BBC Manifold Runner Parameters

Post by **SchmidtMotorWorks** » Tue May 31, 2005 3:27 pm

Good points, I have the same concerns but don't have any idea how much importance to assign to them.

The sequence of how the firing order plays out in a single plane plenums makes me think that some cylinders get some advantage and others are just as certainly hurt by the same cylinder to cylinder relationships.

Wouldn't it be great to get a high speed movie of what goes on in a plenum?

Does anyone know how much reversion there is at the runner opening at 7,000 rpm?

As for the pulses going back up the carb, I suppose that may also be a problem too, but I kind of doubt it. On some motorcycles I have looked at (thinking Yamaha) I have looked down the carb throat when the headers were off the engine and I could see the light on the top of the piston. I guess if there was a problem with the port/runner going straight up the carb throat that those motorcylce engines would suffer too.

On the other hand I have seen engines on the dyno with a big mist above the carb, I don't know much about it but I guess it is fuel being pushed up through the carb by some reversion.

Maybe this means that some sort of plenum housing is needed like some motorcycles have?

Post by **SchmidtMotorWorks** » Wed Jun 01, 2005 1:28 am

OK here is what I have come up with from the dimensions that people have sent to me.

The average runner length is about 5.7

Please excuse the half finished blending, I will clean it up after the concept is nailed down.

Does it look worth a try? It will take a summer of evenings and weekends and about 4K on materials for the patterns.

Here's the outside

Here's the inside

cboggs · Post by **cboggs** » Wed Jun 01, 2005 2:10 am

Jon,
What do you think the fuel distribution will be like with the
runner entries located like they are?

Curtis

Post by **SchmidtMotorWorks** » Wed Jun 01, 2005 2:22 am

Jon,
What do you think the fuel distribution will be like with the
runner entries located like they are?

I am hoping that it would be better than manifolds where the air must trun sharply to enter the port. My reasoning is that the fuel would tend to stay suspended beter when flowing in a straighter path.

The other think I am hoping is that the reduced crowding of the runner openings might releave some interference or fighting for shared air like cyl 1&2 and 7&8.

Does anyone know of any testing to determine the difference in power of different cylinders in engines with single plane manifolds? Probably headers effect this too unless they are 180s.

shawn · Post by **shawn** » Wed Jun 01, 2005 2:59 am

I just love those CAD renderings. Do you have one that looks down into the plenum from the carb flange?That transition from the roof of the port up to the carb flange is pretty important for distibution.
Shawn

67RS502 · Post by **67RS502** » Wed Jun 01, 2005 7:18 pm

Wouldnt making the plenum longer (front to rear), and spreading out the runner entries from each other make for more power? Making the runner "deviders" very rounded, with a big radius by spreading the runners out should work. Just makin a suggestion, so would this be a good idea?

Post by **SchmidtMotorWorks** » Wed Jun 01, 2005 10:23 pm

The runners are spread .375 now, (about the same as the ports on the head.)

I don't know what makes good or bad fuel distribution so I would appriciate any explanation of that. I have heard that the long log plenum manifolds on european sports cars from the 60s and 70s had fuel distribution problems.

Here are some more images as requested.

As I explained some of the blending is hacked for now, I will clean it up after I stop making other changes as blending is the final step in most models like this and changes often ruin exisiting blends.

let the criticisms rip, I can take it, and would rather make changes now than later.

cboggs · Post by **cboggs** » Wed Jun 01, 2005 11:23 pm

My guess is that all the "in-board" or short runners are going to be rich
and the "out-board" or long runners will be lean.

The shorter runners are directly under the carb venturi and have a strait shot while
the longer runners require the A/F mix to turn into them. Short runners
are very " line of sight"

I also think you might need more plenum volume, it looks very small to me.
The small plenum might run well on the track but a little larger plenum
would make more power.

I'm not sure what to think about the fact the the plenum has no "floor" or flat
on the bottom. From some older manifold tests that showed, .. it's a like
a hurricane and tornado in there, . pure chaos, .. I'm not sure if the charge
smacking into the floor is a good thing or not, ..

Perhaps a call to Tim Hogan would help, .. I know he's built some sheet
metal single four intakes kinda like this, .. but I don't know what the plenum
or runner entries look like.

Curtis

67RS502 · Post by **67RS502** » Thu Jun 02, 2005 9:22 am

Its just an idea, but I mean move the runners about 1" apart, so you'd have a nice
big 1/2" radius at the runner dividers. This would mean that no 2 runners would be
siamesed together. It would be almost like a single carb tunnel ram the way you have the
vertical runners at the plenum. So if you spread the inside runners 5/8" apart more for a
total of 1", and do the same thing between the inside and outside runners, your plenum
would only get 1 7/8" longer (front to back), so there would be a lot more plenum volume,
(dont know if that would be good or bad in your application), but now the runner entry
would be more like a velocity stack, with a big smooth radius at each runner, and being
farther apart they wouldnt be able to affect each other as much.
Although fuel distribution may need to be looked at.
Just an idea, as I'm just a nobody compared to you guys... but I find much of the stuff
discussed here very interesting.

todd8541 · Post by **todd8541** » Thu Jun 02, 2005 9:27 am

I did an intake once and had some slight fuel distribution problems on the outside runners. I think with some crossectional shape changes you can reduce or cure the issue. The best way to determine this would be to do a sla of the intake and wet flow test it. ALthough I know this could be expensive in the initial capital expense it could reduce costs later if you do the tooling and realize you have a serious issue. I would do the sla with thick walls so you it will allow you to shape the runners as necessary. You can use putty or bondo if some areas need to be filled in.

Todd

Post by **SchmidtMotorWorks** » Thu Jun 02, 2005 12:32 pm

Its just an idea, but I mean move the runners about 1" apart, so you'd have a nice
big 1/2" radius at the runner dividers.

Hmmm, interesting idea, this longer fron to back would mean that the distribution concernes that others have rasied might be worse.

Maybe I will make a model like that this weekend to see what it looks like.

Anyone else want to comment on that big radius idea?

BTW, that shape will be much more difficult to cast but I am up for it it of makes sense.

cboggs · Post by **cboggs** » Thu Jun 02, 2005 12:52 pm

I think the long plenum idea is a very good one, ..
most sheet metal comp single four intakes are built this way.

look at this comp manifold from another post here on speed talk, ..
these things work, ..

Curtis

Post by **SchmidtMotorWorks** » Thu Jun 02, 2005 1:14 pm

OK I can make a model like that but for BBC, do you think the runner openings should be round?

cboggs · Post by **cboggs** » Thu Jun 02, 2005 2:04 pm

SchmidtMotorWorks wrote:OK I can make a model like that but for BBC, do you think the runner openings should be round?

Jon,

I think in the interest of keeping the plenum a reasonable size you
might not be able to fit a round opening. Try making the plenum
some where between what you have, .. and the comp intake pictured above.
If you make the plenum long like the above comp intake I think
the plenum will be too big for the intended application.
Will your CAD tell you the volume or cross sectional area in the
plenum?

Let's see what that looks like.

Curtis

todd8541 · Post by **todd8541** » Thu Jun 02, 2005 2:10 pm

Have you thought about making the runners taller and narrower like the ls1 intake ports. Then make the top and bottom wall of the intake ports a full round (with no flat portion). Then transistion them to the shape you have at the intake to head interface. This should allow you to move the outside runners closer inline with the carb. Also it will allow you to maybe add some space btn the runners for a greater radius as mentioned earlier.

Just some thoughts
Todd