kawi600

Im not sure if anyone can help me out with this. Might be more technical than most folks get with these old motors.
Im looking for a before and after experience with a stock 12v modded with a cam, some fuel upgrades and head porting. Maybe I can figure out whats up with my build by comparing setups?
When I started modding this truck I had about 14mpg city, 18mpg hwy. Now Im down to 8mpg city 14hwy. truck smokes at idle and AFC adjustments wont clear that up. Off-idle smoke wont improve beyond a certain point either.. something is just whacked.
Ive tried messing with the fueling system, the transmission, etc. Im running out of ideas.
Latest idea is that the stock head and cam design are setup to promote swirling of air in the cyl, at the expense of optimal flow. Adding a cam changes the lift and the lift rate (Id guess) and potentially the amount of swirl with lower velocities at lower RPM. incoming airflow relative to RPM seems like it has a big effect on where power and economy are. Doesnt sound like much but theres a lot of engineering-level articles online that sure make it sound real important for part-throttle, low-load use in diesels.
So, if by going all-out and porting the head and opting for flow versus swirl, is there anything that can be done to offset the losses on the bottom end?
Maybe a supercharger / turbo setup? I may need to raise port velocity at lower RPM to get the swirl back now that the valve ramps up faster.
Turbos normally used in these trucks dont make that kind of air down low in the rev range.
Truck is a DD and spends a lot of its time puttering around in the city unfortunately.

jstone44720

The supercharger/turbo thing is now an almost bolt on kit. I've seen them that use mustang cobra chargers off the shelf. Pretty neat on the wow factor, but I have not seen any real power numbers off that setup. With an overdrive pulley set 5 psi at idle isn't unreasonable.
The constant velocity might be what you are looking for, but I cannot speak to whether or not it will help your MPG.
Good luck.

John_P

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kawi600:

Let me see if I can give you some input that might help you Sir.

I think you are familiar with my 1996 Dodge CTD 12 valve that I have modified slowly over the last 7-8 years arriving at the 800 H.P. level it is at now. Along the way I saw similar issues related to various mods I chose. FWIW,.....right now I see about 14-16 MPG in town IF I keep my foot OUT of the throttle. On the highway, I can get 18 MPG if I also take it easy.

But with that being said, as I slowly built my truck, I also saw poor mileage due to mods that were not "tuned" with everything else that I was doing.
One thing was the stall speed on my torque converter. Initially, it was at about 1800 and I dropped that down to 1600 which drastically helped the take-off. Secondly, I have had to adjust my timing along the way to not only get better mileage but to help the truck perform better. Many diesel shops and individuals think the higher the timing, the better a 12V will run.
Well I went as low as 16 and as high as 28. I found that when I got above about 25, not only did my mileage go down, but my power also dropped off!
I finally tested different settings and have it set at about 22 degrees right now. Injectors are another issue I played around with. Also, during my last rebuild we did pretty extensive head porting as you mention and this engine definitely runs better and the power is alot more "crisp" if that makes sense.
Finally, I also saw a slight change in the turbos I was running although I m running twins vs a single and I believe they help alot too. Although your truck is more stock than mine, IMHO, all of these things I listed can affect performance and mileage on the 12V engines.

I hope this helps you some kawi! There is more to all of this than you think and I am an example of how many factors affect mileage and power.

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kawi600

John_P, 4x4 and auto on yours too I see.. I also have a low stall converter.
Builder wont give me the specs but they will say its a little higher than the DTT I had, whatever their 'low stall' converter was in '04. This is a billet stator versus steel. Seems to behave a little different.
Just rebuilt the trans again since my DTT one imploded. Got a nice 4 disc lockup in it.
Whats on your pump and do you have a cam in it? Ive got Hamilton's 181/210 cam and the pump is a 215 fresh out of the shop with .181 delivery valves (not sure what seats I assume 181's too), 3k gov springs, mild AFC mods. 25-35psi of pressure to the pump, no air in the lines.
I have tried a couple injectors. DDP 75's which are 5x.012 and some 7x.010's I have on there now. I was hoping better atomization might help but it didnt.
Ive tried timing up to 20* and it moves power up top in the rev range, and the bottom end gets even worse.
I agree, so many variables in there to get mixed up and cummins spends lots of R&D time making these motors, pumps and turbos play well under different types of use.
Im going to try backing off on the fuel or upgrading to twins depending on whats with the motor. Not much I can do with low end air supply and it will have more effect with a ported head =(

John_P

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kawi600:

Yes Sir, I have a 4X4 with an automatic transmission. I am running the "ATS Five-Star" (5 Disc) right now with that low stall.

You asked about my P-Pump. I have a 913 P-7100,.....or to others also called the "215 Pump." My pump was built and bench flowed by Scheid Diesel and believe it or not still has the 12mm barrels and plungers. However, it does have a custom Scheid 5K spring set-up, 181's and one of the custom variable cam plate set-ups. My aneroid is active and max flow right now is about 400, although it is not set that high right now.

Like you, I have tried alot of different injector set-ups. We have a set of Scheid's Custom 5X.018's right now. I have been running my pressures higher kawi and that seems to help everything.

I think you are on the right track on looking into porting your head. I also think that twins would help you too. I originally used an HX-35/12 on top and a Schwitzer S-400 on the bottom. That set-up worked very well and it was very peppy in all areas of the powerband.

kawi600

Thats a lot of fueling and still getting that sort of mileage. Guess something else is causing my problems.
Did you add to the squish when you did your head gasket or is it all stock height? Any porting work done?
Hamilton offers a real nice 12v head I have my eye on, but Ill probably do twins first.