Arrr yes indeed i spose so eh. Now, thank you all very much for all your input, its very much apreciated

 

Hey Guys, woke up early and decided to see what's going on. With my cup of fresh ground & fresh brewed coffee (and a Newport - yeah sorry to say after 20 years I started again - please don't hold that against me!) here I am, ready to spew forth with my 20 years' worth of working on Vettes knowledge.

Beginning in 1985, the L98 Vette went to 'MPI' (Multi-Port Fuel Injection) which was designated 'TPI' (Tuned Port Fuel Injection) and a 'MAF' (Mass Air Flow) sensor. From 1985 thru 1991 the L98 had a 'MAF' sensor and no 'MAP' (Manifold Absolute Pressure)sensor. If the 'MAF' goes south the engine will barely run and you'll limp home at best. At WOT (Wide Open Throttle) the 'MAF' is not used and the 'ECM' (Electronic Control Module - the engine computer) goes to a set of look up tables called Power Enrichment. The 'MAF' sensor is a Bosch hot wire and the fine platinum wire is subject to breaking.

In 1990/91 the L98 Vette used a 'MAP' sensor (no 'MAF') and speed/density fuel calculations. The 'MAP' sensor is a 1 BAR and thus does not recognize the addition of a turbo or supercharger. Non the less these vehicles can be made to operate very nicely with a blower and the 1 BAR MAP sensor when the chip (E-Prom) is properly recalibrated and the appropriate injectors are used.

In 1992/93 the LT1 Corvette continued to use a 1 BAR MAP sensor for speed/density fuel calculations and an E-Prom for programming the ECM. No MAF sensor.

In 1994-96 the LT1/LT4 Corvette went to a 'PCM' (Powertrain Control Module) and 'flash' memory (no removeable E-Prom) along with a new and better 'MAF' sensor (virtually indestructible) along with the 1 BAR MAP sensor. If the MAF goes south (or you unplug it) the 'PCM' reverts to speed/density and except for a trouble code and higher emissions you barely notice the difference. The MAF makes it much easier to add a blower because the PCM uses the MAF information at WOT and to a degree takes care of adding the extra fuel for you with the blower. In 1995/96 (OBDII) get beyond about 5 or 6 pounds of boost pressure and you set a Code 103 (excessive air flow) and the PCM reverts to the speed/density calculation. This is not good if you added a blower because now the PCM is now not adding the extra fuel it was before setting Code 103. This is easily addressed with a new PCM program.

A MAF sensor measures mass air flow (not CFM [cubic feet/minute] which is a volume measurement) and as such does not care whether it is placed before or after the blower/turbo.

OK I'm on my third cup of coffee and 7th butt and if my computer crashes you will not be seeing this post !!!

Now a thought question for you guru's. I place a CFM meter on a good running normally aspirated '350' and see 800 CFM at WOT @ 6000 RPM. Now I add a blower or turbo and at WOT @ 6000 RPM I make 10 pounds or so of boost or manifold pressure. I also now have a CFM meter before and after the blower. I take a set of readings at WOT at the 6000 RPM. What will I see/read on my CFM meters (I'm not looking for an exact set of calculations because I haven't given you enough info for that)?

BOV's and surge valves. When the throttle snaps closed after a WOT run on an engine with a centrifugal compressor (turbo or supercharger) the compressor goes into 'surge'. The air has no place to go so it actually reverses direction. After reversing the pressure collapes and the air goes forward again. This repeats many times until the impeller slow down and eventually the 'surge' stops. It's a rather cool sound but it's awfully hard on the impeller, impeller seal, and bearings. If the engine has no 'MAF' sensor, add a BOV and the problem (surge) goes away. The BOV is open whenever there is manifold vacuum and the compressor stays out of surge because the excess air is simply vented to atmosphere. However, if the vehicle has a MAF sensor, and is positioned such that it 'sees' the wasted air we have a no-no. That's because the MAF sensor will add fuel for the wasted air and your engine will run pig rich - another no-no. If the MAF is positioned upstream of the MAF you can get away with a BOV, as long as it is not too close to the MAF. The MAF is sensitive enough to see oscillating air even though downstream of the BOV if too close.

If the MAF sensor 'sees' the BOV flow then connect the discharge of the BOV to the suction side of the blower/turbo and the BOV becomes a Surge Valve and the MAF is now happy. The BOV (now a surge valve) has to be big enough to keep the blower out of surge at all times or the MAF will go crazy and the 'driveability' of your vehicle will suffer - you'll feel light throttle surging - as though the engine were too lean.

