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Question: I have always wondered about certain aspects of heavily modified sled pulling engines, specifically the 5.9L Cummins. I read about these Cummins engines that make well in excess of 1,500 hp, and they all seem to have very low compression pistons. Arias 11.5:1 units seem commonplace on these engines. My question is: How do these low-compression pistons help performance? Are they low so that super-high levels of boost (100 to 150 psi) can be run without grenading the engine's internals? How does the fuel light off at such a low level of compression? If you put such low-compression pistons in a stock 5.9L Cummins would it be able to start, much less run well?
Garrett Nieber
Ann Arbor, Michigan
Answer: Interesting question. Even on street trucks, many people are experimenting with reduced-compression engines. While lowering the compression might sound like an odd way of making power, it can work. If we go back to our high school physics class, we'll see that a high volume of oxygen is what we're really after in a diesel engine. Denser, cooler air is one way of increasing the volume of oxygen in the cylinders, which is why intercooling works so well on diesels.
| White smoke during start-up can be the indication of very low compression combined with very large injectors. While 1,500 hp might sound enticing, remember that these are $50,000 engines designed for short bursts of speed—a street truck engine must be built in a completely different manner.
Increasing boost is another way of introducing more oxygen into the engine, but this practice has its limits. If cylinder pressure is too high, it can literally rip the engine apart. In a sled pulling engine, an extreme amount of fuel must be introduced in a short amount of time in order for the engine to make the 1,500 to 2,000 hp. Although few sled pullers are willing to divulge their secrets, it's our guess that the lower compression allows the fuel to be injected and start burning very early in the power stroke without causing cylinder pressures to spike out of control. We know a lot of sled pullers who run more than 40 degrees of timing, which means they are getting more time to inject the fuel and a much longer burn cycle than a comparable street engine. A longer burn cycle that maintains a high amount of cylinder pressure (thanks to high boost levels) means more power.
We've seen this work on sled pulling engines, but on a street-driven truck, running compression that low would be a pain. We're not going to say it couldn't be done, but you'd have to rig up some sort of in-cab, ether-injection system to start the engine. Diesel engines that have 15:1 compression or lower generally have a hard time starting in cold weather without the help of ether, and when they do start, they'll erupt in a cloud of white smoke. In super-low compression rigs (like the 11.5:1 Arias engines) they'll often lose fire due to excessive fuel and low intake air temperatures. Once up on boost, however, the intake air is so hot that the lower compression ratio doesn't matter all that much. For an all-out street effort, we'd say 14 to 15:1 is about as low as you'd want to go. Plus, we've seen plenty of stock compression engines make 700 to 1,000 hp, which is about the most power you'd be able to use on the street anyhow.
Question: I have a '96 Dodge Ram with a 5.9L 12-valve Cummins engine and an automatic transmission. It has more than 210,000 miles on it and my transmission is finally in need of repair. I would like to know how to improve my stock transmission on a budget. I hear about monster transmission builds for guys with gobs of power going through them, but I am on a budget. My truck is stock now, and I don't plan on doing anything more than slightly bigger injectors, helping it breathe, and possibly water-methanol.
Joshua Munski
Via Email
Answer: As owner of Project Rust Bucket, relating to budget builds is easy. While full-billet transmissions costing $6,000 to $8,000 can be great for mega-horsepower competition vehicles, your average trailer-tugger doesn't necessarily need all those expensive hard parts. Your '96 Dodge has the relatively simple 46RH transmission, which can benefit greatly from a few mild upgrades. For starters, your transmission must be upgraded according to the power level you are looking to produce. In your case, it looks like a transmission that could handle 300 rear-wheel horsepower would be as much power-handling capacity as you would need. The stock converter is usually the first thing to go, and in the case of your truck, the stock converter most likely is worn out. Although the converter could be cut apart, refreshed, and a new clutch disc installed, by that time you'd be better off just upgrading to a new unit. If you want to go really cheap, single-disc converters can be made to last at power levels slightly above stock, but if it were our money, we'd spend the extra bucks and upgrade to a triple-disc. Other than that, some Alto clutches and a TransGo shift kit will keep your transmission reliable for years to come. While your price may vary, a budget rebuild should cost about $2,000 to $4,000, depending on whether or not you remove and install the transmission yourself-with a good chunk of that money going toward a new torque converter. If you're doing it yourself, double- and triple-check that your transmission has enough fluid in it after the rebuild by putting it in and out of gear (to fill the converter with fluid). We'd recommend buying a new transmission cooler and blowing out the lines in case your old cooler is filled with transmission gunk. Places like Goerend Transmission, ATS Diesel Performance, and North American Diesel Performance can all provide you with everything from a converter to a complete rebuilt transmission, or you can have a reputable local shop in your area perform the work.
| An upgraded torque converter, shift kit, and basic rebuild will make most worn-out transmissions last for hundreds of thousands of miles, as long as the owner keeps the horsepower within about 100 hp of the stock power levels.
Question: I have a problem with my '01 Dodge Ram 3500. It's hard to start unless I keep the fuel tank full and the front end of the truck downhill. I think it's a valve on the injection pump letting air into the line, but my mechanic disagrees. Is there a way to check it? I have 90hp injectors, an Edge computer module, 4-inch exhaust, and a 150-gph AirDog fuel pump that is providing 25 psi to the injection pump.
Matt Turner Via Email
Answer: The VP44 injection pump is known to have starting issues, especially if the truck is modified to produce more power than a stocker. One of the main problems is the stock lift pump can't keep up with fuel demand. If the injection pump sees 0 psi for too long, irreparable damage will result. A pump like your 150-gph AirDog is a good solution to remedy fuel starvation problems, however, additional problems can arise. Sometimes, if you're using a draw tube instead of the factory sump, the tube can suck air if you go below about a quarter of a tank. Another common problem (which sounds like the one you have) is that running too much pressure to the VP44 pump creates too much force against the fuel solenoid in the pump, and the truck won't start. A simple test for this is to disable the power to your external lift pump before you try and start it. There will still be enough fuel in the lines to supply the injection pump (as the VP44 has a built-in lift pump), but the truck won't have the excess pressure on the fuel solenoid.
| Have a hard-starting VP44 truck? Chip Fisher of Blue Chip Diesel recommends at least 5 psi of fuel pressure at wide-open throttle, but no more than 20 psi at idle to the injection pump for long life and optimal performance.
