MFI injection troubles .
 

Hey guy`s . I just installed a 2.6 (66mm x 92mm) liter twin plug engine with 10.5:1 comp . The MFI pump is an aluminum type 019 model (authentic RS) . The engine is useing S cams and 42mm rsr headers up to a GHL 2.5" outlet muffler . The throttle stacks are bored out magnesium units (69`-71`) with solex 2.7 RS throttle bodies. The injection pump was just rebuilt ($2000.00) by Gus at Pacific . The injectors are new (part# unknown at this time) .
I just installed the engine and at first had not enough fuel psi at the injector pump inlet (8-10 psi) . I found that the restrictor valve in the filter console was not restricting (broken spring) . I replaced that and now have 12-15 psi depending on engine load and rpm (is it supposed to fluctuate ? ) . As of now the power is close to scary but it runs poorly when rpm is held steady once above 1500 rpm . I drove the car home today . My 12 mile ride home includes a 6 mile stretch of open traffic free highway . I found that there seems to be no loss of power of any kind ...But when cruising along steadily the engine seems very lean and consistanly burbles and backfires out the exhaust . To prevent this I just keep on accelerating and then letting off . My question is . Is there a mid range mixture adjustment that I can set ? Does this sound like a mixture problem ? I have a chassis dyno and an exhaust analyzer but want to know how to set the mixture for part throttle(off idle) situations , before I spend the time hooking the car up .
In case any of you are wondering , i have the idle set at 950 rpm , the timing is at 28 deg at 6000 rpm and i balanced the airflow at the thottle bodies allready. I believe im ready to make the next step but just dont know how . After searching the web and this website I found many step by step procedures . Problem is that none have a picture showing where this adjustment is . Also at idle my CO is running about 4ppm and at 3k rpm is about 2.5 ppm . Any help would be much appreiciated .

Kurt Williams

http://forums.pelicanparts.com/uploa...1103157579.jpg

Interesting question. My stock system does the same burbbling at part throttle under light load (cruising at 65mph, or so).

Indications on my car was that it was rich. I adjusted the rack with the tool thru the fan, but haven't had it out yet to compare. I'll be watching this thread.

Kurt,

Welcome to MFI.

First things first.

So long as it isn’t too lean or too rich, go get some miles on it. Get everything settled in and establish a base-line to start from. Only then start the CMA procedure. Just because the engine machinery is fresh, the procedure is equally appropriate. Don’t discount anything.

Please report back:
Cranking compression?
Cylinder leak?
Ignition curve? (28 sounds too much, I would start at 24 @ 6000)
What is the CO at various RPM and throttle positions?
How did you actually measure the compression ratio? What variation among cylinders?
What sparkplugs (top & bottom)?


Not to worry about hooking it up to an analyzer. That will tell you in an instant which way to go. Occasionally the symptoms of too rich and too lean can overlap. A tester will tell – very worthwhile.

Here is CMA:
Check Measure Adjust (CMA)
http://www.scatliff.mb.ca/pelican/MFI_Check_Measure_Adjust.pdf


Best,
Grady

Grady im glad you are here to help.

"Please report back:
Cranking compression?
Cylinder leak?
Ignition curve? (28 sounds too much, I would start at 24 @ 6000)
What is the CO at various RPM and throttle positions?
How did you actually measure the compression ratio? What variation among cylinders?
What sparkplugs (top & bottom)?"

All I can tell you right now is that the compression ratio is estimated . The pistons are mahle 92mm RSR units . Heads are 2.7 units machined for the injectors. I was told that 10.5:1 should be correct . What do you think . This is assuming the cumbustion chambers are unaltered. I will repost the answers to your other questions tomorow after I test it . Btw is there such an adjustment for mid range ? Where is it ? Picture ? Many thanks.

