howracer

i would be interested to know if any of you have experience as to how much, if any, a mild streetport, streetport or raceport changes your compression test results versus stock ports.



either numbers from the mazda strip tester, a compression gauge, or idle vacuum at a particular rpm either in inches or MMs would be welcome.



actually, it might be helpful to all if everyone posts what they have along w mods.



howard coleman

Maxt

My experiences so far...

13b na 6 port, bone stock ports 185,000 km's compression low 90's pulls around 17" or so vacuum..

13b na 6 port I checked for another FC owner with 20,000 km's on it rebuilt by a local builder with 3 mm seals 10" vaccuum, 60/65 compression, supposedly ported but I doubt it..

My S5 13bt jspec when it was stock, 90 psi both rotors, pulled 16"

S5 with Large streetport hit a high of 100 psi per rotor with the first batch of RA seals, and an eventual low of like 15-30-15 on the rear, and 0-0-0 on the front when hot, when that motor was fresh it still pulled 16" vac even with a large streetport..

S5 13bt with Mazda 2mm and partial bridgeport, over 100 psi cranking both rotors, 10 to 12" vac

S5 13bt partial bridgeport with new RA's in it , a few hours on the motor cranks over 100 psi, right now its pulling 12" vac, and is very crisp sounding..This is round 2 with the RA seals, hopefully all goes well this time..



I have found that porting doesn't have that large of play on the cranking pressure of the motor, it does of course affect idle vacuum with overlap, internal measuring and seal fitting, is where its at...To date, locally we have not had one 3mm motor meet minimum compression spec, not ven close...Maxt

howracer

thanks for the reply max...



it is interesting that you state there is not necessarily a direct correlation between cranking (around 250 rpm) pressure and idling vacuum. what is your new motor idling at?



as to 3 mms. i had an engine built by Pettit for 40,000 miles that had a small streetport and 3 mms and just prior to dropping a hoseclamp down the lim it was pulling 17 inches.



might it be that the quality of the groove remachining might be an issue with some 3 mm conversions? i have heard that there are very few trusted vendors as to the groove machining process.



BTW, i consider the piece you wrote on bridgeporting to be one of the most interesting and provocative pieces on either of the two forums and would welcome any additional comments you have now that your new motor is up and running.



howard coleman

RETed

Vacuum pull and compression numbers are not necessarily related.

Compression is more dependent on compression ratio (of the rotors).





-Ted

Maxt

Yes in a perfect world in the same housings with the same port closing times this would be true, however notice that Mazda gives the same minimum spec for both motors , turbo and N/A...My rule of thumb on piston motors has been 25 psi for each point of comp when correcting for camshaft changes, but it doesnt hold out on rotaries...



Howard:

Yes the quality of the milling of the 3mm rotor itself has alot to do with the compression numbers and the low rpm sealing of the motor. The limit for the compression on factory 3mm motor was 80 psi min at sea level, 250 rpm, I recall my old 12a, had compression in the low 90's after a rebuild, but motors that about 3 or 4 of us have tested, all with milling done by the same place, yield very low compression numbers, even with the cranking speed and elevation correction, which here works out to a min spec of 75 psi or so, so I will agree it matters on the quality of the milling alot, on a rotor I have here, the milling is offset and not entirely straight, its actually s shaped but looking at the factory min specs, notice the limit for the 3mm engines from mazda was also 5 psi lower than the 2mm motors, I do believe mazda moved to 2mm for improved sealing and they do seal better on the whole..The numbers we have been seeing on basically stock port 3mm motors is compression #'s in the 60-70 range and vacuum in the range from 10-14" at best at 900 rpm idles with a/fs in the 12-12.8:1 a/f, they need alot of fuel to idle and make vacuum...I am going to do a set of 3mm rotors this spring myself with the aid of a good machinist I know, then I will be able to really judge the 3mm versus 2mm for myself with parts that are accuratly done and properly assembled..

I think my 1 page Bridgeport rant was erased with the hacking of the board..Ah well...at least it was up for awhile..

