Inlet manifold options/mods for a Vortec 350 on propane.

Pinger said:

The MPFI - not GM then?
I can expect to find the original GM SCFI in my 1999 Vortec - ie, MPFI wasn't a GM fix introduced during the production run but an aftermarket one?

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I believe the fix came about because the poppets would wipe out the cats before their 8 or 10 year warranty was up. It was a GM fix and it came after they ended production on the GMT400. The 4.3 lived on though and the later variants rolled off the line with the MPFI injectors. They snap in place of the poppets and do not alter trajectory. They also impede airflow just like the stock setup.

alpinecrick said:

Although there was GM bulletin recommending the MFPI as a fix I'm not aware of an AC Delco labeled MPFI spider. But Delphi makes one and at the time Delphi was a GM subsidiary. Delphi is no longer owned by GM but I think they are still a GM supplier.

And yes, you probably have the original CSFI spider in your Suburban.

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Fairly certain my upgrade spider was an AC Delco. Upgraded to it back in 06 when I put a new engine in at 57K after intake gaskets destroyed the stock 97 one. Sat for nearly 2 years and the poppets were stuck on varnished fuel.

Supercharged111 said:

I believe the fix came about because the poppets would wipe out the cats before their 8 or 10 year warranty was up. It was a GM fix and it came after they ended production on the GMT400. The 4.3 lived on though and the later variants rolled off the line with the MPFI injectors. They snap in place of the poppets and do not alter trajectory. They also impede airflow just like the stock setup.

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The MFI update spider was factory in the 02-07 Mexico trucks and I believe the 02 Vans.

L31MaxExpress said:

Fairly certain my upgrade spider was an AC Delco. Upgraded to it back in 06 when I put a new engine in at 57K after intake gaskets destroyed the stock 97 one. Sat for nearly 2 years and the poppets were stuck on varnished fuel.

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Probably were available back then, just haven't seen 'em in more recent times. I've had such good experience with Delphi stuff their spider is probably the equivalent.

Supercharged111 said:

I believe the fix came about because the poppets would wipe out the cats before their 8 or 10 year warranty was up. It was a GM fix and it came after they ended production on the GMT400. The 4.3 lived on though and the later variants rolled off the line with the MPFI injectors. They snap in place of the poppets and do not alter trajectory.

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Thanks - and for the other posts too guys, I'm starting to get a grip of this - at last!

Supercharged111 said:

They also impede airflow just like the stock setup.

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And that's my main concern here - especially as my incoming air is laden with LPG which is heavier than air and thus to a degree can suffer the same mixture distribution problems associated with carbs. Any ideas on how the airflow impediment can be eased while retaining the CSFI system (which I want as back up)?
All I can think of so far - if it's even feasible - is to try raise it clear. My rear resonator is gone (replaced with a flush fitting pop-off valve - an LPG thing) so I have some height there. My guess so though is that the open mouth of 'shroud' that sits around it on the upper manifold will then create turbulence so no real improvement unless I could some how fill that void. Goes without saying I'm paranoid about introducing anything there that the engine could ingest and cause utter destruction!

On the varnishing of the poppets. My motor has only done 40,000 miles and much of that will have been on LPG. Also, I use a decent fuel additive designed specifically to keep gasoline systems clean so the stock CSFI is what I want to retain as back up. Just re-jigged a bit to improve air flow within the plenum.

It only matters from 3000+ RPM. You can get the plenum spacer. Some people like them, some think they're a waste. It gives the air more room to maneuver around all that stuff in the way. I think Airdeano from Fullsizechevy had his hands on designing that if you want to google a blast from the past.

I think you're making a mountain out of a mole-hill.

The gas mixer and the manifold is going to have enough turbulence to keep the gas from separating from the air in the intake manifold.

I bet the gas-mixer is more of a restriction than the Vortec injection unit.

Schurkey said:

I think you're making a mountain out of a mole-hill.

The gas mixer and the manifold is going to have enough turbulence to keep the gas from separating from the air in the intake manifold.

I bet the gas-mixer is more of a restriction than the Vortec injection unit.

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Very possibly true.
But my thinking here is to try and make any improvements I can in the manifold while I have that apart changing the lower gasket. I can always attend to the mixer later as it's external. And there'd be no point in improving the mixer if the spider block was still an impediment which this thread has confirmed it is. That 3000-4000rpm range in 3rd gear is exactly where I need it sweet for short sharp overtaking on the tight roads around here.

To a degree, I'm thinking out loud on this thread but I've learned a hell of a lot more than I knew before.