Kurt Williams

I should also add that my thermostat is installed but not hooked up. Also the microswitch and solenoid are not used either . I do not experience any decel backfire regardless of throttle position or clutch pedal position when down shifting . The engine was set up that way . Im using a original type cold start fuel spray enrichment system using the solenoid on top of the filter console as operated by a momentary switch from the dash . Cold starts are near perfect . No fast idle lever either .


Kurt Wiliams

If it's backfiring through the exhaust, its rich.

Follow Grady's advice and let it "settle in" before you start "adjusting" it.

David Duffield

What was the logic for excluding those features and then tuning around their absence? If they really were redundant Porsche would not have included them. If you want the engine to run like Porsch intended, I suspect that you'll need then installed.

For example, you mention the microswitch is not used. My understanding and experience is that the purpose of that is to shut off fuel when the throttles are closed and the engine speed above idle because in that condition the engine would normally run richer then needed, waste fuel and backfire. Since the car doesn't backfire now with the microswitch disconnected, I wonder if it is set too lean for part throttle running. If the engine is set too lean for part throttle running (but OK at idle and WOT), you could very well have a lean surge condition. Does it go away if you richen it up? Does the backfire on over-run return? If it does you'll then need the microswitch installed.

How did you lean out the mixture as the car warms up if the thermostat is not hooked up? I have found with a lean mixture the car will 'buck' and shock the drivetrain at steady state, and with a very rich mixture the car will surge and burble at steady state trottle positions (cruising). Which condition to you have.

PS: If you are running very rich this seems to give greater power @high deltaRPM/deltatime conditions than the standard 12.5-13.0/1 air/fuel, but not for steady state HP.

Kurt, the thermostat installed but not hooked up will still change the mixture. If you are going to keep the housing in place, remove half the discs and put them away somewhere: this will keep the rod from moving as the engine/MFI pump get warm or as OAT changes.

Next, I respectfully disagree with John WRT the speed switch and overrun solenoid: Mine doesn't work and I've never heard the fuel cooking off in the exhaust on the overrun, e.g. when throttle is closed at 6000 rpm under load (if it was every going to do it, this would be a good time for it).

Find out where the mixture is – rich or lean? A MFI 911 will run reasonably well when way too rich. That has other consequences like washing the oil from the cylinders and diluting the oil with gasoline. This is particularly bad for a fresh engine.

With the thermostat not hooked to the hot air, not in place, or without all the thermodiscs, the system will run exceedingly rich.
“MFI thermostat spacers”
p. 1 end has CMA extension, p. 3 has nozzle, fuel flow, and fuel pressure.
http://forums.pelicanparts.com/porsche-911-technical-forum/169445-mfi-thermostat-spacers.html


I would recommend you make a point of getting EVERYTHING in the original MFI system working properly. Many times when something like thermostat or overrun-shut-off are left not functioning properly, you end up chasing your tail with the adjustments. If you feel you don’t want to use those parts of the system, disconnect them later, after you have the engine running properly and fully broken-in.

Best,
Grady

Kurt, in answer to your earlier qiestion about what compression ratio you have:

I do not claim to be the expert Grady is, but I have been calculating 2.6 short stroke compression ratios recently in preperation for my own possible buildup. I have not had the opportunity to CC the volume of the piston domes (still waiting for them to arrive), but calculated it based on known data points taken from Bruce Anderson's book. Page 139 has a graph of 92mm RSR piston CR's with RSR heads (76 cc volume) and 2.7 heads (68.1 cc volume). Both of those are 70.4mm stroke configurations. Using that information I calculated the piston dome volume at approximately 29.65 cc. IF THIS IS CORRECT, than a 66 mm stroke motor with a 68.1 cc head volume and a deck height of .040 inches, would get you a CR of 10.7:1. If deck height is .050 inches, CR would be 10.3:1. A deck height of .045 puts you right at 10.5:1. So it all depends on how your engine was put together.

Take this all with a grain of salt, because I'm new to this myself. Maybe someone else can confirm my calculations with hard data (CC'd pistons and heads).

