Bee Jay's RPM Air Gap Mod
08-01-2010, 06:28 PM
#21
Team Owner
IMO.................. If you look back at early hot rodding days. The famed LT1 dual p manifold was the first to incorperate the cut down divider. The reason was to allow the felt smaller volume at lower rpm and if needed at higher RPM draw from all 4 barrels.
Way back in the 80's I always wanted more power out of my little 355 L-82 based motor. so being progressive I bought ht weiand single plane that you could even slide in a divider plate. I think that it is still sold today.
I think that cam timing and if you are carbed plays a bigger roll in how a motor actually runs. Not the manifold design.
Just a personal observation from 25 years of single planes and tunnels rams driven on the street
Way back in the 80's I always wanted more power out of my little 355 L-82 based motor. so being progressive I bought ht weiand single plane that you could even slide in a divider plate. I think that it is still sold today.
I think that cam timing and if you are carbed plays a bigger roll in how a motor actually runs. Not the manifold design.
Just a personal observation from 25 years of single planes and tunnels rams driven on the street
08-01-2010, 07:23 PM
#22
Race Director
MH: Would the increased displacement of your engine be a reason the AG and the Vic jr are so close? All those cubes need extra cfm, the Vic jr does flow more. Most tests on 350 SB show a Vic jr is a little much for the smaller milder SB. A 427 @ 8K will need mucho cfm! LOL
R
R
The 427 gets the Vic Jr. I also had AFR port match it to the 227cc Eliminator heads and seems like they did a good job as I paid to have my heads flowed with an without the intake bolted to the heads and with the intake in place there was only a 3% loss in CFM over without the intake. I was impressed
08-02-2010, 01:32 AM
#23
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The butt dyno didn't feel much difference but I did notice that in fourth, overdrive, with the lockup switched on, the car idled on up to 85 mph effortlessly. Hardly no throttle at all and the car is cruising at about 2500rpm, 85 mph. Just barely off of idle. Also the vacuum did pick up from 14" to 15". That's the most my car has seen at idle in a long time. But no noticeable performance feel difference. I'm curious about the increase in vacuum though. The air fuel ratio meter said the same usual stuff too.
Bee Jay
Bee Jay
Last edited by Bee Jay; 08-04-2010 at 12:23 AM.
08-02-2010, 06:41 AM
#24
Race Director
if you are that worried about them backing out, loctite them in or even a small dab of epoxy on top of them. but like you said, being an inch or so long kinda keeps the possibility of them coming out...
08-02-2010, 07:16 AM
#25
08-02-2010, 10:35 AM
#26
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I drove it to work this morning. Still the only big noticeable difference is how the car can cruise (75-85mph) barely off of idle. Also, on a hill I take on the highway to work, I usually have to unlock the converter to keep my agressive pace, this morning I didn't have to unlock. Also, the air fuel ratio meter seems a little more steady. Usually it can swing around pretty good. I need more butt dyno time.
Bee Jay
Bee Jay
Last edited by Bee Jay; 08-04-2010 at 12:23 AM.
08-02-2010, 01:32 PM
#28
Race Director
Sound like the divider gave you a little more low RPM torque which sounds reasonable. I bet above 2500 RPM or so there is no difference, what is the advertised low RPM point of the cam. Also it won't make as much HP at 6000RPM but again nothing your really going to be able to feel 
This dual plane manifold and other single planes like the Vic Jr are designed to give you a few more ponies in the high RPM range. They are really racing manifolds, because when it comes down to it 5 more HP at 6000 + RPM could mean a win over a loss in a heads up drag race
The only way to find out for sure is to put it on a chassis dyno with and without your divider in there, back to back on the same day
This dual plane manifold and other single planes like the Vic Jr are designed to give you a few more ponies in the high RPM range. They are really racing manifolds, because when it comes down to it 5 more HP at 6000 + RPM could mean a win over a loss in a heads up drag race
The only way to find out for sure is to put it on a chassis dyno with and without your divider in there, back to back on the same day
08-02-2010, 04:29 PM
#29
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Sound like the divider gave you a little more low RPM torque which sounds reasonable. I bet above 2500 RPM or so there is no difference, what is the advertised low RPM point of the cam. Also it won't make as much HP at 6000RPM but again nothing your really going to be able to feel
This dual plane manifold and other single planes like the Vic Jr are designed to give you a few more ponies in the high RPM range. They are really racing manifolds, because when it comes down to it 5 more HP at 6000 + RPM could mean a win over a loss in a heads up drag race
The only way to find out for sure is to put it on a chassis dyno with and without your divider in there, back to back on the same day
This dual plane manifold and other single planes like the Vic Jr are designed to give you a few more ponies in the high RPM range. They are really racing manifolds, because when it comes down to it 5 more HP at 6000 + RPM could mean a win over a loss in a heads up drag race
The only way to find out for sure is to put it on a chassis dyno with and without your divider in there, back to back on the same day
Bee Jay
08-02-2010, 05:50 PM
#30
Melting Slicks
What you have done is defeating the purpose of the cut down divider and effectively changed what makes the Air Gap such a good manifold, one of the best out there.
