Please help me figure this out...


I want to make my truck as fast in the quarter as possible. To do this, especially with my 3 speed transmission, I need to make the engine revolve as fast as possible.

So the questions:

1. Everybody, even Piers, says that a 4200 RPM valve spring is perfectly safe as long as you have 60 pound valve springs. Is it true that you only need the springs? Are the stock pushrods and rockers strong enough for the heavier springs?

2. How does RPM relate to exhaust gas temperatures? The way I see it is that more revolutions will put more exhaust into the tail pipe, which will result in more backpressure and higher EGT's. Is this true?

3. How does RPM affect turbo boost? I would assume that the increased (and hotter?) amounts of exhaust gas at high RPM would cause the turbo to spool faster. Would the stock turbo be overspooling at 4200 RPM? I can imagine that you would need a wastegate to avoid this?

4. How in the world can the injection pump take 4200 RPM? I know that Bosch sells up to 5200 RPM springs for it, but if I recall correctly those springs were for the pump when it was on a smaller engine... If the pump has no problems injecting a stock amount of fuel at those RPMs, does it have a problem with POD's or larger injectors?

5. How does the high RPM affect the transmission? Higher RPM equals more horsepower, but you are keeping the same amount of torque, so I would think that the transmission wouldn't have any durability problems.

6. Also regarding transmissions, how would the transmissions shift points be affected? I don't know snot about valve bodies (or much else), but would the stock tranny always be trying to shift at the stock redline point? I don't know if you would really gain anything by going all the way to 4000 rpm in first or second gear, though. I can imagine that with the massive amounts of Cummins torque you just want to get into 3rd gear (1:1 ratio) as fast as possible.


Thank you for helping my project here. (I'm finally back in town with my truck for the first time in a few months, so now I get to start having some fun! I've only put 300 miles on it since I bought it a few months ago... )

 

Just a few guesses, I'll leave the rest for better informed people.

There may be a problem with volumetric efficiency at higher RPM's, getting air in and exhaust out past the valves. A different cam profile and porting/polishing will certainly help.

I would guess you'd have a problem with EGT's before you have a problem with overspeeding the turbo. Bigger turbo housings help here I think. 18-21 cm2 would be in order. Also a 4-5 inch exhaust.

You might be better off with more fuel and a higher rear end gear. Keep the engine down in RPM's and just add torque. Although a 3200 govenor spring will help with the fewer number of gears. The torque converter will also smooth things out some.

Look into a custom built tranny to take the extra torque. Give special attention to the TC. Higher pump pressures and higher friction bands/clutch plates will make things slip less.

Keep us posted.

Edwin

 

There doesn't seem to be a whole lot of power way up high. A 3200 RPM spring will be plenty to keep you from running out of RPM at the end of a 1/4 mile strip.

You will be able to raise the shift points a little with the TV cable. You will know you've gone to far if the RPM seems to "hang" before upshifts - meaning that the engine isn't making as much power at the higher RPM.

 

Besides that info the faster you spin that motor the more the insides want to be outside. The parts - rods, pistons, piston pins and crank, are not made to for high rpm- too heavy. Things like lubrication, cyclinder head flow/valve size, exhaust flow (or lack of), all these things plus others will hinder eficient and long term High rpm. It can be done but bring your wallet

 

Is that because it takes the engine a long time to get up to RPM, or because the engine makes squat for power at high RPM?



I guess ported heads and a better cam would help get up to RPM faster due to the reasons Edwin posted above. Or I could just powerbreak up to 2000+ RPM and launch...
I can also imagine that a really small turbo would help considerably to...


The deal is that I know lots of people drag with the Torqueflite... At the moment, my calendar has a picture of a 1400 HP Super Street 526 Hemi Daytona with a 727 in it, for example...
The difference is, of course, that a 526 Hemi gets to 10,000 RPM very quickly. I know that my engine also can get to 10,000 RPM, but only once. Realistically I can get up to half of that, at the most.

The Cummins makes lots of low-down power, which is why it takes off the line so fast. The problem is that the top-end is lacking, even with turbochargers, because of the low redline. The key to the Cummins is to get it up to a high RPM as fast as possible. And then tuning the transmission to take advantage of that powerband.

 

IMHO, the key to the cummins is to tune the drivetrain to the limited rpm band the engine was designed to operate in. It's why all the 5 speed guys want the 6 speed and why all the auto guys want the tc lockup 'magic' switch--to keep the revs between 1900 and, say, 2300. The b series cummins just seems happiest there. The real ticket would be a single speed tranny with a torque converter so smart it'd progressively lock to hold a pre-set rpm...and you could select different lockup curves depending on your driving habits. Kinda like a direct drive dragster, I guess. I hear they really go through clutches, though
g

 

They rev quick enough. The problem is a lack of high RPM power. Not enough air and fuel available, plus you need so much timing that it's counterproductive at lower RPM. There's a reason that high RPM diesels (like the VW TDI) have small cylinders. It takes time for the flame front to travel through the whole combustion chamber. That was also the whole point behind IDI, it takes less timing to run high RPM due to the small combustion space.

You can build them up to make power at high RPM, but you sacrifice low end torque. The serious pullers are alleged to be turning as high as 6k, but if you've seen pulling videos you know what it takes to get the turbos spooled. Not exactly dragstrip friendly, and impossible on the street.

Unless you are running out of RPM at the end of the 1/4 mile there is not much point to exploring the upper limits. What Greg says is true, ideally you would want to hold the engine at peak HP for the entire run. Winding it out way past peak HP will not make you quicker.

Even if you start to run out of gear around the 1,000 foot mark with the 3200 RPM spring (not likely) you won't hurt E.T. much. MPH will be off a little. Your biggest challenge will be getting good 60 ft. times without 4wd.

If you think the top end is lacking, you need a ride in a well bombed truck. These things will pull your head off in top gear. Proper gearing to take advantage of the torque is the key, and with the 3.07's you are well on the way. You won't need OD for a while yet though. The truck probably can't get much over 70 by the end of a 1/4 mile yet.