Recalibrate MFI to new cam specs and CR?
 

Hi everyone!

I have a 1973 911T with MFI. Former owner says engine got upgraded cams (Solex) and increased compression ratio of about 8.5:1 with stock MFI.

My question is: With the above upgrades, doesn’t my MFI need a recalibration to match the new specs?

Car runs great with plenty of torque in the low and mid range. Im very happy with it overall. My problem is I experience some on/off slight hesitation in the 4k & above rpm range. Could this be caused by an MFI needing calibration?

It’s got CDI+ ignition (dual spark) and a recurved dizzy.

I would appreciate any insights.

Thanks!

Randy

The mfi gurus will chime in. Short answer is yes, but when people start doing this and that to them then they become somewhat franken motors, like mine. TB's, heads etc. come into play as well because you have hot cams (solex) which are pretty much between E and S cams in a T car. Logic states that you need head work, stacks, etc. but that gets real expensive real fast, throw in MFI rebuild and recalibration and now you are really up there money wise. Some of what you are experiencing can be dialed in with an a/f meter and knowing how to adjust the mfi itself. I have a 72T, stock heads, rs pistons, E pump, solex cams. Am I getting all out of it, nope. But it runs frickn great and has now for 30 years. If money is a non factor, the have at it. Lets not forget exhaust, you would be surprised what different exhausts can do for these mfi cars. I think I have had at least 5 now.

Technically you need to recalibrate MFI for a camshaft change, since that changes the fuel curve requirement for the engine. (Changes like compression ratio and upgraded ignition would tend to require more fuel evenly across the RPM range - for those, simple enrichment changes are fine.) I believe in practice many who make changes like your engine has, get away with enrichment changes using the main circuit enrichment and "idle" circuit adjustments. If you want to do better, you can get a widebarnd O2 sensor to help tune. There is a great thread here on the forum that Jeff Higgins started about tuning MFI. I'd guess you could make adjustments for your engine using a wideband O2 and get it running very well.

.... and, it may already be about as good as you’ll get it. If you wanted to try something that costs nothing yet can tell you quite a bit about your state of tune, the age-old “run it hard in 3rd gear, cut the engine and coast to a stop, pull a plug on each side of the engine and ‘read’ the plug color”. No harm in trying, with obvious 21st century caveats about doing this safely. Google will yield photos of what you are looking at and want to see. Wide-band O2 sensor will, however, be among the best ca. $300 you can spend... takes away guesswork and sometimes, worries.

Best of luck,
John

+1 on the Wideband. You are running blind otherwise, especially above 4krpm, you don't want it to be running lean.

The other thing you can try (without needing special tools) is to richen the main circuit a few clicks and see if it runs better.

Quite satisfying to validate my hunches. Will now study options you suggested. Thanks to everyone for your inputs! 😊

How about a picture of that 73?

I will heartily second (or is it "third" or "fourth"?) the recommendations given above - get some sort of an A/F meter and start checking. There really is no other way. I bet you will find that your "T" space cam with Solex cams in the motor will work just fine. Compression really does not affect fuel requirements, so that one is kind of moot.

Just as important will be the test parameters you set. I have about a 20 mile loop that I run, which incorporates what I feel satisfy me with regards to a broad range of throttle and rpm conditions. I record individual run files by having a willing accomplice ride along with me to both operate the meter (I use an LM 1 that can record each run file individually) and write down just what that file represents. Mine only records rpm and A/F; I did not use any kind of a throttle position sensor, so keeping accurate notes was all the more important.

For example, "file 1" might be a full throttle pull up a long hill from 2,000 rpm to redline. "File 2" might be a steady state, part throttle cruise on kind of undulating, hilly terrain at 3,000 rpm so that various throttle inputs are required to hold that speed (1/4, 1/2, 3/4 throttle). "File 3" might be the same drill at 3,500 rpm, then "file 4" at 4,000 rpm. And so on.

It might be obvious that it is possible to record an awful lot of data doing this. Lots of run files showing A/F vs rpm and lots of notes telling you how any particular curve was generated. It can get a bit overwhelming... Thankfully, there is a "shortcut"...

What you are really trying to do is to keep the motor from hurting itself. The most likely way for it to do so with an MFI system that is apparently running well but is "out of tune" A/F wise is by running too lean under load. This is what you are trying to avoid at all costs. High rpm detonation is the most dangerous, and often will not be heard as the classic "pinging" noise.

So, the "shortcut" is to simply have that willing accomplice ride along and keep an eagle eye on that A/F ratio for you as you run the car through your test routines. If it goes too lean under load somewhere, simply pull over and richen up the main rack a couple of clicks. I like mine to absolutely never go any leaner than 13:1, but much prefer 12.8:1 or so. Keep driving and checking and adjusting the main rack until the leanest the A/F goes, at any rpm under load, is no leaner than that.

