I know that people swear by "decreasing compression and increasing boost." What I have often wondered is what effect some basic performance mods would yield. That is, what about things like porting, and radiusing the combustion chamber? Several people have mentioned that the block casting will not take the power. What are the major limitations? Are we talking web design and layout of the structural members? Is the rotating assembly the weak link? Anyway, back to porting... it seems only logical that the desire to add more fuel should be complimented by the addition of air. Where in the gasser comunity porting and radiusing yield higher velocity and better flow, in the diesel communtiy it would yield more volume and lower temps. The reason I say this is because the primary concern of a forced induction engine should be the intake air temp, which is a product of the latent heat of compression. Latent heat of compression holds that with added restriction there is more energy transformed to heat by the colliding atoms. The theoretical stuff gets way too intense to properly discuss here, largely due to the lack of imagery available on the subject. I have a friend who is an engineering grad student at UW Madison, who has access to some pretty impressive engine simulation software. I will ask him to do some analysi, but the question still remains... would porting and radiusing be beneficial?

Thanks, Sean

No,


but custom cast heads could solve some valveing issue. The blocks that people are building for power start with splayed mains on a 599 block if they can get it. the lower compression pistons allow more boost volume, the increase in flow you're looking for. Intercooling for air density. I saw a 7.3 PS turbo looked hardly used, guy wanted $150 for it.

In my opinion the turbo 6.5 and most turbo charged engines for that matter would not see a power increase worth the time and money put into porting, polishing, etc. I agree with QM that a custom head casting would benifical and trick if you have the resorces to do that.

Are any of the aftermarket low compression pistons available forged pistons?


The stock pistons are so soft you can put a light scratch in it with your fingernail. It's so soft the piston I just pulled out with a crack in it was only cracked on the face and not the sides. This worked out for me because it didn't scratch up the cylinder wall but I can't help but think a forged steel piston wouldn't crack as easy.


I wouldn't stop at the heads. The head and intake and the turbo and exhaust all innerelate and could be further intergrated and simplified/refined for a larger volume manifolds.

Porting, no - best for naturally aspirated engines to improve air flow into and out of, particularly at high rpm, 5000 and up


Forced induction has fresh air charge ready at intake valve at instance of opening


Port matching, yes - n\a or f\i to prevent flow turbulence, particularly effective in gasoline engines where inlet fuel charge is combined with inlet air charge, prevents centrifugal separation of fuel - heavy - from air charge


Diesel combustion chamber usually in piston, already radiused - prevents hot spots.


6.5L RICARDO indirect injection prechamber in head, separate design for n\a and f\i requirements


Porting\polishing won't hurt 6.5L, just does not help much in low rpm configuration - 3500max @ ~450cfm


6.xL blocks were designed for 135hp max, were shorted on nickel content with that power level in mind.


195hp and up quickly reveals shortcomings of lower than normally low nickel content - three center mains webbing drops right out of block.


AMG - Hummer engines - redesigned the blocks slightly, restored proper nickel alloys to increase strength.


Cranks are nodular iron castings, with rolled fillets in rod and main journals to improve strength - for 135hp.


Never intended to produce 195hp, they suffer breakage at 1-2 rod journals when harmonic balancer goes away, and 7-8 rod journals when dual mass flywheel goes away


As Cummins and Navstar added turbos to their medium truck engines for power, GM tacked on a turbo without addressing the block and crank issue, or that the water jacket and head passages were designed for 135hp, or that head-to-block clamping only gets 5 oddly staggered bolts - shared - per cylinder, limiting Boost pressures.


Without precautions, you get several pieces of cast iron junk.


With proper planning, some factory upgrades and aftermarket goodies, you can get an 18:1 street-motor, good for 300 reliable hp at ~15-20psi Boost - requires charge-air cooling.


6.XL high compression ratio was chosen, along with indirect injection, for 'driveablilty' consideration, with ease of cold starting and smooth drive-away, highly important in the light truck consumer market


Reducing the compression ratio allows higher Boost pressures while maintaining cylinder pressures equivalent to hi c\r config - more oxygen content per unit volume = more power from fuel with 20% more energy per unit volume than gasoline.


Lower c\r may require somewhat longer glow times for cold starting.


That's my story, and I'm stickin' to it...........http://www.dieselplace.com/forum/smileys/smiley2.gifEdited by: gmctd

So, Basically I am looking at finding some parts to make a lower compression engine? If I were to drop the c/r down to 17.6:1, and install an i/c I could "safely" run 15-20 psi boost? Or would I still need to look at block reinforcement? I see the point about only needing ~450 cfm. I was still thinking in terms of scfm. What is the firing pressure running now with 21.5:1 c/r? Would the ecm still work, or would I be looking at a new eprom at the least? All these things to consider, and more. I love the truck, but it leave something to be desired when pulling a loaded car hauler up a hill. I looked at a new GMC d'max today and thought of all the stuff I could do with that kind of money.

Since you can pickup a set of eight 18:1 pistons in your desired bore with rings for $550 USD on ebay motors, it would seem feasable. I paid that much for rings, head bolts, and a gasket set.


