Monday, May 02, 2005


New Motor Time

I'm seriously kicking-around the idea of replacing the TBI (throttle body injection) 305 in my '91 Caprice wagon (named to John Phillip's top-ten most desirable cars list in the latest issue of Car & Driver!) with something possessing a bit more power. 165 HP just ain't cutting it in a car that weighs around 4500 lbs.

The thing is, I want more power without sacrificing economy. It currently gets nearly 20 MPG in my normal mixed driving, even though it's tired enough at 159,000 miles to consume a quart of oil every 600 miles. Now, this task isn't impossible - the later ('94-'96) Caprices came with the much more powerful (260 HP SAE net) LT1, and were rated at 17/25 on the EPA cycle - a hit of only 1 MPG. And the current Hemi-powered Magnum achieves similar economy numbers, albeit it with cylinder deactivation.

TBI engines are hampered by the location of their injectors, which requires a downstream manifold runner shape that's a relatively straight shot to the intake port of the head so that the fuel does not fall out of suspension. Additional, the throttle body itself is severely limited in airflow in the stock form. So I need to use technology to achieve my rough goals of 250 rear-wheel HP (about 300 at the crank) and 20 MPG.

Fortunately, GM already had fairly modern technology back in the late 80s, in the form of the TPI (Tuned Port Injection) engines. Long intake runners combined with port injection meant big torque (and thus horsepower) at lower revs - good for daily drivers and tall gearing. Unfortunately, it was only used in Corvettes and "high-end" F-bodies (a slight contradiction in terms) such as the infamous Camaro IROC-Z/28, and was never employed in B-bodies or pickup trucks. Too bad, since it would have been an ideal system (note that Ford employed a modified version of the Mustang 5.0's EFI in their pickup trucks during this era, and won the horsepower war for many years).

The main drawback of the TPI system was inadequate high-RPM airflow for performance applications, but that's much more a problem in a Corvette than in a station wagon. And there's plenty of induction parts out there to correct this problem. Also, the injectors are controlled in a batch-fire manner, where all are fired at once, instead of the more modern sequential firing sequence implemented on '94-up LT1s. For all practical purposes, this is a pretty minor issue.

I plan to swap in the correct ECM for a TPI engine (likely one based on mass airflow, or MAF, instead of speed density - this will make for easier tuning and less senstitivity to modifications), make the appropriate modifications to the existing wiring harness (since most of the interface is the same, I'll probably leave nearly everything but the injector wiring in place), and then figure out what sort of motor to put underneath it.

Not sure exact what I'm going to build up here, but I think I'm sticking with a stock 350 crank, a 350 block bored 0.030" over (yielding 355 CID, but more importantly, a round and properly-honed bore), cylinder heads appropriate for my power goals (ported L98 TPI heads, later-model GM Vortecs, or a mild aftermarket head like Trick Flows), and an perhaps a stock inatke manifold, or the Holley Stealth Ram (which unfortunately hurts mid-range power a bit while helping things up top). I've got two sets of 24 lb/hr LT1 injectors to replace the stock 19 lb/hr units, which will ensure sufficient fuel flow at peak power while being perfectly manageable at idle. A new fuel pump will be required to get the necessary 3 bar (42-45 PSI) of fuel pressure.

Should be fun - let's see if I get around to it this summer!

EDIT: Screw it - I should just go forced induction on my TPI system, using a SyTy PCM. Too cool.

i am in awe of your skills. is it worth the effort, though, rather than just buying what you want, used?
Through the effort comes education, which indirectly (or sometimes directly) improves my performance at my day job. This stuff also happens to be fun :)

It's also noteworthy that I'm typically building something that isn't available in the exact form I desire.
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