Exhaust back pressure - Diesel Truck Forum - TheDieselGarage.com
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post #1 of 3 (permalink) Old 03-26-2010, 02:06 AM Thread Starter
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Exhaust back pressure

Ok. I would like someone to do a little teaching for me.

I use to think exhaust back pressure was the restriction of exhaust flow, out of the truck. The only way to change that was to change turbos, downpipe, echaust itself, ect. To lower the exhaust back pressure you needed to make the system more free flowing.

Now I also understand how the turbo puts backpressure on the motor in relation to boost. More boost more back pressure. You want the most (or right amount) of boost with the lowest back pressure. With in reason.

Now heres where Im lost.

I have been running the Spartan 250 for a little while now. I have been super happy with the tune. It does everything I wanted. EBP from 10-20lbs of boost is around double the boost. From there up the ratio goes down. That seems normal to me.

I decided to try a few other tunes sinse I have soo many there, and with my towing season right around the corner, figured I would try a littel smaller tune so it would be "easier" on the motor.

Loaded up the 210 tune. all around good tune. Ran good, shift well, little sloppy now and then. I did not give it a ton on driving before swapping it out. The EBP was much higher at part throttle than the 250 tune. at 10-20lbs of boost, The EBP was 3 times the boost. Closer to double as the boost went up. This made the motor seem to work much harder and use more throttle just to normally take off from a light. It was also sucking down the gas much faster. I would guess 2-3 mpg less. I did not run a full tank through to check.

I was thinking it was just that tune, I tried the 210 tow tune with the same results, just firmer and quicker (lower rpm) shifts. Which I liked.

So what is really confusing me is how a tune can change the EBP? And if it can, why would they not make all the tunes with lower back pressure? Which tune is really easier on the motor? The 250 tune runs hotter on the exhaust temp, but the 210 tune really seems to work the motor more. So if the 250 tune is cool enough to haul my trailer. It seems that would be the tune to run.

I did also notice the exhuast hiss was much less with the 250 tune with a lot more rumble. Sounds much better.

New Ride 08 F-250 CC SB FX-4 6.4 auto loaded job 1, Spartan 310,RCD 485,525, Erics Xstreet,KEM250 DPF delete,downpipe, intercooler pipe, 4" mbrp, timbrens

04 F-350 6.0,auto,SC,SB, 8ft Fisher X-Blade, Hideaway Strobes, XM Radio,Alpine Head unit, X-codes, Edge Mini spring pack, Edge adjustable track bar, 315/75/16 Toyo A/t,Prodigy brake controler,MBRP 3.5" downpipe,5"stacks, powermaxx turbo, Xrace tuned

Old truck-3/27/03 build F-350,CC,SB,DRW 6.0, Auto, Black on black, Blacked out bumpers/ grill, Moon roof, fifth wheel.

old sick, 1/7/03 build
2003 F-350,SC,SB,6.0, 4" lift 37 MT/R
http://community.webshots.com/user/traxx400-date
http://s601.photobucket.com/albums/t...0Ford%20F-250/
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post #2 of 3 (permalink) Old 03-27-2010, 03:06 AM
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Two things, youre not comparing apples to apples with ebp vs mgp or manifold gauge pressure, exhaust back presssure is measured in absolute or 14.7 higher than gauge pressure at sea level, so for all intents and purposes, subtract 15 psi from exhaust pressure at all times. Furthermore if you want apples to apples you can watch map or manifold absolute pressure thus all is equal comparatively, but it tops out at some 37 psi or 22 psi gauge. The tune changes the back pressure by changing the settings of VGT vanes, that is they close tighter to make turbo spin up faster and to higher pressure in some tunes, than in others, generally speaking the higher the tune the more aggressive it is but not in all cases. If you floor your truck off idle and btw it idles at 90 some pct duty cycle, it goes to max duty cycle, cant remember number pry 99 then as boost comes up closer to desired ecu setpoint the vgt duty cycle is backed down which lowers ebp and moves vgt duty cycle ever lower till it hits about 7% the minimum, at this duty cycle the exhaust is pretty wide open through turbos, maximizing exhaust flow, and lowering sound giving that v8 sound again. you can teach yourself all about it by monitoring these things...

Ben
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post #3 of 3 (permalink) Old 03-27-2010, 05:11 AM Thread Starter
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I guess everything you said makes perfect sense. More or less what I was thinking with a lot more technical typing. I guess I am just making more general comparison then really specific.

What I do know is the high part throttle back pressure of the 210 tune really sucks down the fuel and makes the truck feel like its straining to move. Is there any reason to do this?

I guess i just like knowing why my truck does what it does and how the tuning effects it. One of these days maybe I will just get in and drive.

New Ride 08 F-250 CC SB FX-4 6.4 auto loaded job 1, Spartan 310,RCD 485,525, Erics Xstreet,KEM250 DPF delete,downpipe, intercooler pipe, 4" mbrp, timbrens

04 F-350 6.0,auto,SC,SB, 8ft Fisher X-Blade, Hideaway Strobes, XM Radio,Alpine Head unit, X-codes, Edge Mini spring pack, Edge adjustable track bar, 315/75/16 Toyo A/t,Prodigy brake controler,MBRP 3.5" downpipe,5"stacks, powermaxx turbo, Xrace tuned

Old truck-3/27/03 build F-350,CC,SB,DRW 6.0, Auto, Black on black, Blacked out bumpers/ grill, Moon roof, fifth wheel.

old sick, 1/7/03 build
2003 F-350,SC,SB,6.0, 4" lift 37 MT/R
http://community.webshots.com/user/traxx400-date
http://s601.photobucket.com/albums/t...0Ford%20F-250/
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