throttle spacer, air gap, or...

[Frankenchevy]

i want to get the correct manifold for a holley sniper efi to connect to vortec heads.

it's been said that dual plane manifolds without an air gap or spacer can have too much variance in AFR from one side of the manifold to the other. there are a few solutions. i can buy an air gap intake which is a little more expensive, i can buy a super common edelbrock performer from CL and mill an air gap or install a spacer to essentially achieve an "air gap".

either way, i think a shorter spacer helps with any of the above configurations for all over power. if you go long on the spacer, i'm aware that i'll sacrifice bottom end. i definitely do not want that. i've heard that .5-1" is pretty optimal for helping without losing bottom end. phenolic spacers okay versus aluminum?

can anyone verify any of this info? any positive experiences with the big name dual plane intake manifolds, specifically an "air gap" style?

don't want to spend a bunch of money on this part of the setup if i can help it. i can pick up a manifold on CL for next to nothing and a spacer is $30. but i'd rather spend a few hundred bucks and not get some sloppy casting on a cheap manifold. suggestions?

 

[1987 GMC Jimmy]

I know someone who has a lot of experience with Air Gap. Hope he sees it... @rich weyand

 

[1987 GMC Jimmy]

But if not, here's some of his literature.

http://www.fullsizechevy.com/forum/...719-Induction-system-for-350cid-with-headers=

 

[75gmck25]

I think you are mixing 2 or 3 concepts and fixes together.

- Air Gap manifolds have an air gap under the carburetor flange that is supposed to cool the fuel. However, this hurts cold start driveability because there is no heat to the carburetor, and you may get icing of the fuel. An Air Gap manifold is often dual plane (Edelbrock 7501).

- Dual plane manifolds are recommended for street engines. However, based on testing by the Engine Masters guys, single plane manifolds may work better with AFI. However, they did not do any really verification of why, so the exact reason is up to debate. I think it may be due to the single plane feeding a more consistent manifold vacuum signal to the EFI ECU, but I don't really know.

- Carb spacers increase the volume of air below the carburetor, which has shown more top end power in some engines, with some decrease in low end torque. You always want an insulator, and a thicker spacer may improve the performance of your engine. Some racers try different spacers and make comparison runs on the dyno or track. There are so many variables that there is no broad good/bad answer.

You may want to try a used dual plane Edelbrock Performer Vortec to get the engine running, and then experiment with different spacers to see if it makes an improvement. The other option is to start out with a single plane manifold like the Torker or Victor, but they are less common used and you will probably pay quite a bit more.

 

[Frankenchevy]

But if not, here's some of his literature.

http://www.fullsizechevy.com/forum/...719-Induction-system-for-350cid-with-headers=

I read the linked thread completely. It was mostly about timing and getting heat into a carburetor. Maybe it was a different thread you’re thinking about?

 

[Frankenchevy]

I think you are mixing 2 or 3 concepts and fixes together.

- Air Gap manifolds have an air gap under the carburetor flange that is supposed to cool the fuel. However, this hurts cold start driveability because there is no heat to the carburetor, and you may get icing of the fuel. An Air Gap manifold is often dual plane (Edelbrock 7501).

- Dual plane manifolds are recommended for street engines. However, based on testing by the Engine Masters guys, single plane manifolds may work better with AFI. However, they did not do any really verification of why, so the exact reason is up to debate. I think it may be due to the single plane feeding a more consistent manifold vacuum signal to the EFI ECU, but I don't really know.

- Carb spacers increase the volume of air below the carburetor, which has shown more top end power in some engines, with some decrease in low end torque. You always want an insulator, and a thicker spacer may improve the performance of your engine. Some racers try different spacers and make comparison runs on the dyno or track. There are so many variables that there is no broad good/bad answer.

You may want to try a used dual plane Edelbrock Performer Vortec to get the engine running, and then experiment with different spacers to see if it makes an improvement. The other option is to start out with a single plane manifold like the Torker or Victor, but they are less common used and you will probably pay quite a bit more.

I’m going efi, so much of the carb related stuff is irrelevant. I live in an area that has an average morning temp of 40-50 degrees in the winter. Cold it is not. Today I have have a low of 51 and a high of 66.

