Imo000

After installing the supercharger, the engine developed a condition that resembles as if the MAF is disconnected. I've posted another topic to check the pins for the MAF. Today I've checked the wires from the MAF connector to the CPU, and everyting is fine. After some trial end error testing, it became evindent that the MAF only funstions when it's completly uncovered. What I mean by this is, that if I install the 3.5" to 2.5" reducer over it the engine will not run. The sligltest upset in air turbulance over the MAF shuts the engine down. It will fire up but with great difficulty and I have to pump the throttle pedal in order to keep some of the cylinders fireing. As soon as the reducer is removed the egine runs perfect. I've tried to reset the computer with everything instaleld (including the SC boost pipe) but the problems still remains. Any upset in the air flow into the MAF causes the engine to stall. I don't understand why the MAF is acting like this. I've replaced the MAF with a spare one I had from another car but they both act the same way.

All the centrifugal 928 SC installs I've seen have the MAF under boost. Is there a certain way that they have to be installed or set in order to handle boost?

I'ver been trying to figure this out for more than a week. This is the only thing holding me back from taking it out for a test drive.

Jim R.

The maf handles boost quite well-nothing special done on install. I did have mine refurbished by John Speake when doing my install. Any ageing of the maf will be exaggerated by boost. Make sure there is no air bleeding from the system after the maf meters it. I'm not too familiar with the 85-86's, but if there is a rubber elbow between the throttle body and the maf, make sure it is tight and leak free(all air into the intake). Beyond that, there are guys who know alot more about this than I do.

Good luck, looks like a fun project.

Jim

Tony

Theyve been sucked on and blown through with no ill effects as far as ive heard. They have also out put more than 5voltswith no issues

Which version are you running CS or twinscrew?

Is your O2 sensor hooked up and functioning?

Gretch

The CSC installs I am familar with do not use a reducer anywhere near the MAF. Maybe your problem is caused by turbulence resulting from transitioning too quickly from 2.5 to 3.5.........Murph or Carl will prolly give you some advice, and even sell you the unique boot the have for their install......I dunno.....

Imo000

Thanks for all the suggestions.

Andy sent me a message last night suggesting the air turbulence as a cause of the problem too. I’m starting to believe that this is the actual problem. Yesterday, I’ve bought a 3.5” elbow at the Mack dealer and only used a small portion of the straight sections in the install. Today, I will redesign the intake section and incorporate the 3.5” mandrel bent elbow. I hope this will smooth out the air flowing through the MAF and eliminate the problem.

The SC I’m using is a centrifugal type (an older Paxton SSG Novi).

The ’85-86s use the same MAF as the S4 and ups do.

doug928

IIRC the MAF requires laminar flow across the element (ask john speake) I believe that is part of the purpose of the screens and if you are using smaller diameter tubing that is not mandrel bent it could effect the MAF depending on how you have it set up.

Imo000

Thanks for all the help, the problem was with the air turbulence entering the MAF. I’ve installed the mandrel bent 3.5” elbow and the reducer was moved to the end of the new pipe. The engine idles perfect and all the previous issues are resolved.

Now I can start getting an FMU and a blow off valve.

Herr-Kuhn

Yes, you need to watch for the transitions....MAF is very sentitive to that...otherwise you can blow through, though the preferred method is draw through for cleaner signal to the brain. On the TT cars, I have a transition before the MAF....it is sensitive to exact placement of the MAF to the transition, you need fully developed flow across the MAF for accurate readings! Mine run draw through. The hot wire can compensate for the density change in blow through situations and will still give you a mass airflow reading that is in line with what is flowing. The only issue with draw through is the turbulence as well as the added heat on the wire...if you are intercooled, that is not as bad as it sounds. Still the best is a draw through setup and it is exactly what you will see on any factory cars out there with hotwires.

On the TT S4 I might go with blow through, but I have not decided just yet. I think the hotwire would last longer and give more precise readings in a draw through configuration where the temperature isn't fluctuating with the boost levels. Blow through is done everyday though and works quite well.

Mongo

hey you guys not to hijack this, but would placement of the MAF before or after the supercharger have any effect on this too? Shouldn't it ALWAYS be place before the supercharger especially on centrifugals?

Imo000

From what I’ve heard over the years, draw through is better. Both of the SC cars I had before were speed density (no MAF). However a lot of times it’s easier to set up a blow through system. In my case there isn’t enough room behind the SC to install the MAF, and a lot of the centrifugal SC 928s are blow through too so it should be ok for me. With a heat shield on the exhaust manifold, there is barely enough room to squeeze a small cone K&N in there let alone relocation of the huge MAF.

When I was searching for a solution to my earlier problem, I’ve realized how much heat a centrifugal SC can generate under full boost. I felt the increase on my hand as I was revving the engine with the boost tube off the MAF. The increase is huge, a lot more than I expected. At higher RPMs it feels like the same as when the hot air defrost is on in the car at full blast. I will definitely install at least an air-to air IC very soon.

Does the hot wire MAF register the intake air temperature and adjust accordingly or is that the AIT sensors job?

Herr-Kuhn

Both are used to compensate...

Everybody thinks the CS blows cooler air than say a turbo...but the heat is a function of the compression....it is thermodynamic law, it must heat under compression. A good efficient turbo wheel will make about the same heat as a similar CS wheel...yes, they will get hot.

I'd like to see a CS with a draw through MAF...I think that would be unique.

Imo000

Ok, thanks for the info. I wasn't aware that the hot wire MAF register the intake air temperature too. Good thing I've asked.

John Speake

Imo000
The MAF has its own temperatrure compensation for air inlet temp built in to the venturi assembly.

The reason you had a problem with the restrictor in place was probably that the output voltage from the MAF was reduced below 2.5v at idle. Under those conditions, the LH ECU will go into "limp home" as you observed...

I've been away for a few days, so only just picked up this thread.

Imo000

John,

Thanks for the input, but in my case it was way forst then limp mode. The engine would not run at all. The car was undrivable. It was a challenge just moving from the diveway to the garage. In any case the problem definetly was the reducer being way too close to the MAF. Now, relocated at the end of a 3.5" mandrel bent elbow it all seems to work just fine.

John Speake

Ok, understood.