Intake length

[DieselTech11]

I'm very new to forums so I'm now sure if I'm doing this correctly. I have a predator 212 with a dyno cams mod 2 cam, 18# springs, billet rod and flywheel, and I'm interested in getting the GX390 carb from nrracing. My question is that they offer a long and short intake for it and I wasn't sure which would be better.

Any help is greatly appreciated. Thank you.

 

[Stangrcr1]

As with any combustion engine, long intakes tend to promote torque and short for max HP.

Me, I go for torque as that is what actually turns the sprockets, HP is just a number derived from torque....

 

[minidragbike]

I prefer longer

 

[Newoldstock]

Not only does MDB prefer longer but hs engines turn at very high rpm.

Tuning the intake, matching the shape of the port, the right valve and correct carb....

These are all part of a very detailed plan.

I prefer curved.
That way the carb doesn' get knocked off....

I know I am leaving power on the table but not having the optimum set up but it more practical.

Unless you need to race like MDB I think its better to use the intake that fits best with the least restriction and compromise.

 

[minidragbike]

Not only does MDB prefer longer but hs engines turn at very high rpm.

Tuning the intake, matching the shape of the port, the right valve and correct carb....

These are all part of a very detailed plan.

I prefer curved.
That way the carb doesn' get knocked off....

I know I am leaving power on the table but not having the optimum set up but it more practical.

Unless you need to race like MDB I think its better to use the intake that fits best with the least restriction and compromise.

As long as the engine is creating enough velocity, the longer intake works great. Curved even works fine, and used a lot in kart racing.

The stock Briggs WF comes with a 4" long curved intake, works nice.

 

[Stangrcr1]

I am glad to know that the LA drag spot peeps check out some of the other threads on the site... :thumbsup:

And speaking of velocity, how do you keep velocity with massive ports? High rpms.

 

[Cornishrooster]

I use a piece of curved radiator hose and a intake that's cut in half. It's the only way it would fit on the bike. They have a tutorial on affordablegokarts.com.

Cheers
Mjke

 

[65ShelbyClone]

The length of an intake directly affects its resonant frequency. When the intake pulse reflects back into the port, it can help with volumetric efficiency if the wave arrives at the right time. 19-20" is about right for an engine with torque peaking in the 4800rpm range. Since that is impractically long for a minibike, I would go with whatever the longer intake option is.

Me, I go for torque as that is what actually turns the sprockets, HP is just a number derived from torque....

Torque can likewise be calculated from horsepower at a given RPM by changing around the equation. Engine torque is force over distance. Horsepower is force over distance with respect to time.

And speaking of velocity, how do you keep velocity with massive ports? High rpms.

And/or more displacement.

 

[Stangrcr1]

not exactly

The length of an intake directly affects its resonant frequency. When the intake pulse reflects back into the port, it can help with volumetric efficiency if the wave arrives at the right time. 19-20" is about right for an engine with torque peaking in the 4800rpm range. Since that is impractically long for a minibike, I would go with whatever the longer intake option is.



Torque can likewise be calculated from horsepower at a given RPM by changing around the equation. Engine torque is force over distance. Horsepower is force over distance with respect to time.
No. Torque is force around an axis. Here is the Wiki definition.
Force over distance is defined as Work.
Force over distance with respect to time is defined as Power, but not Horsepower.
Your definition of Horsepower is relevant when applied to a horse pulling a load over level ground. For rotational purposes like an engine, Here is the Wiki definition.
HP = (Tq*rpm)/5252
In other words, you have to have torque to calculate HP.
Reversing the equation may be close, but the dyno measures the Torque and calculates HP. The clutch, sprockets, etc are turned by torque.



And/or more displacement.

The displacement I agree with.

 

[graygoose420]

I've been using longer intakes with great results. Don't know the science behind it but I know it works. I went 1/2 an inch longer that the normal Agk . Improved with different setups (different carbs, valve size, piston size , and different stokes)LOL.

