Does anyone agree on safe boost levels?

[AndyL]

Hours of searching didn't really leave me with an acceptable safe boost level for my 99 6.5. Heath recommends 7, people on the TDP range from 6 to 11, the two guys at the local Diesel Shop both run 22 (with stock compression)! As everyone else, I want max performance without having to change head gaskets every few months. Or do I keep raising it until I run into Exhaust temp problems?

I have a Turbo Master and need recommendations.

Thanks

Andy

[ 12-16-2003, 09:42 AM: Message edited by: AndyL ]

[More Power]

Just my opinion....

The GM-x series of turbochargers are efficient up to about 12 psi. Above that, the exhaust backpressures between the turbine and the heads rise exponentially. The turbine is too small for high boost pressures and power levels.

So, it becomes increasingly counterproductive to run boost pressures above about 12 psi. Some use up up to 15 psi with the Gm turbos, but above ~12 psi, the backpressures rise through the roof, which drastically raises EGT and intake air temperature.

In general, if you're using a GM-x series turbo, limit boost to 10 psi or lower if you do not have an intercooler or up to 12 psi or a little more if you do.

The Peninsular non-wastegated turbos can run at pressures of up to about 20 psi of so, without restricting exhaust flow nearly as much.

There is a relationship between boost pressure and compression ratio, with regard to safe limits. With stock pistons, I'd feel a lot more comfortable with 10-12 psi. I know of at least two 18:1 6.5's that run 18-20 psi on a regular basis using a Peninsular turbo without any problems - thus far.

MP

[cruzer]

When I purchased my exhaust from Banks it came w/a longer arm to install on my waste gate lever which put my boost to 10-12 psi. The lever was for mechanical inj. and a Banks design. Banks instructions also said to watch EGT and add fuel if you had room for higher EGTs. 22 sounds nuts.
Burn baby burn.

I may be wrong but I think the turbo master instructions says to dial it in by EGT. I put my load on and got on one of the biggest hills I know I travel on and set it by EGT. I didn't feel comfortable going over 14psi w/a IC. W/O an IC I'd go 10-12lbs. My 2cts.

[AndyL]

10 to 12 sounds reasonable. How does an aftermarket exhaust like Bill Heath's effect the situation? Currently I am running the stock 99 exhaust with CAT.

[cruzer]

Do it!!!! My banks kit was 3" exhaust boost lever
gages and a good air cleaner. From what Ive read on this page a 3 1/2" is probably better. The turbo master is better than the lever. It really flatten the hills of KY. I got a lot more power. Remember if your putting more air in you have to get in out. Get a good aircleaner too and look at Kennedy's web page on "tech tips" for some other things concerning air delivery.

I can only tell what I felt w/my 92 mech injection, but you should get similar results
w/your 99. Hopefully someone else will chime in because I think you can do something w/your electronics also.

I didn't think you could put a turbo master on a 99?

[rjschoolcraft]

I ran 13-14 psi without an intercooler. However, I couldn't really use all of that because the PCM would pull back on fuel and boost when temps came up. With the intercooler, I still run 13-14 psi (probably 15-16 psi pre I/C) and have a lot more power. I am planning to install a back pressure gage soon to put some real numbers on things. So far, I'd have to say that I haven't found the cliff with regards to ebp. My engine runs cooler the more boost it gets. As long as boost pressure stays higher than ebp, things are happy. BTW, I am still running stock compression.

I plan to do more experimentation after installation of the ebp gage and after taking delivery on my new Tech II. At some point, I would like to install a bigger turbo. I just noticed that my GM-4 waste gate bushing is showing signs of wear.

[gmctd]

Adding to the above posts -

Of utmost importance, here, whatever advice you take, is an EGT gage.
Never start playing with engine control before you have a monitor on the engine output.

A boost gage is important, also, but EGT is the life of your engine.

Drive by the EGT, no matter what the boost level.

You can fry the engine at 8psi, especially after removing PCM control via aftermarket boost controllers.

Coolant temp is fairly indicative of problems, but EGT is where it all starts and ends.

