Hey Everyone,

So I was talkin to StrokinIt the other night about turbo boost, and getting those PSI's to around 30 pounds.... I was looking at my guage while half asleep on the way to work this morning and started to wonder...How much boost is too much boost, and, where's the normal range?

Mine usually never gets above 15PSI, even towing on passes (here in CO there are lots of them, and steep). Is that good, bad? And EGT's, on that same token. Being that I have always learned and been taught that less is more, I am not so sure now. IS it a good thing that I show low PSI and EGT's while haulin *** and towin?

On another note: my turbo is extremely loud. Sounds like a leer jet. I had read on some of the other threads that early '99 models came with really loud turbos, is this true? I don't know anything about turbo's beyond where it is and what it looks like. Is it possible I have an aftermarket?

Sorry for the lame post and all of the questions, just trying to undersand this truck. Some thing's the manual and magazine can't tell you.

Thanks all!

 

On the stock turbo, most folks believe that 25 psi is a good upper end number to go by. Above that, it's mainly just adding heat which is bad for the turbo, and doesn't do anything for power. If you mod the turbo with an ATS housing, that number can probably be bumped up to about 30. Aftermarket turbos are another story, and often can go much higher and still be efficient. And as far as too much boost being hard on the engine... that's really unknown too. There's not one set number, as there are so many factors that contribute to damaging the engine. Just FYI there's one 7.3L PSD from Utah that gets to around 100 psi... but it's nowhere close to stock.

What you said about less is more is true in some cases. Say, if you go from the stock turbo to a van turbo... due to the new turbo's characteristics, you'll actually run a bit less boost, but more power/more air being moved.

And yes, the early 99 turbos had a different impeller (compressor/intake side) wheel than the late 99+ models. That early 99 wheel is sold aftermarket as the "wicked wheel" and is a more efficient wheel than the later version, in that it has a better compressor MAP as far as the surge line goes. In other words, it's less likely to surge, and that's one of the main reasons folks add them to the later year trucks... to help combat surge. That wheel also has a side effect of being pretty darn loud with whine/whistle. Furthermore, the early 99s didn't have the foil sound insulation wrap on the hot side intercooler pipe, which was added on later trucks to help deaden that loud turbo noise.

Hope that helps some. Don't be afraid to use the search function, too, as there's a TON of good info in past discussions to be learned from.

PS - it's about 2 am so if anyone comes along and sees any errors I've made please point them out.

 

that all looks good to me, Ryan.

rule of thumb is not to exceed 40psi on stock headbolts.

Lisa i WANT your avatar.

 

Thanks Ryan, very helpful. It confirms everything i've read, but no one has actually said (from what ive read) how much boost is too much boost) I was also wondering if I am pulling too little boost? But I am rarely going over 15 PSI and haulin *** so I guess I wont worry about it.

Kris - I made that avatar. Want one? Anything particular you want it to say?

 

I just pulled a 20ft TT to Oregon from Indiana. On long grades with a DP 40hp Tow tune, 4" Turbo Back Straight Piped, and 6637, I get 15 pounds boost pushing it and holding at 1100 egt. Any more boost and the pssssss sound breaks up and turns to a thss thss thss. It doesn't sound good.

 

Sorry, double post.

 

The infamous turbo stall. Get a WW and it will completly clear up with the setup you have.

 

Yep, that's what I figured. I was thinking BB Turbo this January

 

Even better!

 

Okidokey, not. I need some schooling on this. If your boost gauge is mounted let's say at the AIH delete plug, and you change no other plumbing besides the turbo, then does it not stand to reason that the only way to move more air through the same pipe will result in higher boost?

I don't want to get into wastegates and ARs and efficiency #s, just the numbers concerning actual boost. Seems to me the only way to get more air and less boost through the same pipes is with a cooler charge. Where am I missing this?

 

That's right, the boost number may be less, but there is MORE air, as it's denser air. Same thing with why our stock turbos in stock form, above 25 psi, may have higher numbers of boost, but it's mainly real hot air, not dense at all (and bad for the turbo).

This doesn't mean that higher boost numbers don't always mean more air, because often they do, but there's not really a linear relationship between the boost number and the actual volume of air that's flowing.

 

I think the point is that higher boost pressure might be a feedback result of the exhaust restriction caused be the less efficient stock turbo. More manifold pressure does not necessairly mean mor power, it might indicate a restriction problem farther down stream.

 

Huh?????????

 

The drive (exhaust) side of the turbo creates exhaust backpressure. That's what drives the turbo. The less efficient turbo creates more exhaust backpressure, which in turn leaves more "residual" pressure in the cylinder after the exhaust stroke, which will allow less intake air into the cylinder, which can make it apper that you have more boost while in reality you are restricing air flow.

 

if you do not know this, boost is just a measurement of restriction. the restriction in this case is the intake valves, holding the air between the compressor outlet and the cylinders. the constant discharge of the compressor will keep pushing on the cylinders, and that is a restriction you have there. above the 25psi mark, the compressor gets out of it's efficiency ring, and it just superheats the air. sure it may be pumping more air, however it's hotter and there are fewer molecules of o2 in the same space as a cooler charge..

freeing up the exhaust side will make the flow alot higher ,because there is less restriction in the exhaust. therefore the engine can get rid of the exhaust easier and therefore accept more air on the intake stroke.