Brooke

John Cramer. You might be right -- I was using it as a "for-example". There are lots of guys on this BBS who have had issues because the microswitch wasn't working. I'm not saying you can't run without it, but you have to admit that it was put there for a reason.

I think Grady and I are trying to make the same point.

The easiest way to adjust mixture on the MFI is to adjust the linkage to the MFI. Gus usually has these things dialed in when he sends them but you have a very custom set up. I would start there and connect everything else that Porsche intended to work with the MFI. Also, did you send your injectors to Gus as well?

Chris
73 911 E

Actually, you should not touch the linkage rods at all. The midrange adjustment is the same as the high-speed adjustment, which is through the main mixture adjustment screw.

The idle adjustment I believe only affects the mixture from 1000 RPM and below. I'll look through CMA and the other lit to verify if I can.

Chris;
That linkage is a really "blunt instrument" for tuning an MFI. You're basically adjusting the MFI rack position by fairly large amounts. Using the adjustment screws is a much more precise way of doing it. My experience is that you want the linkage adjusted to the factory spec of 114 mm's (going off of memory) and anything different then that messes things up more then the other adjustments will be able to fix. This is why the linkage is one of the first things that CMA says that you should adjust.

Kurt, listen to Grady. Hook up the thermostat and the make sure the microswitch is hooked up and working. Have the system set up the way it was originally designed and have the basics covered. That way you eliminate potential problems and don't have to wonder if maybe its the microswitch or a lean condition for example. Once the thermostat is hooked up the way its supposed to be with the proper amount of spacers, the thermostat will do its job to lean the mix as the engine warms up so you are going to have dramatically alter the adjustment at the pump (enrichen). As Grady stated, then you want to find out where the mixture is, rich or lean once you get the engine to operating temperature ( with the thermostat connected!) This is actually a very tricky proceedure, you have to do the measuring as per check measure adjust as the measurements have to be observed while engine is under load.

Kurt,

If you do decide to get all the MFI components working you may find the link below helpful. It, in part, discusses how to get the thermostat working when using headers... i.e. when there is no hot air tube coming off the left heat exchanger.

http://forums.pelicanparts.com/showthread.php?s=&threadid=148173&highlight=drawin g

Today was way to busy at the shop . I didnt get the time to hook the car up on the dyno and get the CO readings , nor did I check compression on the cylinders . The engine is allready broken in (about 8k miles) so thats not an issue . I bought the engine from engine builder Al Bass in Canada . It belonged to one of his customers thats having him do a 3.4 short stroke . Anyways the injection was allready setup properly in the past without the micro switch and thermostat . From what I understand this is common on hi performance applications . The engine is a little fussy for the first minute or so but runs (idles) perfectly smooth at 975 rpm once slightly warmed . I have no problem waiting for it to warm up . Since the car has headers it will be a burden to hook up the thermostat . Also I see no reason at all to hook up the fuel cut solenoid and microswitch . Grady im glad your here to help as well as some of you others . Its great to know there are others willing to help . I wish there was someone (talented) local to me because I have so many projects and no time to put them together . I hope that tommorow I can gather the info that Grady requested . Then we can move on from there .
BTW where is the adjustment for the midrange ? Anyone have a picture ? I wont be touching it until I know it needs it , but would like to know where it is . Here`s a picture of the engine new home. Just swapped in (after I took this picture) a beautiful set of early sport seats. Anyone need a set of white leather manual 80`s sport seats ?

Kurt Williams

http://forums.pelicanparts.com/uploa...1103242415.jpg

I actually ran my car for sometime without hooking up the thermostat. It can be done, but your going to have to set it very rich to compensate on cold starts and then deal with the attendent problems of an overly rich setting, particulary clogging your vacuum ports with carbon.

Ed why not deal with cold starts and have it set perfectly for warm running ? Its really a summer car anyway . For instance what do you recommend for cold starts when using triple webers ? See my point . I would never consider setting it rich to compensate cold starts . The oil would get contaminated and cause undo wear to my engine . As stated above I am allready running without cold enrichment and it seems fine . After setting the CO we will see . Maybe then it would be a problem.

