GT's 78

I have a RPM Air Gap intake with a Quick Fuel 750 carb. I need to put a 1/2" spacer under carb. My question is do I need the spacer with holes or the square opening?

johnt365

It would be interesting if someone had some dyno numbers to support this, but my understanding is that the 4 hole is for mid range torque and the open style is for top end HP. So for the street I would try 4 hole, for a drag car, open.

Hopefully someone has some dyno results because I have often wondered just how much of a difference.

This may help http://www.popularhotrodding.com/buy...r/viewall.html

GT's 78

I'm afraid my goal is a little more simple than the gains in HP v/s torque although this may be a bonus so I would be curious about both. I did not have enough clearance for my vacuum port at rear of intake with double pumper fuel bowl covering it. So I need in raise the card 1/2".

johnt365

Is all that going to fit under a stock hood?

GT's 78

I've been told it will without the spacer, using a dropped based air cleaner. I'm hoping it still will.

Matt Gruber

send back the air-gap and get the non-air-gap. unless it is a summer only drag car, or is summer only 383+.
Then use a right angle vacuum port, or move the port(drill & tap new hole)
with a thick, 1/4" 4 hole gasket. Or do whtever u want. it will still run, just not as good, and u may never know the difference(unless u remember my warning when it runs like poorly on a cold start, climbing hills, needs more spark advance and a richer mixture)

gkull

The best is the 4 hole wood spacer. Wood is a thermal barrier so you never boil the fuel in the carb bowls.

As to an air gap manifold. I have never seen any solid data that they actually change the incoming fuel charge temp. significatly. It is mostly a BS sales gimmic.

Just think about this. aluminum is an excellant heat transmitter or conductor. Little or no air cooling is happening under the air gap runners. The other item is time. How long does a cylinder filling air chrge actually spend inside the manifold in a running motor when at even just 3000 rpm the motor is firing 25 times per second.

A cold air intake to the carb does much more than a stupid air gap manifold

Matt Gruber

i made my own 4 holer out of 2/10" plywood, 5 yrs ago. painted flat black, extends under the bowls to deflect heat. no need for any other gaskets, the wood has enough "give" to seal after a few heat cycles.
The cut down divider is what nearly ruins the air-gap for the street. Connecting the 2 planes lets exhaust come in at part throttle! Anytime a carb needs a rich idle mixture, like on 4 corner carbs, look for exhaust being sucked in during overlap!. To make air diluted with exhaust, ignite, takes LOTS of fuel! When the 2 planes were connected(open gasket) my 355 used 2 gallons per hour just to idle now it is just 0.9 GPH. I don't mind using extra gas to accelerate, but to idle? no way. And having to downshift in slow traffic was a PITA. Way less part throttle power with exhaust being sucked in. Had to downshift way too often.
George, nice seeing u again on cf.

gkull

Matt, the poster might have a non egr manifold. The other stupidity of thinking that the air gap works is because of the cast in exhaust cross over. It is getting exhaust heating across the manifold.

30 years ago I bought a manifold with no exhaust cross over. Then heads became available to not even have the passage. EGR is something I also got rid of.


http://www.jegs.com/i/Edelbrock/350/...rentProductId=

Matt Gruber

if the OP loves an air gap, he could buy the Weiand 8501, this has a FULL DIVIDER.
The edelbrock air gap does not have an EGR provision, the cut divider gives it built in EGR and LOTS of it! This EGR does NOT come from the xover passage(there isn't any). It come from any cyl in one plane that is on the intake stroke, that suction crosses over the cut-down divider and meets up with any cyl on the other plane that has both valves open(overlap). that suction draws in exhaust.

garygnu

is there a big vacuum port at the back of the carb ?if so you can use that port for brake vacuum ,and not need a carb spacer.

