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auh auh is offline
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injector upgrade

How easy of an up grade is this and how much power will I gain?

I've heard that inorder to detune our cars in the US porsche gave us week injectors.
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Old 03-24-2005, 08:28 AM
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Piece of cake to change injectors. You can get injectors from a late 944NA, turbo, or S2 and they will all plug and play. You won't really gain anything but you can do it. Do some more research on ROW cars, it was more along the lines of compression, emissions, and timing that changed, not so much injectors. There is no easy way to HP on the NA. The keys to HP on an NA are MAF, cams, and headwork. Anything else you can think of might net you 1/2HP at 7K rmp so it's pretty much useless.
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Old 03-24-2005, 08:47 AM
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Azn is right, The ROW (rest of world) cars had different fuel/ignition maps then the US one's Giving the ROW more power. Changing the injectors of a NA is easy but its pointless unless for some reason your trying to get more fuel, which unless your getting more air in there is no point. Most of the high HP 951 guys change to bigger injectors because the stock ones just cant support the Fuel need for that HP level and will start to exceed the 80% duty cycle limit. AUH ae you trying to get more power from your 944???
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Old 03-24-2005, 12:52 PM
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sure am trying to get more power. Only thing is I'm trying to do it on a buget which I see is not easy...lol
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1974 914 1.8 Sold...
1984 944 my baby...more of a pain in the @$$ than takin' it with a cucumber, but I still love it. THE CAR YOU PERVS!!!!!!!!!!!!
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Old 03-24-2005, 12:54 PM
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HAHAHA, I gave my budget up a long time ago. Try getting a new perfomance chip or a ROW DME, I think you could just swap it in but Im not sure. Also I have heard good things about WEB CAMS #274 grind
http://www.webcamshafts.com/

Its gonna be really hard to make more power on a buget, you wanna go fast you gotta pay

HP=time +$$$$$$$$
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Old 03-24-2005, 01:01 PM
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If 951 injectors are plug and play, would they be enough to allow the DME to add enough fuel for a supercharger?
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Old 03-24-2005, 01:31 PM
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YES anything higher then about 6psi needs bigger injectors then the NA's In fact Speed force racing sells them with there stage 2 SC kit. Of course you will need something to tune the injectors in or the DME will have no idea it needs more fuel. Since the NA DME's have no way to understand boost they cant not adjust for it, unless custom chips are made or there is a manual way of tuning like a ARC 2 type setup
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Old 03-24-2005, 01:40 PM
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So are you saying the MAP sensor doesn't actually meter the air ingested by the engine? If the supercharger feeds from air metered by the MAP sensor why wouldn't that be enough info for fuel metering?
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Old 03-24-2005, 04:33 PM
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Well, I didnt know you would be running a MAP sensor, But there is still a problem, unless you tweek the fuel curve with a chip or a ARC2 setup the DME only has the NA fuel/ignition maps to look at so it will see what the MAP sensor is telling it and go to the closes map it has to try and compensate. This is the reason why Speed force racing includes what I think is the ARC2 so you can trick the DME to putting in more fuel. Im not sure what setup you are running, If it came with a MAP sensor then Im sure they gave you the proper tools to tune with. Im talking more along the lines of someone just bolting on a supercharger and thinking the DME knows whats happening. The stock DME has no way to know what boost is or how to deal with it, Infact the stock NA DME has a hard time dealing with the extra air from a MAF setup. Thats why there are signal massagers like the ARC2, Also I know that GURU RACING has some sweet MAP and MAF setups that would be great to use if your making your own SC system.
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]87 924S Gaurds red- SOLD after 11 years of ownership
Old 03-24-2005, 07:25 PM
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The problem is that the Stock NA DME was never programed for Boost, It was just made to handle the stock setup. Once pressure goes over atmospheric (boost) It has nothing to compare readings to. Its like if you went to school to learn how to build 911 engines and then some one wanted you to build a jet engine (with no info on how they work) You would give it your best try but it would be no where near running because you dont have the proper skills to do it.
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]87 924S Gaurds red- SOLD after 11 years of ownership
Old 03-24-2005, 07:31 PM
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Yeah, trying to make the stock DME work with forced injection is difficult, not impossible. You'd replace the AFM with a MAF that can measure the extra flow. Then squeeze the entire stock fuel-map down into 1/2 the data-cells on the chips. Then program the empty space with fuel-values for boost.

FWIW, the ROW fuel-maps are much leaner, -5 to -10% less fuel than the USA maps across the board. These are actually more precise mappings that matches actual air-flow volumes. The USA cars rely on the O2-sensor to pull back the fuel into proper stoich. ranges.