[More Power]

The Project 6.5 is running a GM-8. Back in early 1999 during the initial performance tests, I varied boost pressure to determine its effect on performance.

What I discovered was that 0-60 times were the best with 12 psi of boost pressure. Performance degraded at 15 psi, along with somewhat higher EGT's and more smoke during the first half of the run.

As Ronnie noted, intake air temperatures increase with boost pressure. This is why it's important to run an intercooler above about 10 psi.

These are very general guidelines. Performing your own careful performance tests and engine parameter measurements could help you decide what works best for your combination of components. Your results may vary.

MP

[pannhead]

i spike at 15 and can hold 13....like others have said there is a BALANCE.....as posted above EGT gauge very important when playing w/boost and fuel...good luck and have fun ;)

[Marty Lau]

Andy L.

If you don't have a Pryo follow Bill Heaths instruction on the legnth for 5-7 lbs boost. He recomends 14 with Gages and his Chip. He also talks about the dinishing returns above 14 pounds.
More say 12 lbs.... okay. Intercooling or mist injecton or something to cool the charge air some where above 10 -14 psi.

[britannic]

YMMV, but cranking compression is at 380psi minimum, so 7-22 psi extra isn't that much extra pressure for a diesel. As has already been stated, the real enemy is heat from more fuel and/or timing advance.

Make sure boost is optimized for minimum backpressure and then set your fuel for a safe EGT.

I run boost pressures upto 20psi, but have found performance is just as good with 14psi. Anything above 12psi will really benefit from mist injection and/or an intercooler.

[Cowracer]

Correct me if I am wrong, but if you increase boost by 10psi, you would see more than a 10 psi jump in compression pressure, right?

Logic would dictate that with a 22:1 comression ratio, a 10psi increase in boost would mean an additional 220psi in comression pressure. That WOULD be a problem if not designed for it.

Or am I looking at it wrong?

Tim

[grape]

I think you guys have reached the limits of the gm series turbos. There are compressor wheels and housings available that move enough air at under 10 psi to make well over 300 hp. Minor modifications might be in order however.

[AndyL]

Originally posted by Cowracer:
Or am I looking at it wrong?

Tim Sounds correct to me.

[a5150nut]

Grape

You now have my attention. Can you give more info or sources??

[DogDiesel]

I had a GM4, upgraded to GM-8 and now I have the Peninsular Performance Turbo, a non-wastegated model.

First. I can rapp my engine to 4500 (rarely do) and only about 13-15 max with this thing. Mine is intercoolerd. I've heard a couple talk about the same turbo boosting over 20PSI.. I would like to see that. Must be some serious fuel!

Second. When I pull hard and am pulling grades, I encounter overboost and some turbine stall, just like a wastegated turbo. However, seldom and only minor. So for this Peninsular turbo to boost 20+PSI... Things that make me go "HHHMMMHHMMM..."

Finally, any boost over 5 PSI and I have clean exhaust. When pulling hard, I get a big gray plummel of exhaust, so it's doing turbo stuff, and there is plenty of power.

I agree with MP, any boost over 12 PSI is point beyond deminishing returns. This is in total agreement with the many conversations I have had with Matt and the guys at Peninsular, and the Depot.

Now that I have a non-wastegate, I won't go back. Yes putting around empty, the wastegate is smoother and peppy. But for pulling, the non-wastegated turbo whipps across the RPM band.

My 2 cents.

Wayne
~~Near Baghdad, IRAQ~~
My winter vacation...

[gmctd]

As you know, Wayne, several things play here.
Engine rpm, final drive ratio, demand, injected fuel, etc.
Boost is generated by exhaust gas velocity thru the turbine.
Velocity is increased by greater flow thru engine rpm, and\or by heat thru power demand - more fuel.

The boost delay you've described since your Peninsular upgrade is 'turbo lag', where foot off the fuel, clutch in, shift gears, hit the fuel pedal reduces engine rpm, which reduces flow and turbine rpm. A wastegated turbo is more efficient at recovering and maintaining power levels here.

