I have a theory that the real reason 6.2L and 6.5L heads crack so often (it would seem) is that with less boost (0- 5 psi), stock timing and a heavy load the engine has to burn more fuel to make the same power as with a higher level (12-18 psi) of boost.
This heats the engine, EGT's rise, coolant temps, with little boost there isn't much cooling effect from the intake/exhaust overlap. CC temps skyrocket.The longer you throttle the worse it gets.
The ECM is constantly lowering the WGDC the more you throttle, effectively lowering your boost levels. Now add to this a weak vac system and I can see how an underboost condition could cause an overheating truck faster than too much boost.
A spring will never let this happen. They just make more boost.
Higher boost keep's the coolant temps lower and egt's lower.

Boost is used to make an engine capable of burning more fuel and making more power.
On a n/a engine, we'll say a 6.2, the Injection pump is calibrated to deliver the amount of fuel the engine is capable of burning, based on the displacement of the engine, and the amount of air it is able to draw in and push out from the factory intake/exhaust. Spec for 6.2 (4979) and 6.5 (5088) n/a both max at 56cc in their respective HD models.
On a Turbo engine, we'll say 6.5 turbo, injection pump is calibrated to deliver quantity of fuel the engine can burn with forced induction at factory boost levels, which is about 64cc (4911 and 5521). HP increase over the n/a model is ~40. More fuel, more air, more power, means more heat.
Going beyond factory boost levels in and of it self, doesn't help anything. More boost means higher IATs, more air to be compressed in the chamber, and more backpressure to spool the turbo faster. Same fuel, power lost, and more heat created by resistance.

Hm, I guess the stock exhaust does contribute a lot to that. It seems to become insufficient at about 2000 RPM with stock boost pressure and light load.

There must be something like a break-even point up to which more boost does good --- I'd guess you reach it at about 4 PSI with stock exhaust. After that, more boost gives you much more heat because of the backpressure --- and increased power will be harder on the engine in aspects other than heat, too.

BTW, after reading that you got a cracked piston, I started to wonder if I should do any boost fooling at all. With cruise control engaged, boost drops to 0 as if the ECM does that intentionally --- maybe to get better mileage. Can you tell if higher boost has an effect on mileage and if it's worthwhile for longivety with an improved exhaust?

When fuel in the chamber is not completely combusted because of lack of heat because of late timing, poor atomization or lost compression, the exhaust will be white smoke from raw unburnt fuel.

When the fuel is not completely combusted due to lack of Oxygen, the exhaust is black because the hydrogen atoms in the fuel only need one 0xygen to each pair and they combine more easily and quickly than carbon which are left to fight for the remaining oxygen molecules. They hydrogen joins with oxygen to form H20 (water) leaving the carbon atoms that don't have enough oxygen to simply be exhausted as black carbon or C0 (carbon monoxide) instead of CO2 (carbon dioxide) which is clear.

My truck is not the same as stock. I've removed every restriction ,intake/exhaust, that could go and ported nearly every surface internally for better flow( 'cept the heads, but peninsular ports their 6.5L heads for performance and so shall I).

I'm comparing my pyro temps and ECT's to when my vac pump wouldn't make more than 5 psi boost no matter how much throttle I gave it, when it completely failed and I had 0 psi for a about a hundred miles and now with my little friend on the WG and the vac pump removed.

With a failing vac pump, I routinely saw the ECT's climb during WOT for freeway speeds and hill climbs, pyro ran up tp 1000* F. When it went dead and I had 0 psi EGT's and ECT's rose higher, and quicker during some WOT tests. With my spring I saw EGT's drop in half from low vac EGT's, ECT doesn't budge racing uphill WOT or slightly less(15+ psi boost range), EGT's hardly climb passed 600*F and

during normal driving 70-75 mph @ 5-6 psi EGT's fall below the low indicator(300*F).