The main reason the European cars had more HP was due to the higher-compression pistons, and a lesser amount from having no catalytic. If you want quick easy HP, just swap in a set of Euro 10.6:1 pistons.

The trick with increasing HP is to flow more air through the engine, not ffuel. It's the numbers of oxygen molecules per intake-stroke that counts, you can always dump in more and more fuel, but once you have enough fuel to mix and burn with each oxygen, that's the maximum amount of power you're gonna make. Extra fuel will have no oxygen to burn with and will go out the tail pipe as pure hydrocarbons.

Remember that hot-rod saying from the '60s???

Last edited by DannoXYZ; 03-25-2005 at 12:27 AM..
Old 03-25-2005, 12:14 AM
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I still don't understand why it matters whether or not the air is above atmospheric. If the volume of air is metered, why does it matter if it is compressed? The sensor is simply measuring the amount of air, and the DME adds the proper amount of fuel for the volume of air. I would think that the spark curve would be agressive for forced induction, but why would the stock set-up be inadequate to measure the volume of air and add the appropriate amount of fuel?
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Old 03-25-2005, 03:36 AM
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To be aggressive with the timing advance you need to use a knock sensor (or two) as the S, turbos and 2.7 do. I would think that going to a higher compression would require a knock sensor too.
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Old 03-25-2005, 07:24 AM
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"I still don't understand why it matters whether or not the air is above atmospheric. If the volume of air is metered, why does it matter if it is compressed?"

That's because it's the actual numbers of oxygen molecules that matter. In normal atmosphere, that's about 19-20%. The trick with measuring air going into the engine is what to measure. The old method was to measure air volume with the flapper-door AFM.

However the numbers of oxygen molecules per cubic foot of air changes with the air pressure as well. So if you measure 1-cubic foot of air at sea-level flowing past the AFM, it may have X number of oxygen molecules. But at altitude, that same cubic foot of air will have only 80% as much oxygen or 0.8X. But, the AFM still measures the SAME AIR VOLUME and injects the same amount of fuel. This would be 20% too much fuel (what was that '60s saying again?). So an altitude sensor is used along with the AFM in a two-way calculation. The AFM sends raw CF flow, the altittude-sensor sends its data and the computer figures out an appropriate amount of fuel compensated for lower air-density..

The temperature of the air also determines the number of molecules of oxygen per cubic foot as well. On a cold day, the air is denser and a CF of air will have more oxygen molecules than a super-hot day, which may only have 95% as much oxygen per CF. So the AFM has another sensor in it, a temp-sensor. This 3rd piece of data goes to the computer as well and it will re-adjust the fuel-calculations based upon air-temperature.

So at a minimum, the fuel-calculations is a function of f(CFM-flow,altitude,air-temp)

----------------------------

BTW - a MAP sensor does not measure air-flow or air-volume in any way. It only measures existing air-pressure, a static value. A MAP sensor would put out the same signal with the engine at WOT@2000rpm as the engine at WOT@6000rpm. However, the CFM flow of air going into that engine is 2.5x higher at 6000rpm than at 2000rpm. So you would have to integrate MAP with respect to time in order to come up with the CFM flow rate, so CFM-flow = f(MAP,t1...t2) then plug into the previous equation above. The DME Motronic does not do this integration calculation, so you'd need an additional computer if you want to use a MAP sensor.

Last edited by DannoXYZ; 03-25-2005 at 09:11 AM..
Old 03-25-2005, 08:53 AM
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First, I want to be 100% upfront. My terminology was wrong earlier. I have a stock '86 2.5 NA with a factory set-up (AFM?) I was under the mistaken impression that there were 2 types air meters for efi MAP and MAF, and that since the MAF was the hot-wire set-up that stock is a MAP. Sorry for not knowing even what I have under my hood. Also, my questions are simply to clarify my understanding, or correct my misconceptions. I am curious about the potential of an Eaton supercharger, and have some questions.

