Inlet manifold runner length calcs

[John Dixon]

Wondering if anyone has calculations for estimating the required runner length for an independant throttle body setup? Found plenty of stuff for exhaust but inlet seems harder to come by.

[chassis 33]

Try getting a copy of Design and Simulation of Four-Stroke Engines, by Gordon Blair (assuming its for a 4stroke, but he also does a 2stroke book)

Also www.billzilla.org/engineintro.htm

and there is another one reccomended to me by a mate who did this as his dissertation at uni, but i cant remembre for the life of me what its called, something like design of engine manifolds by someone Bone, its the bone bit that stuck in my head, os i had a bit of a schoolboy giggle about it!

(edited to add...www.grapeaperacing.com/GrapeApeRacing/tech/inductionsystems.pdf)

Hope that helps?

Regards
Iain

>> Edited by chassis 33 on Sunday 2nd October 22:14

[aldi]

There's a thread here on that very subject, seems to get good about 1/2 way through...

[GavinPearson]

You're better off working it out by volume, then the length comes as a function of that.

Basically, if you want best top end power then you want 3xEngine Capacity as the inlet manifold volume. If you actually want driveability at the bottom end, make that more like twice capacity.

For a 2 litre 4 cyl engine an inlet manifold running 50 mm throttle bodies has an inlet area of 4 x PI x 25 x 25 = 4 x PI x 625 = 2500 PI = 7850 mm 2.

That equates to a single throttle of 50 mm, so that needs to be packaged - you effectively need a plenum around 60 mm in diameter the length of the engine, then the runners in the engine need to be accounted for, then the bodies you are using likewise need accounting, for, so the rest is a matter of simple maths.


>> Edited by GavinPearson on Monday 3rd October 04:49

[350matt]

A little something I prepared earlier... Matt
Exhaust and intake length tuning

You will also need to determine the capacity of your engine, by using the number of cylinders and the capacity of each. To discover the best length and diameter of pipe for your engine, you will need to know the exhaust valve timing; that is the number of degrees before bottom dead centre that the valve starts to open. with this knowledge you can then set about making a tuned exhaust system by working out the following formula and applying it to your own engine:
850 (180 + N)
L =(------------- )- P
RPM
where L = length of pipe in inches
N = degree before BDC exhaust valve opens
P = distance from exhaust valve to manifold
and RPM = desired RPM For peak power

The diameter of the pipe needs to be calculated so that the volume of the exhaust pipe attached to each cylinder is twice the volume of each cylinder. The exact diameter of the pipe, incidentally, is not critical and should only be used as a guide in determining which standard sized pipe diameter should be used.
As a guide, this is example calculation. If the exhaust valve opens 110 degrees before BDC. The engine capacity was 700 cc per cylinder and the desired peak power RPM is 6,500

850 (180 + 110)
L =(--------------) - 3 in = 887 cm = 34.9 inches.
6,500
To find the diameter of pipe, the following formula can be used:
π r^2 L = 2 * 700cc
π r^2 * 223 cm = 1400cc
r = 1.4 cm
Diameter = 2.8 cm or 1.1 in
The nearest standard stainless steel tube available was 1.5” Ø.
If a primary pipe is oversize the consequences are far less severe than being undersize and unless a pipe is massively oversized it will have little detrimental effect.
To reduce the weight and complexity of the finished system, the exhaust pipe can be of a two-into-one type, with the joint at half length. The single pipe then finishes the required length and then the two pipes can then be joined together into a single collector and the exhaust silencer is then fitted onto the end of this single pipe. Typically this will not produce as much power as a well designed 4 into 1 system but is often far more convenient to package


intake RUNNER length ii
ECD = Effective Cam Duration
RV = Reflective Value = 2 for 2nd reflection 3 for 3rd etc – the earlier the reflection = the stronger the tuning
ECD = 720 - (Adv. duration - 30)
Speed of sound = 396m/sec
intake runner length = ((ECD/360) × (1/(tuned rpm /60))* speed of sound)/(2*RV)
( ((470/360) x (1/6000 /60))*396) /(2 *6) = 0.43mtrs




All of the above calculations will get you somewhere close to the perfect length ie within an inch on the exhaust, but this can only be confirmed by dyno testing, consequently these numbers are a rough guide only.

[John Dixon]

Thanks guys, some food for thought there. Have always planned on doing some dyno time to tune the intake once the engine is built anyway, but nice to know roughly what's needed as it'll dictate the position / type of air boxes to some extent.