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

I was going to just tag this to the thread that I've taken the quote below from but decided rather that threadjack I should start my own one. Here we go!

east302 said:

He has swapped the original poppet injectors to the MPFI. Here’s an article that goes a little into the difference:


https://www.fourwheeler.com/how-to/1012or-csfi-to-mpfi-vortec-engine-fueling-fix/

Click to expand...

I've read the linked piece and can see the reasoning (but elsewhere while researching this stuff I read that the poppets actually deliver a more targeted spray than an injector can which is why GM chose the set-up but can also see that there is still an element of that in that conversion) but as I'm not going down that route my questions relate to air flow.
I'm going to number the questions and anyone with any thoughts on any of the questions, please chime in with your thoughts.

1) Am I correct in thinking that the delivery tubes are routed to be clear of the runner mouths so as not to impede air flow?

2) Does the spider block impede air flow to the rearward cylinders?

3) Has there ever been a manifold for a carb set up that had the the throttle body located non centrally (as opposed to forwards on the CSFI Vortecs?

4) Does the TBI (even though it isn't a Vortec) have its throttle body central or off-set?

5) Between Vortec CSFI, TBI, and carb manifolds are there differences that relate to where the fuel is introduced. Ie. does a TBI or carb manifold have features that are there solely to aid even mixture distribution to the cylinders - something that isn't a concern with the CSFI system?

6) What is there in the way of a gasket between the upper and lower manifold halves on a Vortec?


The reason I ask all of this is that my Vortec is running on LPG (propane) and the LPG is introduced via a mixer venturi upstream of the throttle body (at the engine end of the trunking that carries the MAF sensor). Thus, mixture distribution isn't necessarily catered for by a manifold that relies on injecting fuel into the individual runners. The LPG is already in gas form when introduced but is still heavier than air and therefor can potentially suffer the same inertial effects that gasoline from a carb can. I'm looking at that spider block and pipes and thinking that isn't helping the cause at all.

My intention is to replace the inlet manifold gasket in the next few months and I'm only retaining the gasoline system as a 'get me home' provision should I run out of LPG before I can get to an LPG filling station (they are few in number and often have restricted opening hours). I don't even need the gasoline for start up - it'll happily fire up from cold on LPG.

So what I'm thinking about is (A) a way of simplifying (primarily getting rid of that spider block and pipes) the gasoline system right down to emergency only use and/or (B) should I consider another manifold (carb) with possibly a central throttle body to aid fuel (LPG) distribution and if that is viable, am I looking at both halves or is there an upper half that would mate with the existing lower. (My understanding so far is that the only lower manifold suitable for the Vortec heads is the marine one (but that they are hard to locate) but what about an alternative upper one?

On top of all this, I want to retain (preferably/presumably) my existing throttle body (with its TPS), don't want to lose cruise control and have in mind also the aggro to benefit ratio in that I don't want to have to (overly) fabricate unique parts, relocate other components (clearance) or introduce unreliability.

So, if anyone can help with any of the above please throw your ideas in - I'm open to all ideas. At the very least I'm keen to achieve plan (A) ie be rid if all the gubbins inside the manifold but I understand that the injectors must be downstream from the throttle plate to run with the existing PCM which I really don't want to change or mess with as I have a separate ECU for the LPG system. I'm also thinking about ditching the EGR (which I wouldn't countenance with a diesel but for a lightly stressed gasoline engine is OK IMO). I know this can lead to detonation but bear in mind the octane rating of LPG is higher that that of gasoline.

Thoughts?

Pinger said:

4) Does the TBI (even though it isn't a Vortec) have its throttle body central or off-set?

Click to expand...

It's central to the cylinders. The injectors fire synchronously meaning every other intake event one injector fires then the other. I was curious myself about which cylinders they feed so, I took some white zip ties and ran them through the left side (looking at it) to show what cylinders it feeds. The black zip ties show how the right injector feeds those cylinders.


With the firing order 18436572, you can see how they alternate to feed the cylinders. There are a couple passages that help to balance out the flow.



In case you're interested the EGR valve is fed by cylinders 4 & 6 through the 2 small ports you see between the intake ports. BTW they tend to plug up after so many miles. If you fail NOx you may have to pull the manifold and clean them out.


I thought the EGR was fed from both sides but the driver's side is not bored for it.

Ya learn something new every day (I just did), ha ha.

PlayingWithTBI said:

It's central to the cylinders. The injectors fire synchronously meaning every other intake event one injector fires then the other. I was curious myself about which cylinders they feed so, I took some white zip ties and ran them through the left side (looking at it) to show what cylinders it feeds. The black zip ties show how the right injector feeds those cylinders.

You must be registered for see images attach

Click to expand...

That, with its central throttle body location is what I think would be better for my LPG system - which, in essence, is like a carb (but with additional mixture control from the O2 sensors).
If there was something similar that would mate with my Vortec lower manifold and accept its throttle body...

PlayingWithTBI said:

With the firing order 18436572, you can see how they alternate to feed the cylinders. There are a couple passages that help to balance out the flow.



In case you're interested the EGR valve is fed by cylinders 4 & 6 through the 2 small ports you see between the intake ports. BTW they tend to plug up after so many miles. If you fail NOx you may have to pull the manifold and clean them out.


I thought the EGR was fed from both sides but the driver's side is not bored for it.

Ya learn something new every day (I just did), ha ha.

Click to expand...

