Hello,

Does anyone have dimensions for the intake flange on the 12a?

I'm getting a flange laser cut so I can make a custom intake for my drawthru setup. I can get this done cheap through my work.

In a few days I'll be able to get measurements myself, but I'm hoping to be able to start drawing this up sooner than that



Also, as far as design goes, I saw that the primary port is purposefully smaller than the secondaries and that it's pretty important to not change that, so I plan to leave that as is. But is there anything I should keep in mind as I design this?
I'm planning to pretty much copy this (Image from RotaryShack):



The differences would be the intake would face the other way and both runners would collect into a single pipe that would be plumbed to the turbo.

 

The intake ports are smaller by design to reduce the affects of reversion from the apex seal sweeping past after the exhaust is pushed out. It causes a back flow up the intake for a short period. If the ports are matched this is much stronger into the intake and can seriously hurt performance.

 

Why not get a 12A intake gasket and use that as a template. The gasket is pretty close albeit a bit large on the port side.

 

I got one on the way

That's really interesting. I'm glad I know about it now though. I found a bit of information here:
https://www.rx7club.com/1st-generati...5/#post1044131



So, I talked to a guy at work and we discussed what I want to do for a bit and we ended up deciding that a better idea would be getting the stock intake manifold and grinding out the inside of the carb flange so that everything is angled to help airflow, then get a carb flange and weld a pipe onto it and take that to the turbo compressor outlet.
I think that's what I'm gonna end up doing.


Also, completely unrelated, but potentially interesting anyways. I was trying to learn more about drawthru, so I was trolling through the VW forums. They had a comparison between blowthru and drawthru. On their cons for blowthru they mention you can't run much boost because your limited to what the carb can take. On the drawthru they mention you can run a lot of boost because the fuel will be chilled.

There's guys on there running upwards of 24psi of boost on drawthru with no intercooler (even saw 30). That's insane. I'm just hoping to get 10-14psi.

 

I think I could pressurize my Nikki up to 20 pounds or more. My transmission would then hand grenade

What kind of carbs where they talking about not being able to be pressurized up to 16 pounds. I imagine if the stock solex pic 34T couldn't take it. And lots of guys will blow through the pic 34T cause its easy to do and works.


Issues I have with draw thru is the amount of areas for vacuum leaks, and fuel pooling. Also no intercooling capabilities. With my blow thru setup I don't have to worry about AIT's my intake is cold because I get the same cooling effect from atomizing fuel that a draw through does only my charged air is at ambient temp prior to atomizing.



Eh, draw thru is different though not gonna lie. Good luck.



What are your plans for ignition? You've gotta try out E-85 its sooooooo much fun on a turbo car.

 

A lot of the guys are running holley's but I don't know exactly which ones. I'd have to dig through the forums again and look. I also haven't done much research into how much boost Rx7 drivers are running on blowthru Nikkis.

With drawthru you have
Carb -> Turbo -> Intake
With blowthru you have:
Turbo -> Intercooler -> Carb -> Intake
So I'm not exactly sure where the extra area for vacuum leaks comes from?

The pooling is definitely a concern. I remember that from the other thread I started. My plan is just to have the carb be higher than the turbo which will have the outlet angled down to the intake... hopefully avoiding pooling. We'll find out though.

I've become convinced that not being able to run an intercooler with drawthru is not a downside. The fuel is actually chilled - you're not injecting hot air and fuel into the intake. So if anything, an intercooler would just keep the fuel/air ambient and present much more risk than benefit.


Thanks! I hope it works like it does in my head

I don't actually have many plans yet. I was gonna convert to DLIDFIS and probably still will. What's the process like for converting to E85? Just advance timings?
Is that what you're running?

 

You get vacuum after the carb regardless of where it is located. So if you have a blow thru setup your carb is sitting on an intake manifold that's bolted to an engine. You can leak vacuum between the engine and the carb. On a draw thru you have a carb attached to a turbo where the joints between the turbo and the carb can leak. The turbo can leak vacuum. Then the manifold that connects the turbo can be an issue.

It seems, at least for me, that there is less room for failure in a blow thru setup. Being that the only area that experiences vacuum is the intake manifold.

When carbureted turbo setups whether blow thru or draw thru with have an innate cooling effect when they deliver fuel. Its just in the nature of how a carburetor atomizes fuel. Even N/A carbs do it.


Even if you do some kind of direct fire setup you're gonna have to limit the amount of ignition timing. Its not as hard as it sounds. Just pop the distributer off and goober a little weld in the advancer slot. Grind the slag and slap it back together.




E85 is Freaking awesome sauce. Just tune your carb to run it. It uses about 30% more fuel. So your turbo reaches higher speeds 30% faster. There is NO predetonation so you can advance as normal (I haven't fooled around with that yet). Its like instantaneously noticeable the difference. I mix a little in my tank every fill up. I do it partially to correct for my poor tuning.

