boost question

[CJ_4]

i was wondering how much you can turn up your boost before you need to increase fuel. i know with a gasser you have to increase the fuel for every psi of boost you go up, but diesel's air/fuel ratios are different than gassers, so how would this work? also, i think i remember seeing that 20 psi is about all the stock turbo will put out. is this correct? i have the GM-8 turbo. thanks for any help

[DmaxMaverick]

A GM8 turbo is capable of 20 PSI, but getting it there is more than a matter of just tightening the wastegate. You'll need a ton of fuel to get it there, and anything above ~14 PSI starts to diminish your returns...Efficiency suffers, and starts to take more power to turn the turbo than you will gain. Stock fueling will gain you only about 12 PSI. Boost on a Diesel engine is dependent on fuel. The wastegate just limits the boost to protect the engine, and the ECM will defuel to control the boost and EGT's.

The reality is, a stock engine won't tolerate that much boost. About 12 PSI is max for high performance. More than that, and you'll melt it down. Exhaust back pressure will send your EGT's through the roof. You can build an engine to take that pressure, but you won't get there with a stock engine, stock IP and GM-8.

If you want to get that much boost, you'll have to start with an engine built to take it, and another turbo and fuel system.

[Bnave95]

DMax
What makes my truck different,When I lost the Elec. boost and jump the pump to have full vaccum at the waste gate I was able to max the boost gauge-15*+. Course the Max-e chip might be the difference with fuel rate.

[moondoggie]

Good Day!

"...diesel's air/fuel ratios are different than gassers..." Air/fuel ratio is a term you can forget, when referring to diesel engines. A diesel engine has no throttle plate, so it draws all the air it can, all the time. When she starts to make black smoke, you're starting to get too much fuel; you may also be at a very bad EGT, which for the sake of your engine should be confirmed by an EGT gauge.

Blessings!

[DmaxMaverick]

DMax
What makes my truck different,When I lost the Elec. boost and jump the pump to have full vaccum at the waste gate I was able to max the boost gauge-15*+. Course the Max-e chip might be the difference with fuel rate.

Whatever your maximum pressure was, is your limit with the fuel you were throwing at it.

Which turbo?
What were the conditions? Ambient temp, load, grade, engine temp, EGT, IAT, WMI, speed, RPM, elapsed time, etc.?

[ronniejoe]

With a stock pump, there will definitely be a limit to the amount of boost that you can make. However, increasing the operating boost to that limit is a good idea for several reasons. Number 1, the compressor on the turbo is more efficient than the intake tract of the engine. This is born out by the decrease in egt that accompanies an increase in boost with all other factors unchanged. Number 2, the engine will make more power because its overall efficiency has been increased. This means less power is lost in pumping, etc. Number 3, excess air is generally a good thing with Diesel engines. Some operate with 50% excess air.

The reason there is a limit to the boost you can make with a stock pump is because of the fuel rate. It takes power to drive the compressor. That power is scavenged from the heat and (primarily) pressure (read engergy) that is in the exhaust gases. The energy available to drive the compressor is directly related to the fuel rate and amount of air provided to the cylinder along with the efficiency of the turbine. If you increase the fuel rate, you'll be able to achieve higher boost pressures if you keep the wastegate shut.

There's more to it than this, but this is a good primer.

[CJ_4]

[About 12 PSI is max for high performance.

would this be changed if you get an intercooler and comp. chip? my plans are to run 10 psi until i get and intercooler and chip, then running about 15 psi. would this be okay to run it like this?

one other question, would it be bad on anything on my engine if i just capped the vacuum hose going to the waste gate and made a mechanical waste gate with the spring?

thanks for the advice ya'll already gave

[DmaxMaverick]

[About 12 PSI is max for high performance.

would this be changed if you get an intercooler and comp. chip? my plans are to run 10 psi until i get and intercooler and chip, then running about 15 psi. would this be okay to run it like this?

one other question, would it be bad on anything on my engine if i just capped the vacuum hose going to the waste gate and made a mechanical waste gate with the spring?

thanks for the advice ya'll already gave

Cylinder pressure the stopper. An intercooler may help some, but not to an appreciable level by itself. It won't change the efficiency of the turbo. The turbo as an operating envelope it must be operated within. Once you get outside that envelope, its efficiency is counterproductive.

If you intend to run more than 12 PSI, or tow, a lower compression ratio is necessary. Even if you keep your EGT's in check, you will still have issues with cylinder pressure.

[ronniejoe]

I successfully ran up to 18 psi below 2500 rpm with stock compression. EGT was nice and cool. Above 2500 rpm, the back pressure blows the wastegate open and the compressor operates on a constant speed line on the map. Boost falls off as engine speed increases. The best I could do was 10 psi at 3500 rpm. That is with the vacuum line going straight to the wastegate actuator.

[Bnave95]

With my truck I can pull 10# boost with a light foot,heavy foot goes to 13# with black smoke.This Eng. has never been a non-smoker.:D
Come to think about it,once I got the truck and was stock I did'nt have a smoker. Changed pump and mods,now she a smoker.;)

[CJ_4]

thanks for all the great advice ya'll gave, but does any know about the effects of capping the vaccuum line goin to the waste gate. i tried this once, but i got a fuel leak while trying it, i've heard of other people doing this, so i dont think this is what caused it, but i was just wondering if it could damage any thing

[DmaxMaverick]

thanks for all the great advice ya'll gave, but does any know about the effects of capping the vaccuum line goin to the waste gate. i tried this once, but i got a fuel leak while trying it, i've heard of other people doing this, so i dont think this is what caused it, but i was just wondering if it could damage any thing

No boost. Vacuum is needed to CLOSE the wastegate. Without it, the wastegate will be open all the time.

If you force the wastegate closed (like wiring it shut), you will have issues with overboost, which will lead to other very unpleasant catastrophies. Not a good idea, outside of diagnostic purposes.

[CJ_4]

No boost. Vacuum is needed to CLOSE the wastegate. Without it, the wastegate will be open all the time.

If you force the wastegate closed (like wiring it shut), you will have issues with overboost, which will lead to other very unpleasant catastrophies. Not a good idea, outside of diagnostic purposes.

sry, i didnt make clear what i did. when i capped the vaccuum hose, i used a spring to work the waste gate and took the stock set up off, thus having no need for the hose

[CJ_4]

bump to top

[Patrick m.]

the best performance you will get out of the GM-8 is around 12-14 psi. Go over that and you are pushing it.
I have used other turbos that give better power at much less boost. (you just have to wait a bit for it)

[Scooby]

The spring method is kinda like Mr Heath's Turbomaster, which works very well in my truck, especially with his PCM. Just the Turbomaster alone did bring my EGT's down, but when I added the new computer, the truck came to life. It works well for me in FLorida, but I am sure if I had hills to deal with, I would get impatient and want MORE POWER.