924Board.org

[ideola]

Can someone explain to me why our beloved 2.0 NA purportedly performs better with carbs than the CSI fuel injection? Other info I've read indicates that the head is the constraint, so if that's the case, why does carburetion improve performance???

Also, I was reading the recent posts on the EFI conversion Leadfoot is working on. Sounds *expensive*. Are there other options for increasing fuel injected-based fuel delivery that will improve performance???
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erstwhile owner of just about every 924 variant ever made

[Lizard]

with converting to carbs you do yes see about a 10hp increase in power, the fuel milage drop dramatically, and overall drivability isnt as good,
with converting over to EFI you would probably only gain about 5hp on an NA engine, but drivability would be ALOT better,
one of the main reasons that carbs gain the horse they do (imo) is due to the shorter intake runners, the exhaust on these cars is tuned right from the factory for more of a top end car, so if you shorten the intake runners this will give it more top end, at the sacrific of bottom end power,

to confirm that though you would really have to see dyno sheets with stock CIS and then Carbed cars, I havent seen any dyno sheets myself
if anyone has pics of dynosheets and would post them side by side I would love to see it, and confirm this.
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3 928s,

[ideola]

OK, that makes sense...couple of follow up questions...

1. Drivability isn't as good...can you be more specific? Is it just lack of btm end power, or is there more that suffers?

2. Are there EFI kits available like the carbed kits you can buy, or do they have to be cobbled together like Leadfoot is doing?

3. How much HP advantage does a turbo have?
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erstwhile owner of just about every 924 variant ever made

[-nick]

Leadfoot's numbers are in Aussie dollars, so multiply his numbers by 3/4 to get US $. Also, you can use a megasquirt EFI which is considerably cheaper ~$400 US assembled (for the controller&harness).

I don't think anyone has dyno'ed a stock car with CIS, carbs, and EFI to actually see the changes. Those 5hp/10hp numbers are more guesstimates than anything.

Finally, there are no EFI "kits". Although, I'm getting a pretty good parts list together to make the conversion as painless as possible. Regardless, you'll still need to fab up a fuel rail and change the throttle body to one with TPS as well as mounting new sensors on the intake/etc and quite a few other fabrication obstacles. Then there is tuning which either requires hours of expensive dyno time or a wide-band o2 (or both). This is not a bolt-on by any means.

Turbo's offer an enormous power advantage. For reference, I'm aiming at a very driveable and reliable 250hp machine after about $1500-2000 worth of work this winter. I think Paeco lists something like 220hp out of their $10k N/A engine. And that's for an engine whose life is counted in hours not miles!

nick

[Mikri184]

There doesn't seem to be any data as for how the carbs preform on the 924, and a proper carb set up shouldn't cause any driveability problems except at start up as much dual carbs set up will not have an electric choke, so warm up is spent with you managing the fuel. I agree with Lizard that the power increase is good for the shorter intake SO fuel has less time to travel. I haveknown a few people who have successfully installed carbs on thier 924's and have been very happy. As for the EFI, I have yet to see someone get this completely installed and show some numbers.
And yet the CIS set up is a very good system and can handle alot of power. And if working properly can be very problem free. Hope this helps, Good Luck
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Life is to short to be looking for something?

[Wolf1]

There are two main reasons that a carb set-up "could" add extra power.

1. As mentioned, the runner length. This determines the the "sweet spot" for your particular engine. For example; if you get 100% VE at 3500 rpm with cis, that is where your peak torque will be found. Say you shorten the runner length and remove that restriction and now get 100% VE at 4500 rpm with carbs. HP is a calculated number of torque times rpm if you move your peak torque up in the rpm band the hp number will be greater. However, a peaky power curve is not always better than a broad power band.

2. The other reason a carb set-up "could" increase hp is; Less over-all restriction in the intake tract. The cis fuel dist is a restriction, no way around it. That is the reason that turbo guys are switching to MAP and MAF sensor systems on their 951's. As well as more infinite adjustment and metering of the incoming air charge.
Yes, I know that 951's do not have cis. But the barn door style AFM is a similar restriction.
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88 951 stone gray
80 931 Red and surgically enhanced
72 914 (old toy) $old
2 1/2 924's for spares..

[Lizard]

in regards to the drivability of the carb setup being less, carbs are alot more particular and dont rev up quite as smooth as a CIS system,
yes the horse #'s that I have stated are mainly a guess,

in regards to a turbo, I would recommend that you go with a factory turbo or 931, you can get aftermarket kits for the 924 but I would recommend the factory 1 instead.
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3 928s,

[My924gtc]

[Vince Ponz]

First things first. If you want a better and faster car buy a 944. As someone who has had carbs on the car for over 22 years I speak with a little knowledge.
Benefit of carbs is that you can now modifiy the engine to extract maybe 145hp out of it. Cam, bigger valves, springs, etc. With the stock setup I don't think you can get 145.
Gas mileage drops with carbs. Once setup you never have to do it again.
You also will get a sour spot around 1800 rpms if you accelerate slowly.
Modify a 924 and the best you can achieve in my mind is about 145 without larger pistons and displacement. The 944 will have the same HP and better comfort and handling.
The cost to achieve this 145 is about 2500US over and above the cost of your car. The 944 would cost about the same as both dollars added together.
Having said this I still like my 924 modified car better than my 944.......on the track. I have a lot of suspension etc work done to it.
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"Never let them see you sweat"
77.5 924 modified track car
79 931 Euro stock
88 924S SE
87 911 Targa stock

[ideola]

I've already ruled out the 924S and 944 because I don't want an interference engine. Don't want to be doing timing belts every 30K miles.
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erstwhile owner of just about every 924 variant ever made

[-nick]

Very valid reasons. 944's have their own quirks too.

Lizard's suggestion to go with a factory 931/turbo is for a couple reasons- the build quality is much better than aftermarket kits, the transaxle is stronger, you will get more power out of the better design, you get other upgraded factory parts such as brakes/springs/swaybars/etc, last but not least- you will spend as much on an aftermarket kit as you will just buying a 931 to begin with. If you just want to tinker, you might be better off adapting a supercharger to an NA. More just to be different than anything. HP is more expensive and involved on these engines than on most others.

nick

[Lizard]

nick is very correct as to why a factory turbo,
parts are stronger/upgraded, more capabilities, and a better head design
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3 928s,

[CBass]

Without a doubt a CIS fuel distributor is more restrictive than a MAF sensor, but I have yet to see any sort of proof that it's actually a restriction to the 924.

Keep in mind, the biggest restriction in the engine is where the air/fuel has to get through the ports and around the valves. In my opinion, the money you'd spend on carbs or EFI would be much better spent getting some head work done. Some good bowl work and a more aggressive cam will do a lot more for both top end performance and driveability than carbs or EFI will.
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'81 931 in various states of assembly