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Att dyno results

I think the reason that the 6.5L has trouble making up in the torque department has to do with a variety of little things. The biggest being the precups. They are a huge limiting factor in getting the flame front and explosive force at TDC compression to get to the piston. That small opening that opens up to the cylinder bore is rather restrictive in comparison to a direct injection. Then factor in things such as timing, available fuel, higher IAT due to the lack of a CAC, compression ratio's, and Bore / stroke. Its a variety of things, but I really think there is amazing torque potential available if someone could find a way to take the precups out of the equation.
 
ak diesel driver said:
well that would explain some of the torque diff but def not all of it
Click to expand...

I think that's a big contributing factor, but I have to agree maybe not all of it. Has anybody ever done a side-by-side comparison of a 6.9L /7.3L IDI precup against a set of Diamond 6.5L precups? I'd love to see measurements / specs of the two side by side. Perhaps that "oh-so-important" hole exiting to the cylinder bore is larger on the IH than compared to the 6.5L
 
My brother had a 3.9 liter 6 cylinder engine that had small precups ,very similar to the 6.2 precups and it had awesome torque very much like a cummins. He currently has a 4bt at 680 ftlb torque and it runs like the the 6 cyinder did. It had 22-1 compression ratio.
 
6.9 was 22.5:1, the 7.3 was 21.5:1

6.9 had two different pre-cups, but the later is an 18cc cup, the 7.3 uses a 20cc cup. 7.3 injectors are .060" further out of the heads. The 7.3 changes were for emissions purposes.
 
The point I was trying to make,was if you tune for top end power,low end torque can disappear. It's actuially easier to tune for low to midrange torque and power,since the injector pump can pump more fuel in that range. With electronics this might not hold true,since the timing could be perfect at all rpms.
 
I'd sure like to know what I have for precups but until the engine needs disassembled, I won't know. About the only thing I do know is the engine was replaced under warranty for the 1st owner. They could have put anything in there as GM isn't gonna worry about what they consider a minor issue.
 
these are mine:

3-angle valve grind plus a 2* interference. Cups were shaved with the heads, opening the mouth a little bit.

IMG_1401.jpg


IMG_1398.jpg


IMG_1397.jpg


these are the pistons, mine are shaved .080"

IMG_1540.jpg
 
In the dyno run, did they put it in 4th and hammer it or ease into throttle? With the tranny dividing RPM 1st and 2nd are going to feel pretty powerful, compared to what dyno results in 1:1 ratio show. It is the later model trucks that have high torque rating from GM at 1800rpm, 438ft-lbs at the flywheel I assume. They are the diamond precups, GM8 turbo and electronic injection that can set timing higher right off the bat and across the range.
 
Ok the physics of the internal combustion engine dictates that below 5500 rpm, torque always is greater than horsepower, and at 5500 rpm torque and horsepower are equal, then after 5500 rpm torque is always lower than horsepower.

So how do the two correlate below 5500 rpm? how do we get 438 horsepower at 1800 rpm? What would horsepower have to be to achieve this number. I just don't see the 6.5 having 438 foot pounds of torque at 1800 when the max horse power out of these engines was 190 to 220 max.

Then the equation for this monster 6.5 truck is HP = Torque x RPM ÷ 5252


438 x 1800 / 5252 =150.114242193 Horsepower, now I don't think that the 6.5 has the mechanical brute strength to get that kind of torque at 150 horsepower. I just don't see it given the dyno numbers for 6.5 L truck with the gm 4, no where close to the number. The link below is VERY informative.

http://www.epi-eng.com/piston_engine_technology/power_and_torque.htm

Maybe we should move this discussion to another thread titled GM 6.5 liter torque and horsepower discussion. Moderators?
 
It doesnt matter what the horsepower is. Torque is a function of force and distance. A long crank arm (stroke) and a high force (burning lots of fuel) will make big torque. Torque is not RPM dependent, but horsepower is, and at higher RPM you can burn more fuel in a given time to make more total power. Having torque early just means your injection pump is capable of outputting fuel early on and you have the oxygen to burn it. I think the DB2 is more RPM dependent than the DS4. As RPMs increase transfer pressure and advance increases allowing more fuel to be injected. Thats partly because of higher pressure and partly due to earlier injection likely rides on a steeper part of the cam ring lobe, allowing more travel of the plunger in the given injection event. So cam ring profiles are a big factor of where you get the most fuel. That happens to be how GM made the "high output" DS4-5068 spit more fuel at low-mid RPMs, with a different cam ring.
 
Yeah, during the dyno runs they said to barely get into 4th gear, and start to accelerate like normal and when it hit 1500rpm, to pin it and not let off until the motor governed at 4000rpm.
 
6.2 turbo said:
Right around 4000 rpms something happens,not sure what it is. I noticed lower boost and drive psi helped some,and more timing. Timing makes the biggest difference,and lowers drive psi at the same time. At one point I had decent power up to 4700 rpms,but lost power and torque below 3000 rpms.
Click to expand...

You have to adjust the governor screw on the DB2 to get over 4000 RPM. Otherwise it shuts the throttle. Also I have found that it runs out of fuel with any fuel system issue like plugged tank sock, weak lift pump, etc in the higher RPM's. The RPMS raise internal pressure and the vent wire bypasses more fuel back to the tank. There comes a point where pressure falls from fuel use and fuel return being more than the lift pump with restrictions can supply.

So two things limit your upper end power. Fuel and the governor. This is shown by the boost falling off in the upper RPM. Less fuel= less boost even on a GMx in the upper RPM range. Although that is fuel starvation when I noticed that.
 
See, I want to turn the fuel up after I dyno it but I see a few issues. I still can't burn a lot down low, fuel mileage might drop, and it would really only be useful in the higher rpm. and I am not even sure it would help spool the turbo more? Thoughts?
 
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