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Supercharged N/A with CIS
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leadfoot  



Joined: 11 Dec 2002
Posts: 2222
Location: gOLD cOAST Australia

PostPosted: Sun Apr 26, 2015 11:00 am    Post subject: Reply with quote

Basically each injector opens at a set pressure, once this is overcome by the difference between system-control pressures.
If manifold is atmospheric 14.7 then the injector will have no problem opening at 70psi aystem - 52psi control = 18psi approx
If manifold absolute is 14.7+8psi boost= 22.7 ergo its going to struggle at the higher rpm, thats where the wur makes the differential opening pressure higher by dropping control pressure equivelent to boost sensing, adjusting system higher is another way but the WUR is most effective.
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Raize  



Joined: 18 Sep 2013
Posts: 88
Location: UK

PostPosted: Mon Apr 27, 2015 1:14 am    Post subject: Reply with quote

OK, so what I need to do is:

1. Get a turbo WUR for it's boost sensing ability.
2. Make said WUR adjustable.
3. Shim the system pressure up to increase the maximum possible fuel.
4. Adjust the WUR to give a higher control pressure to achieve correct fuel/air ratios.

This will let me keep using a 924 N/A fuel distributor.

I tested my injectors previously at an ultimate total flow rate of 800cc/minute at atmospheric pressure. So the improvement in ultimate flow rate will be equal to:

(New system pressure - boost pressure) / Old system pressure.

From here I can calculate exactly what system pressure I need for a particular level of boost and HP.



So...

What parts are likely to fail with a system pressure increase?
Has this kind of thing been done before with CIS-basic?
What's the highest system pressure that's been successfully achieved (I saw a 930 with 7.4 bar!)?

EDIT: I'm not even sure this is a good idea. The air bowl shape will be wrong and the pipe will have to do a U-turn to get to the SC intake whereas with the downdraft it would only have to do an S-bend.
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morghen  



Joined: 21 Jan 2005
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Location: Romania

PostPosted: Mon Apr 27, 2015 5:01 am    Post subject: Reply with quote

What i would do is install a wideband first.
Get a working 931 WUR and DO NOT TOUCH IT.
I would simply install it, hook it with a vacum line from the intake and test drive carefully with low boost levels.

I expect that it would do its job nicely and nothing else would be necessary.
I would not touch the system pressures, nor i would directly modify the 931 WUR. I really think that there are chances that it would just work as it is.
If, and only if after the initial tests i could not dial in the AFR i want, only then i would set about modding components.
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leadfoot  



Joined: 11 Dec 2002
Posts: 2222
Location: gOLD cOAST Australia

PostPosted: Mon Apr 27, 2015 8:46 am    Post subject: Reply with quote

+1
931 WUR - leave as is
931 s1 ignition dissy
Leave system pressure alone , change only if you need to when dyno tuning fo A/F, knock sense and correlate your wideband gauge at the same time...
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chris79  



Joined: 29 Jan 2014
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Location: milan

PostPosted: Sun May 03, 2015 5:37 am    Post subject: Reply with quote

Raize where do you think place intercooler in your project?
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Raize  



Joined: 18 Sep 2013
Posts: 88
Location: UK

PostPosted: Sun May 03, 2015 10:00 pm    Post subject: Reply with quote

If I was going to use an intercooler I'd just have it in the normal place in front of the radiator.
Not sure whether to intercool, water-inject or just have a hot air blower.

I've been looking at Arvid's M62 thread and this is the same place I'd put the blower.
http://www.924board.org/viewtopic.php?t=33291


So the idea would just be to have a downdraft Turbo metering unit directly above it. Because my car is RHD there is no steering shaft on that side, so I can mount the blower lower if necessary for clearance.

An N/A metering unit is going to require a ridiculous 180 degree bend to get the air to the SC inlet so the turbo unit is much more practical.

EDIT: Just realised the fuel line ports on the turbo fuel distributor don't line up with the hard N/A fuel lines and there is no way to rotate it to make them line up. So I'd need to fabricate completely new fuel lines for everything except the four main injectors. Probably easier to have a ridiculous 180 degree bend in the intake piping, then.
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Raize  



Joined: 18 Sep 2013
Posts: 88
Location: UK

PostPosted: Mon May 04, 2015 5:10 am    Post subject: Reply with quote

I have read that the injector flow will increase as the square root of any fuel pressure increase. So a system pressure increase from 70 to 90 PSI will give me a 13% increase in fueling - taking the 800cc/min I measured earlier up to 904cc/min.

904cc/min is 86 lb of fuel per hour. Assuming a BSFC of 0.5 this is enough for exactly 172 bhp (would have been 152bhp without the system pressure increase) - BEFORE the parasitic loss from the supercharger.

I have no idea what the actual BSFC would be.




Another way of looking at it:

86 lb of fuel per hour requires 1032 lb of air per hour (at a 12:1 ratio). This converts to 225 CFM.

If a 924 N/A engine was to operate at 100% VE, the air flow would be:

121ci x 5800 / 3456 = 203 CFM.

The figure normally guessed at for VE on an old 2 valve engine is 85%, so that gives us 173 CFM.
(Personally I believe the 924 engine must get higher than 85% VE to make the power it does N/A, this would make things even worse!)

225 / 173 = 1.3 (pressure ratio)

Or in other words, 0.3 bar (4 PSI) before I exceed the fuelling capabilities of even a heavily modified N/A CIS. That's dissappointing.