What cinched it for me was working on the old man's truck and giving it a test drive. His truck never heats up much when accelerating but the ECT hit 210 in seconds and being used to getting 15-19 psi and hearing that jet spool-up past 100,000 rpm, I noticed it not there. His vac pump is weak and boosting at 4-5 psi maximum wot. I started thinking how my truck used to heat up the same way before the spring install but with all the other mods in place.


I am now, of the opinion that an unloaded 6.5L can take full turbo With no IC with a spring to release the top exhaust back pressures as long as pcm is aftermarket or fooled without the fuel cutting back. I've not experienced anything like it. I can make the WG chatter from a dead stop or at 70 mph and everywhere inbetween. It is a vibriant illustration of when your at peak EBP. If I were towing , it'd be diferent and I plan on towing some measured trailer loads in the near future with the scantool watching IAT's while the ambient temps are moderate.

Also, I was reading a similiar story on www.gm-diesel.com (http://www.gm-diesel.com/) forums where I go by another name. Less general understanding of the 6.5L there but alot of names here are filling that void. 0Lee for example.

Ever stood in the kitchen and watched your (in)significant other fry chicken in a pan of water?

Huh? Whazzat you say? Not possible? Thinking about a 'replacement'?

Water boils at ~220deg at sea level - that's the max heat you'll ever get out of a pot of boiling water.

Your 'chef' (wisely) uses cooking oil, with a boiling point of ~1000deg or more.
The oil can attain whatever heat is put into it, up to and including boiling point.

Which is good for us, because chicken, and steak, and bacon, and etc, frys at ~325deg - more meat in the pot, greater the thermal input required to maintain that frying temp.
The oil can handle it.

Don't worry, kids - ole gmctd aint quite gone around the bend.........yet.

Ever notice how you can squirt on the benzine to get the charcoals glowing?
Some of it - squirted on the red glow - flames up, burns black and smokey, where some of it, close to the heat, but not in it, sort of wafts off in the breeze as a white vapor?

Temperature of combustion without enough oxygen to sustain combustion vs temperature too low to result in combustion.

White Diesel fuel vapor is un-ignited fuel, where fuel vapor never reached combustion temperature, possibly from late injection, cold cylinder\head\cc from cold weather - several factors contribute.

White smoke means cool exhaust gas temperatures - EGT's.

Black Diesel fuel vapor - black smoke - is where fuel was ignited - above combustion temperature - but oxygen was depleted before all fuel was combusted.
This stuff can cause plenty big problems.

Remember one of the facts from the domestic sitcom, above?
Oil can attain, and retain, much greater thermal level - temperature - than water.

Atmosphere is ~20% oxygen, the rest is junk.
Diesel fuel is a light oil.

Diesel fuel combustion temperature is up around 1625deg F - when all fuel is combusted we get waste product from the fuel and the leftover junk from air.

Whatever the actual composition, none of it resembles - oil .
Oil that can retain high levels of heat - heat that can transfer to anything the oil touches - pistons, cylinders, heads, valves, exhaust ports, manifolds, turbine blades, etc.

The combustion residue scoots out the exhaust port and into the exhaust system, leaving little but some soot to show it had ever been there.
Being of low density and relatively mass-less, it's temp is dissipated rapidly.
Takes a lot of it - high power, high flow rate - to effect higer temperature into it's surroundings.

Black smoke - unburned Diesel fuel - oil - is at combustion temperature levels (1625deg F) as it is forced out of the cylinder by the up-coming piston.
Everything it touches heats up - the greater the volume, the quicker the heat increase.

Black smoke means high exhaust gas temperatures - EGT's - AND high engine coolant temperatures - ECT's - bad news all around.

Restrictive exhaust system slows up the black vapors, allowing more of the inherent heat to transfer into the piston, block, head, and etc.
Bad news ain't gettin' any better, is it?

Now - about those head cracks - the cracks occur between the intake valve and exhaust valve, in those end cylinders where engine coolant is not directed into the head on both sides of the pre-cup, glow-plug, injector area.

