WHYTLIE

A guy on another board (RX7/LS1 swap owner and previous Import driver) claims that changing the diameter and length of the intake tubing can create changes in the torque curve and hp numbers and change peak.

This doesn't make sense to me. I figure as long as there is no restriction, it should have zero effect on power. I figure a difference in length from the TB to the intake valves does make a difference, but not prior to the TB.

Am I right or wrong here?

I am also planning on ditching my K&N FIPK and running a custom inlet setup. Don't know if I should go much larger on the intake or just match it to the same size as the MAF and put a large K&N filter on the end to not create a restriction.

Thanks for your time.

Injuneer

The fact that air is flowing through the tube means that the pressure at the engine end of the tube is lower than it is at the air filter end. The more pressure you lose in the intake tube, the less pressure you have in the cylinder = less power made.

Pressure drop is determined by length (increases in direct proportion to length), diamater (changes by -VERY roughly- the square of the square of the diameter), and any bends or sudden changes in cross-sectional area.

So, yes, any change in the intake tube can change the engine's power production. Its not a large amount, IF the system was designed large enough to begin with. But surely you can see that if you put a 1" rubber hose in place of the 3" plastic tube, the engine will lose power.

WHYTLIE

Surely I can, that is why I stated that if the tubing was not a restriction, would changing the length or size make any power difference or change the torque curve or peaks?

Say going from a 3.5" inlet that is 3 feet in length to 4" inlet that is 4 feet in lenght? Or a 4" inlet that is 2 feet in length, etc?

Brian

1racerdude

The K&N air intake will flow well over 1300CFM. Ya need to get the 12" long filter to flow with it and a 1300 mono blade TB and ya will be set.
Takes a little work to fit the fender but it will fit.

Injuneer

But, BY DEFINITION, the tubing is a restriction. The fact that air is flowing through it causes a loss of pressure. Make it large enough, or short enough, and it will become an insignificant restriction, but it is still causing a pressure loss, and reducing the density of the air in the cylinders = less power. Making a piece of tubing twice as long will double the pressure loss. Adding bends to the tubing will increase the pressure loss.
Making the diameter larger decreases the pressure loss, and making it longer adds some back in. Just playing with numbers, increasing the diameter from 3.5->4.0" decreased the pressure loss by about 25%. Changing the length from 3ft->4ft increased the pressure drop by 33%. So, your larger, longer pipe has more pressure drop than the smaller diameter, shorter one. (These are "rough" numbers for illustrational purposes only).

If you increase from 3.5"->4", and shorten the length from 3ft->2ft, you will reduce the pressure loss.

What you really need to look at is the magnitude of the pressure loss. You have 14.7psi available at the air filter to push air into the cylinder as the piston drops. If the ducting is costing you 1psi pressure loss, its reducing the potential power by 6.5%. If your intake is already so efficient that it is only costing you 0.1psi pressure loss, you are losing less than 1% of the power potential, so minor changes in diameter or lenght won't we visible.