Phil Zell

That is exactly why I bought a Weiand Air Strike intake. The port is further out on the runner and a double pumper will fit

GT's 78

What happens in a single plane manifold with the exhaust gases? I have second guessed this disision, but it's to late now, it's bolted down so I've bought it. This is not going on a stock engine. It's a 383, forged balanced rotating assembly, 220 cfm heads, solid roller cam. I could always epoxy the divider. Would that make the air-gap more friendly on idle and use less gas?

gkull

I always had a single plane on my 383. I hope that you have 220 cc intake port heads not 220 cfm

Exhaust reversion has many factors to consider. In a low rpm motor that only idles.......... I too might use an intake with a divider. Now a solid roller with big duration will have low rpm reversion as a trade off for high rpm power. As for me and my single plane solid roller motors......... I only idle at the stop light and i shift to keep my motor over 2000 rpm

Matt Gruber

that about sums it up. keep it 2000+. not hard to do, i did it for years.
BUT, i'd not go back to that wimpy low end ever again. i'm spoiled. Around town in traffic, i just cruise along with traffic at 1200-1600, climbs overpasses easily without down shifting.
When i do get the chance to floor it, it goes 60 mph in 1st, in drive! Excellent power. True dual planes rule! A modern dual plane like the 7101 or the weiand 8150/8151 can support 550 HP! For normal street use, a cut down divider DP or a single plane is simply the wrong part for the application. Those living in the desert SW, like gkull in Nevada, have lots of fast, open roads. So i can see where he would not care about low rpm power. But, the OP needs to ask only 1 question. Is he over 550 HP often enough to kill low speed torque? Is that a trade he wants to make? That is when a cut down divider/SP can be an advantage, over 6300 rpm, 550HP. At the Silver State Challenge, i can see using a SP. That is a RACE. The air-gap and SP are good to use for racing. But have poor manners in ordinary driving. Long warmups, poor low rpm power, frequent downshifting, worse mpg, even backfiring thru the carb driving off on a cold start in cold temps. I had all these problems, and i will never go back.

kcarlasc

I just had the same issue.. Jegs has a right angel adapter that is suppsoed to fit under the carb.. mine is on the way..

http://www.jegs.com/i/JEGS+Performan...15985/10002/-1

gkull

Back to what the poster ask. the 4 hole wood spacer is the best type made. the 4 hole increases the signal to the boosters and the added height keeps the fuel atomized because it adds length before hitting the bottom of the plenum before having to make an abrupt 90 degree turn towards the heads.

As to what Matt is saying. He is correct that a divided dual plane is the best idea for a street driven car. It comes down to the whole package.

Crepitus

Matt did a great wright up about idle/light cruise with a fully divided duel plane. The fully divided single plane should also help with emissions testing. Over 3500-4000 the big shared plenum wins. For the inquiring mind; try something like the Air Warrior stealth with and with out a 1/2" 1 hole spacer. IMHO the question about where your engine spends the most time is not important. Even a Top Fuel spends most of its time at idle, that does not dictate that a Prius engine will be the best choice. Under what conditions is top performance most important to you? What are you willing to compromise? Kinda got OT but what the hell.

Matt Gruber

Everyone should make an informed choice. Even drag racers over on speedtalk argue over which is faster in the 1/4. SP vs DP about a 50/50 split. There is no clear winner. I was surprised at that, i thought the SP would be best. Seems the 5000+ stall guys like the SP. Stick and lower stall often go DP, but there are plenty of exceptions.
There is no compromise on street cars under 550 HP, 6300. There is NOTHING lost by a fully divided DP. Only nice benefits. It really annoyed me that i had to warm it up for 2 minutes before driving off below 50F. But, i see 1 neighbor warming up his truck, so he would be OK warming it up. I just want guys to know about these little annoying quirks. Easy to avoid them if you get the best parts for the application.
Guys, thanks for the vote of confidence

lars

If that's all you're wanting to do, forget the spacer. For a vacuum port for an accessory that is not continuous flow, simply drill and tap for a port into an accessible manifold runner that does not interfere with the carb. For a port that flows continuously, such as PCV, you can drill and tap right into the side of the base plenum, just below the carb surface.

I drilled the back surface of the plenum for PCV, but you can also go right into the left or right side. You can do the same with power brake line and accessory lines: