Adding Jack Brakes to 8V71

richard5933 · February 05, 2018, 04:05:52 PM

I've been wanting to add Jake Brakes to the 8V71 in our 'new' 1974 GMC 4108a since we got it in the fall. Now that I'm actively searching for a working set of Jakes to transplant into our engine, I've got a few questions. The bus has a bone stock drive line including the DD 8V71 and 4-speed.

First, is it really worth the effort? I know that they are beneficial when driving through the mountains, but when I asked Luke about it before buying the coach he said that it was rare for East Coast buses to have had them back then. I did ask about driving through the mountains in the east (Blue Ridge, Appalachian, Alleghenies, etc) and he said that drivers did it for decades with nothing but the drum brakes that came from the factory.

Second, assuming that my hunch is correct and it's worthwhile to add them, just how difficult is it to get this to work. I don't mean physically how difficult, because I'm probably going to have a professional do the work. By difficult I mean how many other systems downstream will this affect? I know I'll need to rework the rear engine cover to make room for the taller valve cover, but I don't know how it affects other systems like the fast idle, etc. How many other things besides the actual Jake brakes will I need to plan for?

Maybe someone that's added Jakes to a naturally aspirated 8V71 can help me out here.

Also, I've seen the set that's available on eBay right now. Not a big fan of buying from eBay after getting burned a few times in the past. Any other suggestions on where to get the needed parts to put together a complete set of Jakes?

Thanks!

chessie4905 · February 05, 2018, 04:40:50 PM

They require longer bolts, higher fuel crossover tubes to clear units. You can go to the Jacobs engine brake site and download instructions and brochure and see what is required.

luvrbus · February 05, 2018, 07:03:27 PM

The fast idle buffer switch is $$$ and finding the bridges for a 8v71 is a task and they are costly if you can find a set.LOL when you get ready I have the the bridges and a couple of the fast idle buffer switch along with the complete set - the fuel lines .Watch the Ebay seller they will sell you a set of 71/92 Jakes and they will have 92 bridges

HB of CJ · February 05, 2018, 09:16:17 PM

Nothing but nothing sounds as sweet as the 8V71. The only sound sweeter is a 8V71 riding a nice crisp Jake. Single stage. Off or on. If it does nothing else it will make driving your Bus Conversion so much more fun.

richard5933 · February 06, 2018, 02:25:11 AM

Quote from: luvrbus on February 05, 2018, 07:03:27 PM
The fast idle buffer switch is $$$ and finding the bridges for a 8v71 is a task and they are costly if you can find a set.LOL when you get ready I have the the bridges and a couple of the fast idle buffer switch along with the complete set - the fuel lines .Watch the Ebay seller they will sell you a set of 71/92 Jakes and they will have 92 bridges

I take from this that the bridges on the 71 which I need are different than the ones on eBay for the 92?

scanzel · February 06, 2018, 03:55:49 AM

The guy in Texas that advertises has the jakes and tall valve covers but they are being removed from military 8V92's.

richard5933 · February 06, 2018, 04:55:17 AM

Quote from: scanzel on February 06, 2018, 03:55:49 AM
The guy in Texas that advertises has the jakes and tall valve covers but they are being removed from military 8V92's.

Will these fit a civilian 8V71?

luvrbus · February 06, 2018, 06:41:04 AM

Everything will fit except the bridges the 71 uses a shorter bridge than a 92 series because of the valve spacing but they will be 24v, the fuel lines won't do you any good unless you change the fuel stands and injector connections the military engines use a o-ring type fuel line

bevans6 · February 06, 2018, 09:50:02 AM

I've swapped jake brakes from one engine to another. I took them from an MCI 8V71 and put the on a NATO 8V71T. Some comments...

The bridges and adjusting the bridges correctly are pretty important. Mine were worn, and I had them re-faced. The bridges are the bits that connect the two exhaust valves under the Jake head and are operated by the rocker arm to push down both valves at once.

My jake fuel tubes were the O-ring type, and the NATO engine had the flare type. I swapped the O-ring stand pipes and injector fittings over to the NATO engine. I didn't torque the stand pipes to spec and had a fuel leak that I chased for a long time, until I took every thing off (again) and started from scratch. That time I found the torque spec buried in the manual - 45 ft lbs.