As for the intercooler, think truck salvage.Edited by: quantum mechanic

Your reference to scfm - 14.7psia and 60deg base - refers to relationship of actual swept volume (bore x stroke x rpm) to relationship of 20cuft at 1psia vs 1cuft at 20psia.


Do Baro x c.r., then Baro + stock Boost x c.r. for 21.5:1cr, then 18:1cr, to give an idea of base cylinder pressures vs Boost increase allowed by reduction in c.r. to 18:1.


lLess operating load overhead - same pumping pressure for increased Boost - means better efficiency, increased power - more oxygen per pumping cycle.


Every little bit helps with this engine...........Edited by: gmctd

if you do dive into the block to swap pistons, many over on www.gmdiesel.com (http://www.gmdiesel.com) seem to recommend the stud girdle kit. People were saying that the 6.5 block was never intended to put out over 135hp so that the nickel content of the block was lessened thereby giving you the bottom end problems..... Word of mouth over at GMdiesel.com...

You have to machine the block for the stud girdle kit/splayed mains. Considering that the avg block could use something to hold the bottom end together, it's cheap insurance.

The new 6.5 n/a sold here in Western Australia are rated at 170HP at 3400.
320 ft lb at 1700.

Most people here sware by them. ( or is it at them!) :)

http://www.brunswickdiesels.com.au/new/technical.php

you have to machine for the splayed mains but i don't think the stud girdle kit requires it.

Uh, forged steel pistons? Surely you did not seriously mean that? Maybe you meant forged alum pistons? The stock pistons, or low comp alternative is fine for the 6.5L. What other kind would you want, beside steel ones?

Are any of the aftermarket low compression pistons available forged pistons?


The stock pistons are so soft you can put a light scratch in it with your fingernail. It's so soft the piston I just pulled out with a crack in it was only cracked on the face and not the sides. This worked out for me because it didn't scratch up the cylinder wall but I can't help but think a forged steel piston wouldn't crack as easy.


I wouldn't stop at the heads. The head and intake and the turbo and exhaust all innerelate and could be further intergrated and simplified/refined for a larger volume manifolds.

Yep - even the big rigs get only cast pistons with a cast-iron crown..........

Given enough time and money, most aftermarket manufacturers will build custom sets of forged pistons to your specs. Extra strength and durability couldn't hurt. Research some companies for price and feasibility. It may surprise you considering how expensive new factory pieces are these days.

Penninsular Diesel and Diesel Depot carry 18:1 6.5L pistons at ~105 each.

GM oem replacement pistons are ~85 each, iirc.

Fred, thatsa buncha miles you got on them trucks, old son...............I am impressed.

Particularly, that '82 with 4.10's, cranking 31-3200rpm at 70mph for 22years.

Maintenance? Parts replacements?

I'm going to shave the top of the pistons on my '93 down to 19 to 1 cr.. I will have to find a place to recoat the ceramic. that way it will only cost me a headgasket and bolt set to rebuild it.

How are you gonna re-balance the rotating assembly to compensate?

I'm going to weigh the pistons and get them as close as possible to the fraction of a gram I can measure to. I'll only remove the crank if cracks or wear are present in the main journal area other wise it's staying as is.

do you have to rebalance it after adding the low compression pistons that you can buy?

No - they are to oem balance spec.

Big BUT, here - if you are upgrading for greater power output levels, would pay to have recip\rotating assy balanced

Hmmm...I know your right but .22" of aluminum x 8 isn't much. I'll match it on the rod cap!!LOL

I'm going to weigh the pistons and get them as close as possible to the fraction of a gram I can measure to. I'll only remove the crank if cracks or wear are present in the main journal area other wise it's staying as is.Could you do a before and after weighing and only remove the same exact amount from each piston then any other balance method on factory crank won't be throwm off?

May not be much laying on your bench, but at 3500rpm takes on block-cracking, crank-breaking significance.

Corresponding weight reduction must be compensated from crank counterweights

Fred, that'sa buncha miles you got on them trucks, old son......I am impressed.

Particularly the '82 with 4.10's, cranking 31-3200rpm at 70mph for 22 years.

Maintenance? Parts replaced?

Etc.

Bow - counterweights for rod journals are balanced for the weights of the piston\rings\rod pair on each journal.

Lighter weight pistons require corresponding removal\redistribution of counterweight mass.

When investing big money in power upgrades involving the reciprocating\rotating assembly, re-balancing is necessary, as factory balance was only to acceptable spec, and could be at the ragged edge.

Find the lightest piston and match the other seven to that piston. Each piston should be checked to deck height, as each rod and throw has different dimensions. Cutting the piston domes is not advisable as this will alter the squish area above the piston. This clearance should remain unaltered. Perhaps the dish can be milled instead. Reciprocating weight is less critical than rotating weight. Most race engines are "over" balanced, with more weight on the counter throws than normal. Lighter pistons are more desirable than heavy pistons.
Do not take weight off the rod cap to compensate. That is rotating weight and should not be changed.

Bow - counterweights for rod journals are balanced for the weights of the piston\rings\rod pair on each journal.

Lighter weight pistons require corresponding removal\redistribution of counterweight mass.

Yeapper I thought that was what I said but I guess not as well