MFI injection troubles .

[pjv911]

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

[Zeke]

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.

[Grady Clay]

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

[pjv911]

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

[pjv911]

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

[Oldporsche]

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

[jluetjen]

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 .

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.

[69911e]

Originally posted by pjv911
I should also add that my thermostat is installed but not hooked up.
Kurt Wiliams

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.

[304065]

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).

[Grady Clay]

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.
MFI thermostat spacers


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

[rswannabe]

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

[jluetjen]

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.

[cmcfaul]

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

[304065]

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.

[jluetjen]

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.

[ed martin]

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.

[derek murray]

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.

Please help - Headers & MFi

[pjv911]

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

[ed martin]

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.

[pjv911]

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