The cut down divider is to allow this dual plane manifold high RPM performance while still allowing low end power too.
I doubt you will notice any difference unless you do a back to back chassis dyno session with and without your spacer insert. What you will find is approximately the same low end torque but you will lose some HP around 6000RPM and above with the insert in the manifold.
I did a back to back test between the Air Gap (dual plane ) and a Vic Jr( single plane ) and found the low end torque and HP to be pretty much the same until around 6000RPM where the Vic jr pulled a couple more HP out of the motor.
I would have used the RPM Air Gap ( I did for a year or so ) but I switched to Vic Jr when I found out contrary to popular belief the single plane didn't "lose" any TQ compared to the dual plane Air Gap and I like the looks of the Vic Jr better.
So it really came down which manifold I liked the looks of better because this motor was only really capable of about 6000RPM so the performance of one or the other didn't really matter.
Now my new 427ci small block is capable of ~8000RPM and the Vic Jr was the only manifold I would put on
So these are the results of 2 specific manifolds and may or may not hold true for any other dual or single plane manifolds.
The cut down divider is to allow this dual plane manifold high RPM performance while still allowing low end power too.
I doubt you will notice any difference unless you do a back to back chassis dyno session with and without your spacer insert. What you will find is approximately the same low end torque but you will lose some HP around 6000RPM and above with the insert in the manifold.
I did a back to back test between the Air Gap (dual plane ) and a Vic Jr( single plane ) and found the low end torque and HP to be pretty much the same until around 6000RPM where the Vic jr pulled a couple more HP out of the motor.
I would have used the RPM Air Gap ( I did for a year or so ) but I switched to Vic Jr when I found out contrary to popular belief the single plane didn't "lose" any TQ compared to the dual plane Air Gap and I like the looks of the Vic Jr better.
So it really came down which manifold I liked the looks of better because this motor was only really capable of about 6000RPM so the performance of one or the other didn't really matter.Now my new 427ci small block is capable of ~8000RPM and the Vic Jr was the only manifold I would put on
So these are the results of 2 specific manifolds and may or may not hold true for any other dual or single plane manifolds.
Just so I'm not only answering a question that wasn't asked and offering critisism where it wasn't requested, I will say that the divider eliminator looks pretty nice and the proper carb spacer (or carb for that matter) will keep any of the fastenters from going anywhere.
I also run the AFR 195, albiet ported a bit, and would run to about 7500rpm at the track. I had to make everything I could out of 1st and 2nd to avoid going into 3rd with my 3.55 gears.
Last edited by VETDRMS; 08-02-2010 at 05:54 PM.
08-02-2010, 08:29 PM
#31
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I'll just toss my .02 into the hat here. I did test an RPM Air-Gap with the Victor Jr. at the track and in every respect the RPM Air-Gap lost out to the Victor Jr. I posted a test thread somewhere around here a long time ago, I'll see if I can find it.
Just so I'm not only answering a question that wasn't asked and offering critisism where it wasn't requested, I will say that the divider eliminator looks pretty nice and the proper carb spacer (or carb for that matter) will keep any of the fastenters from going anywhere.
I also run the AFR 195, albiet ported a bit, and would run to about 7500rpm at the track. I had to make everything I could out of 1st and 2nd to avoid going into 3rd with my 3.55 gears.
Just so I'm not only answering a question that wasn't asked and offering critisism where it wasn't requested, I will say that the divider eliminator looks pretty nice and the proper carb spacer (or carb for that matter) will keep any of the fastenters from going anywhere.
I also run the AFR 195, albiet ported a bit, and would run to about 7500rpm at the track. I had to make everything I could out of 1st and 2nd to avoid going into 3rd with my 3.55 gears.