My thinking is: Air filter to meter to compressor to engine. Since the stock AFM measures the air leaving my air filter, why would that information change because it was being fed into a compressor prior to being ingested by the engine. Even though the air is being compressed (and heated) the original oxygen content stays the same doesn't it? If so, that oxygen is what determines the fuel demands, so why can't the stock computer process that info? I don't see how altitude/temp issues would be a more significant concern than without the compressor.
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Old 03-25-2005, 10:29 AM
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The stock DME can handle that up to a certain point, I think someone said ~5psi for a stock setup. Think of it as a reference chart, AFM reads X amount of air going in and then injects Y amount of fuel. When you add a blower to this equation then you are adding more air. The problem you run into is at a certain point the table runs out of corresponding values so the DME is essentially maxed out and you can throw as much air as you want but the fuel is going to be the same as before.
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'89 Porsche 944 S2 - Parted

Looking for a nice 944S for a daily driver.
Old 03-25-2005, 01:46 PM
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Exactly what AVN said
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Old 03-25-2005, 01:51 PM
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"If so, that oxygen is what determines the fuel demands, so why can't the stock computer process that info? I don't see how altitude/temp issues would be a more significant concern than without the compressor."

Because at sea-level with a certain X-CFM flow-rate being compressed to 7psi may end up being the same X-CFM flow-rate across the AFM being compressed to 5psi at 5000ft. The lower-density air would still be read as the same volume and flow (X-CFM) by the AFM, but it will only contain 80% of the oxygen as before. The lower-density mixture will also be compressed to a lower boost-level. But the AFM doesn't know any differently.

Well, there is a slight change in the AFM's flapper door's due to the lower kinetic-energy of the lower-density air, but it doesn't fully compensate for the reduction in density. Only an altitude sensor can provide the additional data for proper compensation.

The other issue is one of spatial displacement that causes a time-delay. The exact piece of air that the AFM is measuring, is NOT the same piece of air that's being compressed and not the same piece of air that's going into the engine. This cause a time-delay where the DME is playing catch-up to the actual air-flow conditions.

When you first step on the throttle under WOT, the increased air-flow is not immediately picked up by the AFM. That's ok, because an acceleration-pump enrichment algorithm is provided by the DME based upon TPS-angle and velocity. However, when you let off the gas, the throttle-body as shut off the air-flow going into the engine, but the compressor and air-column is still moving pulling air past the AFM. The computer thinks air's still flowing when it actually isn't. It takes a little time for the air-column to slam into the closed throttle-plate and back up out too the AFM and stop air from flowing. This cause a rich-spike, flames out the tailpipe and a little pop when you let off the throttle quickly.

This is just fuel, we have to address ignition. Increased air-flow density ffor the same volume will cause increased heat. To have ignition be a couple degrees away from the knock/detonation limit for safety would require some way to adjust igniton away from stock values.

"If so, that oxygen is what determines the fuel demands, so why can't the stock computer process that info? "
"The stock DME can handle that up to a certain point, I think someone said ~5psi for a stock setup."

AZN is correct. Now, if you're going to be boosting the NA, you have to deal with its increased flow rate. The stock AFM has only a certain maximum flow-rate it can measure before the flapper-door is fully open and the voltage-output is maxed-out and fixed at around 4.6-4.8v. If you use say... 5-6psi boost, the AFM will most likely be maxed out and send a fixed signal from 3500rpm onwards. It will then trace the red line through the fuel-map instead of the stock green one:



So basically once the look-up point on the 3D map is smashed against the edge of fuel-map due to the AFM being maxed-out early, your fuel-values will be fixed on whatever's programmed into the chips. So 7psi boost will get exactly the same amount of fuel as 4psi boost resulting in a lean condition. Using 9psi boost will get even leaner because the same fuel will be injected, but 10% more air is being pushed through. You can't even use a piggyback signal-massager like the ARC2 because increasing the output voltage once it's maxed-out against the edge of the fuel-map will do no good. It will just hit the edge of the maps earlier at 3000rpm instead of 3500rpm.*

Now if you were to get larger injectors, you'll be way, way too rich in off-boost conditions like idle and partial-throttle/mid-range cruising. If you were to get 50% larger injectors, you MAY have enough fuel under full-boost of 7psi, but you'll have 50% too much fuel at idle and partial-throtle, resulting in a 8.0:1 fuel-mixture. The car won't idle or run very well with that much fuel.
-----------------------------------------------------------------------
* it's actually more complicated than this, but this is a rough outline to illustrate the concept.

Last edited by DannoXYZ; 03-25-2005 at 07:18 PM..
Old 03-25-2005, 03:29 PM
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Not sure if a normally aspired DME would drive the 951 injectors, 924 has low impedance & 951 about a midrange impedance.
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Old 03-26-2005, 02:48 AM
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So, is it the AFM that cannot accurately measure the increased volume of air, or is it the DME that cannot process the signals from the AFM?

Also, early in the thread it was indicated that 951 injectors can be used in a 944. I'm assuming they would cause the same problems with a NA AFM/DME regardless of application?
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Old 03-26-2005, 03:13 AM
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