[Gmecm] PE lean issues on a ?747 TBI

[David Haggard]

1-- The ECM's program extrapolates the needed settings between the table RPM's and above 3600. This is helped by the settings in the #2 fuel "VE Adder" table. I found for my setup with the variable fuel pressure that I could flatten the #2 fuel table above 3600 and get what I needed.

2-- I don't know if an injector ever goes "constantly open." But I do know that on modified engines it is possible to overrun the injector's maximum flow ability. For mine with the higher, variable pressure, I found that the perfect injectors are the ones from the 350 Caprice "police" engine. The original 305 injectors couldn't come close to delivering what the engine wanted. When I upgrade to a 383 sometime in the (hopefully) near future, I may be forced to go up to the 454 injectors that originally came with the 454 TBI I'm using. But a flow-matched pair, not the originals!

2 continued-- The stock TBI can only flow about 440 cfm. That's fine for a stock 305 or 350. But when you open up the heads and valves and upgrade the intake manifold, it's common to use a 650-750 cfm 4-bbl carburetor for engines from 302 up to 400 cubic inches. I personally take that as a good guide for the size of TBI to use.
   With my ported heads and manifold able to flow 800 cfm, and the red-line of the engine raised to 6000 rpm, even my 305 was getting air-starved above 5000 rpm. The engine really needed the TBI from the 454 pickup engine, which flows between 650 and 670 cfm. That will carry over very well to my future 383. It just may need larger injectors to supply the fuel that the 383 will want.
   The bottleneck I just discovered is the air cleaner. Even though I've opened it up and feed it from a cowl hood, the filter itself is too small. K&N's web site has a formula for calculating your air filter size requirements. Using their formula, my 305 needs 110 square inches of K&N-type air filter. The 383 will want 140 square inches. My stock air filter is only 80 square inches, and it was a paper element. (Gee, maybe that's why the 'Bird can do 60-foot times of 1.5 seconds, but runs the quarter in a sad 14.2! The times are definitely pointing at a high-rpm power drop-off, and with my build, the only explanation is air starvation from filter or heads. I hope it's the filter! Heads are expensive.)
   So it's now got a new K&N the same size, and I've got one of their "X-Stream" filter lids on the way, for a total air filter area of about 180 cubic inches.

But I digress. I get off on examples and sometimes go too far. Sorry 'bout that.

David
'88 Formula Firebird -- "SR-71 Black 'Bird"

> -----Original Message-----
> From: gmecm-bounces at diy-efi.org 
> [mailto:gmecm-bounces at diy-efi.org] On Behalf Of Per Lonnborg
> Sent: Friday, July 08, 2005 9:22 AM
> To: gmecm at diy-efi.org
> Subject: Ang: RE: [Gmecm] PE lean issues on a ?747 TBI
> 
> Thank you very much David, Jay, Joe and others for your 
> answers, now I can move on!
> 
> I?ll check the fuel-pump, filter, flow, pressure and the 
> charging system to begin with.
> 
> Maybe it is that I?m "on the edge" on fuel delivery right 
> now. The links on adjust fuel pressure were very interesting! 
> Seemed quite easy to adjust fuel pressure.
> 
> Two more things:
> 
> 1. David, you mentioned The "Power Enrichment AFR vs. 
> RPM"-table, I noticed that it only has 9 tables(0-3200RPM in 
> steps of 400RPM), does that mean that what I put in the "3200 
> RPM" cell will also affect RPM above, say 3600-5500RPM?
> 
> 2. Is it common on stock TBI?s (exept for changes in the 
> calibration) with modified engines that the pulse-width 
> reaches it limit, i.e. "constantly open", with I guess burned 
> up injectors? Or is it more common that the stock trottle 
> bore itself restricts airflow before 100% injector PW?
> 
> I?ll let you know the results when I?m finished testing!
> 
> /Per
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