adjusting jake brakes affects performance?

[bevans6]

I thought I would start a new topic, since this is of particular interest to me.  Reading all the threads on this in the archives, something popped out at me - comments that Jake brakes on a 8V71 don't have the best performance, and a comment that they can be adjusted to have great performance.  Strongly implied was that adjusting them for great performance was a black art practised only by certain wizards of the two stroke trade...  Now, I freely admit that I can't tell the difference between my jake brakes on or off.  When I test them by snapping the speed lever on and off, I can hear them in the exhaust note.  I can tell the electrics are working, and they read the correct resistance on the ohm meter.

Having read the manuals, it seems to me that adjusting the jake brakes, from absolute scratch, would consist of adjusting the valve bridges, then adjusting the valves, then adjusting the jake brake  clearance from bridge to the slave piston.  What am I missing?  Is there some biasing on the adjustment that makes a big difference, like instead of .064", adjust them .002" tight or loose and gain some benefit from that?

thanks very much,  Brian

[luvrbus]

Brian, it is very important that you know what cam and valves you have before changing the setting on a Jake they put a lot of engineering into the adjustment of Jake's to cover all bases. 


good luck

[bevans6]

So would your advice be to simply check the adjustment and if it's out restore it to factory specifications? 

Dumb question - where do they get their oil feed from anyway?  I don't see it in any of the diagrams.

Brian

[luvrbus]

Brian, they have special bolts and feed from oil ports in the heads.


good luck

[bevans6]

Clifford, thank you..  Another dumb question or two - are both bolts per module drilled for the oil feed, or just one, and does the drone receive it's oil pressure from special bolts also, or is it bridged across from the supply module?

I was really sorry to hear about your home problem, I'm very upset right now over that.

Brian

[JackConrad]

The "drone" receives its oil from the "master" through a small tube the run between the 2.  I don't feel much response from the Jakes on our 8V71 when in high gear on a flat road, but in a lower gear, going downhill, I feel it much more.  Jack

[TomC]

If you're in Bakersfield, Ca have Don Fairchild adjust your Jake brakes.  I drive from Los Angeles just to have him do this.  Before I had him do the adjustment, coming down the northbound Grapevine with my car behind, I'd have to apply light brake pedal several times to control speed.  After he adjusted them, I had to switch the Jake from full to half and back to keep from slowing down too much. The Jakes on a 71/92 series really are effective if you have someone that knows how to precision adjust them.  When I was driving with my big rig with 8V-92TA, I could come down the Grapevine at 28mph and never touch the brakes-and man what a great sound that was!  Good Luck, TomC

[luvrbus]

Brian, you are not going to get the braking as TomC he has a low geared bus it works just the way Jacks says with 3:73 gears and less with 3:36 gears.
 I need to be in 2nd and 3rd for good engine brake 4th is worthless on my 8v92 with 3:33 gears and I do know how to adjust a Jake.

good luck

[Barn Owl]

Modern Jakes on four strokes can have more braking HP than engine produced HP. So in comparison, the two stroke Jakes are not nearly as strong. That is a bigger deal if you are dragging around 80k lbs. My bus is 26-27k lbs. and the Jakes are very effective. I have been down many 8 to 10% grades in the Rockies and never had to touch the brakes. Proper adjustment is important and I believe i have been told they should be done with the engine hot if you adjust them tight for maximum braking effort. The reason is that if done cold and the tolerances are tight, when the engine heats up and the metal expands, you will damage the engine. If anyone knows more about this I would like to find out.

[Barn Owl]

On interstate roads, high gear, long steep grades, my bus with Jakes applied, will not accelerate but will maintain speed, without applying brakes. As others have posted, it seems to become more effective as you go down in gearing. Keep in mind that speed has a lot to do with the perception of your Jake performance. Every time you double your speed it takes four times the energy to stop. So there is a lot more work that has to be done at 60 mph than 30 mph. The Jakes are still producing the same braking HP at a given engine RPM, it only seems like they are not as affective in high gear/high speed even though they have a lot more work to do, and are shedding energy just the same.

[TomC]

My bus has an effective rear gearing of 4.01-which is not so far off from 3.73.  And considering, I only have three speeds, and most buses with T drive have the additional 1st gear, the braking in a normal bus can be a lot more in 1st gear then I could ever have.  Yet, with a properly adjusted Jake, I had great braking in the Sierras on 8-10% grades in 1st gear (like second on a HT740). Good Luck, TomC

[bobofthenorth]

While I tend to agree with Clifford that 2-stage Jakes on a 2-stroke are just a waste of wire, my Jakes are nevertheless pretty effective.  There seems to be a wide variance in what gets signed as a 6% grade but towing the Exploder and grossing around 40,000 combined I can generally hold a 6% grade in 9th with only the Jakes.  We top out around 70 MPH in 9th so I can run the tach up around 2200 and stay under 70 MPH but like I said, there is a big variation in what gets posted as 6%.  For some "6% grades" I need to drop to 8th and slow down to around 55.  They aren't "slap your beak against the window" 4-stroke Jakes by any stretch but I can definitely feel them holding.

[bevans6]

I think the question that gets over looked is the speed and gear that you are in when you want the jake brakes to work.  I tend to think of them for rolling down big grades at highway speed, and I came down I77 from Fancy Gap going south, which is the steepest grade I usually drive on that is more than a mile long, just having to touch the brakes every 30 - 40 seconds to keep my speed in check, which was 60 mph in 4th gear.  Not exactly a lot of leverage to work with, for the reasons others have noted.  But there is another hill I take that needs second gear, and I find there the bus maintains speed or even slows down just fine.

My jakes are probable fine, but I think I should know how to adjust them anyway, so I'll figure it out!

Brian