IR lengths and plenum sizing for carbs

[BenE64]

Currently revising my intake manifold to suit 44mm IDFs.
I have searched but cant find where i read some info about the runner lengths, ie to suit recomended lengths by pipemax. Is it valve to bellmouth for the harmonic tuning?

If i were to build a plenum on top with a large panel filter inlet being fed from reverse cowl or would individual air filters with no plenum be the better approach?

If you have the plenum mounted to the top of the carbs with the trumpets protruding into the plenum how does this go with fuel pooling? I know it is not recommended to have places for fuel to puddle in a manifold. If i get the reversion people talk about with webers at certain rpms how will this go with fuel puddling inside the air filter plenum?

If there is threads with this info let me know i may be searching the wrong terms.

Thanks

[modok]

I'd say the tuned length is both, maybe 60% to the vent and 40% to the trumpet. No definite rule but very long or short trumpets probably won't be as good as moving the carb in/out as well. Usually use fourth harmonic to the trumpet but not always, depends on everything else. I am not sure of pipemax's harmonics are the same as actual or not in this aspect.

On a four banger putting a plenum on them and tuning the inlet to that can smooth out the low end without doing much harm to the top, but you don't see it done very often because it's a lot of work, if it fits at all.

[hoffman900]

Currently revising my intake manifold to suit 44mm IDFs.
I have searched but cant find where i read some info about the runner lengths, ie to suit recomended lengths by pipemax. Is it valve to bellmouth for the harmonic tuning?

If i were to build a plenum on top with a large panel filter inlet being fed from reverse cowl or would individual air filters with no plenum be the better approach?

If you have the plenum mounted to the top of the carbs with the trumpets protruding into the plenum how does this go with fuel pooling? I know it is not recommended to have places for fuel to puddle in a manifold. If i get the reversion people talk about with webers at certain rpms how will this go with fuel puddling inside the air filter plenum?

If there is threads with this info let me know i may be searching the wrong terms.

Thanks

Yes, it's valve seat to about a halfway point on the velocity stack bellmouth radius.

I would not recommend sticking a carburetor intake runner with a floor below it, for the reason you stated before. If things are correct, it shouldn't be so much that a lot will puddle. Look at doing something like this:



I think at this point, it's best to call it an airbox. I'd look into boxing it in and having the open end to the cowel. That way you're not sucking in hot engine bay air.

[modok]

The fuel fog above the carbs can turn into a wetting problem if there is not enough room above the stack and the fog hits the lid. You can have the bellmouths above the airbox floor as long as there is enough headroom as needed for the application so the fog does not hit the lid excessively. Usually the longer the runners and wilder the cam timing the more headroom is needed. A clear cover will let you see what is happening inside

[ptuomov]

How many cylinders, what displacement, what rpm, how much overlap in the cams? I can’t help, but someone who can will probably need to know that stuff.

[BenE64]

I'd say the tuned length is both, maybe 60% to the vent and 40% to the trumpet. No definite rule but very long or short trumpets probably won't be as good as moving the carb in/out as well. Usually use fourth harmonic to the trumpet but not always, depends on everything else. I am not sure of pipemax's harmonics are the same as actual or not in this aspect.

After hours of research and googling I've located the carb mounts location in the runners at precisely where i could get them to fit and not foul the linkages

The end point of the trumpet is at the 3rd harmonic I think i dont have my notes here with me. Its either 3rd or 4th within a few mm's. The trumpets are standard 65mm long from memory. The "plenum or air box" roof will be around 40mm above the trumpets

On a four banger putting a plenum on them and tuning the inlet to that can smooth out the low end without doing much harm to the top, but you don't see it done very often because it's a lot of work, if it fits at all.

[BenE64]

How many cylinders, what displacement, what rpm, how much overlap in the cams? I can’t help, but someone who can will probably need to know that stuff.

I was leaving it fairly generic as I didnt think it would matter.

