A larger downpipe won't help much, IMO, if the small turbine is the bottleneck. At these low power levels (<200hp) I really don't think you're going to get any more gains by modifying the exhaust system. The middle muffler is already a straight-through affair, you'll probably get zero extra flow by removing it - just save a few pounds of weight.

I find it interesting that peak EGT's didn't change much with lower boost. Perhaps the rate of EGT increase is slower with less boost? Hmmm. If you're not getting smoke at WOT with less boost, then the engine obviously doesn't need the extra air to make the power. Excess pressure means higher IAT temps and a little extra backpressure too, but perhaps not enough to make a huge difference (maybe if you tried to run 20psi+ it would).

The Finns mentioned keeping exhaust backpressure, pre-turbo, to no more than twice the boost pressure. Even the factory service manual says pressure should be no more than 30psi (stock boost spec is 12-14psi for an OM603), and although this was for testing the trap oxidizer, the principle remains the same. I'd love to test my pre-turbo backpressure to see what it is but I don't have the tools to do so. The only cure is a larger turbo, AFAIK... which will have the drawback of extra lag and slower spool-up off-idle.

Decisions, decisions...

:o

For anyone that has removed their EGR completely, that blocked off tube in the exhaust manifold is a nice spot to tap for pressure (tap into whatever you used to block the tube off).

It would be intersting to check it on a stock setup...I guess I could move my 0-30psi boost gauge over to the block-off.

:)

Great info guys...glad to see this thread is sparking some action. :)

I'll be adding a bung for an exhaust pressure gauge the same time I tap for the pyrometer.

I'll be using a KKK turbo so I guess it won't translate over well to you Garrett folk.

I wouldn't be so quick to assume that....why?

Becasue the T3 is the turbo used on the more heavily modified hondas...(1.6 liter) and even on the T-25 which is smaller the recomended system size is 3" and thats on a engine 1/2 the size.....even a 2.5" on that makes a measurible reduction in spooling and total power. Now before you say apples and oranges a lot of these hondas that run this have redlines under 8K and are 1/2 the displacement our diesels are.

Quick to assume what - the lag issue? Or the exhaust issue? :confused:

If the outlet of the turbo is only 2.25" (which I think it is - can't remember) and you attach a 4 inch pipe to it, you're still stuck with a 2.25" restriction no matter what you do with the downpipe. We've already had multiple people remove their exhaust restrictions (cat, resonator, and/or muffler) with NO increase in power, just a slight reduction in EGT (if anything). Until someone can prove a tangible power gain, I'm personally not going to chase down exhaust mods as long as I'm under ~200hp (translation - with a stock injection pump).

I'd be more than happy to be proved wrong, though!! Anyone willing to fab a 3" pipe onto a stock turbo and see what happens?

:o

Until and unless I replace the insanely restrictive and convoluted stock exhaust manifold on there, I ain't bothering to modify much of anything in the exhaust stream. IMHO, the entire exhaust from the head back needs to be replaced in order to get the beast breathing right. I don't think a larger turbo is gonna be worth the effort unless it's bolted to a less restrictive custom header, and would need a larger ID pipe behind it anyway--big job. I'll be doing this when I upgrade the pump to run larger elements--> big job = big bucks = big fun.

you will have a higher velocity at the turbo....which right at the point the pipe increaces the backpressure that is detrimental to the turbo is reduced.....you can run a turbo engine with no exhaust with no damage or performance hit to the engine...

the problem is to the driver.

want to talk with a LOT of people who have proven a 3" exhast from the turbo back works great on a 1.6 engine with a T-3 here is a link...the theory is no different with an engine twice its size.

http://www.honda-tech.com/zeroforum?id=16

Agreed. There's room for massive improvements (the Finns proved that beyond dispute!) but most of the changes have to be done in unison to realize the power gains.

Ah yes, but what size was the stock Honduh exhaust? Did they already have 2.25" all the way back, and got power gains going from 2.25 to 3.0? A simple test would be to connect a pressure gauge to a fitting on the downpipe and see what kind of backpressure there is in the exaust system. I'd bet it's very low in our stock diesels with modified exhausts.

:cool:

Yes they did get gains going from 2.25 mandrel to 3" mandrel...downpipe back...not just cat back...

Peak HP up in the low double digit range and quicker spooling....try a search on that site....its to honda tuners what this site is to Benz owners...one of the best.

On the WRX I owned, it had a turbo smaller than these stock. It had a 2.25" stock downpipe, very lame. I threw on a 3" unit (just the DP not catback as I did that prior) and picked up over 22whp.

FWIW - Holset HX35 and HX40s only have a 3" outlet yet run 4-5" exhaust systems.

603 or 617?

As you know turbo motors need high pressure on the intake side of the exhaust side and low pressure on the exhaust side. Small inlet pipe equates to higher inlet velocity. Most every diesel motor I've seen can flat out haul with a stock manifold(s)... sure you can gain more from porting it or a tubular header but it's the last place I'd look for power.

Good point...we are actually talking about two different engines intermixed on this thread....may not be easy for everyone to know specificly who is refering to what engine...

H2O2 is dealing with the 603 , I am dealing with the 617....

Well, I am observing something interesting. When I ajusted the full load 1.5 turns the alda needed to be adjusted because it was really smoking people out when the pedal was mashed. So I backed down the alda a turn and the smoking went away. There was no changes in my 0-60 and boost was at 15psi. I decided to turn the alda up 1/2 turn to see if it smoked much. It did but not much.

Here is where it gets interesting. There is a 1psi drop in second gear at full pedal then goes back to 15psi upon shifting to third and then stays at 15psi.

Then I was driving 3/4 of the way into the pedal and I noticed that the boost never dropped at any time. It only drops when the pedal is to the floor. With the pedal mashed and at 3/4 this can be reproduced over an over again.

It seems as if the extra fuel is affecting the boost pressure.

Any comments/suggestions?

I'm going to experiment with dropping boost a few psi.....I am fine until outside temps hit 90 degrees F....then my EGT starts to soar.....