Hoping I'm wrong somewhere. Any comments?
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morghen  



Joined: 21 Jan 2005
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PostPosted: Mon May 04, 2015 5:31 am    Post subject: Reply with quote

Raize wrote:
Any comments?


Yes, you could have been driving that thing by now.
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Raize  



Joined: 18 Sep 2013
Posts: 88
Location: UK

PostPosted: Mon May 04, 2015 6:07 am    Post subject: Reply with quote

morghen wrote:
Raize wrote:
Any comments?


Yes, you could have been driving that thing by now.


But probably not for long until a piston got holed from detonation due to lean running.

It's not really viable to take this project to the next stage as the theory proves that it simply won't give a meaningful power increase (too much is lost to the SC itself) with a N/A CIS and for the cost and difficulty of using a Turbo CIS I might as well convert to EFI anyway.

At least I learned some things, I guess.
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chris79  



Joined: 29 Jan 2014
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Location: milan

PostPosted: Mon May 04, 2015 6:38 am    Post subject: Reply with quote

But when the turbo start the volume of air that lift the plate is changing, right?
How can you calculate if quantity of air aspirated is not too much or too few for moving the plunger of distributor and increase the flow?
I'm interested to this project, I would like made the same road.
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morghen  



Joined: 21 Jan 2005
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Location: Romania

PostPosted: Mon May 04, 2015 5:03 pm    Post subject: Reply with quote

I think your theory proves nothing about the feasibility of this project.
Your calculations and estimations are "quick and dirty" at best.
By the way, for 200HP you need 300cc injectors so do your math.

The fact that you expect the engine to go grenade the second its on boost perplexes me....I mean...you could just not floor it from second one and actually take it slow at the begining keeping a close eye on AFR levels...and if really its lean at some point then get 931 injectors or a 931 metering head. Those would be minimal expenses as these parts are available for cheap. Further...you could buy CIS parts from other turbocharged cars and go even cheaper but like i said, you would not need them.

You have no idea what you are embarking on when converting to EFI.
I have said this before and i'll say it again, there are SO MANY 924s that got parted and/or are not working & sitting in garages/driveways because of EFI conversions.
Unless you REALLY know what you're doing it will probably take you a couple of years to get it to work right.

You might be better off enjoying the NA, or getting another car.



Raize wrote:
morghen wrote:
Raize wrote:
Any comments?


Yes, you could have been driving that thing by now.


But probably not for long until a piston got holed from detonation due to lean running.

It's not really viable to take this project to the next stage as the theory proves that it simply won't give a meaningful power increase (too much is lost to the SC itself) with a N/A CIS and for the cost and difficulty of using a Turbo CIS I might as well convert to EFI anyway.

At least I learned some things, I guess.

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chris79  



Joined: 29 Jan 2014
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Location: milan

PostPosted: Tue May 05, 2015 1:22 am    Post subject: Reply with quote

this can work for turbo conversion ?

http://www.ebay.com/itm/Porsche-924-Turbo-Exhaust-Manifold-9311115940r-/251855326989?hash=item3aa3bf4f0d&vxp=mtr
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Raceboy  



Joined: 01 Mar 2004
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PostPosted: Tue May 05, 2015 1:55 am    Post subject: Reply with quote

No, 924 Turbo exhaust ports are bigger and the mating surface is also different (Turbo has fire rings).
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Raize  



Joined: 18 Sep 2013
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Location: UK

PostPosted: Tue May 05, 2015 4:57 am    Post subject: Reply with quote

morghen wrote:
I think your theory proves nothing about the feasibility of this project.
Your calculations and estimations are "quick and dirty" at best.


I can't find any real figures to go by so I'm forced to make estimates. The only real information I have is:
N/A K-Jet flow rate at 70PSI: 200cc/injector/minute (measured MYSELF)
931 K-Jet flow rate at 97PSI: 270cc/injector/minute (This guy: http://www.924board.org/viewtopic.php?t=32521 )

I agree that an EFI conversion is not the way to go here.

I would like to make this project work but it NEEDS to stay simple and it NEEDS to work in theory otherwise I can't justify any spending on components. I know that things that work in theory often don't work in reality... so if it doesn't work in theory, chances are it definitely won't work in reality.

I really need to know exactly what effect increasing system pressure will have on the flow rate - if it does follow the rule that "flow rate increase is equal to the square root of pressure increase" or if the flow rate increases more than that for some reason. But I can't afford to risk damaging the fuel system on my daily driver just for an experiment.
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morghen  



Joined: 21 Jan 2005
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Location: Romania

PostPosted: Tue May 05, 2015 5:55 am    Post subject: Reply with quote

Well you can quit before starting if you relie on theory which relies on thin air.
I'll tell you something about CIS and specifically about the 931 system...people underestimate it. Honestly its SUCH a good system...i mean these cars still make decent power and still have decent fuel consumption compared to most cars on the roads today.

From the factory it comes on an engine that makes 177hp on the S2 and 170hp on the S1. Countless people have intercooled their 931 and increased boost. Many 931 make 200-220hp.
I have gone further, adding a bigger turbo, a K27. This K27 makes 1.1bar on my 931, it pushes a lot more air than the factory K26.
It takes the car to @260km/h and it is a quick car.

What do you think i changed to the fueling system?
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