Engine braking- How many RPMs?

[datrbone8]

We spent a week up in the White Mountains and had to go up and down quite a few 6, 7 and 8 degree inclines. Coach is indicated below w/ Caterpillar 3126B engine and Allison transmission.

Had absolutely no troubles but I was wondering about the RPMs when engine braking down the hills. Under ordinary conditions I usually run at 1,500 to 1,700 RPMs. Coming down the RPMs sometimes go up to 2,500 before I use the brakes to slow it down a bit.

There's no "redline" indicated on the tachometer but I wonder what is a healthy upper limit when engine braking.

[jimbo2013]

U should be downshifting, what ever gear you went up is the same usually going down.

No exhaust brake?

[Mr_D]

That's about right, same as our 2000 DSDP with 3126. I even called CAT about it, they said the rev limit ishigher when the e-brake is on.

[Koop]

At 2,500 RPM the Cat 3126 has a mean piston speed of 2,075 feet per minute. IMHO, this is no problem at all.

[sknight]

According to the PDF below 2640 is max and 2500 is the governed RPM.

I like to downshift where the max speed is about 80-90% of the engine's RPM max. My engine has a max of 3400 which means 2700-3000 is where I like to stop.

So I'll let it coast on the engine until it hits 2700ish then I'll hit the brakes fairly hard to slow it down, get it 10-15 below the limit then get off the brakes so they can cool. That way I'm not dragging the brakes which builds up much more heat than just using them and then letting them cool off.

Since the Cat PDF says you're governed to 2500 I'd try to keep it to 2200 or so then slow it down.

http://www.google.com/url?sa=t&rct=j...73612305,d.aWw

[datrbone8]

Thanks- That's kind of what I was figuring but nice to have the peace of mind. We're headed north to the big mountains next summer and just wanted second opinion. When the engine speed starts to get near 2,500 while exhaust braking I use the chassis brakes sporadically to slow it down further to around 2,000. I figured on and off the chassis brakes keeps them from getting too hot.

[Koop]

Ok, we can ignore science. Mean piston speed and acceleration is the governing factor. 2,075 feet per minute is pedestrian. Let your system work for you.

[ahicks]

When going downhill, if the engine over speeds in anything but 6th gear, it's going to up shift to the next highest gear all by itself (in a self preservation move).

Knowing that, I'm generally going to slow the coach with the service brake to the point where it will go into the next lower gear when I see it approaching 24-2500.

So we're going down a hill in 5th gear, and I see the rpms building slowly up into the 24-2500 rpm range. Realizing 5th isn't going to hold the coach on this hill, I apply the service brake pretty hard/briefly to slow the coach to about 2000-2100 and shift down to 4th gear. If that doesn't hold (the rpms again climbing to 24-2500), I would repeat that same process to get it into 3rd gear. I've never been on a hill where 3rd didn't hold, but I would repeat the process until I found a gear that would hold without increasing rpm to the point where the trans would force a shift up.

I'll add that when driving in the hills or mountains for a while (second day maybe?), it won't take long to figure out what gear you need to be in for every downgrade sign you see from 4% through 6%! At that point, relaxing going down a hill becomes MUCH easier. The blood can return to your hands, and you actually have time to look around and see the sights! Until that next switchback anyway! ;^)

[sknight]

Originally Posted by Koop

Ok, we can ignore science. Mean piston speed and acceleration is the governing factor. 2,075 feet per minute is pedestrian. Let your system work for you.

Piston speed is only one part of the equation. Through valvetrain weight (Lifter, pushrod, rocker, valve, retainer and spring) the opening ramps of the cam lobes and the available spring pressure, the valvetrain does more to determine free RPM than unloaded (Not actively firing) piston speed does.

The max RPM the valvetrain can take before losing control of the valve isn't terribly higher than the factory RPM rating, hit one valve and that could really ruin your day.

Cat says 2640, there's no need to get higher than that and a safety margin is smart to have.

[aztortuga]

I have a c7 cat and with the exhaust brake I will use the service brake to keep it below 2300 or so. We run alot of mountains here in Oregon and this has worked well over the last 75000 miles.

[OLYLEN]

I think Cat is smart enough to have a safety factor built into the 2650 braking limit. But as others as the RPM will not hold the speed I stab brake to a lower get speed. Oh I have been on a few grades 2nd gear was needed to hold speed.

LEN

[ahicks]

I'm thinking the safety factor is built into the transmission - with the fact it will up shift at some point when excessive engine rpm is detected?

Or I suppose you could say the protection is in whatever is telling the trans to up shift. I don't know a thing about the engine ECU. I suppose that could be doing it as well. Whatever....

[MaverickBBD]

With my exhaust brake on, the Alison is 'requesting' 2nd gear. It will continue to downshift to there as r.p.m. permits. That is why I have to remember to turn off economy mode when the exhaust brake is activated.

[jauguston]

You can safely let the rpm increase until the transmission automatically upshifts which will be between 2600-2700 RPM ( mine is close to 2800). You can as another poster said brake firmly down to the point the transmission will automatically downshift. You after experimenting will know how fast you can go before the transmission will auto upshift. Mine will go 35 mph in 3rd gear and upshift to 4th if I let it go any faster. There is NO WAY you can do anything that will allow the engine to over speed. The computer (ECM) takes care of it.

Jim