Explain porting to me
 

I am getting a 87 TII which needs a rebuild. I was thinking that while I get the rebuild done I should consider the possibility of doing some porting as well.

Can you explain to me how porting would affect my power band? I have heard that it hurts low end power but helps with high end power. Is this always the case?

How else does porting affect the characteristics of a motor (noise, gas mileage, etc...) and what kind of supporting mods are needed when you port a motor (assuming its not a very agressive port.)?

My goal for this car is to maintain streetability (and low end power) while increasing the overall power. Should I even be considering a port?

I have done some searches for "streetport" and such, but it gives way to many hits for me to sort thru and find all the answers to these questions, so i figured I would just ask...

very informative, i didnt realise how damaging porting could be

K thanks for the link. It sounds like if I don't want to loose much low end torque I should stick with a mild port.

I'm still not sure exactly what a mild port would do to the power band tho...

Also, with a mild port does the idle have to be adjusted at all?

Yeah, I see what u mean... I obviously don't know much about this, but it would seem to me that with all the options available for adding more power when dealing with a turbo setup, doing a very agressive port doesn't seem like a great way to go.

If you know piston engines, it's about the same as a "head job" and a change in camshaft profile.

Changing the (teardrop) shape of the intake port will almost always change the intake opening / closing / duration.

The link given above is applicable to non-turbo applications, and once you talk about forced induction (i.e. turbos), the descriptions are almost useless.

If you're not going to do tackle this job yourself, then it's highly recommended to talk to the shop that is going to do the porting work for you.


-Ted

Well if I did it I would have the shop thats doing the rebuild do the porting also. I just figured I'd try to educate myself about it on here first.

I'll be bringing it to the shop tomorrow, so I gotta make my decision about what kinda port to get (if any at all.)
Considering the car is only BPU, i guess it wouldnt be worth doing anything more than a mild street port. Would I even notice much of a difference doing that?

i don't think you can go wrong with a mild streetport.
yes. you will notice a difference.

Sound was the biggest thing I knowtest. The tone and pitch totally change.

Will my low end a mid torque be affected? Or will it be more or less the same but with more high end torque?

You really need to ask the shop / person who is actually doing the work.

Shown were examples of what a "porting job" would look like, but that doesn't mean that's exactly what you will get.

In fact, the term "streetport" or "street port" is such a general term, we really can't give you an easy answer.

How bad you ask?
If the port job entails earlier intake port opening, this induces more overlap.
More overlap is bad when talking about low-end power.
So this will hurt low-end power slightly.
The results will be generally the same when you cut the exhaust ports to close later - same smell, increase overlap.

An SP will almost always close the intake port later - this will increase airflow, and this increase power, AND it will tend to shift the peak power higher in the RPM's.

If the SP is done right, you'd get power gains all throughout the RPM band.

On the exhaust, if you drastically cut the port to open earlier, low end torque (i.e. power) will drop.
You're trying to get the exhaust gases to exit the combustion earlier, and thus, the gases have less time to do it's work on the eccentric shaft.
I've seen this first-hand, cause I cut my exhaust ports really early and my torque suffered because of it.
I put down 253hp / 207 lb-ft torque on my SP 13BT.
Stock port 13BT's will easily beat my torque figure (I typically see 220 to 230 lb-ft of torque on other dyno sheets), but my power band got shifter way higher in the RPM band.
This is on a 1987 Turbo II 13BT, but results are similar no matter what 13B you're running.

These are all basic fundamentals of intake and exhaust timing (especially when talking about psiton engines and camshaft profiles) - keep in mind, this is NOT ignition timing.
It can all be applied to rotary engines also.


-Ted

Sweet, I'm printing that out. Thats some good info for people who don't know much about the actual porting. As for increasing the actual port size or port opening into the chamber, does this cut down on velocity of the intake charge? What are your thoughts on that? Thanks a lot

Chris

Cool thanks for the info... I'm actually leaning towards not getting any porting done. I like having a torquey low end so I figure why mess with it?
Also, I could spend that money on other things (such as a wide band O2 sensor and an upgraded fuel system) which would ensure the longevity of my motor.