Kurt Williams

Kurt,

Nice looking ’69.

OK, now I have a little better picture. There is a thermostat in place on the pump but it is set to have no effect? In other words it thinks the engine is at temperature all the time? This can be done just by adding shims – you might disassemble the thermostat and look. When everything is cold, you should remove the little access plate on top of the MFI pump just forward of the thermostat. Start the engine and carefully push down on the mechanism with a small tool. See the diagrams in the linked posts. If the thermostat is truly set to not function, there should be no change in mixture.

Is the cold start system functioning? This is the solenoid on top of the fuel filter console, the thermo time switch, and circuitry. You should also check that it opens promptly, shuts off promptly, and does not leak while the engine is running. Even the slightest leak can dramatically affect the mixture. I see the plumbing on top of the air filters; make sure nothing can ever fall into the engine.

I am concerned that it runs OK when cold. That is usually a sign that it is way too rich when warm and without a thermostat to lean it out. The exhaust analyzer will tell. Some can get away with too lean when cold – if you live in Key West, Tucson, or San Diego.

No, there is not a midrange adjustment. That is why it is so important to get the mixture correct. What is the S/N of the MFI pump? You can find the numbers in the CMA.

Compression ratio is usually something not guessed at – unless it is too low. Same is true with piston to head clearance. See if you can get those numbers from the builder.
If you have the occasion to have the heads off, measure them yourself. It could be anywhere from 8:1 to 11+:1. That will have a profound effect on how everything is set.
Until you know the details, I would back the ignition timing off to a conservative 24 @ 6000 and use very high octane fuel. You don’t want to damage those beautiful 2.8 RSR pistons.

Best,
Grady

Feel free to e-mail me at gradyclay@hotmail.com although it is best to keep most everything on the Forum for all to benefit.
G.

The engine builder told me it was 10.5:1 . I can see that there is 2 copper gaskets under the cylinder bases . I suppose this offers more valve clearance as well as a slight drop in compression ratio . I allready tested the valve clearance by spinning it over 8500 rpm , no traction in 1st gear on these cold new york streets . I am so impressed with the power this little bastard makes . For example today I drove home a customers 1980 euro 930 with only 37k miles . My 69` would toast it at any speed . Forget about 0-60 that wouldnt even look like a race . I cant imagine that the power may increase once the mixture is set properly . BTW Grady the car does not run well when cold . It just runs barely . Once its warm (1-2 min) then its driveable . After the oil temp starts reading something (aprox 5-8 min) the car has full power and a smooth idle . Keep in mind that here on Long Island outside temps lately have been between 20-40 deg f .

Kurt Williams

I have the cold start solenoid hooked up to a dash mounted momentary switch . I will look to make sure its not leaking as well as look for the possibility of something falling in . Thanks for the warning .

Kurt Williams

Kurt,

If you're are not experiencing a lot of popping through the stacks and bucking when the engine is cold and until it reachs operating temperature then my guess is your probably aready running very rich. By the way I notice your CO values are expressed as ppm. I believe the values expressed in CMA are in terms of percentages, that is to my understanding,a percentage of CO relative to the total amount of exhaust gases. So I'm not sure how ppm would correlate to percentage of CO. If I recall 4% CO is very low for an old system wereas 2.5 is optimal(new) and around 6% is the norm.

Did I miss something? Where have the CO values been posted?

Well, the CO values are in CMA.

Having said that, I don't know anyone who can get their car to run right with a 4.0% CO value. More like, 5-6 at idle, 7-8 at 3000 RPM.

Kurt, given your setup you may want to seriously consider an LM-1 Data Logger for realtime acquisition of air-fuel ratio data. The output can be graphed against RPM so you don't have to rely on the seat-of-the-pants dyno to know where the mixture is set.