slvr7

Hi all,



I'm just about to port my first engine, and although I've read everything I can find on the topic, I'm still somewhat unsure about how far to go.

It's an S4 13B, with the S5 VDI intake (fully operational & adjustable with rpm switches), RB header, & free flowing exhaust, cone intake, & SAFC. It's been dynoed at 160 rwhp and has a good, torquey powerband. The car is a street car used for autoxing.

I'm thinking about just mimicing the mazdatrix photos of their 13B 6port style. I favour streetability & reliability over all-out HP. But one thing I find is that no one seems to talk about the timing numbers they're using when porting. Should I be extending the primary intake closing to 50*,or 55*? Is overlap less of a concern as rpm builds, and if so, can I open the secondaries a little earlier than 32* (how much)? What's best for the exhaust ports?



I'd be grateful for any & all advice........



Curtis

'86 GXL autox'r

l8t apex

easiest and surest thing to do is buy a template. No guess work and it will work.Dont guess be sure.

83turbo

The Mazdatrix/Racing Beat template for the 6 port is actually quite close to stock S4 port size (for secondary/aux). Nice upgrade for a GSL-SE, but really only makes a big difference on the primary side of a 2nd gen. I kind of think the primary closing point is bumped to about 50 - check out the Mazdatrix porting FAQ where they compare the "1974" port to the template. The 1974 closes at 50. Any more and you've got to watch out for the water jacket.



As for opening, I generally use 25 degrees (RB 4 port template, unsure about the 6 port template). It doesn't seem to hurt idle at 800 RPM. The exhaust ports are probably okay as is, unless the diffuser is being restrictive (I don't know if it is).

GMON

I highly recomend making a plexiglas copy of your port. Use the dowel pins to hold it in place and cut a hole to reach the rotor. Stack a housing on top of a plate and drop in the rotor. Throw in a side and an apex seal. For best results use the e-shaft as well. Then just slowly spin it over so you can see exatly what the port is doing.



Read the porting threads on here as well. Porting is not just the opening but the runners as well.



Good luck and have fun.



GregW

slvr7

Thanks so far, guys.

Yeah, I intend on doing the plexi template thing, and making a degree wheel as well. That's why I'm interested in the timing numbers of various options - I've read all the SAE technical papers from Mazda I can find, the Yamamoto book, and threads here (if a little knowledge is dangerous, then I suppose I'm lethal).



I've read that moving the primary closing from 40* to 50* ABDC will reduce brake mean effective press. under 3500 rpm, but will increase it above that point. I'd like to know 'how much' the trade equation is, though. I'm also wondering if extending the intake duration will also benefit from the pressure spike caused by the dynamic effect. Stock port closing shuts off as the pressure peaks, and loses the downward 'ramp' of pressure. The greatest pressure developed by the the dynamic effect occurs when the residual exhaust pressure 'pops' into the intake tract during overlap, and reaches the other rotor at intake closing. It would also seem then that an earlier opening would make a little more use of the effect, at the expense of charge dilution. Again, trade-offs and how far do you go?



I do understand that runner shaping for flow & velocity relies on 'art' rather than specification, though. Lynn E. Hanover wrote very eloquently in a thread here on the subject not too long ago. So far I've got the manifold pieces gasket-matched and smoothed internally.

One idea I want to investigate is cutting out the brass bushing protruding into the LIM aux. runner for the actutator rod - it represents a large 'boulder' in the 'air-stream'.



TIA



Curtis