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Low compression pistons

That's too little information. We need to know what compression ratio, head bolts or studs, and most importantly what turbo. Also what rpm are you trying to peak torque at, and what to rpm are you going to run at.

Take just the turbo- a larger turbo moves much more air at lower pressure, so a small gm3 could produce a good amount at 8psi, but the hx40 can do the same thing at 4-5psi.
 
Imo, reduced compression pistons are a mistake with idi engines. People lower the compression which makes it harder to start. Then run very high boost pressures at very high volume to make up for it.

This is a common practice almong turbo gasoline engines that rev 8000+ RPM. It's a trend that does not stick well with idi 4000-4500 rpm diesels. The lower compression is so the engine can rev up faster to build boost so that can make full use of the turbo.

As soon as you get your 6.5 L to get work at 6000 RPM then I will concede to it being a good idea.

Gm turbos like the gm4 is a well sought after turbo for 4 cyl 2.5 - 3.2 liter gas engines that rev to 7000 rpm. For a 6.5 at 3500-4000 rpm, not so great. The other thing about low compression high boosted engines is the need to waste fuel to spin them up into the power band. Kind of the opposite of what this engine was designed around.

A better flowing turbo or twin smaller turbos like the factory one will do much better. The regular head gaskets are known to fail under high heat or high boost situations. Use of felpro or victor rienze head gaskets with studs instead of cheap tty bolts. Of coarse much larger & better flowing exhaust is needed to go with that.

Boats are another good option for that because it's work load is not linear to the rpm output. When a truck is towing a trailer at highway speeds, then is starts a hill the work load increases and can drag down the rpm. On the water at cruising speed you don't suddenly find yourself coming up to a 3-5% grade to climb for the next 5-10 minutes. (If you do quit driving and start praying). The sustained easier turning higher rpm saves fuel, but at the expense of top end power needed to deal with changing elevations.

My 2c. Welcome all to show the err of my ways.
 
The idea behind lowering the compression ratio is so you can maintain lower cylinder pressures while moving the same amount of air. With that being said, the stock GM turbos are pretty sorry and run high boost without moving alot of air. An ATT or even an HX-40 can move a good bit more air at 10-12 PSI of boost VS's a GM-X series pushing 20 pounds of boost. The idea is to move air, and maintain efficiency. With the different turbos available now, many have found that lowering the compression just isn't needed for most applications.
 
5-8 PSI is from what again? The limit of the GM turbo's is 14 PSI before they choke. I recall the vac system runs 12 PSI.

On other turbo's ATT about 18 PSI and the HX40II 24 PSI. When I dyno'd my rig 15 PSI on an ATT turbo is the same power on the rollers as 24 PSI from the HX40II due to back pressure etc. This was on the higher compression 6.2 engine vs. the slightly lower 6.5 compression.

3-5% grade is just a mole hill Will. :dummy1: It's 7-15% grade going for miles out here... Lots of burnt up pavement and cratered engines litter these grades.
 
Thanks for all points of view. Being here in Australia the depth of knowledge on these motors is pretty well non excistant. 5-8 psi would be on the stock turbo as that is what I've read in a gm training manual that I came across on the web.
 
5-8 PSI was stock for the 92-93 turbo trucks. 94+ trucks would run roughly 7 PSi of boost at sea level if intake air temps were under 315 or so I believe. many run them up as high as 18-20, but it just isn't efficient up there due to all of the backpressure it has.
 
Just remember going to larger turbo's is trading the low end for top end RPM. The GM turbo will spool off the starter! Then the backpresure ratio gets to 2:1 somewhere over 8PSi, ~ 2200 engine RPM. In effect the turbo is an asthma attack for the engine on any RPM over 2200 wasting fuel and driving ECT through the roof. The ATT turbo is like 1:1.1, but won't light much below 2000 RPM. A BD Spool Valve on the ATT gets 10 PSI at 2000 RPM or higher stall converter to flash to 2000 RPM from a dig are some options. I've tried and posted results for both.

IMO I haven't gone low compression, don't recommend it, and have run higher compression. Compression = power. NOx emissions are one reason why other diesels go lower compression.
 
I have a 94 silverado with factory origional pistons and a modified stock turbo manifold. My truck has a Series 60 Turbonetics turbo and it rules. It does in fact need time to build boost out of the hole, but on the highway she scoots just fine. My turbo is a T4 turbo and imo works great for what I want. Now my engine has no waste gate, except the pedal on the floor. WOT this turbo will get around 17psi, mid throttle boost stays around 8-12 with no problem and the truck will bark a wheel in turns. Hauling with a trailer or a shit load of weight in the bed, the truck does not slow much. I had around 1400lbs in the bed and still got 0-60mph in 17 seconds, now that's not a rocket ship but for a truck this size it is a significant improvement. If what I did to my truck interests you I would be happy to share it in depth.
 
TractorTruck,
Pleas start a build thread. The more info everyone can see helps everyone learn. Even the Einstiens can enjoy reading what they already knew, because they learned it at some point also-reminiscing is great!
 
Will L. said:
TractorTruck,
Pleas start a build thread. The more info everyone can see helps everyone learn. Even the Einstiens can enjoy reading what they already knew, because they learned it at some point also-reminiscing is great!
Click to expand...
sorry bro, still learning to navigate here. I just posted in the build.
 
