apwatson50

Good discussion Hohn, hope your new job is treating you well!

I thought I would just post some info from Don of F1 diesel (helix cams). The following info is just one of his posts from another site, its not my personal info and since he put it on another forum I don't see why I can't copy it here.

"Couple of things happened on the CR: The duration went way down and the lift came up. Especially on the exhaust.

This is why the tappet diameter and the lobe width is so wide now on the stock CR. The lobes on a stock cam are very aggressive. Grossly. That triangle is hard on tappets, lobes and the pressed-in seats.

The Helix by contrast is easy on lobes. The lift on the exhaust is actually a little less than a stock CR, but the duration is much longer. This makes a more egg shaped lobe, not a triangle. It is not aggressive, it is not as anywhere as hard on valvetrain either. The frequency in the Helix VS the stock CR cam is waaaayyyy better too. No funky valve bounce and such the stock cam can give at high boost and RPM levels.

Some numbers:

Stock intake lift: .235
Helix intake lift is north of .265

Stock intake duration : 155 degrees
Helix is north of 180

Stock exhaust lift is .297
Helix is north of .280

Stock exhaust duration is 192
Helix is north of 202

The ramps are smooth, not aggressive. That crap is for the track and has no place in an engine built for durability."

There is more than just some specs there, also some more information on making a cam durable.

Aaron

Ph4tty

First of all why assume that drive pressure is less than boost? Its usually the other way around. Secondly, what is your opinion on emissions impacts on cams vs the different generations in trucks? I always assumed 12v's would see less from a cam than a cr and the vp trucks would lie someone in between.

Mike Holmen

Any body make a roller tappet for these trucks? You can only put so much duration in the cam after that its hard on the tappet and the lobe. One day we diesel guys will have the wizzy gaser variable timing, double cam stuff. I think that the ISX has it in the engine.

The head should also be reworked. The intake side is design mess. Its just kinda dumped into big hole at the front of the engine with a couple bumps to control intake air flow. No wonder that the #5 and #6 have so many issues. Some guys just shave off the stock intake runner and use tubing for each intake cylinder port.

HOHN

Good questions. As is typical of me, this is just my speculation.

I assume that TIP is less than boost because a well- engineered system SHOULD have that and no worse. If you can't get this, you are losing too much heat in the exhaust plumbing or have some other problem in the turbo system. It's likely that the turbos have undersized turbine housings and so forth. A twins setup should be able to hit this, I'd think.

As for emissions considerations on cams, it's not good. Cam specs are one of the things that most drastically shift an engine from how you or I define performance to how the EPA defines it.

Modern cam specs actually encourage reversion. In the EPA era, fresh air is bad because it lets the combustion event occur quickly and drive up NOx levels. EPA says low NOx, so we encourage reversion to serve as a sort of undefeatable EGR.

The factory cams over the years are different, but I'm not really sure how much different. This is something where Don would know a lot more about that than I do. I could *maybe* look all that up at work, but no one has time for that-- least of all me of late.

While the newer engines are spec'd out with stronger EPA limitations, they also have a lot more advanced technology in them in many cases. The newer HPCR cams that have such aggressive lobes must have been designed with an eye towards getting the absolute minimum duration for a given "area under the curve".

When you're trying to meet tight epa requirements, valve overlap is an uncertainty you end up trying to avoid. So you end up with very little overlap and have to cram more air into the cylinder in less time. That means higher boost pressures, I'd think, and less efficiency in general.

Emissions considerations are the inseparable enemy of efficiency. I've never been more convinced of it than I am now that I've been working at Cummins for a while.

Fortunately, Cummins seems to be able to innovate enough to keep a good amout of efficiency under adverse circumstances.

I recently saw some test pictures from an engine abuse test where injection pressures were almost DOUBLE the current HPCR pressure! Yikes!


JMO

HOHN

ISX has roller followers of an OHC type setup. Other cummins engines have a roller follower of a cam in the block, and these followers then push the pushrods and operate the rocker levers.

I don't know of any Cummins engine that uses a roller cam setup comparable to what we'd picture in a big-hp hot rod engine.

JH