Bret's EFI Skunk Works hits a few speed bumps!

UGG!

Ok I'm looking for some thoughts. As some of you know I just replaced the old cast head with a new alloy head, installed a new camshaft along with a set of roller rockers on my old 78 MGB. Problem is I think I may have made it breath a little too good.

Above 2K rpms the engine runs great – however the Moss EFI kit is having a hard time with the lower manifold vacuum at idle. Darn thing actually hunts quit a bit between 500 where it sounds like it’s about to die, then up to about 1200 rpms where it seems to run fine before dropping off again.

Doing some investigative work - I checked the vacuum from the manifold at idle and it's about ~6 inHg during the hunting. But if you artificially raise the vacuum to say ~15 inHg with a vacuum testing device the engine settles down & idles normally at the chosen 850rpm idle speed. Sweet!

Now I’ve left a couple of emails with Moss's tech support gurus of the EFI kit explaining the situation. Mainly because the ECU is considered a “Back Box” and I have no way to fiddle with the MAP settings. Adding to the woes - there is a good chance that because the ECU is "CARB certified" (CA Emissions legal) intended for a Bone Stock Emissions MGB, they may vary well tell me to go pound salt. But I'm hoping they're willing to work with me, even if it means that I’m on my own when I have to take it in for smog testing.

Ideally I’d like to have them either reprogram the ECU for the lower idle vacuum setting or tell me where I can get one of the ECUs so I can do my own mapping even if that means that they’ll disown me, when it comes time for the next smog test in about a year.

In the interim I think I’ve found a work around.

In the Hot Rod world there’s an all too common problem caused when blowers & larger camshafts are incorporated. In these freer flowing setups it’s not at all un common to have issues with the lower manifold vacuum detrimentally affecting power brake boosters. Brake boosters like a nice healthy 15-16” inHg (mercury) to maintain boost. The fix is to put an electronic vacuum pump in line between the manifold and the power brake booster. This set up detects whenever the vacuum drops below 16” inHg starts the electric motor to maintain vacuum to the brake booster. But kicks off when the vacuum rises above that as would be observed heavy acceleration or during highway speeds – in-other-words above 2000 rpms.

I’ve placed an order with Jegs for of these vacuum pumps to continue with my testing while I deal with the tech support folks at Moss Motors.

Any thoughts?

Bret, I think the vacuum booster may work for you, but if it were me the first thing I'd do is swap out the roller rockers for stock pieces. I don't think your cam is aggressive enough to be the problem, so swapping them out should at least let you know if the rockers are in fact the issue or if it's something else. And honestly, I'm not sure the roller rockers are going to give you a HP gain to make it worth it.

Thanks Tim,

Yes its a process. But regrettably I’m starting to think your right. But I’m gonna go ahead & try it with the roller rockers for now with the air pump setup described above.

I’ll let everybody know how it works out.

I think you'll need some type of RPM switch or a throttle switch as well. What if you're spinning 5 grand with your foot on the floor? Very low vacuum and you don't want the ECU faked out.

Is there some type of idle switch with their setup? Maybe your TPS isn't adjusted properly. At idle, most ECU's can tell they are supposed to be idling and adjust accordingly. Sounds like that ECU doesn't know that.

I prefer Megasquirt.

The roller rockers shouldn't matter unless they have a diffrent ratio, if they add lift, that might be to much with the cam. Might have to turn up the idle with the added cam, if the fuel injection is depending on vac then any cam beyond stock won't work very well. How low are you attempting to set the idle, will it idle OK at 1,500 RPM. Why not use some weber carbs, would go nice with the added cam.

Last time I checked, Megasquirt doesn't want to play nice with the B's siamesed head. I think it CAN work, but it's tricky. Doesn't matter, though, as Brett's in California, which also rules out the Weber, I believe.

The MS community has done the siamesed heads. I've seen discussions of it. If I recall, your choices are: Use a throtle body injection system, put two injectors in the common port (one high, one low), and maybe put one injector in the port but wire it to both banks. Not sure about that last one. Probably would depend on duty cycle.

I think the TBI is probably the easiest.

Doc's correct. MS has worked with the siamese heads before. I wonder if the megaspuirt basic system will work with Moss's throttle body?

However as Tim pointed out - I live on the left coast in the land of OZ where anything goes (warranted or not) for the sake of the free to be you & me hug a tree crowd all in the name of better emissions.

But I digress..

The vacuum pump was delivered yesterday and is a bit bigger than I thought it was going to be but I'm petty sure I can tuck it out of the way under the bonnet.

On the Moss Motors front I have a contact in their tech support dept., but am travling and won't be able to get back with him until tomorrow afternoon.

I'm also in the middle of moving into my new home and hope to be fully moved in by the end of the month when I can spend more time on this.

Other than that I will keep everyone informed of my progress.

Thanks, Bret

MS will work with virtually anything. If you can get a plug the same as is on your Moss EFI box, you can wire up the MS unit to that and be plug compatible. That is, unplug the Moss unit, put MS in, tune, drive, enjoy full access to the fuel maps. Time for inspection: Plug the CARB approved box back in, get inspected, swap back again.

Or, do like the rest of the state is doing: Move somewhere else.

Moving isn’t really an option at this point as I just bought a house & started moving in this week in fact. As for most of those folks moving out of the state 90% of them probably ain’t natives anyway. No loss. :grin:

Anyway I plan on researching the MS system a little more to use it pretty much as you described and should the connector not be compatible - I've been told that I'm pretty handy with electronics so making up my own adaptor plug shouldn't be that big of a deal. Another cool thing is the exhaust I have has a the bun hole for adding a 02 sensor so mapping shouldn’t be that hard.

However I’m still going to need to have the vacuum pump to get the Moss ECU to run properly with my set up. Whenever I put it back in.

Heck – I’m thinking that since the ECU is kind of hidden out of sight in the passengers foot well, as long as I have all the proper smog equipment (cat & smog pump) in place, I’m thinking the MS would pass emissions just fine.

Worst case – even if I pass the tail pipe sniffer on the dyno & the visual inspection, to be truly SMOG legal I'm still going to have to have a CARB referee inspect my set up and give his stamp of approval. While I’m confident it will work – I’m not really looking forward to the paperwork.

My understanding of the Moss system is that it's a basic fuel-only system, in which case setting it up with a MS would mostly involve making a wiring adapter, calibrating it for the sensors if they're not GM sensors, and getting it in tune.

Yes & no - the Moss system looks at several sensors (like the manifold vacuum) but unlike most EFI setups it doesn’t use an O2 sensor.

According to my readings on the Moss EFI project, the lack of an O2 sensor was done as a cost saving measure. Now I don’t pose to imply that I know exactly what the Moss engineers where thinking at the time. But it seems logical to assume that keeping the stock cast intake/exhaust manifold, would’ve called for relocation of the cat farther down stream to accommodate a bun for the sensor. This probably would’ve also called for an aftermarket cat and some major pluming work. The other option would’ve been to toss the old cast manifold and a redesign similar to what I’ve done.