Bee Jay
08-02-2010, 09:57 PM
#32
Melting Slicks
I drive in that range too but a 4000+ RPM billet converter really doesn't require a huge amount of midrange. I'm probably close to peak torque at the converter's stall so there is no lacking of mid-range or off-throttle loss it's like an ON/OFF switch to smiles and rubber stripes. 
I did try both manifold though and liked the Victor Jr. more, it's just felt stronger everywhere.
I did try both manifold though and liked the Victor Jr. more, it's just felt stronger everywhere.
08-02-2010, 11:03 PM
#33
Racer
A couple have stated that the Air Gap is there to provide low-end torque at the expense of high-end horse power. If closing the Air Gap at high RPM would provide additional horse power, could a butterfly be added to close the gap and regain the HP?
08-02-2010, 11:58 PM
#34
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Bee Jay
08-03-2010, 08:51 AM
#35
Drifting
Butterfly???
I think he means a butterfly in the "air gap" or slot under the carb, you know, the one you've blocked up. The thought of a variable orifice in there gives one something to think about, doesn't it?
Regards from Down Under
aussiejohn
and, your mod works for you & I like it ... good reporting
08-03-2010, 10:46 AM
#37
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Bee Jay
Last edited by Bee Jay; 08-03-2010 at 10:49 AM.
08-03-2010, 10:49 AM
#38
Melting Slicks
Here are the threads I was talking about, three part comparison from way back in 2005. Wow time goes fast.
http://forums.corvetteforum.com/c3-t...round-1-a.html
http://forums.corvetteforum.com/c3-t...pressions.html
http://forums.corvetteforum.com/c3-t...sults-vid.html
http://forums.corvetteforum.com/c3-t...round-1-a.html
http://forums.corvetteforum.com/c3-t...pressions.html
http://forums.corvetteforum.com/c3-t...sults-vid.html
08-03-2010, 12:32 PM
#39
Le Mans Master
If you didn't want the cut down plenum divider, why didn't you just buy the Performer RPM and close off the exhaust crossover? ACCORDING TO EDELBROCK, the runner design is the same with the only differences being the air gap and the cut down plenum divider.
I use a Performer RPM with the crossover blocked off and a "splash shield" added in the lifter valley to keep hot oil away from the intake, and added a 1/2" open spacer. I had asked my mechanic about cutting down the divider and he said it is better to add the 1/2" spacer. This actually does several things like, adds plenum volume, lets one bank of cylinders draw off the other side of the carb at higher RPMs, lets the intake charge make a gentler turn, reduces carb temps, gets the air cleaner further from the heat, etc.
Additionally, the manifold remains intact and if you don't get the gains you want, you can just remove the spacer.
I use a Performer RPM with the crossover blocked off and a "splash shield" added in the lifter valley to keep hot oil away from the intake, and added a 1/2" open spacer. I had asked my mechanic about cutting down the divider and he said it is better to add the 1/2" spacer. This actually does several things like, adds plenum volume, lets one bank of cylinders draw off the other side of the carb at higher RPMs, lets the intake charge make a gentler turn, reduces carb temps, gets the air cleaner further from the heat, etc.
Additionally, the manifold remains intact and if you don't get the gains you want, you can just remove the spacer.
Last edited by toddalin; 08-03-2010 at 12:42 PM.
08-03-2010, 12:32 PM
#40
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Here are the threads I was talking about, three part comparison from way back in 2005. Wow time goes fast.
http://forums.corvetteforum.com/c3-t...round-1-a.html
http://forums.corvetteforum.com/c3-t...pressions.html
http://forums.corvetteforum.com/c3-t...sults-vid.html
http://forums.corvetteforum.com/c3-t...round-1-a.html
http://forums.corvetteforum.com/c3-t...pressions.html
http://forums.corvetteforum.com/c3-t...sults-vid.html
But what I did here was eliminate the notch in the RPM Air Gap plenum to see if it made a difference in low speed torque and drivability. My torque converter is just a little looser than stock, it's a Buick Grand National replacement. I drove it to work for the second time today and played with the lock up some more. I can definitely switch the lockup on earlier than before. I gave it one full throttle red line second gear kick, it still burns rubber, it still hauls ***, and it still pulls stong to 6500 rpm. So I don't think I hurt it at all. Maybe some one here with access to an engine on a dyno can do a comparison, RPM Air Gap with notch vs RPM Air Gap with no notch.
Bee Jay
Bee Jay