Junkyard TBI

It's been a while since I looked at this, but I thought the only sensors needed were the TPS on the throttle body, O2 sensor, and the MAP sensor (IIRC MAP is built into the megasquirt). RPM pulled off the distributor. Maybe one of the people who have done Megasquirt here could chime in.

 

I need everything, TBI, wiring harness and controller. All I have been able to find on Ebay is Holley stuff.
I am either going to scrap the idea because of cost or try to find a private owner parting a 4.3 out.

 

You need the TPS sensor, O2 sensor may I suggest a wideband a bit more money but well worth it.
The RPM signal comes from the negative side of the coil.
The map sensor is built in but with proper programing the Megasquirt can understand just about any MAP sensor if you choose.
The harness is easy to build I went to local yard and got all the connectors and water proof relays out of the GM's and built my harness around that.

 

IIRC from our earlier discussion, the stock even fire computer sees all the coil firing pulses, and assumes for RPM calculations that they are evenly spaced. With an odd fire engine, the pulses are not evenly spaced, so the computer samples the RPM incorrectly at times. I'm pretty sure the fuel maps take RPM as an input, and if the RPM is read incorrectly, the fuel mixture will be wrong.

 

Granted an oddfire will run on an even fire computer but it wont run right by no means.

 

Are you saying the output pulses from an odd fire HEI tach output are distributed at consistent time periods at a given RPM?

 

I have a tach running from the tach output on my GM HEI odd fire dist. Since it's normalized for a tach i figure i can use that for an rpm input on a regular cpu.

I looked at the megasquirt for quite a while now but you can get a 4.3 '94 astro cpu from advance auto for about $145 or $85 if you have core of some kind. A junkyard CPU will run about $80 around here but they won't even remove the wiring harness for you (they don't want to be bothered with it and won't let you go back and pull it.

Does the stock 4.3 cpu need to be reprogrammed for this?

Since TBI is ported into the manifold and not the cylinder the timing issue should not be an issue.
So, if you're running an odd fire HEI dist on an OF 225 you should not need a magnetic trigger on the flywheel or the timing wheel, correct?

Seems like we discussed this before about a year ago. I am just slowly getting around to it have a TB and aircleaner plus odd assortment of stuff but no CPU yet.

 

Are you sure the output is normalized? Does it only pulse once per revolution of the engine? I'm running a tach off my stock odd fire points ignition and coil and it seems to work fine. Tachs have a pretty slow response time, and I would think tend to average the odd pulses. Just curious if anyone has verified the odd fire HEI puts out an even pulse train on the tach output?

 

As far as the Megasquirt compared to the GM unit, there will be no comparison. The MS will beat the Gm unit hands down. Simply becasue you tailor it to the engine and not use a generic controller. The GM unit will definitely be better than the stock 2G but if you want really good control the megasquirt is the way to go. If someone wants a MS unit let me know and I can tell you where to get one
already put together.

 

As I understand all this (not having done it but doing some research and planning to), Mike (Mcruff) is correct if you use the original PROM chip from the 4.3L or 2.5L ECM. However, any application other than the original engine will require tailoring. The GM ECM with the original PROM will start and run, and it will adapt itself to the new engine to some degree. To get the best performance, you must program an EPROM for the application. You do this by gathering information from the ECM while it is running, and adjusting the tuning tables to be loaded in your EPROM.

I expect the Megasquirt requires the same process, so no great advantage there. However, the tuning process for the GM ECM was not intended to be as easy as possible, where I expect the Megasquirt tuning process was. The GM ECM does provide spark tuning though, and maybe the Megasquirt does not - don't know. GM also offers a 'limp home' mode that means the engine will still run if the sensors fail - does Megasquirt? Again, I don't know. Plus the GM parts are widely available, so field repairs are going to be somewhat easier. Thus, there are tradeoffs for either option.

 

I agree, megasquirt is the way to go and that is what I am planning. I even got the OK from the cost center manager... er.... wife.

 

Megasquirt does allow limp mode, it also uses dynamic ram to program it. Simply hook up a lap top or PC and tune it at anytime including while your driving down the road. The newer version does have spark control bu the guys that have an oddfire don't use it because of the complexity. The megasquirt unit will run up to 2 injectors per cylinder from 1-12 cylinders. It will batch fire or sequentially fire the injectors, run odd or even fire motors. 2 or 4 stroke motors. There is even a tuning program that will allow you to tune from a palm pilot.
There are literally 1000's of them out there now and there are several guys that have started small business's making the units. I have seen several of the units on hotrods at car shows in the last year now.

 

A nit Mike - I'd guess the Megasquirt must use flash memory, not DRAM or SRAM. Neither DRAM or SRAM will sustain their memory without backup power while flash memory will. You don't want your tuning tables going away if you lose battery power! Flash memory has a downside compared to an EPROM though, in that it wears out sooner. Maybe not an issue, depending on how the Megasquirt accesses the tuning tables (ie once at startup or continuously).

Just gabbing - I find the topic interesting.

 

Dynamic ram I am talking about is a type of ram chip used in machinery, the info will stay there unless overwritten. Virtually all CNC's for the last 20 years have used it, never seen a machine have a chip where out. It is not DRAM as in a PC. Doesn't even look the same and costs about 5x as much.

 

Mike, I pm'd you.

 

Just curious about this one. If I install dual exhausts can I, should I, do I need to, install sensors in both exhausts?

If I keep my stock exhaust then I will put one in the connecting Y.