Plenum Volume

[BRENT FAY]

Plenum volume- what would be a good starting point as to know if you have enough? Measure volume of intake tract and divide the plenum by 4 (single 4 single plane intake or even tunnel ram) to end up with at least the volume of one cyl. or more if VE is say 110%? Or would you want a certain distance to the center opening of intake tract to throttle plates to keep fuel hitting plenum floor? Or you just keep adding carb spacers until carb loses it signal strength. Has anyone put a window in the side of plenum or int. tract near the opening to observe the A/F flow (Bill Jones?) on a running engine. On the intakes Jean Dittmer did for me he said to add a 1/2 in. spacer to make up for volume after reworking plenum floor. Or should one take a bunch spacers to track and do A B A B B C B C test. I dont have a dyno.... Just wondering, Brent

[bill jones]

-I have had "windows" in several manifolds and in some valve covers.
-the problem I saw was that you can't see thru the fog.
-One single 4bbl manifold I put the windows in all the intake ports about half an inch from the valve cover.
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-One box type tunnel ram that Edelbrock came out with in the late 1960's I had two 500 cfm 2bbls mounted on 1/2" clear plastic and the front carb was moved forward and the rear carb was turned around backwards and moved towards the rear.
-This allowed me to see clear thru the plenum down thru the runner and I could even see the fire in the cylinder during overlap right thru the open intake valve.
-The light from the fire in the cylinder was just like a timing light.
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-I could see in there just fine at idle and at part throttle but when you'd crack the throttle open real quick or rev the engine up it looked like you were spraying a firehose in there, very overwhelming amounts of moisture and you can't see thru that at all.
-The only time you can see in there is at idle and part throttle and it's basically pretty dark in there where you really want to see.
-I did learn several things from all that but everybody now knows those same things I learned.
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-I used a dial back timing light on the single 4bbl manifold to try to see if that would do anything but the reflection back off the windows was always a problem.
-A normal timing light did show interesting things inside a valve cover on a stud mounted rockers.
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-I discussed this some with Keith Dorton about 1987 and he built a single 4bbl with windows in the ports, and at my suggestion he mounted a bright light up inside the center of the plenum floor with some sort of a glass dome.
-I saw a photo of the manifold the next year in his catalog but I never heard back as to whether or not he gleaned anything from it and I doubt that he did learn much.
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-I was using the box tunnel ram on a 302 SBC where we had a rule that allowed a single carburetor or up to three 2bbls so I chose the two 500's.
-The 302 didn't like the big box manifold at all until about 5500 which was perfect for us as we ran that engine 2000 rpm higher than what everybody else was running at the time.
-Then we decided to make the plenum smaller and ended up making the two carbs nearly isolated from each other with a plenum about like a 3" tall open holer.
-This small plenum was the best overall performance our engine ever ran but by time we got to the small plenums we were running a 327 and from 5200 to 8200 as the top rpm rather than 5800 to a little over 9000 we had run with the 302.
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-I have never seen where a 2" tall 4 hole blended and flared spacer ever slowed any wide open throttle engines down here at 4200 elevation.
-Most every single 4bbl car we put a 2" four hole flared spacer on will pick up at least 1-1/2 tenths to 4/10ths on a drag strip bracket racer.
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-Back when the rules allowed us to do this sort of stuff on oval track cars we had mixed results because the extra power was sometimes hard to hook up and the car would slow down from wheel spin.
-But we could usually always end up gaining from the modified 4holers.
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-Just trying to measure the plenum of a single 4bbl manifold is major task so not many people go thru the hassle, including me.

[Darin Morgan]

Plenum volume- what would be a good starting point as to know if you have enough? Measure volume of intake tract and divide the plenum by 4 (single 4 single plane intake or even tunnel ram) to end up with at least the volume of one cyl. or more if VE is say 110%? Or would you want a certain distance to the center opening of intake tract to throttle plates to keep fuel hitting plenum floor? Or you just keep adding carb spacers until carb loses it signal strength. Has anyone put a window in the side of plenum or int. tract near the opening to observe the A/F flow (Bill Jones?) on a running engine. On the intakes Jean Dittmer did for me he said to add a 1/2 in. spacer to make up for volume after reworking plenum floor. Or should one take a bunch spacers to track and do A B A B B C B C test. I dont have a dyno.... Just wondering, Brent