Severe or\and repeated over-heating can cause the cracks to occur in any of the other positions, but most common cracks are in the end positions.

Few heads cannot be repaired, and the repaired head gives no repair-related failures.

These cracks were mostly eliminated from '97-up, with the oem upgraded heads and coolant system improvements.

That's my story, and I'm stickin' to it...........

This is what I did for my dad's GM-5 turbo. I didn't have to modify anything 'cept the spring.

this is what puzzles me about the 93 up trucks........a real wastegate uses intake manifold pressure to control the opening of the valve........ You guys appear to be using exhaust manifold pressure to blow the valve open, which is different from any other turbo system I've ever heard of.:(

Most all American oem systems used that method, grape, from the '70s - simple and effective.

Some used a spring-loaded exhaust 'gate, with diaphragm-assist and manifold pressure plumbed into the cannister as security.

The computer-ized systems started using vacuum\diaphragm with the spring.

Don't recall seeing much different in the rice-burner systems, up thru early '90s, when I quit paying close attention - I put a Buick GNX\ECM in a Starion, to get away from the tractor sound, while getting some reliable BIG power - horsepower AND torque.

'94-up GM uses diaphragm and regulated vacuum, referenced to Boost pressure.

Some people have gone back to an adjustable spring, available from several vendors.

My truck needs the vacuum pump, so I'm sticking with the vacuum motor control.

But, I'm also keeping a close eye on what you're doing...........

Okay, I have a question here about the intake/exhaust overlap. No granted, this is all in theory and on paper, so operational differences are always possible.

The intake/exhaust overlap even would be a wonderful benefit to this engine, however, for the purging to actually happen, (if I understand it correctly) of air through the clyinder at the end of exhaust stroke, and beginning of intake stroke, then your exhaust back pressure would have to be lower than your intake pressure. Since where are operating a four stroke/turbo instead of two stroke, this seems a little bit tricky.

If you are running 12 psi of boost on your intake, then your exhaust back pressure will be equal to or greater than 12 psi (pre-turbo, which is what the engine sees) I thinky the only way around this is supercharging your engine, and elminating the exhaust back pressure to get that purging of air though your clyinder.

Flow-thru purging is effective where the Diesel combustion chamber is in the piston - Cummins, Navstar, Isuzu, etc - not as effective where the indirect injection cc is in the head.
I think flow-thru purging is not employed in the 6.5L, with it's flat pistons.

The turbocharger in the low-to-medium rpm 6.5 system works well, as Boost comes up instanteously off-idle, where additional torque is needed to get the load moving.
12-15psi will readily move most loads at hi-way 2000 - 2500rpm, with low EGT's.

Where 12-15psi causes problems is at ~3000rpm, where exhaust flow and velocity are much higher, as engine is developing it's normally aspirated swept-volume flow rate.

You might want a super-charger for hi-rpm racing, but the 6.5 is limited to about 3500-4000 rpm with the cast crank.
A crank known to break when the harmonic balancer gets a little ragged.
I've never seen a Chevy big-block steel crank break, no matter what shape the hb is in.

I am aware of the Whipple-Hummer conversions - it's the cast crankshaft nose that would be of major concern.

A better alternative might be twin turbos - no additional crank-loading.

Free up the post-turbo exhaust system over the factory crimp-job, and the '97 cooling upgrades will allow all the power out of the GM-X that most folks will need.

Larger waste-gated turbos, for family-bus convenience, are available, but most folks do not have the wampum with which to experiment amongst them.

If you could take a ride in TurbineDoc's heavy-half K, you might gain a little more respect for the GM-X turbo - the truck flat out hauls booty.

Okay, I see in your post GMCTD, about twin turbos and no additional crankloading. How does that work? So boost, just isn't boost, but how you make boost?

The GM-X series turbine is sized small to get quick spool-up at low rpm - that becomes a problematic restriction at higher flow-rates.

A small wastegated turbo on each bank would allow quick Boost, with much less restriction at higher flow rates.

