Jeff20B

iTrader: (13)

The FB manifold to be modded.

big drill bit to take off lots of material

making progress

Done! Sorry for the over exposure.

Some Y plates and R5 12A housings.

84stock

iTrader: (5)

Interesting, i would really really like to know what results this experiment gives.

Myself, for 83 I am "thinking" if trying a 1/2" open spacer under the nikki. This works so soooo well on the holleys that I think it may be worth a try...

Jeff20B

iTrader: (13)

In case you might be wondering, the channeled manifold will not be going on the half BP. It would be bad in many ways.

Jeff20B

iTrader: (13)

I was unhappy with the over exposed picture above so I took another.

Glazedham42

iTrader: (4)

Don't run an open spacer on your intake manifold. It won't perform well. I think Peejay discovered this a long time ago. It's buried on the internet somewhere. Basically, if you want to improve your FB manifold, copy what Jeff did above. I've done it a couple of times and it is about as good as you can get the FB manifold.

For some reason the 12A/Nikki doesn't like an open spacer like a Holley. It is actually worse that what Jeff is showing you on his manifold above.

84stock

iTrader: (5)

saved me a lot of trouble, thanks

Glazedham42

iTrader: (4)

Had to go to the internet archive to find it. Here is the basis for doing what Jeff has done above. It also describes what happened when the open plenum spacer was tried. It's a great read. You'll end up really appreciating what Jeff has done above after you read this article.

http://replay.waybackmachine.org/200...penspacer.html

LizardFC

iTrader: (1)

Your thread has another loyal follower. I'm planning on going bridge on my 12a toward the end of the summer so I'm really interested to see how this works out. Thanks for posting all this and keep up the good work.

Question: On the modded FB manifold above, wouldn't you want to taper those flat "bridges" where you just made your channels? Seems like the air meeting a wall like that would cause a lot of turbulence.

j9fd3s

iTrader: (3)

ok so who is going to try keeping the primaries independent for the low rpm fuel metering, and the secondaries plenum style just to get the air volume thru the carb?

Jeff20B

iTrader: (13)

I don't think it works that way. Then again I'm not very sure about which channels to cut and where. All I have to go on is mentioned above.

That said, I'll be testing my 390 when the next 13B is built after this half BP. The last time I tested the 390, it was on a 74 ported R5 13B. The manifold had channels exactly like a 79-80 manifold connecting primary to secondary. The carb adaptor had just one channel connecting both primary holes. This is opposite of what peejay said allowed an edlebrock to scream; he said when the secondary holes in the adaptor/spacer were channeled, it fixed the missing high RPM issue. Well guess what, as I was testing the 390 on the 76 Cosmo, it lacked high RPM! I thought it would have flowed fine because I was using an open hole thick gasket, so there was true communication of all four barrels right under the carb, but maybe it wasn't enough? Plus another possible issue was the exhaust consisting of a stock cast iron manifold and then onto a 2" system until it got to the stock resonator, after which it contacted down to 1 7/8" and through a stock ancient Cosmo muffler. So it was running out of flow at 4k and above.

When I test this carb again, the next engine will also have 74 spec ports and R5 rotors to keep the playing field level. Different manifold, this time separate runners. I'll start with the carb adaptor as is and the open hole gasket. The exhaust will have a header and collect to 2.5" with straight-through components so no chance of restriction this time. I'll also have access to an O2 gauge to really see what the carb is doing. I'll see if I can use a wideband if it's available. I'll then cut a channel in the adaptor to connect both secondaries together. I'll then swap in the channeled manifold and keep an eye on the gauge and seat of the pants dyno to see if there is a difference. I hope the open hole gasket will be enough communication so I don't have to use the channeled manifold as it won't fit in the application where this engine is going (MG Midget). A 76 reverse runner manifold will, with some trimming of the ACV section.

By the way, testing this setup so far on the channeled manifold and connected primary holes in the Cosmo, it allowed for a decent idle but not all that much power until about maybe 1500 to 1800, so you had to be a little more careful than when this engine had a stock Hitachi on it. As the Cosmo weighs as much as an FC or REPU, you had to remember which carb you had in there at the time. The little MG will probably be a lot more fun with this carb as ultra low end isn't as important.