No NOx testing here in the UK - just an annual inspection fail for removed/tampered with emissions kit. If I disabled it by disconnecting the electric valve connection - job done. Just re connect for the test.

Vortecs though take EGR from the driver's side manifold - a PITA if I want to fit headers. But, the guy I will be getting the test done by is a specialist in American motors (two miles from my door in rural Scotland!). Maybe he'd overlook the missing pipe..... I can ask.

After I logged off last night, I realised the manifold there is in one piece - not split.
I'm guessing the Vortec manifold is unique in being in two parts.
Thinking more about it, I think all I want to do (at this stage at least) is get that spider block out of the manifold. I can't see it as anything but an impediment to flow. If I could mount it externally and feed the supply tubes in through where it mounted (and a connection for the FPR) or just raise it a bit.

Still trying to get my head around the spider and where the injectors are located as standard.

Video here the guy describes 'old' and 'new' style. What does my 1999 350 have?
The 'new' he describes is like the modified set-up mentioned on the other manifold thread with the electrical connections away from the spider. I got the impression that was an aftermarket modification but the way the video is sounds like an upgrade from GM.
Will mine be the old or new?

(Looks like the vid will start where it's shown. If not - 1min 40s in).

Pinger,

The GMT400 Vortec small blocks originally all came with a Central Sequential Fuel Injection spider. The poppets were susceptible to particles lodging in the poppets and holding them partially open (once again, changing the fuel filter often and frequently goes a long ways to keeping things running on these trucks).
The fix was a Multi Port Fuel Injection spider. When I suspected my original CSFI spider was leaking I replaced it with the MPFI, but the original CSFI spider turned out to be fine despite having 230K on it.

Supposedly there is no performance gain between the two.

Also, although the fuel system on the GMT400's was kind of a unique "transition" fuel injection system, they work very well. The fuel system can support a lot more hp than what these engines are making. There are a lot of other things to do if chasing more power than spending money on the fuel system. Once the motor hits 350hp or more, than a guy can look at fuel.

In the past there was a company or two making higher flow spiders for these 1st gen Vortecs.

PlayingWithTBI said:

I was curious myself about which cylinders they feed so, I took some white zip ties and ran them through the left side (looking at it) to show what cylinders it feeds. The black zip ties show how the right injector feeds those cylinders.

You must be registered for see images attach

With the firing order 18436572, you can see how they alternate to feed the cylinders. There are a couple passages that help to balance out the flow.

You must be registered for see images attach

Click to expand...

Like virtually all two-plane manifolds, there's an upper and a lower plane. Each plane feeds the outer two cylinders of one bank, and the inner two cylinders of the other bank.

Different manufacturers may change which carb venturi feeds the upper and lower. For example, the OEM manifold may have the left venturi feeding the upper plane, while an aftermarket manifold might have the upper plane on the right venturi. But either way, it's feeding the outside two cylinders on one bank, and the inner two on the other bank.

PlayingWithTBI said:

In case you're interested the EGR valve is fed by cylinders 4 & 6 through the 2 small ports you see between the intake ports. BTW they tend to plug up after so many miles. If you fail NOx you may have to pull the manifold and clean them out.

You must be registered for see images attach

Click to expand...

No, not really. There's no opening in the head or the intake manifold gasket that "feeds" the two small slots in the manifold. The exhaust gas is supplied by the larger, rectangular opening in the middle. Apparently, the two small openings are just core passages that dead-end at the intake gasket. Those two slots--and the passages they create--tie the central exhaust port to the EGR valve. Those slots can plug by the gasket and not affect EGR operation. However, if they plug the entire length of the cast passage, they'd block exhaust flow to the EGR valve. This photo was intended to show the difference in coolant passages, but the lack of an opening for those two smaller slots is also apparent.

PlayingWithTBI said:

I thought the EGR was fed from both sides but the driver's side is not bored for it.

You must be registered for see images attach

Click to expand...

The exhaust doesn't cross-over to the other side, because the manifold is kept at a relatively constant temperature by the coolant passage that runs from the thermostat area to the right rear, for the heater core supply. On older engines, it's the exhaust crossover that heats the manifold.

Schurkey said:

Different manufacturers may change which carb venturi feeds the upper and lower. For example, the OEM manifold may have the left venturi feeding the upper plane, while an aftermarket manifold might have the upper plane on the right venturi. But either way, it's feeding the outside two cylinders on one bank, and the inner two on the other bank.

Click to expand...

I guess the point I was making is the TBI fires synchronously and by comparing how the manifold is made AND the firing order, this shows how it works.

Schurkey said:

The exhaust gas is supplied by the larger, rectangular opening in the middle. Apparently, the two small openings are just core passages that dead-end at the intake gasket. Those two slots--and the passages they create--tie the central exhaust port to the EGR valve. Those slots can plug by the gasket and not affect EGR operation. However, if they plug the entire length of the cast passage, they'd block exhaust flow to the EGR valve.

Click to expand...

My bad, I wonder if they need those ports there for machining or maybe when/if the passage gets blocked you need to clean it through those 2 slots? When I pulled that manifold they were plugged pretty bad, had to chip out the crap in there with a chisel.

alpinecrick said:

Pinger,

The fix was a Multi Port Fuel Injection spider. When I suspected my original SCFI spider was leaking I replaced it with the MPFI, but the original SCFI spider turned out to be fine despite having 230K on it.

.

Click to expand...

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?

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?

Click to expand...

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.