The GM-X would perform very well off one bank at the higher flow rate.
Off-idle Boost may be less.

Now if you could only have both banks go through a GM-X turbo to about 2500 rpm and then swap over to two turbos (one for each bank)....

That would be somthing! My brain hurts just thinking about the plumbing though

Tim

OR - bypass one bank..................

toyota engines in the cart days ran one bank of exhaust to the turbocharger on speedway applications.........all others followed suit shorty after.

Yep - tho in this case, you'd want both banks to puff up the Boost off-idle, then waste the driver-side bank when combined Boosted and n\a volumetric flow rate becomes sufficient to drive the turbine off the passenger-side bank, alone.........

Any advantage to running two GM-x turbos seriesed? Or whould it add to the backpressure?

Should unlock now

Maybe it will go now

Got it post away fellow enthusiasts

The big pulling tractors run two turbos - one huge, for flow rate, driving one not-so-huge, for pressure - in series to get the Boost up to 150psi, but not much need for that here.

Idea here is to get quick low-flow Boost while reducing exhaust back pressure at hi flow, as all turbos present a restriction to any exhaust flow thru the turbine.

Twin-turbos - one per bank - reduce hi-flow EBP, compressor flow rate adds, pressure - paralleled - equalizes.

Output of two 200cfm @ 20psi compressors is 400cfm @ 20psi.

Crank gets more loading from any power-increase - supercharger also increases drive-loading on crank-end not designed for increased loads.

Maybe it will go now
where you want it to go now

Turbo's in series are for when you have two turbos that aren't meant to make the kind of power you want to make. As much as I bash Gale Banks for what he sells........what he builds for himself is right on the technological edge. His little Dakota had 1 turbocharger and made over 700 hp. Turbos in series in my book is because you're too cheap to buy a single turbo with the correct inlet wheel for the power you plan to make. Gale's hx 60 or what ever it is keeps me excited that I might have picked the right stuff for my project. The 60's inlet wheel is a touch smaller than a T88 which shows by the efficiency map to be able to make that power with a little over 20 pounds of boost..........WE DON'T NEED TWO TURBO'S ANYMORE. I have friends that make 1800 hp with a T100 single turbocharger on 330 cubic inches. Turbo technology has changed in the last 5 years by astronomical amounts. The huge single turbo's came into existance due to alot of drag racing gasoline classes which require only 1 power adder. At first that meant turbo's, nitrous, or blower, and most turbo guys ran twin turbo's..........which destroyed the competition. Then all of the centrifigul blower manufactures *****ed because they had '2' power adders with twin turbo's. So they beat them at their own game and created the T series and all the new huge single turbo's.

when i read about the series twin turbo setup i assumed that the small one was for quick spool-up (reducing lag) and the larger was for WOT top end flow.

Very insightful grape,
you're right that a lack of funding keeps one thinking of ways to get around those big dollar purchases. What do these aftermarket unit's go for new?

I've been hauling 2-3 tons of trailer and materials last couple of days. Near WOt EGt's are running 600 deg at 10-15+psi sustained, up on ramps to merge with 70 mph traffic, uphills. ECt's have stayed ~195 deg. It really glides down the road now.

the thumper series or any of the T88 up to T106 is around $1800-$2000 for the unit itself. Huge is right......about 60 to 80 pounds is what one of those things weighs, here is one of my buddies T91.....91 mm inlet wheel. This should prove that 10 pounds with a GMx turbo isn't the same volume of air as 10 pounds with this monster.

Might also be less confusing, and much less exciting to the un-initiated, if you would post the rpm and flow rate required to drive those monsters, as compared to the ~400cfm @ 3500rpm of the 6.5L...............

The thread got locked my 1st attempts to unlock it did not work, as a mod I could post but no one else, QM fessed up he might have broke it looks fixed now.



where you want it to go now

WOT = ?

Looked in FAQ's but didn't find that one?

Wide Open Throttle
or WFO, Wide Freaking Open