Notice I keep saying "under load". Especially if you still have the fuel cutoff solenoid and rpm sensor, trailing throttle A/F ratios on these things will go quite lean. Like lean enough to give an "error" message on my LM 1, well over 18:1. I've never seen that this is anything to worry about. You will notice, too, that as soon as you "tip in" to the throttle, A/F will go right back to "normal".

One final word (for now): this is not modern EFI. It's pretty darn rude and crude in comparison. Even proper, factory matched, original systems with correct cams and all of that show A/F ratios that vary a good deal more than we would expect on today's systems. Expect at least a full point, point and a half, or even pushing a two point variation across the full rpm range at full throttle. The key is, again, that very leanest point in the rpm range - don't let that go too lean. That's what will hurt it.

And yes, these systems run a little dirty. A little stinky. Sometimes a bit smokey with unburnt fuel as they "clear their throats" on their first full throttle run-up. Us MFI nuts (and we are "nuts") see these as "features"...

Wow! I absolutely appreciate your detailed reply and the ‘shortcut’ way of tuning it.

I will try the suggestions starting from the basics and will let you know of the results.

Randy
(another MFI nut 😊)

I'm looking forward to hearing your results.

Oh, and very pretty car, by the way. I know the camera can fool us sometimes, so i have to ask - Albert Blue? Or is it Oxford/Dalmatian Blue? I'm a big fan of Albert Blue myself... ;)

http://forums.pelicanparts.com/uploa...1606193869.jpg

I love that Albert Blue! But mine’s Oxford Blue. This color kinda grew on me now and actually learned to love it’s bold tone. I figured it’s something I need to have when driving these early 911s on the twisties. 😊

The best blu is when it's: 'tobluforu". ;) These are somewhat, kind of, maybe not, like old muscle cars with six pack carbs, crazy cams, manifolds, etc.... Where you spent a lot of time adjusting them pretty much all the time. Even though they seem hard to work on, they really aren't once you get the hang of them. Just a bunch of rods to adjust and screws to turn this way or that, so to speak.
Anyhoo, have fun with the car.

She's a Beauty!!!

All good information and help from Mr. Higgins - who is something of a guru around here.

I'd re-iterate his point about the AFR moving around a bit during use. Don't drive yourself too crazy trying to get to a certain number, the safe range of AFR's is fairly wide. It's more important that you not be running lean and that it doesn't stumble and fart. The factory CO settings correlate to AFR's on the rich side of what would be normal for a modern car.

Jeff and I have discussed this before, but MFI seems to prefer mixtures on the rich side, mine will surge a bit even if the AFR goes as high as 13.5:1. I suspect if I install a vacuum advance on my distributor this could improve, since the slower burning lean mixture could use a bit more advance. Do you have an installed and working vac advance mechanism?

The CDI+ is a great choice - I'm a big fan, and a customer.

Agree!

These cars are nice to look at at any color. But they’re a ton of fun when driven, especially on mountain roads!😊

Thanks to you, Jeff, and all the others who chimed in! 😊

Well noted on the AFR values.

I decided to remove vacuum advance when I had the dizzy rebuilt by Barry Hershon a few years back.

+1 on CDI+ of Jonny Hart. Car runs smoother with that dual spark system.

Regards,

Randy

Yup, we've been over all of this before and, like you say, even 13.5:1 seems "too lean" for optimum running on these cars. I've never thought of the vacuum advance approach - I used to run a straight mechanical advance distributor modified by Barry. Full advance came in at 2,000 rpm, well below where we run these things. Basically an "idle" setting and a "running" setting. But I'm not picky, so the subtleties of working out any stumble or farting with vacuum advance would be lost on me. A more genteel and civilized man would likely appreciate it more... ;)

Yup, Barry is the man. He rebuilt mine and told me to forget the vacuum advance altogether. Both of my sons run them, one on his '68 2.4 liter MFI hot-rod, the other on his '79 SC hot-rod. They run great on strictly mechanical advance, with that same very quick transition to full advance at 2,000 rpm.

In the meantime, I've switched to the Electromotive crank fired ignition setup. It, of course, has no vacuum advance capability. It's far more programmable than I'll ever use - once again, I simply set it for an idle advance of ten degrees and full advance of 23 degrees total at 2,000 rpm. Basically "idle" and "run". Works great.

It seems MFI is particularly well suited to this kind of caveman approach to advance curves. Basically the same advance across the entire rpm range, backing it off just to make it idle.

You will never get the MFI to perfectly map to the new cam. You can get it close with an AFR meter but there will always be a lean or rich spot. Not a big deal. Just know where it is and adjust throttle position when encountered.

Chris
73 911 E
DC 30 cams