RXDad

iTrader: (2)

Had a similar experience with my 4 bbl TB FI set-up on my 12a with street port.
Originally, I was using this manifold with primary to secondary plenum. It works great at mid to high RPMs, but idles rough.



I switched to a RB 12 stock port manifold and the idle and on the primaries was smooth and strong. High end was not as strong and it took huge amounts of acceleration enrichment to overcome a lean condition when the secondaries opened.



Rotaries like some communication between primary and secondary ports, but lose some smoothness on idle. Probably why there was two channels on the early SA manifolds and only one on later manifolds.

RXDad

LizardFC

iTrader: (1)

Jeff, that kind of exhaust restriction on the Cosmo would definitely kill your top end. Especially on a ported motor. Interested to see what kind of results you get with a more open setup and a sensor.

That's more of what I was talking about with tapered channels, RXDad, although you really took it to an extreme! Ported right into the screws

RXDad

iTrader: (2)

My manifold was adapted for a holly spreadbore carb, hence the addition of an adapter block and extreme widening of the channels. I added the center divider to isolate the rotors.

Amazing thing is that it runs very well, much better than the RB in the mid to high rpm bands without too much loss in the lower.

RXDad

Jeff20B

iTrader: (13)

Yeah, I'll be test driving it in the REPU. In there I can test various intake manifolds and it has a narrowband gauge already. I'll swap in the wideband if it's available. I'll start out with a similar exhaust to what was in the Cosmo (because it's already built). Exhaust manifold, 2", then into two Rotary Engineering glasspacks in series (one long 2" exhaust). Then I'll fab up a long primary with a header and collect it to 2.5". This will flow 100% better.

Getting back to the half BP, I think I have the specs to aim for. https://www.rx7club.com/showpost.php...49&postcount=9
I'll still probably do 5mm like I originally planned, for longevity as a thinner bridge is more prone to cracking. The info above is for race engines which get torn down at the end of every race season.

j9fd3s

iTrader: (3)

the homologation paper even has pics

Jeff20B

iTrader: (13)

Are they online?

The bridges I scribed out are taller than 41mm. I think this is fine because the Group B cars had bridged primaries and secondaries, but since mine are bridged secondaries only, I can do larger bridges and not worry about weak low end.

Jeff20B

iTrader: (13)

The right tools


close up


rear plate


some progress

Jeff20B

iTrader: (13)

more progress


front plate


progress


Now time for dremel action.

peejay

The Group B cars had 48 IDAs and MFR peripheral port 13Bs. At least, the "Evo" models did, which were the only ones ever to see GpB competition.

I think the GSL-SE was the "homologation" model. 200 cars of a type had to be made for homologation, and 20 Evolutions could be made that were modifications of that type. So the GSL-SE homologated the 13B engine and basically everything done for the Evo was either an approved variance or was unrestricted.

Disclaimer: This is speculation.

I plan on printing out the relevant page of the RX-7 GROUP A HOMOLOGATION PDF (why no that is not a subtle hint) and just duplicating the homologated ports. Why? Well, it worked for them, and should work at least as well as making crap up on the fly like I've always done before...

Jeff20B

iTrader: (13)

Thanks for the info, peejay. Group A you say?

I got the half BPs done. The forum isn't letting my upload pics so I'll try again later. I can describe them. Eyebrow length came out to slightly longer than 41mm. I had to bevel the ends so they ended up longer. Bridge varies from 4.5mm to 4.8mm with 4.7mm being the average thickness (hey, it's my first time). Eyebrow width is around 6.9mm as I didn't want to cut too close to the coolant seal. Turns out I could have gone wider being an 85 or older with the coolant seals in the housings. This is not a race engine though, it's for the street and they're WAY bigger than the jobs I've seen on 86+ engines.

Man I see what you're saying about 86+, you gotta keept the bridges small on those. Kinda weak lol. Another thing I don't understand is why BDC makes his famous half BPs so small. ***** ports! No offense intended with that comment, but he does recommend a stock flywheel because his engines loose so much low end torque. Sounds pretty weak to me lol. My engine should retain all of its low end torque. Is there some communication in the T2 TB or something?

