What

[TurboDiverArt]

Hi All,

Lately in the past month or so I have noticed that I have lower boost at cruise. I cruise for 30 miles at a clip on the same road every day at 70 MPH. My Suburban has stock size tires and 3:73 gearing. I haven

[rjschoolcraft]

When in stock trim, my Suburban produced ZERO boost at 75 mph with 4.10 gears. At 70, it was around 2-3 psi.

BTW, your description of boost falling off at higher rpm is normal and is a function of the programming. The wastegate duty cycle is reduced allowing the wastegate to blow open and reduce boost pressure. Your description matches the way mine used to function almost perfectly.

[TurboDiverArt]

Thanks Ron! At least nothing is wrong. I would swear though that I never had zero boost at 75 though.

Art.

[charliepeterson]

You don't need any boost at 75mph unless the engine is working (ie. hills, fully loaded). If you had a good reader on the truck and watched the fuel rate, all the parameters would be in spec. The engine is getting all the air it needs for what it's doing. When you add more fuel the boost will come up to match it.

I can cruise at 70mph on a flat road and my boost is flat too. When I drop the hammer, the transmission drops down, fuel rate and boost climb quickly as well as the speed!

[rjschoolcraft]

With my current modification, I run about 5 psi boost at 75 mph solo. EGT is much lower than with 0 psi. ;)

[TJ]

I'm running about 8 lbs with my turbo master and high revs at 75. EGT's are nice here....

TJ

[TurboDiverArt]

You're leading into my next question. When I had a vacuum leak I would cruise at 0 boost all the time. I noticed higher EGT's. Once I fixed the vacuum leak my EGT's dropped a little.

If you cruise under boost, wouldn't you get better mileage? Just wondering if at say 70 MPH, would you get better mileage at 5-6 psi vs. 2.5?

I've considered putting on a turbo master and have it set to 8-9psi. I can't stand the fact that the boost drops off above 3000psi, especially when towing, Yugo's pass you by...

Art.

[Turbine Doc]

Turbo Master is a good mod, but without reflash you may need to fool PCM boost input with either a kitted fooler such as JK sells or build one yourself as detailed in the reference area. PCM is expecting a given boost level for a given rpm may set a code if you exceed the expected range, also don't max out TM setting without WMI or IC cooling as then you might get up against Hi IAT limit.

[markrinker]

I run about 5-6 lbs under the same speeds described above. EGTs with no load are around 500. If I dialed the Turbo-Master down to 0 boost, would expect 650 EGTS.

Not sure if it saves fuel...

[TurboDiverArt]

Originally posted by Mark Rinker:
I run about 5-6 lbs under the same speeds described above. EGTs with no load are around 500. If I dialed the Turbo-Master down to 0 boost, would expect 650 EGTS.

Not sure if it saves fuel... When you cranked your boost up, did you also add fuel? Your 94 has an electronic IP, right? I know you twist the IP to change the injection timing. I read mention about cranking up a screw to add fuel. Is this

[rjschoolcraft]

The "screw" is on the mechanical pumps. Fuel is increased on the the electronic pumps through programming. There are other ways, but less practical.

Increasing the boost will help the truck make more power even without adding fuel. It improves the volumetric efficiency of the engine. EGT will be much lower. This is true to a point without intercooling, but even more so with intercooling. I've experienced the benifit of increased boost with and without intercooling.

[Spindrift]

Ditto what RJ said, plus...

Prior to my new programming, I used the TM to get me sustained boost of 7-8 psi. Any boost higher than that and I would throw a code.