Thread: Turbo to intake sizing and exhaust sizing

Plenty of crew on here to help you out on that question I think steveG put up a good thread on exhaust benefits of 3 and 2.5 inch systems. Read that, in summary I think 2.5" if fuel rate is left standard open up the fuel screw and 3" is a benefits. Think about what gains you wish to achieve as there a lot grunt to be had in a isuzu it's more about what meets your needs decide that first and search the threads on here and go from there.

Don't even consider 2" for the exhaust. The least exhaust back pressure you have the better for performance. What garry... said makes fair sense.

For the compressor outlet to manifold, I would use 2".

For the compressor inlet, again the lower the pressure drop the better for performance. I would use the largest that fits between the filter and the compressor inlet and a transition reducer at the compressor.

I'm not considering 2" for exhaust, but was trying to find the value in going from 2.5" to 3" considering the small turbo outlet. But since I've slept on it and read steveG's 2.5" and 3" comparison thread, I think I can answer my own question... Make 3" fit and I'll be happier with the result.

Now I need to stop scratching my head at how little room there is between turbo and vac pump and make it happen.

Originally Posted by Judo

I'm not considering 2" for exhaust, but was trying to find the value in going from 2.5" to 3" considering the small turbo outlet. But since I've slept on it and read steveG's 2.5" and 3" comparison thread, I think I can answer my own question... Make 3" fit and I'll be happier with the result.

Now I need to stop scratching my head at how little room there is between turbo and vac pump and make it happen.

For the bend that connects to the turbo, you do not have to use a complete 90* elbow.

Start with a 90* elbow, but measure the distance that you need to clear the vacuum pump to see where you need to make the cut. You will most likely end up cutting along a line that is not radial from the centre of the bend, and so the shape of the inlet is not a true circle, but that won't make much difference - it may need a little massaging to the flange.

Just confirming you are moving your turbo to the passenger side?

For your clearance.. I can take some measurements if you like.. with my dump pipe I had to go around the manifold and the alt:



It is hard to see from that photo but it actually bends back under the turbo a bit, just enough to go in front of the starter motor. Depending on what manifold you have and how long your turbo is you might have to do something similar.

It seems the turbo you have was fitted to VL commodore turbos and 280/300Zs ???

Can you remove that cast rear exhaust housing or is the wastegate incorporated into it???

I would go 3" for the exhaust and 2" expanding gradually to 2.5" to match the intake.

flagg, that photo makes it look so simple. I went the angled isuzu turbo manifold. I'll let the pics do the talking re: clearance.

Originally Posted by isuzurover

It seems the turbo you have was fitted to VL commodore turbos and 280/300Zs ???

Can you remove that cast rear exhaust housing or is the wastegate incorporated into it???

I would go 3" for the exhaust and 2" expanding gradually to 2.5" to match the intake.

VL commodore you say? If you say so! I wouldn't have a clue!

I have considered removing the rear housing, but yes the wastegate is incorporated and I can only assume I will snap the bolt heads off (they look very corroded) leaving me with an even bigger headache. Unless I know where to source a replacement rear housing with wastegate, I'm not sure what I'm trying to achieve by removing it...

Original plan was to create drawings and have an exhaust shop make stuff for me, but I'm thinking the best idea is to make measuring someone else's problem too... Get it running with no exhaust and drive it to an exhaust shop. (Should run better than a 3" right? ). If I'm not making the exhaust anyway, it's probably best if they can just measure everything in place. I'll just give them a license to be creative to make a 3" fit... I couldn't care less if they have to "massage" it to shape... As long as it's 3" and functional.

I was just concerned they might look at it and say "3" is not possible". Only one way to find out I guess!

Anyway, I still have time to change my mind a few more times... I need to prioritise better. Next job is to source and install a wingless sump cover. Then do some plumbing so I can see if it spins on ignition, THEN exhaust!

Is it possible to move vac pump either up or out from the block with a different belt bit hard to determine from pics

AM