Responsive compounds AND low EGT's while towing/climbing hills at high altitude.

[Big Bore]

A few weeks ago I was talking to Curtis after monitoring my EGT's climbing a VERY steep grade at around 9000ft elevation that has a speed limit of 50mph, which with 3:73's puts my RPM's very low in OD (I think it was 1750) which in turn will start bringing the EGT's up even running empty. Boost at that speed is only 5psi, but it's not lagging at all, I can accelerate at will with minimal throttle input. If I run the same grade at 60mph, EGT's are not a problem and I start hitting that sweet spot. Tunes are not the issue at all, even at the low RPM I need only about 1/3 throttle and there is no smoke. As an aside, being able to hold that RPM in OD on that grade is really a testament to Curtis tuning abilities. I am constantly amazed at how well this truck shifts and drives. I use the same tune (HOT) for everything, and the way everything is scaled the shifting responds to the load and throttle input seamlessly whether empty or loaded with 5 cords of wood going over Hoosier pass on switchbacks. All this with Swamps 175/30's. Bigger nozzles would definitely make a huge difference as proven by Curtis, and that is definitely coming soon.

Anyway, the discussion shifted to compounds. It has always been my impression that compounds allow you to keep EGT's down in a situation such as the one I'm describing because the atmo would be pre-stuffing the drive turbo, making up for the lower atmospheric pressure at high altitude, and that at 1750 rpm the 38R is still making some boost and could spool the atmo without creating lag. Curtis was concerned there would be more lag than with a single because the atmo would be causing the drive turbo to lag while trying to spool the atmo.

This is exactly why I keep going back the idea of a 4202, or an S series equivalent (DzChey tried to steer me towards an S series awhile back and I'm coming around to the idea) with a very loose exhaust housing as an atmo. I've been going through Cheyenne couple times a month, I need to stop by Dz's and buy him lunch and pick his brain I think.


In any event, I thought compounds were a way to help prevent lag and the accompanying high EGT's, not cause it?

 

[Hotrodtractor]

Your EGT issues at a lower speed is simply because of turbo efficiency under that load and engine RPMs - since you are running a fixed geometry turbo (a little push because of the other thread LOL) you need to change your operating conditions - either travel faster - or travel in a different gear (downshift). And your problems will go away with your setup.

While it is true that adding an atmosphere will slow down the spooling of an existing turbo - it should also be noted that the atmosphere turbo could and should start spooling and start feeding the high pressure turbo with air so the net result is lower EGTs, faster overall spoolup, etc... but there are also many setups out there that do not work as they should or were not set up to operate in this fashion.

I often see my EGTs DROP going up a hill with a trailer on the back as the extra load can shift the operating range of the turbos around and it actually can get more efficient - but I also have the additional variable of the VGT.

 

[Big Bore]

Yes, as soon as I downshift the EGT's drop. I just prefer the lower rpm's.

I should add that the EGT's were 1250 at 50mph/1750rpm/5psi/8500ft elevation/6% grade. I don't think that's bad at all. I would just like to see better. (I said 9000ft earlier, the grade starts at 7750 and climbs 900ft to roughly 8650)

The turbo is not lagging much if at all, I can easily accelerate without downshifting and without smoking much if any, depending on how deep I go in the throttle.

 

[TARM]

With a load on that grade that is low rpms IMO I can understand why EGTS are up.

 

[Big Bore]

With a load on that grade that is low rpms IMO I can understand why EGTS are up.

That was empty.

 

[TARM]

Ok that makes more sense. I was thinking damn what is he worrying about he is lugging the crap out of that thing LOL

 

[Big Bore]

Ok that makes more sense. I was thinking damn what is he worrying about he is lugging the crap out of that thing LOL

lol yea, and it's an auto, right on the verge of downshifting but was holding OD nicely with no smoke. Oh well, I'm off to find out about VGT's.

 

[lincolnlocker]

Your EGT issues at a lower speed is simply because of turbo efficiency under that load and engine RPMs - since you are running a fixed geometry turbo (a little push because of the other thread LOL) you need to change your operating conditions - either travel faster - or travel in a different gear (downshift). And your problems will go away with your setup.

While it is true that adding an atmosphere will slow down the spooling of an existing turbo - it should also be noted that the atmosphere turbo could and should start spooling and start feeding the high pressure turbo with air so the net result is lower EGTs, faster overall spoolup, etc... but there are also many setups out there that do not work as they should or were not set up to operate in this fashion.

I often see my EGTs DROP going up a hill with a trailer on the back as the extra load can shift the operating range of the turbos around and it actually can get more efficient - but I also have the additional variable of the VGT.

are you saying to use the vgt as a the high pressure or atmo turbo?

 

[TARM]

High pressure

 

[Hotrodtractor]

are you saying to use the vgt as a the high pressure or atmo turbo?

Or as a single even.

 

[TARM]

I would not mind running one as a single but I would want it a bit larger. Something in the gt42 range . They are some performance VGT turbos out there aren't there?

 

[Big Bore]

Or as a single even.

Thats where I'm going first. When I can have spool up better than my 38r, in a 4294 turbo, I'm sold. Once that's dialed in then I'll start looking into compounds again. Jason do you know what size the stock Duramax turbo is, or how does it compare to a 38R?

 

[Big Bore]

I would not mind running one as a single but I would want it a bit larger. Something in the gt42 range . They are some performance VGT turbos out there aren't there?

Yes, 4294, I posted a pic and stats in the other thread.

 

[Hotrodtractor]

Thats where I'm going first. When I can have spool up better than my 38r, in a 4294 turbo, I'm sold. Once that's dialed in then I'll start looking into compounds again. Jason do you know what size the stock Duramax turbo is, or how does it compare to a 38R?

Off the top of my head the stock Dmax is a 66mm or so (could be slightly off - its been a while) - the one in the pictures I posted is a 72mm charger.....

 

[TARM]

Then the one in the pic if its 72mm is just about ideal for what I would want. What was the make and model for that turbo? What do they tend to run $$?

 

[Hotrodtractor]

Then the one in the pic if its 72mm is just about ideal for what I would want. What was the make and model for that turbo? What do they tend to run $$?

Its a newly released custom built turbo available through Danville Performance - its a bit spendy compared to other chargers - and I would highly recommend getting the setup running with a stock charger before thinking about that route.

 

[Big Bore]

Off the top of my head the stock Dmax is a 66mm or so..

Thats compressor inducer correct? Which would put it at same size as a 38R. I might try one of those first to mock things up and test on the truck, looking around on Ebay they are pretty cheap. If I can talk Jake into making a mount I will probably have to send it to him.

Just did some digging, stock Duramax compressor is 88mm (exducer?). 38r is 88mm exducer/66mm inducer/56 trim

 

[TARM]

That makes sense. Doubt they are much more than the gtx42 I am running now. But still I would hate to get it and then find the VGT solution is not working out for me the way I hoped.

I would think that would make one hell of a manifold charger for a large system and yet still give you a decent bottom end too boot. A 72 with say a gt55 or s500 imp: I had already had thoughts of doing something like that with my current gtx4294 as even with the 1.15 I have been quite happy with its bottom end. Been looking at a slew of comp and turbine maps for the last year kicking things around.

So maybe I will try out a stocker 66mm on an extra truck first and see how it goes.