924Board.org

[Bockscar]

I have a couple questions about port matching the intake on the 2.0 engine -

First, after porting and polishing the intake manifold ports, theoretically, more air will be allowed into the engine because the ports are wider and smooth. However, on the turbo models of our engine, the intake piece from the turbo to the throttle body is metal and cannot be widened (as far as I know). Despite making the intake ports wider, and the pipe between the turbo and the throttle body allow the extra airflow that port matching will theoretically provide?

Second - Our fuel injection system is mechanical and the amount of fuel sent to the fuel injectors is based on how much air flows over the throttle plate. However, at WOT, wouldn't that be the theoretical max amount of fuel being sent to the injectors? Following that question, if the intake ports were extended beyond whatever the fuel injection system is tuned for, wouldn't that mean we would see a leaner air/fuel ratio because more air is reaching the engine than originally designed from the factory?

Just some thoughts I came up with on my way to work. Let me know what you all think and let me know if my logic isn't sound

[leadfoot]

To answer your question, this is in the 924 technical section
"According to the Blue Bible of Bosch FI (Bosch Fuel Injection, by Charles Probst, SAE), the K-Jetronic has been shown, by experience, to be capable of handling without major modification nearly double the output of the stock engine in most of it's factory applications, including the 924. It is a very solid and reliable system, when the components are in working order, and will stay in tune with a regularity that amazes those used to working with old carbureted cars. It truly was an advancement, for its day. It was even used on the 924 Carrera GTS, a street-legal stripped-down monster capable of some amazing performance, though in that case a fuel distributor from an early 928 was used, to provide the additional fuel delivery, being plumbed from 8 output lines back into the 4 injectors in the intake."
Stu
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1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...

[Rich H]

For your first point there is a lot more to consider, bigger is not better it's only different.

For torque you want high speed inlet air speeds which means long thin pipes which gives better cylinder fill. for power you want very short fat pipes which offer minimal drag.

Add to that wave tuning and you get a comlicated compromise.

Smooth is also not that good, rough inlet manifolds encourage turbulence which keeps the fuel mixed better and away from the walls.

Step changes in ports are mostly bad though so port matching isn't a bad idea at all. But don't go mad polishing, an slightly rough finish is fine.

Also for turbo applications inlet manifolds make less difference anyway, just turn the boost up a bit...!
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1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec

[ideola]

Port matching isn't so much to provide a higher volume of air (which is a static, fixed-in-time measurement); it's intended to provide a higher FLOW of air. Think of it like this: through (roughly) the same space, and over the same amount of time, a properly ported system will allow more air to be stuffed into the combustion chamber. The increase isn't from the space being bigger, it's from improving the flow. Improving the flow can be accomplished by removing restrictions, increasing velocity, and eliminating things (like casting seams and mismatched ports) that restrict flow and cause turbulence.

Increasing velocity comes from a nice funnel shape toward the inlet valve (so-called Bernouli effect), so you have to be careful not to remove too much material.

I've had two different setups professionally ported, one by European Motorworks, and one by a full-time head specialist at Roush. In both cases, the work was focused on smoothing the inside surfaces, and cleaning up the radiuses on the head ports, as well as the tight bends inside the intake manifold. And of course, matching the ports to each other where the components connect.

Personally, I wouldn't worry too much with the size of the tract between the turbo outlet and the throttle body, unless you plan to do something about the asthmatic K26 compressor. Opening it up downstream of the turbo is a bad idea because the air will slow down. My bet is that the shape and size of that intake tract is pretty well optimized for the size of the compressor on our K26 turbos.

Now, if you're going to build a hybrid custom turbo with a compressor capable of much higher CFM (i.e. FLOW), then yes, maybe a bigger intake tract and throttle body would help; but I don't believe so on a stock system. There are some old threads, I think by -nick IIRC, where guys that have gone to custom intercooler and intake setups have commented that they regretted going larger than 2.25" - 2.5" on the charge tube piping.
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erstwhile owner of just about every 924 variant ever made

[Min]

[Bockscar]

Great answers guys, thanks!
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Brian
'79 924 - Restoration In Progress
'81 924 Turbo w/EFI - Scrapped

[Mike9311]

I have been lurking on this while the fellas gave you great info. Without me adding to the technical side I want to give you my tech tip I use on everything.

First with port matching you simply want to remove the material required and no more (following the above info ie a tiny 45deg angle to match ports is not what Min means!) to match the port and second don't simply use a gasket to match the ports as your template.

This is the example:
Get some cardboard type paper the same thickness, if possible, to the gasket (not always necessary depending on what you are matching). Tape it to the head in only a few places. Then use a sharp knife, razor, or such to cut out the port shape in the cardboard as it is held over the port openings by the tape. Put weatherstripping adhesive on the intake and bolt on to the head with the cardboard in between. Let sit to dry. Now remove the intake. The tape will rip since the cardboard is now glued to the intake. This is the exact position the intake will be bolted to the head when you final mount and the cutouts you made will now be clearly projected on the intake. You'll know right away if the ports match and how to act accordingly with a dremel tool. I have even used this method to port match oil pumps to block openings where it is mounted.

I will dig up pictures for you but that will take time (also why I have not responded right away). I just wanted to get this info into you hands. Hopefully its understandable?
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1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA ohne 650 mit 471
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car