IR lengths and plenum sizing for carbs

[BenE64]

Maye you can get nics "Stef" or "induction apprentice" to give you some pointers on the runner length, runner diameter, taper, and the number of plenums and volume in each. You could PM them, or google some threads by them, read them, and then respond?

Google searches that might be useful:
stef intake site:speedtalk.com
induction apprentice intake site:speedtalk.com

Thanks for the leads, i have read quite a few of induction apprentice's posts, there are some i took careful notice of with the 3/4 cyl overlap. Now if i haven't competely got it stuffed up! The firing order on this engine means each carb will see a intake 120* apart then a 480* gap before the first throat see's an intake again, if i do put a divider in it will mean 240* between each adjacent throat.

Cheers

[pcnsd]

I had the day off so here is a basic spreadsheet calculator for harmonic lengths. It should get you close. Please note that harmonics are self amplification lengths. Pressure waves are what is happening. There is some loose correlation between tuned wave distance and harmonic resonance distance. 2nd wave and 3rd harmonic are reasonably close. The further from primary the weaker the event.

Inlet resonance.xls

[BenE64]

Thanks I had a play. Now i have the option of 3rd harmonic at 12.5in, 7th harmonic at 11.9in or the 30th harmonic at 12in! Or any harmonic i can tune to in a 3in range of 12in i suppose?

[modok]

WHAT? altering the length 1/2" won't do hardly anything. Must be a typo somewhere.

[pcnsd]

Thanks I had a play. Now i have the option of 3rd harmonic at 12.5in, 7th harmonic at 11.9in or the 30th harmonic at 12in! Or any harmonic i can tune to in a 3in range of 12in i suppose?

"The further from the primary the weaker the event". I'll add you want it a long as you can reasonably fit in the space available. Do you find error in the math?

[BenE64]

Sorry that was the different out puts from the excel the other calcs and pipemax!

Poor humour obviously. There is obviously many methods to reach similar outcomes but i am confused to how there can be so many different results or am i completely stuffing it up?

I put in the same hp and torque rpms and go some similar measurements just called different harmonics?

[modok]

12" There will be 3.5-4.5 peaks at redline, I don't know exactly, but air is air and pipes are pipes, you know, if the math says otherwise the math is probably wrong. If you pair it down to a fairly small plenum volume then i don't know, that's a more complex system, need a simulator.

[BenE64]

Thanks Modok, it will be straight IR once done. The trumpets can be shortened later to fine tune. I have gone as long as possible on runner lengths and was attempting to get the lengths to coincide with the harmonics on pipemax, The air box will still need some mulling over.

I started reading scientific design of intakes and exhausts last night. Try and get a better understanding. Till my wife said my geekyness was a turn on......no not really i just fell asleep.

[MadBill]

I should re-read that book. Smith's writing style irritated the hell out of me 50 years ago; be interesting to see if I've mellowed at all...

[modok]

Don't read the book, or, only read it if you WANT to drive yourself nuts.

Read AG Bell's book, and play with a engine simulator that shows pressure traces.

[BenE64]

I thought it was hard going! Ive had a couple go's at reading it and lost interest. I just ordered the AG Bell book.

What is a decent SIM i have read a few threads before and hadnt got a good feel of a way to go. ENGMOD4T was one i was looking at but was a bit shy to buy a program for that money with out being certain.

I have a big hydraulic pump here unused i have actually thought about building my own dyno just so i could learn. Not sure the neighbours would appreciate my "experimenting" they were complaining about the air compressor and die grinder running flat out for 8 hours yesterday.

[modok]

The best part of the P. Smith scientific design book is the graphs. He has the details of the setup and the pressure trace it produces, and that's ALL true. The author does struggle with building a model to explain what he was seeing.

used to be GT power, Lotus, and....anther i can't recall, but now I think there are a lot more. I'm not up to date, but, I figure it's been out there long enough I bet they all reverse engineered eachother by now.

[user-23911]

- Induction System Tuned Lengths - ( Cylinder Head Port + Manifold Runner )
1st Harmonic= 31.713 (usually this Length is never used)
2nd Harmonic= 17.999 (some Sprint Engines and Factory OEM's w/Injectors)
3rd Harmonic= 12.566 (ProStock or Comp SheetMetal Intake • best overall HP )
4th Harmonic= 9.890 (Single-plane Intakes , less Peak Torque • good HP )
5th Harmonic= 8.025 (Torque is reduced, even though Tuned Length)
6th Harmonic= 6.751 (Torque is reduced, even though Tuned Length)
7th Harmonic= 5.827 (Torque is greatly reduced, even though Tuned Length)
8th Harmonic= 5.125 (Torque is greatly reduced, even though Tuned Length)
Note> 2nd and 3rd Harmonics typically create the most Peak Torque
4th Harmonic is used to package Induction System underneath Hood

Maybe you should be asking about the theory used to determine that the primary length is 31.713?
Then the 3rd at 12.566.......
Obviously wrong just by using a little bit of common sense.


I can give several examples of runners far longer than that on very similar if not identical engines that make far more power than your carbs ever will.
Just ask yourself how long the runners are on a stock VG30 and how much power it makes.
Or rather how much area under the torque curve.......a more realistic figure.
Which determines how quick it goes from A to B.

[digger]

So how long are the VG30 intake manifolds ? How much power ? Looks like peanuts what I recall not that length is a strong function of power....

[BenE64]

- Induction System Tuned Lengths - ( Cylinder Head Port + Manifold Runner )
1st Harmonic= 31.713 (usually this Length is never used)
2nd Harmonic= 17.999 (some Sprint Engines and Factory OEM's w/Injectors)
3rd Harmonic= 12.566 (ProStock or Comp SheetMetal Intake • best overall HP )
4th Harmonic= 9.890 (Single-plane Intakes , less Peak Torque • good HP )
5th Harmonic= 8.025 (Torque is reduced, even though Tuned Length)
6th Harmonic= 6.751 (Torque is reduced, even though Tuned Length)
7th Harmonic= 5.827 (Torque is greatly reduced, even though Tuned Length)
8th Harmonic= 5.125 (Torque is greatly reduced, even though Tuned Length)
Note> 2nd and 3rd Harmonics typically create the most Peak Torque
4th Harmonic is used to package Induction System underneath Hood

Maybe you should be asking about the theory used to determine that the primary length is 31.713?
Then the 3rd at 12.566.......
Obviously wrong just by using a little bit of common sense.


I can give several examples of runners far longer than that on very similar if not identical engines that make far more power than your carbs ever will.
Just ask yourself how long the runners are on a stock VG30 and how much power it makes.
Or rather how much area under the torque curve.......a more realistic figure.
Which determines how quick it goes from A to B.

I posted earlier the lengths of the standard manifold.

From valve to plenum the average was 46cm. On my VG33e. Standard engine is 170hp at 4900rpm. There is nothing after that with out doing something stupid like removing the long restrictive manifold top. There are better VG series manifolds out there. But not currently in my possession.

I think one of the highest hp naturally aspirated vg30 based engine that i have seen had the downdraft webers in the photo i pictured earlier. I have had plenty of people inform me of how shit carbys are thanks. Im happy with the subpar results they will no doubt give. Cheers.