Firstly - thanks to the forum contributors over the years; without the TGO forums I'd never have attempted this in the first place...

I have a '71 914 with an older (80s vintage) Renegade SBC conversion performed several owners ago (i.e. I have zero information beyond what I can see looking at it and the seller's claim that it was built to "about 350hp") that I'm gearing up to convert from carb to EFI, using a 454 TBI unit and '7747 ECM with EBL.

I'm trying to sort out a few simple things right now:

- The Porsche VSS is a reed switch that reads 8 pulses per wheel rotation (magnets in the diff). The signal wire is pulled high by the speedo, and goes to ground when the reed switch closes. My math says this will be about 6500ppm. Will the ECM input be OK with the 12Vpull-up? Can I adjust the ECM calibration to accept this pulse rate? (I believe the answer here is 'yes').

[edit: found the schematics for the '7747, looks like it has an internal pull-up to VIGN on the VSS signal so it should be electrically compatible with the 930 VSS]

- I've read quite a bit on the MAT vs. IAT topic, with sources advocating both but without having come to a final conclusion. Given that my initial objective is to get things running properly first, where should I be starting? The options seem to be a) birdcage-style sensor in the intake air flow, or b) a sensor of either(?) sort in the bung in the #8 runner. Engine compartment airflow in the 914 is weird compared to normal setups, and unless/until I route a cold-air intake feed I'll just be using an open filter on top of the engine, so I expect intake air temperature to be all over the place.

- The 454 TBI arrived with what look like 90# injectors (it was part of a package deal along with a bunch of other bits). Consensus seems to be that making this setup idle may be a bit of a challenge, but at the same time it looks to be the minimum injector size to reach a 350hp target without increasing the fuel pressure. Is there value in switching to smaller injectors and risking running out of fuel at WOT / high RPM in order to get the idle sorted first?

 

That's one crazy package you got there, but welcome to TGO!

On the VSS - stock ECM expects 2000 ppm signal. There are couple of avenues you may try. The easiest is to SGI-5 Universal Signal Interface Unit - see link below
http://www.dakotadigital.com/index.c...roduct_id=126/

Another approach is to modify DRAC/VSSB module used in 1990 -1995 full sized trucks, but this will require some DIY. For additional info follow link:
http://www.tbichips.com/drac/

Dynamic EFI EBL converted ECM - VSS pulse rate is configurable. This probably a best option since you will need to have a VRFPR to feed a mystery 350 with 454 injectors (90 lbs-hr). Follow link below
http://www.dynamicefi.com/

To run 90 lb-hr injectors you'll need VRFPR - fuel pressure will have to be around 10 to 12 PSI at idle (45 to 50 kPa) and 22+ PSI at or near WOT (90 to 100kPa). Idle tuning will be tricky if injector commanded pulse width falls below 1.1 mSec. My recommendation is to have you 7747 upgraded to EBL as it will resolve a host of barriers in getting your combo operational. I suggest that you read tuning with EBL thread and instructions on Dynamic EFI website.

//RF

 

Many thanks for the suggestions; as I noted I'm committed to the EBL route for all the usual reasons (better logging, great support, etc.) - just waiting on the box to arrive. I've been reading as much as I can; the Tuning with EBL thread is great, but there is ten+ years worth of stuff so very hard to absorb it all without the brain becoming full.

Thanks for making me go back and re-check the injector math - what you're referring to is much clearer now. Honestly, I have very little faith in the "350hp" number given the margin of error on everything else associated with the vehicle - looks like I should be adding an VRAFPR to the shopping list just to cover my bases.

I have a little experience with manifold-referenced regulators in boosted applications, but I've never dealt with the vacuum environment. It's not clear to me what the relationship between manifold vacuum and fuel pressure will be, so it's not obvious whether I can address the idle concern by moving to a substantially smaller injector (61# seems to be the next substantial step down, with 80# being too close) and raising the nominal fuel pressure to compensate (I should be able to make > 45psi at the ~35gph flow rate). More reading, I guess. 8)

 

