regulator after the carb with 1/2" back to the tank. i've only started it twice since (waiting for a set of main jets to lean it down a bit). i've had it up to 190 degrees (was probably running a half hour in a single bay section of a garage on a high 90 degree day in ohio) and it was behaving. fuel pressure was a little under 7 psi at around 300o rpm when the vacuum was removed from the regulator. i could feel the fuel line and bottom side of the pump were very cool to the touch. the top side of the pump was hot. along with waiting for a couple different jets, i have a phenolic fuel pump spacer that i was going to throw on while i had the fuel line off the front bowl so i could change it. it seemed as though the fuel was staying in the bowl the last two times. i checked a couple hours later and could still see it just below the site glass, so it's still evaporating just at what appears to be a really slow rate. i've had the pump apart to look inside so i could understand the design and there isn't much inside.

i only have a limited set of runs since the change so i'm waiting to see if this is the real deal or not. i should have the carb back together late this week, so i'll probably have it pretty hot again early this weekend while i'm doing some simple checks on the carb. i don't want to be stuck in traffic or at a show somewhere and have the car die. i litterally just posted about this last weekend.

i have lots of pics (and pics of inside the pump)i could post if interested.

 

[PoppyMod]

Hi (Horseballz)
Returning to your original question and nothing against what has been stated or suggested, as they are good helpful suggestions ......
But for simplicity, add a 1/4-1/2" phenolic spacer. It will do a world of good for a couple of bucks in reducing any "heat-soak" related issues. If and when, you want to slide up to a better intake, for a street ride I would suggest the RPM air-gap. But, only as a natural progression in seeking performance and not for a simple over heated carb fuel deal.
Happy Motoring!

 

[horse sence]

i have the performer rpm as well on my 351 the exhaust cross overs are just dimpled pockets ,running a 750 holley it has a 1 inch spacer that i thought was wood untill i had to remove the intake to glass bead it and saw it was actualy fiberglass looked just like wood grain i would like to go to the air gap but i just shelled out 200 for a new box for the msd ,lent it to a friend ad it didnt work any more. i am not going to put out for another dist. so i guess i will run what i brung , one pump of the gass and hit the key and it fires right up . dont have any 115 heat days but it does get to 109-110
here , i have had problems with mech. fuel pumps bleeding back into the tank i put a fuel filter with a check valve in it and it put a stop to that.

 

[BuckeyeDemon]

here , i have had problems with mech. fuel pumps bleeding back into the tank i put a fuel filter with a check valve in it and it put a stop to that.

when i had my Carter mechanical opened up, there was a tiny hole in the center of the one way valve that flows fuel to the carb and one tiny hole in the center of one of the one way valves (there are two) that pulls fuel from the tank. i'm only guessing these holes were the way they limited the total pressure of the pump. i have been wondering if there were other possible reasons they did it, i'm not sure. so in that configuration, they will automatically drain back.