Plenum volume is interrelated to so many other variables such as, Engine size and rpm. Engines indented use such as Drag Racing, oval track, truck tractor pulling, boat racing, off shore boat racing and does it have an automatic transmission or five speed. These are all questions that need to be asked before deciding on how large the carbs and plenum should be. Its also the questions I ask when designing the manifold itself. If your asking for a magic number or formulas for any given engine, it does not exist. Its one of the last tuning factors we explore on the dyno. Its also very easy to get the plenum to large when chasing power numbers on the dyno! You can get a plenum way to big if you just look at power numbers on the dyno. Then, when you take it to the track the engine wont recover on the shifts and wont accelerate very well. An AED competition eliminator engine is good example. They have a sheet metal single four barrel manifold. On the dyno you can just keep stacking vertical spacers under the carb and the engine will continue to make more power. I have seen 7 one inch spacers make a lot of power on the dyno but the engine would lay on the converter and not recover. The farther you get the carbs from the runners, the more the signal suffers. There is a magic spot on every manifold as far as carb height goes. The track has the final word on that.

[John Wallace]

How would that work with a mechanical fuel injection set-up?
The injecters are at the intake port and don't need the signal like a carb.
Does the plenum matter that much for this?

[Darin Morgan]

How would that work with a mechanical fuel injection set-up?
The injecters are at the intake port and don't need the signal like a carb.
Does the plenum matter that much for this?

No it does not. With fuel injection the plenum can be very large and not effect engine performance.

[dbusch]

Darin, interesting you mention that about the plenum sizes and on track performance. Killer dyno numbers but unsatisfactory track numbers. I know someone with a 400" sbf motor with a sheetmetal manifold, a glide, and a 7" converter. It wont stall the converter on launch and shift as much as it should, and then slips too much on the big end. Sounds to me like it needs less plenum volume or longer runners. Not enough mid-range torque and too much hp up top for the converter design. Maybe a different converter would work better, maybe not. My 398" sbf cast manifold engine stalls the same converter 600 rpm more and locks up much better up top. Go figure. Have you seen good mid range torque dyno numbers with excessive plenum volume yet still poor track performance? Maybe you cant even see the problem on the dyno...

[Darin Morgan]

Darin, interesting you mention that about the plenum sizes and on track performance. Killer dyno numbers but unsatisfactory track numbers. I know someone with a 400" sbf motor with a sheetmetal manifold, a glide, and a 7" converter. It wont stall the converter on launch and shift as much as it should, and then slips too much on the big end. Sounds to me like it needs less plenum volume or longer runners. Not enough mid-range torque and too much hp up top for the converter design. Maybe a different converter would work better, maybe not. My 398" sbf cast manifold engine stalls the same converter 600 rpm more and locks up much better up top. Go figure. Have you seen good mid range torque dyno numbers with excessive plenum volume yet still poor track performance? Maybe you cant even see the problem on the dyno...

The problem will not be readily visible on the dyno except for a mild drop in fuel flow. Your buddys problem seems to be more than just plenum volume. Its air speed, air speed air speed. Sounds like he has ports that are to big and a miss matched manifold combination to boot. This happens a lot because people put way to emphasis on air flow numbers when buying heads.

[RGL]

How practical is a sheetmetal application in say a 3000-3100 pound car with a glide . Is it too peaky to make use of only 2 gears, or can the design of the manifold be manipulated to compesate ? or possibly a 3 speed auto ? I know the Hemi's run them at this weight, but its a cross ram , or possibly some of the faster SS classes ? Thanks , have enjoyed reading on thesite so far.

[panic]

Bill Jenkins on SBC with TR (paraphrased): increase plenum volume until more does not help on the dyno.
This is always to big (lazy response, fuel drop-out). Subtract 10% for a good race volume.

My thoughts:
Section the box height horizontally and re-attach
Stuffer in plenum floor (BJ used wood)

[Bob Hollinshead]

There's some good pics in his book

[1972ho]

https://youtu.be/7Iq1B-2paCs Maybe you haven’t seen Jon Kaase video