Jeff20B

iTrader: (13)

I just drove a 12A today that has '74 spec intakes and slightly enlarged exhaust ports. It's gutless below 1500, has driveable midrange, but takes off really well once you hit about 3800 or so and punch it to open the secondaries. It gets up to 7k really quickly. The carb is a modded Nikki with mechanical secondaries, increased accel pump shot and duration, and 94 primary jets (I think they need to be bigger). Secondaries are stock 160 and they're fine.

This engine could work in the baja, certainly, and will be coming out of the 1st gen eventually, but it wouldn't be right for the baja. Too little low end and too much up high. Maybe in a street bug or a Karmann Ghia, but not a baja.

Those secondaries are really something! It feels like another rotor kicks in. It seriously felt, up high, like the 20B did down low, when it was in the GLC last year. I'm not interested in the weight penalty or the horrible complication of the 20B's EFI system in the baja. I'm trying to keep it simple (stupid). Therefore the 20B will go into the 1st gen, the 13B will get built and find its way into the baja as planned, and the 12A will float off into limbo.

So that's what '74 spec ports will do for an engine. Still quite driveable as a daily driver, but has a little reward waiting for the driver each time he ventures into the upper rev range. It's not a jungle cat like so many streetported and aftermarket carbed "peaky" engines are, and I'm fine with that. No need to build an engine that you can't drive within its sweet spot all the time.

I'm just wondering whether there is a way to bridge the gap between the monster torque of a 20B EFI with the simplicity of a carbed 12A but not suffer from the drawbacks of each; to kind of overlap the good qualities of each setup and minimize the bad. It would have to be carbed for simplicity, provide stump pulling torque at low RPM, and adequate high RPM power but not peak too high.

I've come pretty close according to my best calculations. I started with a special J-spec bus or possibly a delivery van engine from the '70s with the same port timing as a GSL-SE, minus the aux ports. In other words if you disabled the 5th and 6th ports on a GSL-SE, that is basically how this engine would behave, except it was carbed and therefore the torque, driveability etc would be a little less. 4500 RPM would be the practical limit. Sure it has low end torque, but not much power to be had with such a limited rev range.

So I sought out ways to improve the mid and high range power without sacrificing the ever important low end. I concluded that I could go with normal size primary ports instead of the small NO sized ones of the GSL-SE and this engine's stock intermediate plate, and still maintain the same luxurious low end as before.

How?

I got hold of a Cosmo manifold with reversed runners which flows the primaries into the front and rear plates while the secondaries flow into the intermediate (middle) plate. The longer runners help torque and can probably mask or cover for the larger stock ports in the front and rear plates. By "larger" I simply mean stock '76-'85 12A ports (the NO sized primary ports of the old intermediate plate really are small looking compared to a 12A). I don't want them any larger than stock because there is still a chance their size, even though they're stock, could potentially hurt low end, even with the longer runners. I won't know until I fire up a similar engine shortly.

This "similar engine" is the experimental engine I built this week and installed in my MG Midget rotary project. It has '74 spec intake ports all around and stock exhaust ports which open sooner and close sooner than the GSL-SE ports mentioned above. Anyway this engine is in a small light car so low end torque wasn't of paramount importance. However midrange and driveability are. If that 12A I drove today is any indication of what to expect from this 13B in the MG, whoa Nelly I better grab on to something!

It too has a Cosmo manifold because it's the only type that would fit the engine bay. Also the side plates are the same type Y castings from '79-'85. Only the intake port timing has been increased a few degrees and the exhaust port timing is stock for US-spec old carbed 13Bs (mostly for noise control).

Anyway it's a similar engine. There is a chance the long primary runners will make up for the larger ports and behave more like stock '76-'85 12A ports. Wouldn't that be something? I'll have my answer once the engine is fully broken in, but I bet I'll be able to tell on the first drive, which should happen as soon as I get a chance to weld me up an exhaust system.

In closing, if the experimental engine has no power below 1500 RPM because of the ports, or maybe sunspots, neutrinos, yo mama etc, I'll think twice about whether to port the baja engine. I just was hoping I could build the baja engine before shifting gears mentally and working on the exhaust systems of like four cars over the next little while. You know how you get going on something (engine building) and you gotta change direction for a while and then when you get back, it takes a while to get the momentum back up? I wanted to get that baja engine built and installed in my REPU before even thinking about exhaust. The REPU needs a complete exhaust system, the other REPU needs a partial exhaust system, the Cosmo needs a partial exhaust system, and the MG needs a center section. I'm currently eating, breathing, sleeping with engine stuff right now. Every time I think about exhaust I cringe a little. All my engine parts are out like rotors, apex seals, springs, shafts, gasket sets. Only the rotor housings and side plates need to be painted and the casting flash removed/smoothed. All my exhaust stuff is put away at the moment.

Heh, maybe I'm just making excuses. I should just replace the center section with Racing Beat thickwall pipe and be done with it. Oh, and the shift lever too. I don't care for the Hurst T handle the PO installed. I'd prefer a stock style nylon ****, personally. I think I have a spare around here.

Yeah, I should just do this exhaust so I can drive it around and get some actual first hand experience before I build the other engine. Then when the baja engine is done I can do the remaining three exhaust systems and take a break.

Sorry for the tome. I'm sure you enjoyed reading it much as I did typing it. Holds for laughter.