Use new square o-rings in the oil bridge tubes, and adjust them properly. Know how to check them.

The jakes are switched on and off by the governor, using the same part inside as the buffer adjustment. Fast idle also uses the buffer adjustment. The fast idle air cylinder control also uses the same part as the buffer adjustment, so now you have the buffer, the fast idle and the jake all in one piece and they interact. You need to adjust the buffer, make sure it operates the jake switch at no-fuel, and raises the idle to the correct speed on fast idle, all at once. Can be done, I did it, although my fast idle is a little high.

Is installing jake brakes hard? Not really, it's 90% undo bolts, put parts in place, do up bolts, do quite straight-forward adjustments, do some wiring, do some electrical tests. The other 10% is the 90% of understanding what and why of what you are trying to accomplish, adjusting the buffer switch and the fast idle correctly, understanding how to do that, and so on. You are supposed to have a bench jig to set the bridge adjustment but I did it on the engine. I guess I spent around 8 hours doing it the first time, but the engine was on a stand. The next time I did it the engine was in the bus and I was changing injectors, so that doesn't count...

Brian

TomC · February 06, 2018, 10:32:35 AM

True, buses did not have Jake brakes for years without problems. But remember, most buses in commercial service don't go to the out of the way places we do with our buses. Course, with me living in L.A, I get into the mountains and drive windy two lanes to get to the best campgrounds. And I can say without a shadow of a doubt, that I'm very thankful to having my Jake brakes. I just pull the Allison shift to first gear (1) and I get torque converter lockup in 1st gear as low as 15mph. Believe me when I say, most any mountain road you will be down at that speed.
Last summer coming down from Groveland, Ca (for Yosemite) on hwy 120, it is so steep and twisty, I had my wife follow me in the car so I could negotiate the twists and turns just with the bus. Leaving the bus in 1st, I actually used only the low setting (one cylinder head) on the Jake while coming down. When we were down, the brakes were good, not close to hot.
If you plan on coming west of Interstate 25, then Jake Brakes are highly recommended. Good Luck, TomC

chessie4905 · February 06, 2018, 10:50:11 AM

One option would to buy a used take out 8-71 with jake brakes. That would give you some spare engine parts. Also solenoids would be 12 volt, unless out of a coach.

Lin · February 06, 2018, 11:04:16 AM

We had Jakes installed when we switched from the Spicer to an Allison automatic transmission, so I do not know how driving with a Jake on a 4-speed is. However, I would consider Jakes an essential at this point. There are lots of things one can do without. As my father used to say, "you can learn to sleep on a 6 inch wide board if you had to." Or in other words, "I sure loves my Jakes."

eagle19952 · February 06, 2018, 11:36:47 AM

Quote from: Lin on February 06, 2018, 11:04:16 AM
We had Jakes installed when we switched from the Spicer to an Allison automatic transmission, so I do not know how driving with a Jake on a 4-speed is. However, I would consider Jakes an essential at this point. There are lots of things one can do without. As my father used to say, "you can learn to sleep on a 6 inch wide board if you had to." Or in other words, "I sure loves my Jakes."

they are effective on both transmissions just a bit more on the manual and not at all on most Allisons in 1st gear...there are a converter lock up circuit that solves that, iirc, standard fare on some fire apparatus. again, iirc.

Lin · February 06, 2018, 01:29:45 PM

You're right. The 647 we have does not lock up in either first or second, and that was a real annoyance. We did solve that with a valve that locks the converter when the Jakes are on. That, of course, would not be an issue in a manual.

eagle19952 · February 06, 2018, 07:58:40 PM

Quote from: Lin on February 06, 2018, 01:29:45 PM
You're right. The 647 we have does not lock up in either first or second, and that was a real annoyance. We did solve that with a valve that locks the converter when the Jakes are on. That, of course, would not be an issue in a manual.

not sure what he has, my statement is aimed more towards most Allison's and for sure the 740 series...newer than it i have no clue.