Automatic transmission also excels at maintaining power when shifting, as foot doesn't ease up on fuel pedal.
Un-wastegated turbo lag is not as evident with auto trans as with manual.
Auto trans can build boost quicker from start, as engine is loaded thru fluid coupling in the torque convertor.
Load and demand equals more heat equals more boost.

Final drive ratio and engine rpm also contribute to turbine rpm - any truck with 3.42 and O\D gearing will be flowing less cfm than the same truck with 4.10 final drive - 1800 or so rpm vs 3400 rpm at 70mph.

The 4.10 geared truck generates and sustains turbine rpm thru greater flow in cfm - any increased heat generated by demand (more fuel) results in higher attainable boost levels.

This can be tested by simple method of noting conditions vs boost when running in direct, duplicating and noting condtitions when in O\D, where engine rpm is lower.

No argument wastegated vs non-wastegated.
Your stated conditions are correct for your configuration, but not identically comparable to a truck with auto trans, or low final drive ratio, or no O\D.

Take care over there, DogDiesel.

[ 12-22-2003, 03:35 PM: Message edited by: gmctd ]

[Uncle Wally]

I upgraded to a non-wastegated peninsular turbo and chip from JK in August. Power increase was good, not phenominal, but good. I don't have an I/C yet, so I'm sure the installation of that will bring more power. Boost can be held at 15 easily while running light. Loaded it blows that away. My gauge doesn't read over 15, but I extrapalate about 18-20. Spikes in boost are even higher. Stock compression for now. That will change eventually. Highest EGT loaded with 15+ lbs boost was 1100 pre-turbo. I had to take my foot out of it to keep it from going up. That was in the slightly rolling hills of Northern Minnesota. I'm sure the story would be diff in the mountains.

Unfortunately something went in the trans in Oct. and I am waiting until the budget can handle the price of a trans.

Waldo

[britannic]

Originally posted by Cowracer:
Correct me if I am wrong, but if you increase boost by 10psi, you would see more than a 10 psi jump in compression pressure, right?

Logic would dictate that with a 22:1 comression ratio, a 10psi increase in boost would mean an additional 220psi in comression pressure. That WOULD be a problem if not designed for it.

Or am I looking at it wrong?

Tim No, you're right, I overlooked the compression ratio. However, with combustion pressures measured in 1000's of psi, an extra 220psi isn't too alarming, when the cylinder and piston are briefly seeing 3-4000psi or more.

If high boost is desired, then lowering the compression ratio is the right way to go.

The key however, is a turbo that can move the requisite CFM at a reasonable boost pressure without overheating the air. 300HP is definitely attainable with as little as 10-12psi

[ 12-18-2003, 08:43 PM: Message edited by: britannic ]

[DogDiesel]

GMCTD,
Not once did I mention the word "turbo lag" in my comments.
I mentioned "turbo stall" a term which IAW Matt at Peninsular and even Bill Heath, both whom I have had long converstions, explained is when the exhaust backpressure and Pressures in Intercooler and intake equalize and the turbo stalls, it is overwhelmed. That was their answer.
My take is there is more velocity and exhaust than the turbo is designed to pump, the ability to pump and make pressure is the same as intake pressures: stall. Really, we are both meaning the same.
However, the result is the turbo stalls, quits spinning, I guess, because there is deep black exhaust smoke, and greatly reduced power. zero boost.
My GM8 did this on ever grade after warm. My Peninsular non-wastegated does it sweldom, only when I "hammer" it in low gears with a super big load. Feathering the throttle avoids this.
My point was 20+PSI with an intercooled 6.5, even low compression with one of these non-wastegated turbos...well, I would like to see it pull. I'm sure it would do it, with enough fuel, but I think it would stall the turbo.
Before you ask, I have 3.5-4" exhaust and there seems to not be a back pressure problem.
My next point was what MP meant, point of deminishting returns. Clean exhaust is anything over 5PSI.

