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

[Bret]

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 - 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 by 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 and maintains 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?

 

[MGBGT_noob]

Any thoughts?

Megasquirt?

 

[Bret]

Any thoughts?

Megasquirt?

Always a thought - however the Moss EFI kit is the only one as of yet that has gotten it's CARB certification for emissions testing. If I was to go with another brand even if I pass the tail pipe test – technically I’d still have to go through an Emissions referee to be legal.

Lots of extra hoops to jump through. But this may be an option even though this was one of the things I wanted to avoid with my setup.

Oh well I'm pretty sure that if I had gone with a stock cam and stuck with the regular rockers I wouldn't be having this problem right now. But let me tell you I was totally surprised with the performance above 2000rpms.

BTW The sound is totally amazing! :wink:

 

[Nunyas]

I believe you're a pioneer on this frontier, at least in using the Moss EFI kit on these forums. Definitely, a pioneer in using it with the mods you have

Sounds like that vacuum "stabilizer" (or whatever you call it) is the easiest work around for your scenario.

 

[Nunyas]

Oh Oh oh, I just had a secondary thought. You might want to throw your green box on the car to ensure you're not too lean in the higher RPMs with that EFI. If the green box is too unreliable for testing the higher RPM bands, then maybe a trip to a shop with an appropriate Dyno may be in order to get the mix right... assuming you're given the appropriate information needed to retune the ECU...

Since the EFI is intended for a stock car with stock cams, etc, and it doesn't have an O2 sensor on the exhaust, I bet it is not compensating for the higher than normal amounts of air that your engine is capable of inhaling. And I bet there is a good chance that you may be running very lean across the board. Hopefully, it's not "too lean"...

 

[RonMacPherson]

Well, you're right about the diagnosis. The idle vacuum is way too low.

You've gotten an idea for a "patch", but any possiblity of a fix?

Valve timing.

Are you capable of advancing/retarding the cam timing 4 or so degrees to see if your vacuum improves?

How are things out there in the netherlands by San Jac?

When I was a wee one my Great-Grandma lived out there and we would go visit monthly.

 

[Bret]

Hey Ron

Ideally it'd be great if the folks at Moss would call me back to give me a yea or nay if they'd be willing to re-write the ECU’s code for the lower vacuum.

But I may just be stuck with the vacuum pump as the fix if they won't work with me on this. However I really don’t consider this a patch as it’s a common “fix” in Hot Rod circles for accessories (like brake boosters) that need the higher vacuum.

As for advancing or retarding the cam? That is a possibility as I purchased a degreed double camshaft. Right now it's set at 0 degrees (normal). I really didn't want to start playing with that until I was in full tweaking mode after I've got the basic configuration sorted out.

 

[Silverghost]

It seems to me I remember seeing or hearing about a canister that would hold vacuum for things that require more vacuum than comes from the engine at certain times? I recall it being a round black canister about the size of a small coffee can and it went inline on the vacuum line? I may be completely off base here but a mechanic friend had put a bigger cam in his engine and it didn't create enough vacuum at idle so he put this canister in.

Again, I may be complete wrong here but thought I'd throw this out there...

 

[Bret]

Reserve tanks aren't a bad idea but depending on the size they might not hold out if you're doing a lot of crusing at lower RPMs.

In my searches thus far the vacuum pump is the most common fix to low manifold vacuum issues.

 

[Silverghost]

Glad to know I had remembered that correctly...