| Subject: 250 Cat with emphysema? |
Author:
Larry McKay
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Date Posted: 03/ 4/ 02, 9:12pm
This is a long one, so if you're not a Bluebirder into gearhead stuff, you might want to just skip it.
Last October we picked up an 82 FC35 off a lot in Vegas "as is". Nice solid unit with a lot of nice features (original paint in decent shape, good interior, etc.) and a lot of newish expensive items (12KW genset, tires, etc.), but with a lot of little and not-so-little stuff wrong with it. OK by me because the price was very right. Some previous owner, or whoever a previous owner paid to work on it (I shudder to think this could be the case), was a real hack mechanic and electrician, as evidenced by the kind of stuff I'm finding going through it and making it right. Whatever....this is why it was priced so low.
This unit originally had the Cat 3208N, 210 HP @ 2800 RPM, but came to me with a pretty fresh 3208T, 250 HP @ 2600 RPM. It has the original 4 speed Allison MT643 with the shiftpoints still set for the 3208N (too high) and with the loose (high stall speed) converter used with the 210 Cat instead of the tighter one you should use with the 3208T. I'll have an Allison shop reset the shift modulation soon as I get a chance, and will avoid lugging down and slipping the loose converter on grades to avoid frying the trans. She also has the original 5.29 rearend, which gives me a topend speed of 64 MPH at 2600 to drive across Texas with. I'm OK with that because we live, and do most of our RVing, in the mountains of the SW, and the short gears sure help you up the hills, especially towing. A two speed rearend sure would be nice......
The engine checked out perfect at the Cat shop. Injectors are timed stock, etc. But she seemed kinda gutless on the hills, even more so than I expected. She'd turn up 8.5 PSI boost max, and that was down at lower elevation, around 1500 MSL.
Got to investigating air cleaners. Turns out in those days Bluebird shipped 210 and 250 Cats with an aircleaner mounted on top the engine, trying to suck all their intake air thru a little 3.5" round flange on a cold air duct built into the RH lower engine doghouse. Too restrictive; need to open it up some, like double it.
But mine was different: There was a huge (like 18" square) remote industrial-strength updraft aircleaner mounted in the storage compartment just behind the LF wheel (right in the dust on a dirt road.....). The 5" diameter air inlet on the Cat 3208T intake casting was flanged down to 3.5" OD (roughly a 50% reduction in cross-sectional area), and was plumbed to this remote air cleaner with no less than 11 1/2 feet (!) of 3 1/2" tube. This tube included four 90 and four 30 to 45 degree bends (death to CFM flow) and crossed very close to the tailpipe, which undoubtedly heated the air some. But the best part was a 4 foot length of 3.5" OD corrugated steel flextube formed tightly around the exaust turbine housing of the turbocharger, literally touching it in several places and intimately close to it for a good 12", just before the air enters the engine. I can't imagine how much this further heated the induction air at full boost, with the EGT up around 1050-1100F. Ah yes, it's the old exaust-to-air interheater. The setup was pretty well constructed, but whoever did this obviously had no understanding of heat transfer.
I uninstalled the whole remote setup and rounded up an air cleaner housing at a local junkyard for $10. It has a 5" mounting pipe and it fits under the doghouse, just barely. It's off a 1980 GMC schoolbus with a 6.0L gas V8. I made the necessary mounting parts and put it on. Had to cut a couple small reliefs for the corner of this aircleaner in the steel stiffeners under the top of the engine cover. It uses a pleated paper element 5.5" high x 11" diameter. It intakes air thru oval slots in the bottom, but they were too few and too small so I added 1" round holes low on the sides to bring the intake crosssectional area up to equal to the 5" hole in the bottom. It would have been nice to duct in air from the front of the grill, but I couldn't see any reasonable way to add a second ~3.5" duct without performing mayhem on something in there, and that's what it would take, so it's breathing somewhat warmed air from behind the radiator instead. If not perfect, at least the inlet air will be a whole lot cooler than it was!
Took it out yesterday and ran it up a grade that I used to have to drop down to 3rd gear to get over. It accelerated up in 4th. Whoa! Boost pressure stabilized at 12.5 PSI. All this is at 3500 MSL and 60F; it will be interesting to see what she'll do at lower elevation. She used to put out moderate smoke at full throttle on a grade but that's gone now. I was afraid I'd hear a lot of intake honking noise thru the engine cover, but I didn't hear anything different so no problem. I have been seeing around 7.0 to 7.5 MPG but now I can't wait to see how much improvement I get.
I would be very interested to hear, from other Bluebirders, how much boost their 250 Cats produce when stabilized at max power, and what elevation they're at when they see it.
I seem to recall reading, somewhere in specs from Cat, that they spec their engines for HP and torque at standard conditions at sealevel, and with not more than some small pressure drop below athmospheric at the air inlet (ie. inside the air cleaner) to allow for the pressure drop across the element. To accomplish this requires a big element and minimum restriction. These things have straight mechanical injectors, not some hi-tech electronic closed-loop system. Thus the injection is calibrated to just squirt in so much fuel, depending on RPM and throttle position, and it assumes the airflow isn't blocked to get the right mixture. I'm betting a lot of Bluebirds out there have a lot more power and mileage in them, being lost to too-restrictive air intake systems.
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