I'll see if the forum lets me up some pics later, so you can laugh at my ports.

Jeff20B

iTrader: (13)

completed rear port


completed front port


beveled rotor housing


beveled rotor housing


R5 rotor housings


stupid emissions tube, to be removed

cshaw07

iTrader: (14)

emissions tube? what did that come on?

peejay

That's... odd. I would HOPE that BDC is smarter than to claim that a flywheel has anything to do with an engine's output.

At any rate, I can baaaarely tell the difference between a stock flywheel and my SR aluminum one.

There are two plenums, a primary plenum fed by the single primary throttle, and a secondary plenum fed by the dual secondary throttles. Basically a shorter version of the S4 N/A manifold.

The N/A manifold has MUCH longer runners and a much nicer entry angle at the engine, which is why I use it. The VE map does not lie, I needed to add a lot more fuel in the 2000-6000rpm part of the map after I junked the TII manifold. I needed to add more fuel above that point too, but not as much. Basically, my findings are that the TII intake manifold is good at fitting a turbo next to it and an intercooler above it, but it's garbage for moving air into the engine at any speed.

Jeff20B

iTrader: (13)

In one of BDC's videos from a few years back he accelerates from a stop in one of his half BP cars and says something like "see, no loss of low end here" but in the video it sounds like it bogs down a bit. And somewhere else he recommends using the stock flywheel to retain driveability which speaks of a noticeable loss of low end torque because why would he mention it otherwsie? I may have been in error stating that he said it keeps low end torque etc specifically, but I think PercentSevenC will concur that I got the gist of it right as he was thinking of going half BP on a T2 or FD swap in his 1st gen soon and looked into the BDC info more than I did. I recommended against a half BP not knowing the full story of how their TBs flow and whether there is any communication from primary to secondary. You can't have any pri-sec communication on a half BP, otherwise you're just lying to yourself and might as well go full BP or full SP, or live with the loss of low end and less power up high than a full BP. As Blake Qualley used to say, you can't be half pregnant. lol

Older stock flywheels are too heavy for me. I can feel the differences going from say an 83-85 (26 pound) to a light steel flywheel. The later stock ones getting down into the low to mid twenties are ok though, but it depends on the vehicle. REPUs came stock with 30 pound flywheels and a heavy pressure plate, but do best with a 21-24 pound flywheel. At the heaviest I'd say a 26 pound GSL-SE flywheel would be ok, but don't use a Camden supercharger with one because the added rotating mass makes it feel like the OEM 30 pound clunker. There were only two times I was glad I had a stock 30 pound flywheel setup, 98% of the rest of the time, I hated it. I then towed a friend's VW crew cab a little ways with a light steel flywheel and that sealed the deal. No need for the 30 pounder, so go with an S3, S4 or S5 and you're good. Heck I once towed that crew cab about a 1/4 mile with my 76 Cosmo and its GSL-SE flywheel swap. The rear diff gearing is super tall at 3.636 and it took a bit of time to let the clutch out, and it wasn't even broken in, but it did ok.

I'll be using an RB aluminum flywheel in the FB. Good old 7.5 pounds by itself. The MG is getting an RB light steel as I've driven it with a stock REPU 30 pounder (that was stupid) and then a light steel flywheel. The light steel (at 13 pounds) was far better, but the rearend is geared kinda tall so aluminum wouldn't be as nice. The GLC got a super light steel 8 pound flywheel 215mm only, made by GReddy, aparantly. I may swap it over to light steel at some point if the 8 pounder is too light for daily driving. I'll find out this summer. The half BP is getting an 81-82 flywheel.

Interesting about the T2 manifold. Sounds way worse for NA flow than an FD or 13B-RE/20B manifold. Good luck on your next half BP. This time only bridge the secondaries (go bigger than BDC!) and go way out of your way to insure there is no communication from primary to secondary. Also run four injectors, staged, this time. Get a newer MS if you have to. My 2.0 board works great with high z injectors staged to come on at 4k or whatever.

Jeff20B

iTrader: (13)

R5 rotor housings