I've been doing some searching and looking around this forum for info on peripheral ports. Unfortunately there isn't much info about what I'm looking at, which is a relatively low-overlap low-RPM PP motor. As PeeJay on this forum is very fond of pointing out, the PP can be made to have decent mid-range and produce more torque at the same RPM as a side-port engine. Now what if I minimized overlap by running a port that was smaller than the "race" peripheral ports (say 35-45MM ID instead of 55MM)? I could also run that same port size but instead of being round make it somewhat wider than it is tall and so minimize overlap there. Basically I'd been looking at trying to build a PP with as close to stock intake timing as possible. The intake design would be EFI using a dual 50MM TB and port injection. If anyone has any ideas please chime in. The goal would be to increase torque across the board but with a penalty in idle and low-end while still being able to keep stock apex seals (under 8400 RPM).
Thanks,
Grant

P.S. This is NOT for a daily driver, just an occasional driver and CMod autocross car.

 

If you measured the port timing between PP and side port, you'll understand you can't get anywhere close to the short duration of the side port...


-Ted

 

I understand that the overlap would be significant. I plan to minimize the overlap with a smaller port and also by porting the exhaust less extreme then most PP engines. By near to the stock timing I mean just that it will have a small enough overlap to idle under 1500 and be capable of movement under 3000. I may just use a sequential 4-barrel throttlebody make it a semi-PP using the primaries for idle and cruise. Streetability would be nice, but it's not priority. Priority is getting the powerband under the RPM limit of the steel apex's. If I decide I like the configuration I would probably then tear it down and fit carbon seals.
Grant

 

Due to the dynamics of the PP intake port, you get like double the intake port duration versus a side port.

http://www.thecarricos.com/ACRE/Docu...ech84-7-11.pdf

If you try to "squish" the PP down, you end up with this tiny sliver of a PP which becomes ineffective.


-Ted

 

Excellent article, I saved it. When you say it become ineffective, just how ineffective is it? What if I avoided over-porting my exhaust to acheive less overlap? I don't mind a bad idle or cruise, I just would prefer to keep the power in a lower region because building the motor seems to triple in price if anything more than 8000 RPM is to be the norm. I believe that the NSU spyder was a PP and it had very smooth power output and at lower RPM so it is possible, the real question is if it is possible with stock 12A housings. I think I'll try, perhaps not right now but sometime in the next year.
Grant

 

You're asking very valid questions.
I'm afraid I'm not qualified to answer those.

I am very anti overlap, so I won't normally touch anything ressembling a PP.

I do plan on messing with a PP secondary once all my current customer cars are finished...hopefully by the end of '05.

It's hard to condense fluid dynamic theory into a short paragraph, but basically you want to try and keep the port as much of a circle as possible - highest flow potential is in the center, which is a very very condensed version of it.


-Ted

 

Oh well that makes perfect sense then. I was actually just thinking about this and came up with waht might be a decent design for a turbo semi-PP. It would use a single smallish throttlebody on a nive long manifold for the primaries and this would be controlled by the throttle. The PPs would have a dual 50-55mm throttlebody and these would not be controlled by the throttle in any way except to limit them from opening when the throttle was closed. It would use a waste-gate actuator diaphram to open the PPs once the engine came up on a pre-determined boost level (This would have to be adjusted with something like a manual boost controller). Basically you could drive around off boost and never open the PPS but once you hit say 5PSI the actuator would start to open them as the flow increased past the abilities of the primaries. It could make for a rather streetable set-up that would allow relatively low-boost to run larger HP numbers without any large penalties off boost. Could just be a crazy ramble but it is possible if you wanted to spend the time with it.
Grant

 

Easy. I 've done this with several pp's with good results. 43mm intake port(D shape) and average exhaust ports(retaining exhaust sleeves). Overlap similar to MFR housings. Overlap and duration makes power and good airspeed makes lowdown torque. I tested one particular 13bpp a number of times on a steep hill in 5th gear, at 2500rpm I floored it expecting a bog or stumble but it pulled cleanly and evenly(for such low rpm) to 9000. This engine had a very wide powerband from 4000 to 9500rpm.

Factory seals will handle more than 8400, particularly if the ports are no wider than the stock gen 2/3 exhaust port, which they don't need to be anyway.
Factory race inlet ports are 43mm.

 

I ran a half bridged motor for almost a year (before I screwed it up !!!!) and now have decided to try a semi perepheral ported motor , since I have been hearing good things about them . The pp's are about 1.25" I.D. and located where the water jacket would ususlly be (on the older motors) , the outer four side ports and the exhaust ports were opened up as much as possible . I am not sore how it will work , but I expect it to be similar to the half b.P. , may be you can tru this combination and have a bit of both worlds .

 

Thats exactly the set-up I'm looking for, except with a 12A because I like the little buggers! was this a Full PP or semi-PP? I think I'm gonna learn my way around building the EFI first and then take my time building up an engine for the car. Did this car have the bucking problem that I've heard is the case with a bridge-port? That was the main thing I wasn't looking forward to, everything else I can handle.
Grant

 

It was a full pp, no side ports. If you are doing a 12a use 40mm ports instead of 43mm. Any ported engine will buck to a degree, usually on decell at low rpm, all you do is use the clutch a bit more, it takes about 20min to get used to it. Using the clutch a lot with a ported rotary goes without saying.

 

Naturally, what kind of power would the 40mm ports provide? I assume this was a non-turbo car? What if I was willing to go even smaller to a 35mm port, would I be seriously decreasing the amount of power available? I just don't like the idea od gapeing side ports, seems like too much excess seal wear to me.
Grant

 

If you want low end grunt, don't concern yourself with overlap so much as getting the port *closed* earlier.

The intake closing and exhaust opening points determine RPM range. The amount of overlap determines how well it idles and runs at low throttle. The amount of port area (size of window) determines how much HP you can make.

So a nice early closing intake port will have to have "normal" levels of overlap (normal for a peripheral port, or even a bridge - bridges tend to have much more timing than P-ports), or else suffer from a really small port opening. Which might not be that big of a drawback, since a low RPM engine by design won't make as much power since it's not turning the RPM.

 

So I could go with a 35-40mm port and high over-lap but still build for lower RPM? As in what I would do would be to not run the port as close to the tension rod as on most set-ups I've seen? Hmm sorta backwards from what I've seen before but I see what your talking about. Basically the motor may not run beautifully at idle but it won't require huge amounts of RPM to make power. I've read the entire PP thread in this forum, it was very educational but it only addresses purely drag-race type porting. I may do the semi-PP idea as a way of recouping some idle quality, then run a sequential 4 barrel throttlebody.
Grant

 

Welcome to the compromises of porting.

 

35mm ID semi-PP it is! I will still fill the secondary ports with devcon but I'm gonna leave the primaries stock, no porting at all. I'm probably going to convert a holley 650 to a throttlebody with airhorns and run the primaries off of the carb primaries and then run the PP off of mechanical secondaries. ECU will be a megasquirt. I'll probably run some fairly small injectors on the primaries (Stock S4 na?) and then go with some GSL-SE ones for the PP, this should do the power alright. I already have a fairly free-breathing exhaust so that isn't a worry. I will also tune with a dual-table code in the ECU to handle the miss-matched injectors. I may also disconnect the PP and tune just the primaries to begin with. I still have a lot of learning to due with the EFI and that is my first project, making the PP engine will probably have to be put off until next winter. The info I got here certainly seems to suggest that I'm not too far off in my ideas for how to go about building this motor.
Grant