LanEvo
LanEvo Dork
9/11/20 10:19 a.m.

I have two vintage racecars running dual Weber DCOE carb setups: the 190E Cosworth and a Triumph TR4. For both projects, I'm trying to put together a bare-bones wiring harness. All I really need is spark control.

I'm looking at running MegaJolt Lite Jr. unless someone has a better suggestion. I have found sources of trigger wheels for each engine. Now I've got to figure out how to measure engine load. Should I run a MAP or TPS sensor?

What's the easiest way to do this for someone who sucks at fabrication?

Robbie (Forum Supporter)
Robbie (Forum Supporter) MegaDork
9/11/20 10:23 a.m.

The carbs should already have vacuum ports, yes?

Then you just need a little 'T' and you've got MAP. 

Trent (Generally supportive dude)
Trent (Generally supportive dude) PowerDork
9/11/20 10:24 a.m.

Map sensor. The Megajolt has a built in MAP sensor. It is that hose barb on the box. Run a vacuum line to that port and you are done

Robbie (Forum Supporter)
Robbie (Forum Supporter) MegaDork
9/11/20 10:25 a.m.

I should also say in the above that I don't know about how reliable MAP is on small volume intake systems. I've heard it can be very jumpy and not easy to rely on. So even if fabbing and plumbing MAP in is easier, you may still want to go TPS simply because it is a better signal. 

alfadriver (Forum Supporter)
alfadriver (Forum Supporter) MegaDork
9/11/20 10:31 a.m.
Robbie (Forum Supporter) said:

I should also say in the above that I don't know about how reliable MAP is on small volume intake systems. I've heard it can be very jumpy and not easy to rely on. So even if fabbing and plumbing MAP in is easier, you may still want to go TPS simply because it is a better signal. 

You can build a chamber system that has a mechanical filter build into it.

That being said, for a race car, that's not needed at all- vent the MAP to BP, and just run a spark that is for WOT.  Very simple spark curve based on engine speed only.  Maybe have some corrections for very high engine and intake temps.

For many years, Alfa's never had vacuum advance, and they work well enough.  If I were to do EFI, I would add some advance to help fuel economy- but for a race car, 99% of the time it matters, you are at WOT.

TurnerX19
TurnerX19 SuperDork
9/11/20 10:41 a.m.

First off Alfadriver is correct, you do not need to monitor the load, but if you must, with the DCOEs you will need to merge a vacuum signal from every cylinder to get a useable signal. Otherwise it will be too spikey.

Stefan (Forum Supporter)
Stefan (Forum Supporter) MegaDork
9/11/20 10:47 a.m.

On my ITB street car that I ran MegaSquirt on, I also controlled ignition.  I used the MAP signal and I needed to merge the signals from the throats and then add a small restrictor to the line to help smooth out the signal.  Worked pretty well.  The trick was getting the restrictor sized to get a decent signal without it being too lazy.  That took a little experimentation.

Eventually I dropped the MAP use altogether and moved away from straight SD.

I would recommend a vacuum chamber with perhaps a restrictor to help clean up the signal and keep the ignition curve relatively simple and you should be in good shape.

LanEvo
LanEvo Dork
9/11/20 10:50 a.m.

Thanks for the responses, guys. I'm new to this (I guess that's pretty obvious).

Robbie (Forum Supporter) said:

I should also say in the above that I don't know about how reliable MAP is on small volume intake systems. I've heard it can be very jumpy and not easy to rely on.

This is what I've heard as well. From what I've read TPS might be simpler. I just don't know how you add a TPS sensor to an old-fashioned throttle.

alfadriver (Forum Supporter) said:

for a race car, that's not needed at all- vent the MAP to BP, and just run a spark that is for WOT.  Very simple spark curve based on engine speed only.  Maybe have some corrections for very high engine and intake temps.

Wow. That actually makes sense! On the track, I'm pretty much WOT all the time anyway. Any downside?

Tom1200
Tom1200 Dork
9/11/20 12:31 p.m.

The ignition on the Datsun is all in by 4000 RPM, on rare occasions I see as low 3800 RPM, I'm revving the car between 5000-8200RPM.  So basically if your ignition is at full advance below you minimum RPM worrying about the curve is a moot point

I can't see any downsides unless you are trying to run lower octane fuel.

alfadriver (Forum Supporter)
alfadriver (Forum Supporter) MegaDork
9/11/20 12:34 p.m.

In reply to LanEvo :

My Alfa is now almost 50 years old w/o vacuum advance.  Runs fine.

The only downside you would ever have would be lack of fuel economy at part load.  And if you are spending a lot of time not at WOT when racing, you are doing it wrong.... 

