Highlander944

Ok I'm new to this... trying to understand how exacly some of these tweaks to the Porsche 944 Turbo really work.

** Wastgate and Boost bolt questions **

Ok concerning the Wastgate shims and boost bolt. As I under stand this, the boost bolt allows a certain amount of air bleedoff around the turbo. The size of this hole controls the 'max boost' setpoint. The wastgate is a device which bleeds off excess pressure.

Now if you put a boost bolt into the car which doesn't 'open' till max boost is obtiained (some kind of spring loaded needle that could be adjusted by a threaded bolt) then the only reason to shim the wastgate would be if max boost can not be obtained. Is this correct thinking? Furthermore, if you 'messed up' the boost bolt so that it was set to 20psi or something, would the wastgate 'save' your car? It would see best to have a 'matching' configuration. Perhaps the wastgate set a few psi higher than standard boost.

So how good or bad is this thinking?

** MAF and MAP questions **

Ok now for my MAF and MAP thought. If I am thinking correctly the turbo 'rams' air into the engine. The standard Air flow sensor is restrictive. So the turbo has to overcome the mechanical device used to sence air flow. So if you put a MAP or MAF sysetm into your car, it would seem reasonable that this system does not 'dampen' the air raming effect of the turbo, thus allowing for 'full boost'.

Is it reasonable to think that 'over boosting' the stock air flow sensor should accomplish the same result less the responsiveness as the other less restrictive methods? IE, the 'boost correct' for having to push the mechanical gate open is... XXX??

Thanks for your help,

Peter

Tom M'Guinn

The boost bolt does not bleed off boost if that is how you understand it. The boost bolt simply sends intake pressured air to the wastegate via the cycling valve, to open the waste gate. The special boost bolts they sell are just restricted a little with a carb jet, so that the wastegate gets pressurized a little later than it otherwise would, so that the car makes higher boost. The best boost bolt or boost controller in the world will not make up for a bad wastegate.

MAFs and MAPs do allow for more air. Lots of people run them on these cars.

Tom M'Guinn

You might want to go to Clark's garage and read the 951 FAQ as is relates to boost control on these cars. It has a good description of how the system works.

http://www.clarks-garage.com/

Highlander944

I guess I'm able to convey my thought correctly.

I looked at that site. The wastegate DOES function as a boost bleed off. The boost bolt DOES function to control the amount of boost. So my thinking is proper? The exact how, is next.

Now regarding the MAF and MAP kits. Is my thinking correct here? Why would a MAF or MAP kit give you soo much more HP than over boosting? Has anyone ever tried measuring the amount of boost lost to opening the air intake? IE, a boost level of 12psi MAF or MAP == 15psi with stock air flow sensor?

Thanks for the link.

Peter.

Tom M'Guinn

Peter,

Not to belabor the point, but to be clear, the wastegate opens to redirect exhuast gases away from the hot side of the turbo, so that the turbo will spin slower and produce less boost. When people say "boost bleed off" they are usually referring to something like a blow-off valve that allows boost to escape once made. That is why one would normally not think of the wastegate as a "boost bleed off" device.

The boost bolt sends a little stream of pressuized air to the accuator diaphram on the wastegate--the more pressure on the diaphram, the more the wastegate opens. So, with a restricted boost bolt, it takes longer for the accuator diaphram on the wastegate to open, which means more exhaust gases are sent into the turbo hot side, which means more boost is made. I thought you misunderstood the system when you wrote: " The wastgate is a device which bleeds off excess pressure."

The question you ask about the stock air flow meter has been around a while. Some people swear that the stock meter is good up to ungodly HP numbers. I think the reality is that most people run the MAF's or MAP's becuase that is the simplest way to gain control over your fuel map. When you put on a MAF or MAP, you usually need some kind of fuel computer to receive the MAF/MAP signal and translate it into signals the DME (factory engine computer) can use. This also gives you the chance to tune the fuel curve so that you can run more boost, etc. Also, in addition to the factory airflow meter, the MAF/MAP set ups get rid of the long restrictive intake track (air filter box and snorkle to fender). This debate has died down lately, but look at the old rennlist mailing list archives and you can see people used to argue over the value of MAFs.

Waterguy

I'll try explaining these issues:

1. Waste Gate and Boost Bolt Question:

The wastegate is the method used to control turbo speed and thus the boost created. When it opens, it allows some exhaust to bypass the turbo, limiting the boost created. Wastegates have a spring that keeps them shut until a certain level of boost is achieved. They can also be held shut at higher boost pressures using pressure from a boost controller applied to one side of the wastegate to hold them closed at higher than spring pressure. Porsche 944 stock wastegates use a very weak spring (3 psi?). Most aftermarket wastegates are ordered with stiffer springs (0.6 bar to 1 bar, or 9 psi to 15 psi.) Shimming the wastegate preloads or "stiffens" the spring, allowing the wastegate to hold more boost.

Wastegates can be controlled by a boost controller. In the 944, this is the cycling valve (CV), which is controlled by a second engine management computer called the KLR. The CV gets a supply of pressurised air from the banjo bolt ("boost bolt"). It lets the air through to open the wastegate at preset levels. An 86-88 944 Turbo boosts to 11 psi to about 4000 rpm, then the CV starts opening for increasing time periods (duty cycles) to decrease the boost to 7.5 psi at 5800 rpm. The Turbo S holds 11 psi boost to 5800 rpm, then starts lowering the boost.

An adjustable boost controller is used to replace the stock CV. A check valve ("Lindsey Boost Enhancer") interrupts the pressurised air supply to the CV until a preset boost level is achieved, holding the wastegate closed longer for faster spoolup. A jetted banjo bolt reduces the amount of pressurised air getting to the CV to reduce the effect of the CV, resulting in higher boost but no reduction in lag.

2. MAF or MAP

You are partially correct about the AFM working in place of a MAP or MAF upgrade. The stock air flow meter (AFM) does provide some restriction and pressure drop. You can overcome this by boosting the turbo more to achieve the same horsepower as a MAF or MAP, up to a certain point. At some air flow (I don't know how much), the flap is wide open and the AFM is signalling its maximum voltage. Any increase in airflow past that point cannot be measured, so the computer will not add enough fuel and there goes your head gasket. The other big advantage of a MAP or MAF is that their signal output is easier to use with an add-on or stand-alone engine management computer.

HTH

Highlander944

Thank you both very much for your explanations. I'm sorry for my a-typical terminology, I'm new to this type of language. I have so many good resources available to me, I just wish I had all the knowledge to use them. I was schooled in EE and now work at a small machine shop which my Father owns.

Ok.. I did have a bit of a miss understanding. The modification to the bango bolt 'fools' the CV valve into boosting 'more'. The boost is reduced when the wastegate opens.

Thank you both very much for your patients and detailed explanations.

Peter.