Simp's M923 Big Cam Repower

Don't blow up Truck of the Year.

74M35A2 said:

Don't blow up Truck of the Year.

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Least it will go out being worked rather than from sitting in the shop floor till the end of time and rusting the cylinders up like that ISL is gonna do!


Shes installed. Gotta wait for a turbo drain back tube as my other drains were threaded and this one has the typical bolt on tube. Someone wasn't thinking when they built it apparently that everything else is threaded and this one isn't.

Will see what happens. an HT4B can get upwards of 40lbs of boost. I may have to put the aftercooler back on if the intercooler can't take it.

Can you change a waste gate setting to get a little more air out of her?

BenRoberts said:

Can you change a waste gate setting to get a little more air out of her?

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Non wastegated turbos. They are reverse rotation too.

We lifted a 5.9 Cummins head off the block 1 inch at 72psi. Streched the head bolts. Would the big cam hold up to that?

BenRoberts said:

We lifted a 5.9 Cummins head off the block 1 inch at 72psi. Streched the head bolts. Would the big cam hold up to that?

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Yes
https://www.youtube.com/watch?v=GmiVtKUam3k

and the result

https://www.youtube.com/watch?v=8Td1CNBw0Zk

Subscribed!!

simp5782 said:

Non wastegated turbos. They are reverse rotation too.

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Does that mean you have to drive backwards?

That sucks.

Post install - I did not get on it too hard today. It boosts a little slower but you can't really tell it as it pulls way before the boost picks up. I did hit 32lbs of boost at one point. Mostly hanging around 25/30 before I am getting out of it. Even at a 40mph roll itll jump to 30psi quick and i am at 64 and approaching 2200rpms pretty quickly. I will probably be able to increase the fuel rate as she isn't even making a haze at the moment at WOT. EGTs don't even get near 800 even at WOT and sitting in a slump at around 1200rpms in 16th gear itll boost up quickly and not climb on temps... These are all unloaded with just the truck of course.. Will see tomorrow when I put the FMTV on the trailer and take her for a spin. I did have to double clamp the turbo hose as it blew it off at 35psi. Intercooler is rated at 28lbs continuous and up to 40lbs intermittently according to the builder.

Yes I know my Tach isn't working. I just got the parts in for a digital display tach and the digital sender for the PT pump i need to get it done.

video is a little shaky but i have not had time to get my go pro install done. https://youtu.be/XklePNFKCKw

Heck yeah, turn it up.

Same with the air governor. 152psi is not enough (gauge pic).

74M35A2 said:

Heck yeah, turn it up.

Same with the air governor. 152psi is not enough (gauge pic).

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Cat transmission likes having its air. its Turned up for cold climate. In case of a freeze up it gives me about 90 mins of operating time of air in the tanks for normal highway driving if a line freezes or something else. That is 150psi on 4 12gal tanks and 2 5 gal tanks. And 1 7 gal tank. It allows me to get to safety over those piddly 4 tanks at 125psi on a stock truck. Transmission has to have 60psi to operate above 11th gear and 85psi in the 7th to 11 range

HT4B bearing housings were never threaded, there is an adapter kit to go from T46 to HT turbos.

Welp. After a short run weekend before last she had a laundry list of problems... The new CAT i installed had a clutch and rear bearing issue. Basically a driveline vibration started to build up after some road miles. Pulled great up Happy Valley (Mountain climb NC/VA line on I77). Temps got up to around but that was my own fault on how i rerouted the heater lines to dump back into the intake so scratch that idea. Pulled up happy valley though around 57mph carrying an LMTV and grossing out around 58,000. Pyro was sitting at 1050 degrees post and 39lbs of boost. I did have that heat issue also i believe just due to the size of the fan. I was running the Big Cam 34" fan and it was having some love making conflicts with the radiator/intercooler combo so i went back down to an NHC250 fan clutch and the fan off it. Well it just doesn't pull enough air. SO. Yeah changed out to a bendix snub snout fan clutch that is normally closed (thanks to suprman) and added the 34" fan back on today. Not sure of the CFM difference but i am sure it is alot. I need a anemometer to see what kind wind speed it pulls.

Well here is the kicker. Along the haul, head gaskets started to seep oil. Not bad, just rustproofing. The CAT issue with the vibration caused other problems. It ended up shearing the front engine mount bolts off that I had setup as an offset setup. it then shoved the passengers rear mount into the bellhousing and cracked it all the way around the top. Held together for about 450 miles from Northern GA back to Cookeville and then on to Memphis. Got the bellhousing off and yeah it had cracked thru to every bolt just about. Welp back to work. Cab came off again. Spare CAT transmission being sent to me so i had some down time. I ended up basically resetting the whole front half of the truck. Stripped the engine down to the heads. Replaced the head gaskets and all that good stuff to the top. Replaced the bellhousing with a spare one I had. Reset the front engine mount to be straight on down the frame rather than offset to the passengers side. It is dead set center of the truck. This will be an issue for front driveshaft clearance in the future but I will just reshape an oil pan to account for that. Just seemed easier. Reset the motor in the rear to make it more level as well.