The great thing about your digital ignition is that you can be a lot more aggressive with the spark timing and not worry about spark drift due to X reason- 3000 rpm will ALWAYS be X deg of spark- no bounce, noise would be really small (it would be related to the tone wheel resolution).  No hysteresis as the weights get pulled in and out, nothing of that sort.  Set it and forget it.

Stefan (Forum Supporter)
Stefan (Forum Supporter) MegaDork
9/11/20 6:08 p.m.

To add a TPS, you just need a d-shaped shaft on one end and a couple of mounting holes in the proper orientation.

I mean, it wouldn't have to go on the throttle shaft directly....

Putting one on the pedal assembly might be an option.

JoeTR6 (Forum Supporter)
JoeTR6 (Forum Supporter) Dork
9/11/20 6:15 p.m.

We ran a TR6 with triple Webers as Alfadriver has suggested using an older Electromotive HPV-1 ignition.  Worked great.  Even part throttle wasn't terrible, but fuel efficient it wasn't.  The spark timing was very accurate compared to the original distributor setup.

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
9/11/20 6:57 p.m.

One huge advantage of electronic timing vs. a distributor is that you can have a much more complex timing curve.  If you have access to dyno time, there can be pretty significant gains in adding timing after torque peak.

Streetwiseguy
Streetwiseguy MegaDork
9/11/20 9:00 p.m.

Simple would be whatever number makes it easy to start and idle right, transitioning to whatever it likes best above 4000rpm, because that's pretty much all you want.

Dyno time is needed to get much better than that.

Stefan (Forum Supporter)
Stefan (Forum Supporter) MegaDork
9/11/20 11:12 p.m.
JoeTR6 (Forum Supporter) said:

We ran a TR6 with triple Webers as Alfadriver has suggested using an older Electromotive HPV-1 ignition.  Worked great.  Even part throttle wasn't terrible, but fuel efficient it wasn't.  The spark timing was very accurate compared to the original distributor setup.

The fuel economy with individual throats is generally never good.  Just par for the course.

if you can get a TPS or MAP setup on something then you can spend some time rough tuning driving around the paddock/block and fine tune it on a dyno to find some power under the curve which could help in low speed corners or on starts (where you're often playing with the throttle to maintain position but to also try and get a jump on the other person, etc.)

Jesse Ransom
Jesse Ransom UltimaDork
9/12/20 12:41 a.m.

Any improvements in tip-in crispness, light throttle off of apex, or rev matching blip to be had with light load advance?

alfadriver (Forum Supporter)
alfadriver (Forum Supporter) MegaDork
9/12/20 8:06 a.m.
Jesse Ransom said:

Any improvements in tip-in crispness, light throttle off of apex, or rev matching blip to be had with light load advance?

There's more in getting the fuel right than the spark.  Where spark is a problem is if it suddenly retards too much, too quickly- that can be felt and sucks.  But with no vacuum advance, that's not a problem (vacuum advance = tip in retard).  The other issue with spark is if you having it retard too slowly, you will get tip in knock- which isn't dangerous, but really annoying.  

alfadriver (Forum Supporter)
alfadriver (Forum Supporter) MegaDork
9/12/20 8:08 a.m.
Stefan (Forum Supporter) said:
JoeTR6 (Forum Supporter) said:

We ran a TR6 with triple Webers as Alfadriver has suggested using an older Electromotive HPV-1 ignition.  Worked great.  Even part throttle wasn't terrible, but fuel efficient it wasn't.  The spark timing was very accurate compared to the original distributor setup.

The fuel economy with individual throats is generally never good.  Just par for the course.

if you can get a TPS or MAP setup on something then you can spend some time rough tuning driving around the paddock/block and fine tune it on a dyno to find some power under the curve which could help in low speed corners or on starts (where you're often playing with the throttle to maintain position but to also try and get a jump on the other person, etc.)

At part load, your foot acts as a nice controller of power.  Unless there's a lot of time spent going down straights at part throttle, I would really question the effort to get a really good part throttle spark map.

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
9/12/20 9:26 a.m.
alfadriver (Forum Supporter) said:
Jesse Ransom said:

Any improvements in tip-in crispness, light throttle off of apex, or rev matching blip to be had with light load advance?

There's more in getting the fuel right than the spark.  Where spark is a problem is if it suddenly retards too much, too quickly- that can be felt and sucks.  But with no vacuum advance, that's not a problem (vacuum advance = tip in retard).

That's why vacuum advance should be connected to ported vacuum, not manifold vacuum.  (That, and it makes for a much more stable/repeatable idle)

 

Not sure if any side draft style carbs even HAVE a ported vacuum circuit.  Vacuum source would be far too fleeting to be very useful.

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