Great debate.
Wayne
~~IRAQ~~
PS: As far as for keeping my head down, these people guard me like a... well, they take every precaution and so far, I can only fly, no convoys. My vacation would be a lot more fun if I could see the country side and get out. BTW - the weather is nice here.

[rjschoolcraft]

Read the Pull-Off III article from last year. Kennedy's truck pulled the hill (best 6.5 that year) with more than 20 psi boost. I personally have driven this truck and it is impressive on big end acceleration with 23 psi or so boost. Hopefully Kennedy will chime in here with more info on his run up the mountain, but something is disconnecting here. I'm not sure what.

[gmctd]

I agree, and you are correct on each point, Wayne.

Turbo "lag" and "stall" are not the same limits.
Turbine stall can be corrected with adjustment of A/R - different housing and\or wheel, or larger turbo, but at loss of lower rpm response, which is lag.
Correct for lag, increase high-end losses.

Which explains Dr. Lee's statement concerning highest efficiency is obtained in stationary turbocharged Diesels - no large rpm changes under load.
The turbo can be sized for the narrow rpm band in the engine's design for maximum power.

You, and I, and others here know these things from experience and\or training - my intent was to offer info to readers puzzled by various and seemingly conflicting reported figures for upgraded boost levels and performance

Unfortunately, being pressed for time (muddled thinking), it was not clearly stated that I was addressing your earlier, informative posts concerning observed turbo lag when shifting a manual transmission, and why it may not be so obvious with an automatic, which most folks have.

All's well that ends well, tho, as you've sufficiently explained "stall" cause and effect.

All goes to explain why sizing a turbo for driveabliity (response) and power (flow-rate) is not an easy task, with or without a wastegate.

To all, I want to stress again - safe boost levels are indicated by exhaust gas temperature.
Install an EGT probe and gage and monitor it closely.

Take care over there.
Particularly if being invited to the casbah. ;)

[ 12-19-2003, 08:57 AM: Message edited by: gmctd ]

[Mark Bajus]

there are multiple variables at work here.

Big turbos (i.e. Penninsular) work best under load; with no load (hot rodding) they dont work as well (overfuelling ect).

Had a local guy apparently set up 18:1 with big turbo, big pump: he hated it because cold starts werent great, hot rodding wasnt great too much smoke. He went back to 21:1 with everything else the same, now he's happy (not sure about the motor though!!!)

Especially when towing, I notice exactly what has been mentioned: above 12-13 PSI non-intercooled its a waste of time/fuel to hit the go pedal any more: all that happens is more smoke.

[gmctd]

ronniejoe, if you have not, do a search on DOGDIESEL posts, where he upgraded from GM-8 to (iirc) Pen's middle range turbo, for driveability.

Those posts are highly informative, as he also has the intercooled 18:1 300hp 6.5.

Also, iirc, JK is running the big Pen turbo, or similar, on similar engine.

[Animal]

Originally posted by Cowracer:
Correct me if I am wrong, but if you increase boost by 10psi, you would see more than a 10 psi jump in compression pressure, right?

Logic would dictate that with a 22:1 comression ratio, a 10psi increase in boost would mean an additional 220psi in comression pressure. That WOULD be a problem if not designed for it.

Or am I looking at it wrong?

Tim Here is a site that may help!

http://www.goodvibesracing.com/Compression%20Ratio.htm

If you pump in 15psi of boost pressure (one atmosphere) you are doubling the compression ratio + doubling the effective engine power.

[Scooby]

I dont think you would be doubling compression, mainly due to the fact you will have losses thru the intake systemm, especially the valve. Boost air will not be able to completely fill the cylinder in the short time the intake valve is open. You may have double the psi, but that doesnt mean the psi at btdc is doubled before the intake closes and compression begins.

[gmctd]

That's the beauty of charging the intake system - fresh air not only completely fills the cylinder, but with correct valve timing, also blows the old combustion residuals out the exhaust valve.

Precisely how 800cu\in effective power can be wrung outta 400cu\in displaccement, within the same rpm\power band.

No normal intake restriction will impede the new charge, since it is already present at the valve
as it begins to open.

