velocity stack info

[lance1a]

Anyone know about the effects of size-length of stacks on a carb setup? I ran my Mikuni's with 170mm X 50mm stacks and the low end torque was good but it was breathless above 4000rpm. I removed the stacks and went for filters on the carb body and got loads of revs but little low end torque. I am trying to find a happy medium by using a 130mm x 35mm stack with a 45 degree bend but, the join between the stack and the inlet of the carb has an area of about 30mm where the diameter opens up again to 50mm. Will this cause any negative effects on the airspeed? The inlet manifold from the carb to the head is 150mm x 50mm.
Any help is appreciated!

Cheers

[esselte]

Might get more replies in the engines and drivetrain section

[lance1a]

Please would the mods mind moving this to the drivetrain section? Thanx.

[hilly]

For a basic inlet length calculator look here ......

http://www.bgsoflex.com/intakeln.html

[lance1a]

hilly said:

For a basic inlet length calculator look here ......

http://www.bgsoflex.com/intakeln.html

Cheers, but maybe it's me, I don't get that. Are they talking about the inlet runner as a whole (IE: including the filter-to-carb rams and the length of the carb venturi and inlet manifold together)?


What I would like to know is the diameter as well.
250mm (including bike carb venturi) gets me good torque between 2500/4500 rpm with a 35mm ram into a 36mm venturi and a 40mm inlet. But this loses guts over 4500rpm. When I remove the rams I get a significant reduction in torque but a wider rev range, with more power over 4500rpm.
Is there a calculation based on cubic displacement per cylinder that would give the info about what length and diameter of pipe? The site you linked shows that I would only get a 4th phase pulse at 5000/6000rpm at a very low percent increase. 10% at 10-11k is off the chart for my car.

[350Matt]

lance1a said:

hilly said:

For a basic inlet length calculator look here ......

http://www.bgsoflex.com/intakeln.html

Cheers, but maybe it's me, I don't get that. Are they talking about the inlet runner as a whole (IE: including the filter-to-carb rams and the length of the carb venturi and inlet manifold together)?


What I would like to know is the diameter as well.
250mm (including bike carb venturi) gets me good torque between 2500/4500 rpm with a 35mm ram into a 36mm venturi and a 40mm inlet. But this loses guts over 4500rpm. When I remove the rams I get a significant reduction in torque but a wider rev range, with more power over 4500rpm.
Is there a calculation based on cubic displacement per cylinder that would give the info about what length and diameter of pipe? The site you linked shows that I would only get a 4th phase pulse at 5000/6000rpm at a very low percent increase. 10% at 10-11k is off the chart for my car.

The calc will be for the entire intake runner, from the end of the carb stack / bellmouth to the back of the valve

Matt

[Else]

I don't get it either, by that method i would need an intake length of 25 inches for my 5.2 RV8, max revs 6000rpm.

Computation Results:
Input length is 25 inches
For 2nd harmonic, RPM range is from 4699 to 5702 with a pulse strength of 10 percent
For 3rd harmonic, RPM range is from 3531 to 4035 with a pulse strength of 7 percent
For 4th harmonic, RPM range is from 2753 to 3078 with a pulse strength of 4 percent

I was told that 11 inches is about the optimum for my engine, so which is correct?

Andy

[Else]

anyone?

[rev-erend]

I think you are all confusing the intake where the trumpets end (about 11 inches) and where the intake pulse (from the exhaust) / sonic backflow pressure wave ends in the inlet.. it's normal for at air filter to be at this point and have a solid end plate (like a K & N) for the pulse to hit, reverse and travel back..

[350Matt]

Try this as it takes account for your cam details

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 = 17"


Bear in mind these are very rough calcs to give you a starting point, you then tune it on the dyno if you can

Matt

[knighty]

lance1a - have you ever thought about running half the trumpets at the longer lenghth, and the other half at the shorter length, so say for a 4cylinder engine, cyls 1 & 4 would be the short length, and cylinders 2 & 3 would be the long length, this is exactly how by black-bird bike engine was from the factory, only about a 10mm difference though, and I have seen many V8 can-am cars using this technique too.....it should be worth a try

[lance1a]

knighty said:

lance1a - have you ever thought about running half the trumpets at the longer lenghth, and the other half at the shorter length, so say for a 4cylinder engine, cyls 1 & 4 would be the short length, and cylinders 2 & 3 would be the long length, this is exactly how by black-bird bike engine was from the factory, only about a 10mm difference though, and I have seen many V8 can-am cars using this technique too.....it should be worth a try

I have noticed this on certain bike trumpets. Will it not affect the motor adversely in any way.....balance, fueling etc?

[HarryW]

Aren't the intakes that have different length stacks (trumpets) just compensating for different lengths of intake track beneath the stack . Thus evening out the overall length from bellmouth to the back of the valve (that's what they do on the RV8 injection set up anyway).
Uneven lengths of exhaust primary is commonly done mainly due to packaging problems but can have the side effect of fatting up the torque curve though .

[Steve_T]

HarryW said:

Aren't the intakes that have different length stacks (trumpets) just compensating for different lengths of intake track beneath the stack . Thus evening out the overall length from bellmouth to the back of the valve (that's what they do on the RV8 injection set up anyway).
Uneven lengths of exhaust primary is commonly done mainly due to packaging problems but can have the side effect of fatting up the torque curve though .

But presumably reducing the peak?

[HarryW]

Steve_T said:

HarryW said:

Aren't the intakes that have different length stacks (trumpets) just compensating for different lengths of intake track beneath the stack . Thus evening out the overall length from bellmouth to the back of the valve (that's what they do on the RV8 injection set up anyway).
Uneven lengths of exhaust primary is commonly done mainly due to packaging problems but can have the side effect of fatting up the torque curve though .

But presumably reducing the peak?

I assume we are talking the torque curve here Steve.
Then based on the overall area under the plot is a finite thing specific to your engine. It may take some peak torque out but fatten up either side of it, one of the reasons the RV8 Tivs with there unequal length primaries have the table top torque curve imho.
For a road car a good compromise is to tune intakes and exhaust to different points i.e. intake length for power, exhaust length for torque.

[Mr2Mike]

rev-erend said:

I think you are all confusing the intake where the trumpets end (about 11 inches) and where the intake pulse (from the exhaust) / sonic backflow pressure wave ends in the inlet.. it's normal for at air filter to be at this point and have a solid end plate (like a K & N) for the pulse to hit, reverse and travel back..

The depends on wether you want a negative pressure or positive pressure pulse returning. An open pipe reflects pulses just as well as a flat plate on the end, just the pulse are of opposite "polarity".