The VRFPR (for example Aeromotive 13301) varies fuel pressure as a function of the intake manifold vacuum. So, for the sake of discussion, let us assume that your mystery 350 is idling around 700 RPM and absolute intake manifold pressure is approximately 45 kPa (vacuum ~ 15 in-Hg). Under these conditions engine does not need too much fuel to maintain idle - just enough to overcome internal friction and few external loads. With stock injectors 62 lb-hr injectors at 13 PSI fuel pressure most 350's will command around 1.3 to 1.6 mSec injector pulse width in CL(14.7:1). If you increase fuel pressure injector pulse width will have to shorten so that close loop CL is maintained. Injectors have a pulse width limit - how fast can it open before it is closed again. This governed by its mechanical construction, inertia of moving parts and fuel pressure. The shortest pulse that a TBI injector can provide is around 1.1 mSec. Installing 80 or 90 lb-Hr on 350 engine will require dropping idle fuel pressure below 10 PSI to maintain CL. Once pressure drops at or below 10 PSI the fuel droplets get large and fuel mist turns into large stream flow. This is less than ideal. Conversely, you can allow engine to idle pig rich in OL with 80, 90 lb-Hr injectors. Alternatively, if you can source B-body 65 lb-Hr injectors and adjust idle pressure to about 18 pSI (I estimate this will be right at the 1.1 mSec injector limit) while having WOT fuel pressure at around 28 to 30 PSI range. You'll have to do some math and see of you have enough fuel to sustain WOT HP numbers.

All of the aforementioned injector ratings are referenced to 13 PSI fuel pressure.


On IAT - I have used air duct birdcage style, I can not comment on intake manifold temperature version.

//RF

 

Thanks for the breakdown, and for some really helpful ballpark figures; I feel like I have a better sense of where I am aiming now.

Is it reasonable to expect ~12psi fuel pressure variation though? 15"hg is ~7.5psi so if the regulator is 1:1 on the vacuum side isn't the lower limit more like 22psi if WOT pressure is 30?

Going halfway, the math suggests 75# injectors would need ~22psi to meet the WOT number, and that would get idle fuel pressure down closer to 14psi (assuming my understanding above holds). Realistically, I expect to be on the low side of the 350 number, so this would give me room to drop the WOT pressure a bit to get more injector pulse width margin at the bottom end.


Aside from idle vacuum, is there anything I can realistically measure on my current setup that would help with these calculations? I did quite a bit of reading conversion threads but in most cases the writer either disappears halfway, or they're converting a known engine configuration...

 

i'm not too sure on the fueling part since i have never run a HiPo TBI build yet, so without having checked the math or having a gut feeling for the ballpark numbers, i can just say that RFmaster's approach basically seems valid.
using VRFPR and EBL sounds like the way to go. what fuel pump are you gonna be using? might want to check your max pressure.
since you can probably source 350 injectors pretty easily (for cheap) in the states, i would guess you would just start with those at higher pressure and go from there.. if it turns out you really need bigger injectors and/or higher pressure, you can just change that down the road. that's how i would do it.. with a mystery engine setup like that, everything else is a bit speculative.

for the IAT/MAT - what intake are you using? i'm just tuning a 355 with a holley stealth ram and i'm finding the heat soak of its MAT setup, which seems is nothing else than a CTS mounted in the plenum, very annoying to say the least. i'm basically opting for a "bird-cage" IAT style, but that might also depend on how much your intake path will heat up the air on its way to the chamber..

good luck with that porsche conversion happen to have a pic?

 

Actually on my 383 I use 62 lb-HR injectors over 10 to 20 PSI operating range. After a year of driving (it is in my 3/4 ton truck) I have yet to limit VE tables. It is a torque monster - great for towing. I use fuel pressure sensor to monitor regulated FP combined with WBO data (in the same data log) makes tuning a less frustrating activity! Thanks to EBL data logs.

Here is photo of my 383 set-up



I am also have a Renegade intake and 2" TB's which may end-up on top of another 383 or 350. I have not decided yet, need a suitable engine bay to drop into.

//RF

 

Fuel pump will be a Walbro 190, 9.5mm feed, 8mm return. I'm tossing up between the Aeromotive that everyone seems to like and the Fuelab 535/545 on the regulator side; logging fuel pressure seems to argue for more ports on the regulator...

No idea about the manifold yet; I haven't found any identifiable markings on it. I've attached a couple pictures of the engine bay (still with the carb setup) - googling around will get you lots more, this is a pretty common conversion.

RF - again, thanks for the datapoint. I'm off shopping for 65# injectors now.

= Mike

 

f

 

I have a 1987 TPI trans am. Can anyone tell me where the IAT sensor plugs in. I know its in the upper intake but i don't know where the two wires plug into.