I was staring at the 6 port closing time in a mocked up "half motor" the other day,I was trying to work out the exact compression time after e shaft dwell versus the port timing to see what effect it had on compression with port closing time, Michels Frugels(sp?) rough animated screen captures got me thinking about the purpose and importance of late port closing time on the n/a versus the turbo motors.. Just from eyeballing the motors what I see is the late closing time is into the compression phase quite a bit compared to the turbo motor or other 4 port motors, which makes me think that the purpose of the six port opening is not so much for flow inwards to the motor, its for a reversion pulse back up the intake tract to pulse feed the other rotor phase and aid the "dynamic intake effect"...Which then leads me to beleive that really later port closing times are only effective the intake is tuned to make use of it, but also screwing with the port timing closing time on a factory intake tune for a specific reversion timing, is useless as well.. I do believe thats why mazdatrix et al has stated porting the 6 port in a more or less a lost cause, the intake manfild and chamber is specifally timed and tuned to the ports, messing with either just degrades its effectiveness from stock...The only way to go with the port timing on that motor is earlier, then exhaust reversion probably interferes with the compression reversion at some point as well... On boosted motors I don't think the reversion style dynamic supercharging effect has much merit, or need....On the 6 port na motor,I am thinking why people report a loss of low end then when the 6 ports are wired open, is they are losing low end compression, and the reversion pulse is then out of synch with the tuning of the intake tract length and the dynamic chamber, until higher rpms are reached, some say its due to the lower port velocities but I dont think thats it totally and that has a minimal effect.with that in mind, 6 port opening time is not neccarely best load based, but rpm based, the s4 n/a was all back pressure acitivated, and back pressure changed with load, not always rpm, I am not certain, but I think the s5 n/a was all dependent on engine speed, and I think thats the reason why..

So the road I am thinking along is that changing the closing time of the port, will have an effect on compression, and also greatly disrupt the tuning of the intake length and design to the motor, where as earlier port timing would just change overlap, which really is to increase scavenging on the chamber.. Overlap is seen as evil thing in the piston world, mostly because it is said that high overlap camshafts lower compression, this is true only because of the profile of the camshaft lobe and its need to be symetrical in closing and opening due to its mechanical limitations, to open a valve more, the ramp of the lifter has to be gentler, and kept to a certain rate of lift, this means that high lift cams generall move the intake and exhaust event earlier, and also later as compared to stock, which increases the overlap of the cam, but its not the earlier intake event that lowers compression, its the later closing event of the intake valve, thus the lower cranking compression of a larger duration camshft is acutally due to the physical limitations of the mechanism, not the intake opening point itself and subsequent overlap..If you could open the intake earlier,with more valve lift, but close it abruptly, this would solve the need to run higher compression pistons to overcome changes in valvetiming effects on cranking pressure and low end torque, but the intake valve cannot be slammed shut, the mechanical mechanisms of the valve train cannot handle that, roller lifters have changed that a bit, they can open the valve quicker and higher, but the added weight limits rpm of the engine, and thus are not really much a advantage for ultimate all out high hp engines, in hydraulic roller lifter form..

Back to the rotary we are lucky, we can move the intake port timing either way from stock without having to effect each other, thus opening the intake port earlier, makes a lot more sense for greater hp output with emissions aside, messing with port closing time would only be useful if you could absolutley model the compression reversion to suit the intake manifold design, I think especially being true in a turbo motor, on boost, chamber purging yeilds alot more power ouput than having a reversion pulse on a positively charged motor while having trapped spent gases mixed with the intake charge, I think this all has to be kept in mind when tuning as well, a motor with alot of spent gases maintained in the combustion chamber stabilizes the intake charge, and slows the burn rate of the charge aka EGR, thus will be more detonation resistant, but will also be lower in overall ouput as the volumetric efficiency of the motor is reduced overall..

Just some thoughts..Maxt

Old Splatterhand

damn Maxt, talk about mindreading.

i had the same thoughts as you described.

the reason was this: years ago there was that video where SCOOT from japan dynod their p-ported 710 whp FD.

afaik it reached peak hp in the low 7000 RPMs at "only" 2 bar.

i was amazed, because thats pretty low rpm in my book. a stock FD reaches peak hp in the 6000s(right?).

there was also a pic of scoot p-ports floating around. the sideport looked beefy, no doubt., but not extremly radical like some ports you can see here on the forum.

the fact that the port timing was relativly(!) conservativ but combined with pport yields me to the conclusion that it's better to have a short intake phase but with higher flow (p-port), than a huge, long duration- sideport where the air gets compressed out of the chamber back into the intake.

another fact that makes me start doubting aggressive sideport timing is, that the member ErnieT reached some serious hp with a stock motor a while ago.

Just some thoughts here as well, please leave the flame throwers at home.