CRD MPG and turbo

o8k · **Posted:** Mon Jul 02, 2007 11:26 pm

ive read through the FAQ here on this subject and i do agree with everything with in it. but i have to say... I believe one of the primary factors keeping the mpg not at its true best is the turbo kicks in too early. i can be putting along at 50mph w/ barly any foot in it and my MAP is up over 20psi. my TDI VW never does that. the turbo wont bust 20psi unless i get into the peddal a lot more than the CRD. i get 30mpg at 40mph, i get 30mpg at 70mph. it gets really bad after that at 75 to 80mph it hangs out at 25mpg (this is not avg mpg its realtime hwy avgs used to est max theoretical fuel econ) flat road ect ect ect. not to hash over old hat but.... i bring this up b/c im measuring max theoretical hwy mpg. its 30~33mpg no matter what what what, till another factor gets involved like drag(see above). only thing i can do to get it to go beyond 30mpg is to get the MAP low low low....I think the computer is over advancing the turbo for some reason. Anyone got any more ideas on this?

Is there a way to turn off the turbo w/ out breaking anything? Or De-tune it somehow? Then turn it back on when i want more power/torque ect.

o8k

CATCRD · **Posted:** Tue Jul 03, 2007 1:08 am

An absolute pressure of 20 psi is only 5 psi of boost above atmospheric pressure. Your relatively large, blunt vehicle needs to be making decent power to lumber along with a relatively small engine. It's not turbo boost that's keeping mpg low, it's drag, mass, rolling resistance and gearing.

Sir Sam · **Posted:** Tue Jul 03, 2007 1:33 am

CATCRD wrote:

An absolute pressure of 20 psi is only 5 psi of boost above atmospheric pressure. Your relatively large, blunt vehicle needs to be making decent power to lumber along with a relatively small engine. It's not turbo boost that's keeping mpg low, it's drag, mass, rolling resistance and gearing.

agreed, we cant do too much about the the first, but we might be able to do something about the gearing. If we could go with a lower rear end ratio you could possibly improve your high speed mpg, and most definatly improve your top end speed.

Though it would be at a loss of some get up and go, as well as around town drivability.

its all about compromise.

DarbyWalters · **Posted:** Tue Jul 03, 2007 12:01 pm

CATCRD wrote:

An absolute pressure of 20 psi is only 5 psi of boost above atmospheric pressure. Your relatively large, blunt vehicle needs to be making decent power to lumber along with a relatively small engine. It's not turbo boost that's keeping mpg low, it's drag, mass, rolling resistance and gearing.

Yep the MAP gives you "absolute pressure" which includes normal unboosted pressure of ~14.7#s.

nursecosmo · **Posted:** Tue Jul 03, 2007 2:25 pm

An active turbocharger actually helps your mpg while the engine is under load. The reason is that the heat in the exhaust gas is wasted energy. the turbocharger captures some of that energy and changes it back to mechanical energy. The injector will only inject a given amount of fuel for a given rpm at a given accelerator position. When more air is available in the combustion chamber from turbo compression it produces more power from getting heated by the combustion even though all the oxygen is not used up in the burn because only a set amount of fuel was injected according to the fuel mapping in the ECM. when more power is needed (ie. passing a gasser on the trail.) more fuel is injected maximizing the air fuel ratio resulting in a perfect burn. This perfect burn can be increased by adding more pressure from the turbo and more fuel from the pump, however there is a point where the heat generated is too great and things start to melt or the head gasket blows etc.. The factory fuel and turbo pressure mapping is far below this point that is why retuning the computer can give such dramatic increases in horsepower and fuel economy.

chadhargis · **Posted:** Tue Jul 03, 2007 3:10 pm

Yep....I'm loving my chip tune from InMotion!

CRD MPG and turbo

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