My main concern is air fuel ratio being balance on both sides of the intake manifold. Having divided manifolds screw with AFR balance. That’s why an air gap or a spacer may be recommended, so that the mixture between the two sides is better balanced.

Single plane has no divider from the start. So all four squirters are working within the same manifold space. The efi guys that had a dual plane with no air gap had AFRs of 11:1 on one side of the manifold and over 13:1 on the other side of the manifold, so the computer could never do what was best for both sides. They solved it by milling an air gap or installing an open bore carb spacer. The write up is a few years old at this point. So I was searching for up to date knowledge.

I’m not sure single plane is the right match for the truck. I thought they were more for peak hp with tq sacrificed. It’ll rarely go over 3000rpm. Bottom end tq/drivability are paramount.

 

[Frankenchevy]

But yeah...you’re right. I’m mixing concepts, but kind of intentionally.

 

[1987 GMC Jimmy]

Possibly. Sorry about that, man. The biggest takeaway I got from all that, which he talked about on here before, was his homemade heat stove and riser setup and keeping the Thermac intake. You're not gonna be carved so it probably doesn't matter as much, but the TBIs still had this setup so I would assume that there's still a benefit in doing that for EFI albeit a smaller one. There are a few threads on this site that talked about the Air Gap stuff. I couldn't find them, though. I'm just hoping he sees where I tagged him.

 

[Frankenchevy]

I searched too. I’m not a big fan of asking questions that of already been answered, either. Some of the stuff is only been out for so long so the kinks are still being worked out. I have found answers to questions similar to mine but they are a few years old and potentially out of date by now.

I appreciate any your guys help. What we think we figure out today, people might be laughing about in the future. Whenever a company comes up with a new idea for a product, we have to play around with the set up to try and get it dialed. Then as soon as we think we’ve got something figured out, they’ll have something new and more “awesomer” to try out.

 

[Frankenchevy]

 

[bucket]

Couple things here. An open plenum manifold is not an air gap manifold. An air gap manifold has open air space under the runners and above the lifter valley. So, there are air gap manifolds that are not open plenum, and there are open plenum manifolds that are not air gap style.

Also I'm no scientist, but I think when fuel injection comes into the mix, having a "street friendly" dual plane manifold becomes much less important.

 

[vkh]

Think there is a bit of confusion over "air gap". Edelbrock kinda set the nomenclature with their air gap manifolds, they have space between the plenum and the valley to reduce heat soaking into your intake charge. I think what you're looking for is a space in the divider between sides. The performer air gap won't have this. The performer rpm will have the extra space in the divider so the sides aren't completely isolated. The performer rpm air gap will too.

Personally I vote performer rpm. The air gap stuff doesn't make much of a difference. Performer with an open spacer should work well too, but I think the performer rpm is a more elegant solution overall.

 

[twinturbo427]

Key terms being plenum, divided plenum, not air gap.

Weiand makes or made a single plane with a removeable plenum divider. We would trim the divider based on max rpm and throttle response.

Somewhere online is a great article about tbi (all variations) not requiring the vacuum signal produced by dual plane intakes that forms just under the carb mounting flange when the throttle is cracked. The one caveat was that single plane intakes with flat plenum floors pooled fuel under certain circumstances causing poor distribution and erractic behavior.

As long as you arent trying to run a Profiler with Domintor flange it will work. I am leaning towards the Performer rpm due to being readily available used and cheap.

 

[Frankenchevy]

Awesome, thanks for all the clarification. So for non air gap, but without a divider, go with performer rpm...

Think there is a bit of confusion over "air gap". Edelbrock kinda set the nomenclature with their air gap manifolds, they have space between the plenum and the valley to reduce heat soaking into your intake charge. I think what you're looking for is a space in the divider between sides. The performer air gap won't have this. The performer rpm will have the extra space in the divider so the sides aren't completely isolated. The performer rpm air gap will too.

Personally I vote performer rpm. The air gap stuff doesn't make much of a difference. Performer with an open spacer should work well too, but I think the performer rpm is a more elegant solution overall.

 

[Frankenchevy]

Now I’m confused...

Performer rpm:
https://www.summitracing.com/parts/edl-7116

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Performer rpm air gap:
https://www.summitracing.com/parts/edl-7516

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