Doing all the mount reset caused me to redo the rear transmission mount as well as the raise the rear cab mount. I also had to raise the front cab mounts as well. You can see in the one pic of the 3 hockey pucks of how high i raised it over just 1 bushing being in there. It is only there for a mock up to keep it level with the wrecker taking a break.

So yeah. It is pretty much all buttoned up and back together. I did have to drop the radiator some to clear the fan and use some turnbuckle mount adjustments to hold the top of the radiator clear of the fan. will be working on a shroud for it as well. Reset the hood today a little higher as well. I did add an additional 27" x 8" x 6" mini cooler for the flow from the lower hose extra output that will circulate thru it and dump back into the water piping just behind the thermostat. This cooler will sit below the actual radiator in an air channel under it between the frame.

On a another note if you come across a CAT 7155 rebuilt by "Fred" out of ACOM at Ft. Polk I would stay away. That is the 4th known transmission he has touched that has had issues right out of the crate.

You’ve got to have a driveline issue with everything you’ve tore up/replaced since you first started changing motor mount locations.

Is it possible your engine/trans was, or is not parallel with the frame, or at a really weird horizontal angle compared to your axle pinion? I think you’ve already replaced/repaired then removed the several transfer cases, change drive shafts a couple times, replaced intermediate axle, entire bogie, a bunch of transmissions. I think the only thing left is a geometry issue, or someones building bad driveshafts.

And no it’s not a too much power, too many miles, or gross weight issue... not busting your balls, just want to see you get this thing ironed out.

I was always highly suspicious that nobody at Ft Puke knew how to properly overhaul a 7155. I still have one in the crate and my 916 has one that seems to be ok.

Jbulach said:

You’ve got to have a driveline issue with everything you’ve tore up/replaced since you first started changing motor mount locations.

Is it possible your engine/trans was, or is not parallel with the frame, or at a really weird horizontal angle compared to your axle pinion? I think you’ve already replaced/repaired then removed the several transfer cases, change drive shafts a couple times, replaced intermediate axle, entire bogie, a bunch of transmissions. I think the only thing left is a geometry issue, or someones building bad driveshafts.

And no it’s not a too much power, too many miles, or gross weight issue... not busting your balls, just want to see you get this thing ironed out.

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Everything had been fine since i did the CAT 7155 swap as well as the Big Cam swap at first. The truck actually ran fine. I ended up busting the good one cat transmission i was running by accident when I tightened the rear clutch up. I had a vibration issue on that when I put the big cam in I used the same rear transmission mount I had used with the NHC250. It was two bolts thru a cross member to hold the transmission up. Well on the bigger motors the rear mount sits on a spring that just rests on outer frame ears like I have now so it can flex. . This caused the bellhousing bolts to break off back in December due to it busting the rear mount.

The oshkosh 55 transfer case went out due to a mount getting weak and causing the rear bearing to ride funny and burned it up. I had about 45k on the transfer case. I swapped to the longer driveshaft when that happened in NJ. I believe that wore the front pinion bearing out in the front tandem which is why i swapped that. I swapped the whole rear bogie a few years back in 2016 due to what i thought was a vibration but come to find out the front tandem actually had some broken teeth from I don't know what.

This failure here was totally due to the transmission. going down hill at any grade you could hear the front clutch and the rear countershaft bearing just roaring. If i hit the jake you could feel the rear clutch slip and kick the jake off. It also had intermittent moments of not catching a gear on the shift. It would only cycle 3 burst and not lock the shift forks. It also took the oil in it which keeps it normal color for lots of miles and turned it black in 700. So something was burning up.

As far as transmissions. The 2 allisons lasted 30k each. about normal for the heat exposure. My original CAT lasted over 18 months and 120k miles and broke cause of my own stupidity, nothing else. First spare take out CAT i put in wouldn't move at all. Then this new CAT had its issues.

I had the motor sitting offset to one side of the truck like the NHC250 was originally. It is the same motor but the issue of oil filter clearance comes in. That could have played some role in it. Driveshaft angle from the transmission to the front tandem shouldn't play an issue on angle as long as its not a very severe angle. It is a pretty straight go at it on the rear angle. But I went ahead and centered it this time on the original strong mount and reset everything to being center. It is actually sitting pretty even in the frame rails now.

Power could come into play though if it is causing too much frame flex something has to give somewhere and the bolts would be the weak link on the frame side. As far as how it sits now it is pretty close to even at 6 3/4" in the front of the motor off the frame and a shade under that on the rear of the engine. I also used 2 pieces of 5/16 plate behind the drivers side engine mount between the frame to give it somemore space away from the bellhousing as well as 1 plate behind the drivers side.

Yeh, motor set to one side shouldn’t have mattered, just as long as it, the transfer case, and axle pinions are all parallel with the frame rails. That way you only have to deal with your horizontal angles, which can be measured with a cheap Harbor Freight magnetic angle finder. Component height in the frame shouldn’t mater, and the truck dont even have to be that level, just compare the angles to each other. Unless you need some stock base lines, then I can give you the angles from mine, setting on a nice level floor.

Something to think about, you cannot just bolt a flywheel housing on....Is that what broke or the bell housing on the trans? It needs to be dialed in. If not, the flywheel housing will break at every bolt that holds it to the block. I have a tool if you need it.