Jeff20B

They're smaller now than on the old housings. I also chose the D shape to make the powerband less peaky. I think an Atkins supercharger may be in this engine's future too. Either way, I'm glad I didn't raise the entire width of the topof the port as high as the T2.



The ports on the left are the old rotor housings. On the near right is one of the '74-'75 housings. The far right is one of the '79-ish J-spec set with the D ports.

Jeff20B

By the way, the opening of the '74-'75 ports open a little later than the T2 or the J spec ones. I'm going to lower them a little when I port 'em. But before I do, I'm going to fire up the D ported engine and see how it goes.



In this pic, you can see the differences Mazda made to the old style pre-GSL-SE 13B rotor housings. The left is a '78 I think. LOL the previous owner's name is Rick. I've seen quite a lot of this style with their three outcroppings of aluminum, hanging around as many REPUs as I do. The middle is a '74-'75. Notice the lack of a shroud thus allowing B8EV plugs to be used without the need of a thinwall sparkplug socket (not that I'd even bother with them since FC plugs are so good). On the right is the J-spec housing with its semi-circle shroud just like '79 and later housings have. I say it's a J spec because we never got any 13Bs from '79 to '83. I can tell you they're definetly not GSL-SE housings.

Jeff20B

Just another shot of all three ports.

Jeff20B

One more shot. They're not aligned perfectly with each other, but you get an idea of the differences.

diabolical1

i seem to recall reading somewhere that if you definitely will not be using EFI, then the ports should not be smooth as glass. i believe it had something to do with the atomization characteristics of carburetion vs. EFI.



nice job so far.

Jeff20B

Thanks! Yeah I remember reading something about port smoothness earlier this year. The intake ports of the overheated engine were left as rough or rougher than stock when we ported them, thanks to that thread. Now that it's apart for a rotor housing swap, I'm going to smooth the ports a bit and shape them a little better.



I'm even considering a half bridgeport. I just won't be able to use the dellorto on this engine if I do. That's ok because the dellorto will be used on the 20B. The Edelbrock will be used on this engine after it's broken-in. Maybe I should do a half bridge? It'll be limited to 8500RPM because of the iron seals, but from 6500 to 8500, it'll probably feel like another rotor kicks in.



The funny thing about this engine is that it still started right up even after it overheated. It looks like the orange water seal deteriorated near the spark plugs. Otherwise, everything else was ok other than a little bit of crush on the rotor housings.

Jeff20B

Giving it some thought, it seems that this engine should not be half bridgeported. The other 13B might be a better candidate.



Thinking out loud here, I'm puting the dellorto on the 20B, with FI in its future. Then the dellorto will probably go into the MG. The white REPU will get an Atkins supercharger some day, so the engine needs a nice streetport and high oil pressure with a high volume oil pump. In other words, this engine that I'm currently porting will get the SC. I'll plop the Edelbrock 750 on this engine after it's broken-in with my truck's carb. The Edelbrock is plenty big enough for a SC. The other 13B might get half bridged because I'll have more time to experiment on the porting. Or maybe I'll do a 12A?



I have several engines that need specific intakes/carbs. I'd like them to end up like this:



.Edelbrock on Atkins 5" or 7" SC kit on first 13B in white REPU

.Fuel injection on 20B in GLC

.Dellorto in MG

.Truck's carb and Cosmo's intake manifold in Cosmo

.Cosmo's carb on second 13B in red REPU

.I'm not sure about the 12A yet



I think I'll need to do everything in this order:



.Finish streetporting first 13B and use truck's stock carb for break-in

.Dellorto on 20B

.Edelbrock on first 13B after break-in with a peejay-inspired carb spacer

.Streetport or half bridgeport second 13B and use truck's carb again for break-in

.Possibly half bridgeport old '73 12A and use a Nikki for break-in

.Holley 750 DP on second 13B or 12A



If you've actually read all the way through that, you'll see that I'm not exactly sure about some aspects at the moment.

Jeff20B

Time to go finish those rotor housings. 2mm bevel, here I come.

andynogo

You sound liike a man with too many choices!



I get confused with just two project cars...



Those D ports are certainly very common and should work well. The height you have them at now should be ok- taking them down a little as you said would be a good idea- you don't want large overlap especially if you go the SC route. Those old ports were big! I was just a little concerned at the amount of discolouration on the housings above the ports- were the apex seals ok? Was there a nice bevel on the edge of the port?



Anyway, good job and keep us updated, especially that 20B with the two carbys, efi and SC stuck on it!

Jeff20B

I think the discoloration is carbon. I don't think the bevel was done properly on the old rotor housings. I'm going to inspect the apex seals before I reuse them.



So do you think I should move the opening of the D ports a little lower? They'll be NA for probably a year before I can afford a supercharger. Do you think it'll cause the torque curve to move too far up the RPM range so as to reduce low RPM drivability for towing? I guess it doesn't matter with a supercharger.



Hmm, I may move the opening down by 1mm. I'm experimenting with the bevel right now; trying to make it 2mm at 45º, so yeah, maybe 1mm lower should be good.