Hi all,

Just wondering, is it possible to run a TPI system and NOT let the computer do the timing? Could you put say, an HEI in place with a vacuum advance and somehow fool the computer into thinking it was controlling timing when it wasn't, or something?

 

I would think you could get it in the ballpark so to speak, but not good enough to consider it "tuned".

but who knows.

 

Why? You can tune the computer controlled distributor far better, quicker and easier.

 

Don't see why not, don't think the computer using any feed back from the dizzy, it just sends a signal to it. Your eng will run with no signal going to the dizzy like when you set the base timing.

 

I'd have to side with the chief Canuck on this one.

A mechanical/vacuum advance distributor can be set up to advance timing based on engine (distributor) RPM, and on engine load to some extent. However, those are somewhat limited, and the overlapping relationship between them can make ideal tuning almost impossible. One can set up the mass of the weights, curvature of the advance cam, and spring tension to tune a mechanical distributor fairly well. It is even possible to tune a vacuum advance via the springs, orifice sizing, and travel limts, and further tuning can be done if the system has a mechanical advance and retard unit. Integrating the two advance systems can be a week-long chore, however, and it may still never be optimal. A good tuner will get an acceptable pair of curves from each system, and the engine should run fairly well.

An HEI distributor, functioning ESC, and programming allow the programmer to map a nearly endless set of curves for RPM and load, and the advance curves can cross over each other, go negative, peak above adjacent curves, fall well below, regardless of a high/low RPM or high/low load (vacuum) condition. It isn't an infinitely adjustable map, but for all practical purposes, it might as well be infinite. It offers a lot more possibilities than a mechanical setup.

The added benefit of the detonation sensing and subsequent retard is a safety you'll never get from a mechanical distributor. And even the knock sensor attack and recovery rates can be tuned to be more conservative (safer) or more aggressive depending on your whim.

With the EST?HEI you can create a spark timing map, burn or flash a ROM chip, and load it up. Hit the track or dyno, and see what you've done. Thats about twenty minutes if you're as slow as I am, faster for most of the rest of you. Find the trouble areas, burn/flash another, and load/test it again. Inside of a day, you should have a pretty damned well tuned spark map.

Further, if you're running anything with a turbo or axial compressor, or sometimes even a stupid-long duration, forget about a mechanical distributor with vacuum advance. You'll be working on springs and weights only.

Moreover, a good distributor machine is going to set you back a lot more than all the tuning software, ECM hardware, cables, and even a new laptop to take along and tune on the fly.

I'm positive that Glenn and others can expound upon that a lot further, but that's a nutshell sized explanation.

 

Now that the general response is explained, your situation may actually benefit from the changes you propose. Given that you have an '82, and the ECM is probably something like the old 1224810, you may have some real difficulty in tuning your spark. I have been searching for at least a couple years for a mask file to permit tuning of that series of ECM, with no success. Unless you want to change the ECM to a later version (and repin the wiring harness) to enable you to program your own spark map, you are stuck with the original programming. In that case, you may be able to get a better result with a tuned mechanical distributor and abandon the EST system. In that case, you would still have to provide an HEI signal to the ECM, but it would only be a reference to the ECM to keep it from generating error codes and enable other RPM related functions. Not having access to the mask file, I can only guess what those other funtions might be, but I suspect you would need the HEI signal to make the car fully functional. You may even have to retain the EST and dummy the detonation sensor input to prevent setting error codes, although the EST really wouldn't be controlling the timing.

 

Since you're thinking about TPI, definately go with a 1227730 ECM and a small-cap divorced coil HEI with EST, and let the computer manage the spark. (then you can manage the computer. )

 

You'd have to dig in the computer code anyway since the ECM will throw the penalty flag as soon as it sees that the computer controlled distributor isn't present any more. It'll run but I'd bet it wouldn't run anywhere close to optimal, even with a good mechanicallys-supplied spark curve.

 

I don't think it's been covered in the above, so....

In addition to the EST outputs "from" the ECM, the dissy sends pulses (4 per crankshaft revolution) "to" the ECM so it knows when the engine is moving and how fast. To my knowledge (and it probably isn't the best), there isn't a Chevy mechanical/vacuum HEI setup that also outputs the distributor reference pulses, too. It's either one or the other. If you go with the mechanical dissy, you'll need to come up with another way to feed the ECM the engine position signal.

You could potentially mount a crank trigger on the front pulley (kind of like this : http://store.summitracing.com/defaul...=egnsearch.asp ) to feed the ECM the required signals to keep everything happy enough to inject fuel. I'm sure there are other imaginative ways to accomplish this goal for less cost if you played around with swapping/merging some dissy components from both flavors. As mentioned, you'd also want to disable diagnostics on EST related functions in the ECM if you go this route.

All that being said, I'd agree with the general consensus that having ECM control over timing is generally a good thing. But, even with that view, I don't want to squash someone's experiment or desire to think outside of the box!

 

You can do it. It is called Mega Squirt V2.2. I ran it last year. Glad I have computer controlled timing now though. I used a non-vacuum advance distributor, than a Vac. advance, than I stepped up to a computer controlled.

 

Wow, thanks for all the replies guys!

I was really enquiring about this on the behalf of someone else who has a TPI system in an 88 Camaro. The feedback has been most useful so I will pass it onto him.

I'm currently rebuilding an engine with TPI that I will be slipping into my car in the coming months. Watch this space.