Ok, I'm just about to the point in my project to start doing some tuning. Now this isn't on a SBC.... in fact it's not even a GM engine at all. I realize that this is not a "3rd gen" vehicle or engine, but I haven't found a better source of knowledge about tuning GM systems than this board. I have transplanted a GM EFI system (1227747) onto an International Harvester 345 truck engine. This engine is massive. It's physical size is about the same if not a little bigger than a BBC. It's a Y block design and weighs in at a whopping 725 lbs. dry. The rotating assembly is pretty massive as a result.... the forged crank weighs about 85 lbs. and the flywheel is just about 45 lbs without the clutch. Behind it is a Borg-Warner T-19 4sp transmission that weighs in at 155 lbs of which the 125 or so is the mainshaft, countershaft and gears. With a rotating assembly as heavy as that, the inertia it has is pretty amazing... the engine is capable of being lugged down to about 400 rpm without stalling.

This is obviously a slow revving, low rpm engine built more for bottom end torque than for top end horsepower. It starts to float valves at a little bit before 5000 rpm and it's rated redline is 3800 rpm. IH rates their redline at the maximum suggested "all-day" working rpm rather than the maximum rpm before engine parts start separating themselves from each other.

Here's the specs on the engine:

Bore 3 _7/8"
Stroke 3__21/32"
Displacement 344.96 cu in
Compression Ratio 8.28 to 1
Dry Weight 725 lbs
_ _
Intake Seat diameter 1.879"
Intake Head diameter 1.969"
Intake Stem diameter .3725"
Lift .440"
Open (before TDC) 21 degrees
Close (after BDC) 63 degrees
_ _
Exhaust Seat diameter 1.539"
Exhaust Head diameter 1.609"
Exhaust Stem diameter .375"
Lift .395"
Open (before BDC) 58 degrees
Close (after TDC) 18 degrees


Can anyone offer any suggestions as to what kind of spark timing and fuel delivery this thing might enjoy. I have it running now on a 5.7L-manual AHRT .bin and will start to log data in the next couple days. Just wondering if anyone here could offer any insight or suggestions to what I should start playing with first. Thanks....

 

Start with one of the BBC calibrations.
The basic enigne configurations are at least similiar as well as the operating ranges. Back the 70+ K/Pa timing down and 0 out the PE spark to start with. If you have trouble with fuel get some cop car ones. You'll need to use the Injector constant that was used with what ever TB your using.

I just did a thread on chipping a stock application, and it was for a BBC you might look at the shape of the timing and fuel tables I posted in the thread.

 

OK, I'll check into that. I used the small bore TB off of a 4.3L and replaced the injectors with 55 lb/hr (5235206*RPD) injectors from a 5.7L. If I understand you correctly, I should use the injector constant from the 5.7L bin in a 7.4L bin and make the other changes you suggested as a start. I used a 5.7L bin since the displacement was close to mine (5.6xL) but I can see the logic in using the 7.4L since the operating ranges are similar, etc. Thanks for opening my eyes to the overlooked.

 

There's also a recent thread about DIY Prom and a stock application you might look at.

Also, some of the basic engine design stuff.
As I recall, from a long, long time ago, the IH used a large combustion chamber, and this predates any real serious efforts at turbulent anything. It'd be interesting to compare the valve diameters, and bore to stroke, and rod lenght to stroke ratios and see where you are.

BTW, the timing and VE tables mentioned in that thread, reflect a fair amount of time, that I put into doing my truck, which is a BBC.

 

me personally. thinking that engine will both want and not stand alot of spark. having played with the IH motor in school buses its a fussy old girl with its own sort of thing going on. actually id think the 350 $8d tpi Ve curve might be a great fit. mixed with say the tbi 305 spark table $42 truck. its gonna want alot of advance down low but towards the peak TQ ranges it gonna want maybe 30 btdc total. excluding PE mode. id say work for AFR's with that old bird in the 13.6-14.1 range for light throttle and definately cool that big bad boy down with a 12.8 to 12.0 under heavy loads. as said ive payed with the beast its not a good engine. its just very robust. not even rometly efficient.

 

Well, I guess I should further comment on my intended use for this thing. This engine is a fresh rebuild in my 5500lb rock crawler 4x4 so I'm not at all expecting it to be fast at all and it will only see short bursts of WOT. Most of the time it will be at partial throttle (probably around 1200-1800 rpm) where the torque peak is. With an overall reduction ratio in 1st gear/low range of 81:1, I won't be hard into the throttle very often unless I feel like replacing broken axle shafts on the trail. I would agree that for a go-fast project, this would not be a good engine, but for a tow vehicle or 4x4 where low rpm torque and durability is a concern, I would have to argue that it is a good fit. Thanks for the suggestions... keep 'em coming.

 

did you miss my post or something. ??? i told you how to get good reliable power out.youve got a steep shallow tq curve. i also said its a very stout engine its just not efficent and really its not efficient.i wasnt critizing you i was pointing out what i had done with a similar engine in school busses.

 

No, I didn't miss your post. I noted your suggestions and even thanked you for them. They're greatly appreciated. I didn't take you're post as criticism either, I just realized that I hadn't stated the intended use of the engine.... considering that this is a 3rd Gen site and most people here seem to be tuning for ***** to wall top end performance and I thought that I might get better information from not only you, but from others as well if they knew exactly what I tuning for. Just trying to provide as much info as possible about what I'm doing to ensure that I get the best info for my purposes from all who comment. Sorry if it I sounded defensive or snappy.... that was not the intent. Thanks again.....

 

If you still have the original distributor you can check what the factory did for SA. Either a distributor machine or in the engine with a tach & timing light will give you a good baseline to start with.

RBob.

 

I had to modify the distributor to put a magnetic pick-up in to trigger the HEI module. That and I don't have a non-EFI version of the engine to do any testing with. My factory service manual does however, list some distributor test bench numbers. The FSM only gives me four data points to work with for mechanical and vacuum advance. I put those into Excel and plotted them with along with a polynomial curve fit which give me the following.




I can use these two charts to pick off points and "add" the two tables together to generate a spark table. If I do that, I get this...