30 replies to this topic

[LJ RB30]

LJ, BW78, 3" Stainless over the diff, approx 1" lowered stiff springs. Made system myself from scrap from work, new cat & second hand 3 & 5/16 straight through at rear. Shot of the cat is when it had a Supra 5 speed, now has R33 5speed.

[ls2lxhatch]

I think the statement regarding a single 3" in pipe flows 15% more than a dual system refers to a dual 2" system. When the single 3" system became common the typical dual system was 2".

The following calculations use arbitrary figures comparing 3 m of straight pipe flowing 1500 cfm at 50 psi. No allowance is made for different numbers of bends between the dual and single systems. The cfm for the each dual system pipe is 750 cfm.

This airflow calculator confirms that 3 m of 3" pipe flowing 1500 cfm (0.96 psi) has less pressure drop than 3 m of 2" pipe flowing 750 cfm (1.45 psi).



It also confirms that 3 m of 2.5" pipe flowing 750 cfm (0.54 psi) has considerably less pressure drop than 3 m of 3" pipe flowing 1500 cfm (0.96 psi).



I am sure an engineer will be able to point out errors in the assumptions and explain the situation better but at the end of the day I am also now confident that a 2.5 dual system will have less pressure drop than a single 3" system.

Edited by ls2lxhatch, 09 August 2009 - 02:28 PM.

[ls2lxhatch]

I used arbitrary figures for cfm and temperature, the 3300 kg/h is calculated by the program. The gas used in the calculator is air, the difference between air and exhaust in terms of pressure drop should not be significant.

I only changed the diameter and the cfm for the dual system which should make the pressure drop figures suitable for comparison. The diameters are also arguably wrong as the ID of the pipes is not exactly 2", 2.5" and 3" but again it should be a reasonable comparison.

Edited by ls2lxhatch, 09 August 2009 - 04:26 PM.

[TerrA LX]

I think you want the pressure to drop but the flow rate to remain high if you are using a full system.