Spacecoast

After plopping in a newly built 406 (9.7:1 compression, Edelbrock E-Street Heads, Performer Intake, and Thumper Comp Cam) in my 72, I have been having a hell of a time to keep the Holley carb that is on it to stay tuned, especially in this soupy weather we have been having this Florida summer.

After reading a bunch of threads on the topic, I am thinking that my overall driving pleasure with my new motor will be drastically improved with an EFI system. I was thinking of running the following:

Holley Avenger EFI with 75 lb/hr injectors (https://www.holley.com/products/fuel.../parts/550-400)

Mallory Aftermarket Fuel Injection Distributor (http://www.summitracing.com/parts/ma...make/chevrolet)

I figure the Mallory Distributor will allow the timing to be controlled by the EFI but still keeping it with mechanical tach drive (still not 100% sure on this but will keep researching). And of course having to run a return fuel like to my fuel tank, but hoping I would be good to go with a nice street driver.

Was just curious if there was any other input or considerations on running the above with my 406.

zwede

The Holley avenger is a good choice. It's a very capable system that you can still tune more precisely using a laptop if you outgrow the "install it and go" mindset.

I had the old Commander 950 system on my '71 and I'm currently upgrading it to the Holley HP. It's the same computer as the avenger. I haven't started it up yet, still finishing up some wiring.

I don't have any experience with the Mallory distributor, but it looks OK. Says it's using a hall-effect sensor which is good. Many use magnetic sensors and those have such a weak signal most EFI systems don't play nice with them.

I went another route for my ignition: I use an MSD crank trigger, a CDI box and the distributor is gutted (just a rotor). I then added a hall effect sensor in the distributor for cam-sync (one pulse per rotor revolution). This lets me run the injectors sequentially. But if you're going TBI that's not an issue for you.

Cheapest way to go would be to replace your tach with a 75-77 electric tach. That way you don't need a tach drive. Then you can just use any GM small cap HEI distributor ($100 at an autoparts store) and use the OEM 8-pin HEI module. EFI can control timing, you don't even need a CDI box.

Spacecoast

Thanks for the input Zwede.. I'll have to price the 75-77 tach/small cap distributor route instead of the Mallory. That may just be easier and definitely cheaper.

kcarlasc

Check your vacuum... I tried the fast system with a Thumper Cam as well had a ton of issues.. Ended up using the Edelbrock setup instead. According to FAST their system required a minimum of a 10 vacuum reading. May want to check with Holly and your pull too

Spacecoast

Good thing to check before purchasing.. Thanks a bunch

Just out of curiosity, how does your Edelbrock system work on yours?

kcarlasc

So much better than the FAST.. easier to install, better documentation. It wont control timing like the FAST but I put the MSD E-CURVE Dizzy on and they work great together... Even without the vacuum issue.. FAST had the wrong harness in the box, went through 2 brains, 2 handhelds, a MAP sensor and a Temp sender.. all that before it even got fully installed.. We returned it all and the Edelbrock was just better all the way around...

wadebee

I am looking at the same mod as you. I have a 383 crate engine I bought from Smeding Performance earlier this year. They offered a $300 upgrade to the Thumpr Cam. They said it would work with stock converter. Although I got an extra 40 horsepower out of this cam I have had nothing but problems trying to get it to idle with the builder supplied/configured Quickfuel 680VS carb.

Bottom line is that this cam doesn't generate enough vacuum due to the large overlap between intake and exhaust valves. No vacuum means no good carb venturi signal. Smeding's only suggestion is for me to install a higher stall converter and drill out the carb throttle butterflies. I've drilled the butterflies and although it has helped, it is still not enough. Comp Cams says to "give it all the advance I can". I am at 22 degrees inital now and already causing detonation issues with the stock mechanical ignition curve. I installed a J&S SafeGuard to protect the engine from detonation until I can get this all sorted out.

I've pulled vacuum advance out and the only place I have to go now is modifying/locking out the mechanical advance as well. Nobody around here has a dizzy machine anymore to recurve the mechanical. It's just been a nightmare and I am ready to go with a full ECM control system (fuel and ignition) so I can have a real ignition curve and fuel map.

From what I've read, a TBI EFI system is going to suffer from the same issue (not enough vacuum to generate a strong venturi signal). I have been looking at the Holley HP and Dominator setups which are multi-port.

Did you ever pull the trigger on your purchase? How is it going?

zwede

The problem with low vacuum and EFI is not the venturi signal. The problem is the EFI needs some vacuum, as read by the MAP sensor, to properly determine idle fueling. If you have very low vacuum, the EFI sees almost no difference as load is put on the engine at low rpm. There's just not enough resolution and changes get hidden by noise.

Most decent EFI system (such as the Holley HP) will let you run alpha-n fueling at low rpm. This will help with a big cam. Instead of using the MAP sensor to determine load, it uses the TPS (throttle position). At higher rpm (the set point is adjustable), it switches to the MAP sensor.

Still, for a street driven vehicle you don't want alpha-n. It is not nearly as good as MAP. Best thing to do is to pick a cam that has a min of 10" of idle vacuum.