Seems to be much misconception concerning where to use increased diameter exhaust plumbing for the 6.5LTD trucks.

This, for your consideration.............

The crossover pipe must match the outlet diameter of the driver-side manifold, AND the inlet diameter in the passenger-side manifold.

One guy on here lists a 3"dia crossover pipe - mine's bigger'n yours.............yeah, right!!!!!!!!!!!:badidea:

Exhaust gas energy at low rpm is in heated volume, which creates velocity in the turbine for quick spool-up.

Exhaust gas energy at high rpm is in velocity of increased highly heated volume.

Permitting that volume to expand from 2.25" outlet at the driver-side manifold into a large diameter crossover pipe removes some of the heat energy.

Then, the large diameter crossover pipe exposes more surface area to the road draft under the truck, removing even more heat energy.

Then, the large diameter reduction back down to 2.25" at the passenger-side inlet results in turbulence and reduction in flow - a circumstance commonly known as INCREASED Exhaust Back Pressure. :help:

Yeah - gimme one o' them 3"dia suckers, muffler man!!!!!!!!!!;)

The crossover pipe must match the outlet diameter of the driver-side manifold, AND the inlet diameter in the passenger-side manifold.

Now - the 3" dia down pipe matches the turbine exhaust elbow outlet, and since we have only the exhaust volume from ~3500rpm to flow, the 3" is suitable - just keep the bends smooth, with large radius bends.

Increasing that to 3.5" to 4", and up (mines bigger'n yours!) does not offer any restriction to flow, and can be advantageous where multi-bends are required and real estate is available.

Exhaust pipe diameter, post turbine, is owner's pride....er, choice.

:cool:

You know what I'd like to see, is improved flow design exhaust manifolds. C'mon, all 4 cyls on the driver's side flow through 6&8 on the pass side? There's lots of room for improvement IMHO. Does anyone know what the Banks kit looks like when it was a stock option on the 6.2s?

Exactly true, TDG - limiting factor there is the size and shape of the exhaust ports in the heads.

Absolutely nothing can be done, there.

Those are 135hp exhaust ports, and manifolds

Weep, my children, weep.....

Ahhh, and my biggest dream of all, I would like to see a new cylinder head for these engines, Direct injection, 19:1 compression and 24 or 32 valves... But I don't even know who I could talk to about such a venture, if a conversion kit were available, I'd be first in line!

Talk to yer friendly boneyard operator - mention CUMMINS, an' tell'im I sentcha......:cool:

What about increasing the intake diameter to match all the other diameter's that have been increased. For a 4" exhaust with 2.5" crossover what would the ideal diameter between the filter and turbo and turbo to plenum manifolds? I would think 3"-4"

Port-matching, again, QM - match ducting from compressor outlet to charge-air cooler inlet, match charge-air cooler outlet to engine intake duct, without any large, abrupt increase\decrease in diameter.

2.25" compressor outlet diameter should have ~2.5" tubing to 3" dia c-a cooler inlet.
This keeps outlet velocity up, where expansion into c-a cooler inlet creates less of a pressure drop.

Pressure drop across c-a cooler is then result of temperature drop, not sudden changes in ducting diameter, or small-radius bends.

3" dia c-a cooler outlet can be bell-reduced, plumbed to intake with same 2.5" tubing to 2.25" engine intake, with little loss in efficiency, if engine plenum duct intake diameter is opened up with gradual taper, as on JK's site.

Or, increase engine intake diameter to 3", if real estate permits use of 3" ducting from c-a cooler.

Match air-box outlet ducting to compressor inlet size.

Air-box inlet should not be smaller than compressor inlet - preferably larger.

It's all about maintaining smooth flow, without sudden changes - particularly decreases - in cross-sectional area.

Intake or exhaust - same considerations apply - no decreases, specificaly no sudden decreases, or increase\decrease sizing..

TDG--Would there be any advantage to using a 6.2 header on the driver's side head, then to the crossover pipe? Just trying to think of an option for you comment about the manifolds.

I think the garret I bought has a 3" inlet and 2.75" or 2.5" outlet on the chargeair, and the IC's I acquired have 2.5" inlet/outlets. That should keep it relativly turbulance free, and I almost like the idea of reducing the diameter out of the IC back to the upper plenum. It would make it easier to route and wrap with thermal tape and insulation.

2.5" would work well with the low-rpm low-output 6.5 system.

The funniest thing gm did was put a 2.25" crossover on one bank with the exducer of the turbine wheel being 2.125" for both banks combined. But wait, they did give you guys a 1.25" diameter wastegate to get the other banks worth of exhaust out of, I guess.):h

I thought about tapping the WG pre flapper for a pressure baffle.

sorry i didnt get in on this one. left my two cents at used pmd post where i thought we left off about this. my bad.

If I replace a leaky gasket and add a mandrel bent pipe won't I have more pressure at turbo input ? I don't know what's the deal but as soon as I replaced mine I really felt a lot of difference, steadier spooling and more power from engine.

Would depend on how much the gasket was leaking, and the condition\shape of the original crossover.
Mine, new oem, was fairly smooth with little crinkle.

The small turbine grape commented on was to compensate for low exhaust energy at low rpm - the small turbine gives quick spool-up to get the off-line torque up, to get that towed 15000lb backhoe moving.

It then becomes a restriction at higher rpm, where exhaust energy is greater from increased velocity, increased volume, and increased heat.