 

Wait, okay I realized why this is confusing me now. On blowthru, the carb is pressurized by the turbo. It's not under a vacuum. On drawthru, the carb is under a vacuum created by the turbo and engine, so it's just like the carb was sitting on a bigger engine. After the turbo, everything is pressurized and not under a vacuum. Am I misunderstanding you?


I guess I feel differently about that. To me, it seems pressurizing the carb will introduce more room for failure.

I'm again confused about this. In drawthru, the cooling effect happens because fuel is compressed in the turbo then rapidly expands again in the intake/engine. That's why it's chilled. In blowthru, you're pressurizing only air and not fuel. The air gets hot, that's why you need an intercooler. Maybe there's some cooling effect from fuel atomization in the carb, but it's not really the same principle at play with drawthru.



Yeah that sounds pretty cool. I might have to give that a go after I get my turbo setup running in the first place
I still have to build the intake, and bend some pipe for the exhaust and I need gauges. So I still got a bit of spending and labor ahead of me before this is gonna run.

 

Yeah, the turbo pressurizes the carb. But only when the turbo is building pressure. It doesnt' build any pressure at idle or cruise throttle.

The vacuum is created by the engines' displacement creating a void. If the valves on the carb are closed (or really close to closed) they cannot move air through (or cannot move as much air as the void created). This makes a vacuum. This happens on all internal combustion engines that use air to regulate throttle.

So anywhere on the engine between the void and the throttle plates are under vacuum at idle and some parts of partial throttle.



Have you put any thought into deliberatly leaving some flashing in the intake to keep the fuel from droping out of suspension? I heard that a while back that the little bumps in intakes are left there for keeping the air turbulent. But thats just something I heard.

 

Ah yeah you're right. I didn't consider that.

So, well I'm learning ...
Why would there only be vacuum from the carb onwards? Wouldn't there be the same amount of vacuum through the whole intake system assuming no leaks?

I believe this is correct. I have heard it before. If you look at the Cartech intake kit, you can see the manifold from Carb -> Turbo is a <90 degree elbow and the intake manifold is basically a flat plate as opposed to runners leading to each port. I had found somewhere or was told that these manifolds were designed like this to create turbulence and keep fuel in suspension. I haven't put a lot of thought into if and how I will create this sort of turbulence without the cartech manifold yet.

 

I wrote that bit about the vacuum poorly. Yes, all the manifold and bits and pieces are under equal vacuum, BUT when you open the throttle the vacuum decreases. On a turbo car the vacuum turns into boost after the turbo. On your draw thru at WOT you will have no vacuum after the carb but boost after the turbo. On a blow thru the whole system experiences boost.

Its a head scratcher to try to explain.

That vacuum decreas is how a lot of carbs get the signal to start moving different amounts of fuel.

I think you're on the right path getting a 12A intake manifold and butchering it. Just cut off the intake and get a buddy to tig on a joiner to get all of those pipes to collect.

 

Well browsing ebay for rx7 parts, as normal, I ran across an Elford intake and it looks like almost exactly what I need. I went ahead and bought it and just gotta wait for shipping now.

The big problem with the CarTech intake I have right now is clearance from the turbo to the intake manifold because the Rotary Shack headers stick up so high.


But the Elford intake manifold inlet is much higher up, and even has a mounting point where I can mount my carb->turbo manifold.



(Picture from Ian65 here on the forums)

 

Have you decided on a carb? You could pretty easily do a side draft carb. Pretty easy is an understatement. Just bolt the carb to the front of the turbo.... done.

A downdraft four barrel would be more difficult, but if you look for VW draw through setups there are a few that are designed for 4 barrel American square bores. Then you'd just have to figure out a bracket to hold it up and mount that to the turbo.

All this puts a lot of faith in the structural integrity of the exhaust manifold. But rotary shack makes good stuff; at least that's what I hear.


You could use either manifold with that turbo from the looks of it. Just get your hands on some short intercooler turbo piping or exhaust piping (as long as its properly flared out), and put some silicone joiners on the turbo to pipe and the pipe to manifold.

 

I really like the idea of the sidedraft carb straight onto the turbo... What I was gonna do is use the carb -> Turbo flange from the cartech kit.
I might spend a bit more time researching the sidedraft option though. It'd save me having to fab a bracket to mount this to the intake manifold.

 

Lots of people seem to like SU carbs for side draft, but I've seen Dells and other two barrel carbs used too.

That's a hell of an adaptor to reduce that manifold. I'd put a 4 barrel down draft American carb on that manifold and build a bracket from angle iron to attach it to the turbo.

 

Why's your turbo mounted backwards, intake is supposed to face forward