 

[65ShelbyClone]

Okay, I misspoke on some of the definition getting work and the ability to do work crossed.

Force over distance with respect to time is defined as Power, but not Horsepower.
Your definition of Horsepower is relevant when applied to a horse pulling a load over level ground. For rotational purposes like an engine, Here is the Wiki definition.

I defined power in general. Horsepower is a unit of power and is therefore defined the exact same way. Like I said, it is force over distance with respect to time. One horsepower is a force of 550lbs applied over a distance of one foot in a time of one second.

Does a rod journal not rotate a linear distance about the crank axis? That's why π, r, and ω are involved in the rotational equation.

HP = (Tq*rpm)/5252
In other words, you have to have torque to calculate HP.
Reversing the equation may be close, but the dyno measures the Torque and calculates HP. The clutch, sprockets, etc are turned by torque.

I said that finding torque from a known HP and RPM number is a simple matter of solving for a variable. I didn't say it was convention to do so when actually measuring engine output. Besides, If you can't backtrack through an equation and get back to where you started, then something is wrong.

 

[cxbra]

I would build my own intake out of radiator hosing like Cornishrooster said. Make 2 of them, a short and long length and give them both a testing. The longer will probably give you a better throttle responce at low/mid rpm.

 

[ole4]

When designing a tuned length intake, if the length is correct the gains at certain rpm's mainly the tuned length can be significant, however at other speeds it can cause reversion at the carb mouth resulting is mixture being expelled out the carb and then re-ingested in again resulting in a double rich mixture and wet air filter. In my motorcycle racing days it was common to have a intake length of about 10.5 inches from mouth of carb to inlet valve. At about 3000 rpm you could see a cloud of mixture at the carb mouth. Motor could not pull thru this in 3rd or fourth gear if you opened the throttle wide open and would eventually gas foul the plug. Once motor hit about 4300 hang on.

 

[graygoose420]

I would build my own intake out of radiator hosing like Cornishrooster said. Make 2 of them, a short and long length and give them both a testing. The longer will probably give you a better throttle responce at low/mid rpm.

I made mine out of a old fork head tube off one of my bikes . 1st one I made was 1/4" longer . Worked well. 2nd one I made was 1/2" longer and worked better. Been making them like that for me and my buddies for a while now. Also port matched the head. That things the same diameter all the way thru till the port starts to tapper down.

 

[FirePowerMinis]

Some very basic calculators:
Intake Runner and Peak Torque Calculator

More sophisticated ones will take cam timing, valve size, etc in to account.

 

[FirePowerMinis]

Sure, a 502 BBC isn't a lawnmower engine, but this is a good chart, showing that tuning for 2-3-4th wave can alter a torque curve, a LOT

How much efficiency could be recovered with better intake tuning?

Back in 1964, Dr. Blair replaced pages of calculations with a simple formula for intake pipe length:

L = (1100 x S)/N

L = Length of intake tract from bellmouth to intake valve seat.

N = Engine r.p.m. in the middle of the normal power range.

S = number of crankshaft degrees after bottom dead center that the intake valve closes (up to a maximum of 75 degrees).

Using this formula on a variety of early '60s engines, Dr. Blair arrived at projected intake tract lengths ranging from 14.3 inches to 22.5 inches.

A more recent method reported by Julian Edgar in 21st Century Performance focused on reflected pulse intervals:

Second pulse length (inches) = 108,000 divided by r.p.m.

Third pulse length (inches) = 97,000 divided by r.p.m.

Fourth pulse length (inches) = 74,000 divided by r.p.m.

Fifth pulse length (inches) = 54,000 divided by r.p.m.

 

[Stangrcr1]

Interesting info. Thanks!

 

[ole4]

that pic is of the original ramchargers guys. They put that engine in a early 59.s dodge with those exhausts. Great posts thanks for sharing.

 

[Phil1958]

very good thread, excellent technical data :thumbsup:

 

[FirePowerMinis]

The 51-59 Hemi's are a favorite of mine. Built quite a few and that's where my username on eBay and forums comes from