Last but not least all centrifugal pumps (including your water pump) like to push and not pull. Any inlet restrictions to a centrifugal pump will cause a loss of flow. So if the MAF is on the inlet side of the blower it's important to keep the restrictions to a minimum for max boost. This is the reason I located the air filter directly on the blower over the headers on my kits. Remoting the air filter generally causes a loss of power (more so than the filter being over the exhaust). In fact when the Vette is moving the air temperature over the headers is not much higher than ambient because of the way the air flows thru the engine compartment - the negative pressure under the vehicle pulls the air over the engine and down out under. That's why the 'air dam' in the front of the Vette is so important - it helps to create the negative pressure under the vehicle.

OK I'm now tired and said enough. Hope the above clears up some of the questions on this thread. Best regards, Greg

 

Hey Greg... nice post... to try and answer your question from a theory I have, (meaning I am guessing) unless the CFM meters are equiped to measure "MASS" the movement of air through them should be seen as the same. being that a MAF sensor measures how much voltage is needed to run across the sensor wires, the density of the air will play a part as denser air has more effect of manipulating the wire temperature then thinner air. I would also go as far as to guess that on a NA motor, you might see a slight vacuum between the filter and the throttle blades at wide open throttle due to a filter being even the slightest bit restrictive which is why measuring the mass, Grams per second would be a more finite way to tune then if it were to measure in CFM. but if it were a paddle wheel type flow meter, I think it would be blind to pressure and only be able to simply measure the flow rate and the calculated size of the measuring point so both CFM sensors would measure the same amount of "Flow" unable to distinguish between pressurized air movement and air movement at the standard 1 bar for a NA setup. am I close?

Chris

 

We should be honored to have someone who takes the time to answer all of our somewhat repetitive questions as Greg so eagerly does. Thanks!

Just to add a bit of to Greg's great explanation below.

1989 not volume (g/s [grams per second] is a mass measurement, the MAF compressed wave dissipates this MAF sensor is placed before an atmospheric vented BOV (i.e. the BOV is between the MAF and the TB), BOV (i.e. MAF is between the BOV and the TB), but the returning air must still be after the MAF (i.e. MAF -> return from BOV -> compressor -> BOV -> TB)
Any engine whether FI or N/A will only support a given volume of air based on the swept volume of the cylinders (CID) and the rate at which the engine is spun (RPM). However in a FI application, it is consuming the air at a faster rate, therefore both the consumption rate in mass per unit time or volume per unit time are increased. This is why many blower applications exceed the parameters of the stock MAF.

Secondly, the inlet temps in a FI application (when placing the air filter over the header) is typically within 10°F of ambient when the car is moving. This has been verified by real world testing. When stationary, the inlet temps can have a delta in excess of 60°F (depending on the extent of heat soak, fan operation, etc.).

I hope this helps.
Aaron

 

Thanks very much to all of you for that info, but jesus christ that was alot of reading!!!

Wouldnt dream of holding that against you mate, no future in being a quiter!!! If you enjoy it, might as well do it, could drop dead tomorrow mate, smoker or not eh.

 

Yeah, I knew that....

Bows to the east ~offers Greg Gold plated boost gauge~

Love MO

 

Hi Aaron and Icvette. First my hat is off to Icvette for taking his best guess - not a bad answer Icvette! Second, thanks Aaron for chiming in and helping. When one writes these long *** explanations at 4am with only one draft it's bound to need some help. Also thanks for the 'cudos' Aaron - they always make me feel good. For newbies to the Forum asking a question it does sometimes seem like I've seen that one before. There's a lot of good info on the Forum but you've got to be patient and do a lot reading - the answers are there.

Now to the 'CFM' answer. Aaron got it right but it might not be clear to all how to interpret. Let me explain by giving you the example again. If a given engine (eg. 350 small block) 'consumes' let's say 800 CFM (cubic feet/minute) at 6000 RPM @ WOT (which in fact is a realistic number) and makes let's say 400 HP and you then put a blower on it and make 600 HP @ 6000 RPM that engine is still digesting 800 CFM. The CFM flow meter at the entrance to the engine/throttle body reads the same: before & after installing a blower !!! However, if we add a second CFM meter to the suction side of the blower it may read 1200 CFM !!! That's where the extra 200 HP came from. The blower is a compressor and what happened at the entrance to the engine is the charge density changed from 1.0 to say 1.5 (I'm making these numbers up as I write so don't crucify me for being off a little). Again the engine still consumed 800 CFM with the blower at 10 psig but the air is denser/heavier and thus the extra 200 HP. Hope that clarifies how a supercharger works. Regards Greg

 

Yeah mate, very easy to understand. Its always great to get a straight forward answer in terms that can be easily understood, which at times on the forum is a rare find. Thanks very much for taking the time to explain all that and type it all out, i really apreciate it!!