why not save yourself a headache and buy the correct kit for your truck?

 

Does that mean I'm wrong? I really don't know, I was just being argumentive.

 

Ummm I dont know, I was just stating that a big diesel turbo would look HUGE under the hood of a pickup. I honestly dont know, I used to work on big diesels (Cummins, Cat, Peterbuilt, Detroit) I know this- For example the Holset turbo that is used on the cummins ISM engine is huge ( all the single setups are for that matter) 5in inlet, 5.5 outlet. I dont remember the a/r ratings but even in a 1000ci diesel they didnt spool instantly. Granted they still spooled at a low rpm ( to us gas guys) there was a decent amount of lag. My thoughts are that if they have that much lag in a 1000ci engine then if you put them on a 408 they might have a hard time spooling. Now yes the diesel does turn low rpm however they flow a **** load of air at a low rpm due to there size. From playing around I found if unloaded the turbo would generally not spool in the first couple of gears due to no load. Ok I guess I blabbered on enough now.....

 

OP - I'm sorry to take this off topic.

Why does the turbo sound like it's working so much harder when there is a load on the motor. It seems that the engine is still turning at the same speed, so it should have the same amount of exhaust coming out as before the load.

Does it take the motor longer reach a shift point, thus giving the turbo more time to spool?

 

That assumption is not off topic, and it is what's wrong with both of your guy's posts. An engine can move a lot more air at the same rpm. For example, you could be going down a slight grade, and have to let off of the gas pedal to keep 2500 rpm. In this situation, your engine could be using actually less air than at a lower engine speed, cruising on flat ground. In another example, you are towing an overloaded trailer, up a steep hill, and you forgot that you left your parking brake on. The engine will only rev to 2500, but instead of making the 15 HP in the first example, it is under full boost, and making whatever it is capable of at that rpm (300?). It would be so easy to tune an engine if airflow was rpm dependent only.

 

When I said loaded and unloaded I was refering to the truck itself, Unloaded being just the tractor. Loaded being the tractor with a 75000lb trailer behind it. When the turbo spools up you are hearing the air being compressed this is why it gets louder. At a given RPM say 1000, with no boost you have less exhaust gas coming out of the engine, once it is in boost there is more exhaust gas. Positive pressure going in equals greater amounts of air coming out. And yes with a load it takes the motor longer to reach a shift point. With no load the engine goes through the RPM range too fast for the turbo to produce enough air to make boost. Once there is a load it slows down the rate at which RPM climbs and allows the turbo to put out more air then the engine can suck in creating positive pressure.

 

Actually, he's correct in using that as a "rule of thumb." What you said doesn't contradict what he said. But, it's pretty apparent that a 6 liter diesel engine at full tilt at 3000 rpms is going to use a lot less "air" than a 6 liter gas engine at full tilt at 6000 rpms. Obviously details like VE can be nit-picked to death, but the rule of thumb still stands.

Diesel turbos are generally much smaller than a gas turbo fitted to the same size engine because the diesel doesn't move nearly as much air as the gas engine due to the operating range of the engines.

Diesel turbos are also generally designed to operate at higher pressure ratios than what would be used on a gas engine. A for-instance is to compare the compressor maps of a T70 and a T76. The T70 is a modified diesel turbo and reaches peak efficiency at a about a 2:1 PR, or 30 psi. The T76 was designed from the outset as a "large" gas engine turbo and thus reaches its peak efficiency at a lower PR, roughly 18 psi if memory serves.

The seals on diesel turbos are generally not up to gas engine exhaust temperatures, too.

On the "higher load" question - higher load means a greater demand for power which in turn means a greater demand for air. This obviously makes for greater exhaust flow and a higher exhaust heat load. That spools the turbo.

 

You are comparing like displacements. A turbo diesel generally has much less power than the same size gas turbo. Would the turbo from a 500 hp diesel be smaller or larger than the one you need for 500 hp in a gas engine?

 

Right now I'm running a hc1w from an older 5.9l cummins in my 5.3l , it seems to boost very quickly and should be very streetable. It still needs to be tuned before I can see the full results. But if you are looking boost in the higher rpms you may need to look for a larger turbo.

 

No clue. My GUESS would be that the cold side would be approximately the same while the hot side for the diesel would be considerably smaller. However, I'll do some research because I'm curious. Diesels typically run way leaner than gas engines so their lbs/mass air to lbs/mass fuel ratio for a given power level are going to be different than gas engines. How different, I don't know - but I'll find out.