VG33E nissan V6
3.3lt so 550cc per carb throat.
Cam is standard at this stage to get it on the road. Peak hp is around 5000rpm with the horrible standard efi manifold im tuning for 5500rpm.
44mm idfs 34mm chokes.

Cheers

[ptuomov]

How many cylinders, what displacement, what rpm, how much overlap in the cams? I can’t help, but someone who can will probably need to know that stuff.

I was leaving it fairly generic as I didnt think it would matter.

VG33E nissan V6
3.3lt so 550cc per carb throat.
Cam is standard at this stage to get it on the road. Peak hp is around 5000rpm with the horrible standard efi manifold im tuning for 5500rpm.
44mm idfs 34mm chokes.

Cheers

My opinion (not experience, just theory):

If you have 3 equally spaced pulses from both banks, you may want a Helmholtz resonator intake manifold. That is, two plenums that are not connected (assuming you don’t want an active resonance system). If you feed each of the plenums from an air box with a filter, then the connecting zip tube length and diameter also matter.

These tricks help below peak power rpm, I think (but don’t know) that for the peak power at one rpm, you can just feed them all from a large single air box.

In my opinion, the best place to copy some designs from is the Porsche flat sixes. The factory manifolds use all sorts of resonance tricks. Furthermore, those have been tuned by a lot of people with carbs and ITBs.

[modok]

I agree

[BenE64]

images-4.jpg

Well this is pretty much what i am trying to build here. I am not sure howw to begin with building the resonant system. To suit.
All the porsche forums have that i have seen so far is how to delete the resonator? Will need to search further. Bike forums seem more into tuning them will look into it more.

It appears that it is a particular band that you tune for, peak torque or peak hp? Coming in around peak torque to carry out to peak hp or is the band narrow?

Come for answers leave with more questions! Love this forum.

[modok]

Don't think you'd want an airbox for tuning, but for muffling the intake noise. It's not going to be as beneficial for a low end boost pairing three cylinders. You might be able to give it a big mid-top boost but it would be tricky and the carburetors would be confused as heck too.
So, what's the length?

[digger]

whats a good rule of thumb for 3cylinder plenum volume?

[user-23911]

Take a look at pretty much any EFI manifold and the length chosen is the correct length.

If you want to downgrade to carburettors like in your picture above then you need to add intake trumpets to get the length back.
It's easier said than done because the length is similar to that of the primary runners for the exhaust which on an EFI manifold are usually curved.

The chart above is wrong in that only the odd harmonics are used, not the even ones.
Don't know where the chart came from but 2nd, 3rd and 4th are probably (in reality) the odd ones so really the 3rd, 5th and 7th.


edit.......working it out is quite easy........speed of sound 343 m/sec ....... Primary tuned length. That's a half wavelength because the wave flows there and back again.

At 6000 RPM, that's 100Hz or 50 Hz for a 4 stroke single cylinder.
343/50 is the total distance between pulses = 6.86 meters or (6.86/2) 3.43M tuned length.
Nobody ever uses that because it's too long.
The 2nd harmonic is 6.86/4...not used due to fighting airflow instead of helping it.
3rd is 6.86/6.......1.14M (44 inches)
4th is 6.86/8.......not used as above.
5th is 6.86/10......69cm (27 inches)
That's total length, valve to open atmo.

Just ballpark figures but they fit modern EFI engines........not computer simulation gigo.

[digger]

I don't recall ever seeing an efi intake manifold 27" long

[Geoff2]

Webers are great to work with. Get the Weber tuning manual [ genuine Weber ], which has the necessary info you need. The Haines Weber book is also very good, & John Passini's Weber book [ ISBN 9781855207592 ] is the best; he has forgotten more about carbs than most people will ever know....

I would NOT use individual air cleaners, too restrictive. I use a large airbox type with a large single K&N filter, not unlike the one shown by Hoffman900.
I enclosed mine so that that it gets cold cowl fed air & it is covered by aluminised fibreglass heat insulation to keep the air as cool as possible for max HP.