Aussie6500Chev said:
G'day folks, I was wondering what your collective thoughts are on at what boost levels should one use low compression pistons
Click to expand...
long story short a better turbo would do great for that motor. IMO a T3/T40B Turbo is just what the doctor ordered. This is the about the same as the turbo on the Dmax now and is a bolt on. No need to modify and is rated at 400hp at 10 psi and you only need 8 psi of it. It comes with the option of a built in waste gate set to order. Now forgive the long way around, a lower compression ratio is a good thing for high boost on this motor if your doing high power mods and what not. It is not really needed to get good power out of a stock motor. Despite what might be said, really a better turbo is a first stop. I don't recommend treating your engine the way I do mine as I can build mine anytime, and it is what I do for a living. Now, at 16.5:1 you can go as high as 45 psi for short hard romp. 30psi is good and safe for a hard long romp, and 15psi nonstop. 18;1 25psi short romp, 20 psi long romp, 10-12psi nonstop, after that you are back to stock pressures. When I find it there is a good read on how all this works with all the math and everything, I just don't remember where. Hell, I have to dig through my file cabinet and other stuff to find it cause I did print it.
 
TractorTruck thanks for those thoughts. The other info you mentioned would make for some interesting reading. I'm leaning toward supercharging my rig as I don't want to loose out on low to mid range power. My rig is Toyota 80 series GXL wagon. The current set up is a N/A 6.5 with a 700r4 with 4.10:1 diff gears turning 315/75r16 Toyo M/Ts. This gives me 1950 rpm at 100 kph (60mph), which is the max legal speed here. I average 16.5mpg around town and slightly better on the highway 17mpg.I find that its a bit slow to wind up when overtaking hence the desire to add some boost.
 
My hummer was factory natural aspirated, and it was horrible. Then I put on the turbo, turn up the fuel screw and increase the pop pressure of the injectors (i just swapped injectors out). Big difference. Most everyone out there said i would loose fuel mileage, it did not loose it, it got a tiny bit better. I suspect if i had turned up the fuel screw only 1/2 turn it would do even better. You'll be happy with some boost. If you dont do the injectors and the fuel screw you will gain nothing.

I didn't realize you were going to supercharge it instead of turbo charge it. Nice iat advantage and as you mention low end advantages of super vs turbo. That could change my stance for you on the compression. What supercharger do you have? Do you have to change pulleys to alter boost numbers?
 
Will, I'm waiting on a rebuilt injector pump and marine injectors to arrive. I've been told that the injectors have been set to 140bar whatever that is ( haven't had time to put it in english lol). As to the supercharger Im leaning toward the eaton htv 1900. The kit comes with a 8 psi pulley but that is a bit of a misnomer. Im trying to find info on flow rates and pressure ratios to verify the 8 psi claim and at what engine speeds. To change the boost it is just a case of swapping pulleys being mindful of the rotor speeds of course. Im not after a big power tow rig more of an easy daily driver and off roadability.
 
I did a little reading on the htv 1900. Nice numbers until the bottom line imho. Hoping you found a smokin deal on one. That supercharger will definitely beat stock Gm turbo set up, but I see why now your focused on the lower compression ratio. I won't say don't do it, but have to ask why spend that much money on it for the end result.

To run that charger sustained at 8 with a good curve ratio your going to have to drop the compression. That is $ for new Pistons. I didn't look up pricing but I'm going to guess no less than $400? Then the supercharger. Good used maybe $2,500? Call it a other $100 in small parts and your into it $3000. Everything new looks like double that.

For less $ you can put in an ATT, or hx40 series, with a web for the low end. Then buy manifolds and a 4" exhaust. You'll be into it less $ and better fuel efficiency. Injectors are the same, IP changes are the same, rebuild cost of turbo vs supercharger is lower with turbo and turbo is of coarse more fuel efficient.

I spent years juggling turbo vs supercharger for my Hummer. I've ridden in Hummers racing each other super vs turbo that everything else was very well matched. The super charger was slightly faster. As in 1/2 body length in 1/4 mile. The supercharger one also got 1.5 lower mpg. At twice the cost of build, and just wasn't worth it to me.

I did help 1 guy put twin small turbos with dual exhaust all the way back on his, if you see a guy with the angled back exhaust for water fording on both sides, talk to him. Mick is a great guy that moved back to his home in your country. Very fast (for a 1994 hummer).

I still keep interest in supercharged 6.5, as I have a few millionaire Hummer buddies that don't hesitate at 1 second advantage to $15,000 ratios. I however change my own oil out of economic need more than tinkeritis.

Please do keep us informed as you go through your project, and thanks for sharing.
 
Yes ,the bottom line is not very nice, just have to save my pennies for a bit longer. I'm aiming for around 4-5 psi boost at 2000 rpm maxing out around 10 at WOT on stock internals (for now, famous last words haha).Reliability and longevity are my main goals. I'll be heading down to the capital before the years end to check out supplier and hopefully visualise the components and see a motor all set up.
 
From the guy who has owned 6 supercharged engines - try again. Other than a blower that never increase beyond 2 psi, show me a supercharged rig that has 250,000 miles.

Big rigs quit using blowers and superchargers because turbos last longer.
 
I think the 6.5 is a better candidate for a supercharger than most because they are high compression and IDI. It likes to expel more exhaust gasses and doesn't need that much boost. For low and mid range power a supercharger should be good.
With a nifty boost control system that could waste away some boost at cruise would be really cool on a supercharged 6.5.

I'll bet superchargers today would last significantly longer than days of old. Aren't there a few cars running a supercharger from the factory.

IMO there are a couple reasons why boats like low compression better than road vehicles. Since raw cooling systems are not pressurized and no coolant they can't run too close to boiling (have to worry about localized boiling more). My ~2003 3.0L gas Mercruiser boat runs about 170F. Not 100% sure 6.5's run that cool but doubt they can run 195 stats. 2. Boats have a exponential load with speed. The faster they go the heavier the load which matches RPM and allows spooling a turbo. The only time a boat isn't under load is at no wake speed. Accelerating to plane and turns are big loads. I'll bet a hard turn at any appreciable speed is just as heavy or heavier than pulling a grade. The load is less harsh as in impact but its a heavy load.
 
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