Effective compression pressure is CR x baro.

Ram-air understated, I believe......

[AndyL]

Originally posted by gmctd:
That's the beauty of charging the intake system - fresh air not only completely fills the cylinder, but with correct valve timing, also blows the old combustion residuals out the exhaust valve.The 6.5TD camshaft intake/exhaust durations don't overlap do they? If it did wouldn't most of the boost be canceled out by the exhaust backpressure?

[gmctd]

Possibly correct on the first count, partially correct on the second, which is where the 6.5\GM-X combo result in diminishing power levels above 12psi boost.

But not from the exhaust and intake valves being open at the same time. This may aid and abet the problem, but is not the problem.

Valve overlap is done in hi-perf racing engines, such that the exhaust pulses rushing down tuned pipes "scavenge" residual combusted gases from the cylinders, further helping to "draw" the fresh intake charge in. Tuning the system can actually reduce the pressure in the cylinder to below barometric pressure. High pressure will seek lower pressure = more fuel\air intake charge.

'Overlap' a turbocharged engine, the boosted charge air can be used to blow the residuals out the exhaust port. Fresher charge on each intake stroke = more power on each combustion stroke from same ratio of fuel and air.

But in the 6.5\GM-X turbo, exhaust back pressure, from increased exhaust volume (rpm and power) against the wastegated turbine, begins building faster than boost. Much faster.

Check the post by More Power, experimenting with various boost levels during power runs 0 - 60mph.
Or DogDiesel's posts, or Kennedy's. It's all about exhaust backpressure - gains here, losses there.

Small turbine, sized for low exhaust volume at low rpm, gives driveability over max power.

Larger turbine, sized for greater flow at higher power output, gives more power at cost of some driveability = turbo lag.

Large turbine, sized for power = not much streetability for a working truck, but when it turns on, it's 'Katy, bar the door!'

Is 6.5 valve timing overlapped? Dunno - noone has come forward with degreed specs non-turbo 6.2\6.5 vs turbo 6.5 cams.
Dr. Lee visited a noted cam\lifter House in Florida a while back - iirc there was some talk of increasing lift by reducing lobe base circle, but nothing on overlap.

[tom.mcinerney]

Nice thread!

[Peter J. Bierman]

I guess there is some overlap, otherwise the cylinder would not be proppery filled couse the intake valve is not open in TDC, specialy with N/A engines, air need time to start flowing.
It can't be much tho, there is not a lot off space between the piston and the head in TDC.
I'll check it out tomorrow by blowing air in the injectorhole, turning the crank and listen at the intake.
I have the engine open anyway.
I'll keep you posted.

Peter

[rjschoolcraft]

As I reported in 6.5 Performance (http://forum.thedieselpage.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=1;t=006650), I am now running sustained boost pressure of 17 psig in OD under load (pulling a trailer). I don't have a back pressure gage yet, but my EGT is dramatically reduced across the board and the dyno numbers show that the engine makes a lot more power with the higher boost. Stock GM-4 turbo.

[EWC]

The specs for a stock cam are ( from NAPA ) : @ .050" lift , intake open 13 ATDC and closes 11 ABDC , exhaust open 39 BBDC and closes 17 BTDC . Lobe centers are intake 101 and exhaust 120 and the durations are 178 and 202 respectively . The lift is not given in the NAPA specs , but the shop manual lists the lobe lift @ 7.133mm . With 1.5:1 rockers this equates to 0.421" lift @ the valves for both intake and exhaust .

The Crane Cam regrind is : intake open 6 ATDC and closes 24 ABDC , exhaust open 36 BBDC and closes 14 BTDC . Lobe centers look to be 105 and 115 , but the lifts have improved slightly to 0.440" and 0.450" for intake and exhaust . The base circle has been reduced .030" in diameter so longer pushrods hopefully will not be needed . Duration is given as 198 and 202 , but from the specs you will notice the exhaust is not the same as the stock cam . There also is no distinction between turbo , non turbo or 6.2/6.5 in the NAPA listings .