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twentyover
twentyover Dork
8/2/19 7:47 a.m.

Many years ago GRM (may have been when it was still AutoX) I seem to recall did an article removing the FI system from a Miata and replacing it with a couple SU carbs. While I am not suggesting I would do that, I am investigating adding SU's to a Toyota 20R. Does anyone remember anything from this deal? Is it possible to add the article to the website so i can re-read? Tty to get an English car vibe with this deal. Or if anyone has any experience in making this change?

Not interested in Weber's no need to go there.

Or if anyone has any experience in making this change?

 

Longtime subscriber (re-upped my 10 yr subscription last year- I'll probably die of old age before this subscription runs out) and occasional forum reader (I usually focus on builds forum)

 

Jordan Rimpela
Jordan Rimpela Digital Editor
8/2/19 8:09 a.m.

Let me see if I can track down just which issue that might have been from. 

LanEvo
LanEvo Dork
8/2/19 8:24 a.m.

I love dual/triple carbs on older i4 and i6 engines. Easy to tune. Easy to swap. Easy to trouble-shoot. They look cool, sound awesome, and generally have great throttle response.

I won’t recommend Webers, but it does seem like Solex/Mikuni would be the obvious choice for a Toyota 20R

David S. Wallens
David S. Wallens Editorial Director
8/2/19 8:30 a.m.

That sounds like something that Norman Garrett did. I remember him putting SUs on a Miata. Did we run that or did Miata mag? 

As I recall, it looked cool but had some issues. Let me ping Norman. 

David S. Wallens
David S. Wallens Editorial Director
8/2/19 8:50 a.m.

Just heard back from Norman. (Norman was an engineer on the original Miata.)

He still has the SU setup. It's hanging on the garage wall right now. He said the setup was discussed in his book, but he'll share more later. 

mtn
mtn MegaDork
8/2/19 9:09 a.m.

That also reminds me of a featured readers ride that you guys had in probably 2006 or so--it was, IIRC, an NA with the Italia body kit and then the guy just never really stopped modifying its look. I seem to recall carbs on it, but I could be mistaken. 

David S. Wallens
David S. Wallens Editorial Director
8/2/19 10:23 a.m.

In reply to mtn :

Hmmmm. That might have been Randy Stocker's car. Let me check. 

Docmiata
Docmiata New Reader
8/2/19 11:09 a.m.

SU Carbs on a Miata 1.6L

Yes, I put carbs on my Miata as a lark back in '92 or so. I found a pair of rebuildable HS4 carbs in my pile of Brit-car parts and adapted them to my daily driver. These carbs came from a 1.8L MGB engine and were "close" to what would fit the needs of the 1.6L Miata, so that was enough to justify the exercise. I had Graham at Jim Downing's shop (where we sold the Sebring Superchargers out of) make up a beautiful TIG-welded aluminum intake manifold, complete with cross-over pipe/balance tube which is critical for idle performance. Throttle linkage was easy, and I put a manual choke cable on it, mounting the knob where the cigarette lighter hole was in the dash (my favorite part). 

Some notes from the installation:

1. The 1.6L Miata engine pulls around 177 CFM at full song (naturally aspirated) at 7,250 rpm and according to the SU factory specs, the HS4 would be just fine

2. Putting carbs on a Miata eliminates the rev limiter (fuel cut at 7,250 RPM), so it ran to 8,500 quite nicely (albeit at even higher CFM rates, so the carbs began to wheeze a bit up there). The loss of low-end torque through the loss of the factory tuned intake system was partially offset by the nature of the SU carbs (see comment #4 below for why).

3.  I had to trick the ECU into some sort of Throttle Position Sensor (TPS) signal, so I found a plunger TPS (simple linear potentiometer - this one from a 1990 Mazda B2000 truck at MazMart) and mounted it to the side of one of the carbs, actuating it with a small aluminum cam profile I made that fit on a nearby throttle shaft stub. As the throttles opened, the cam depressed the TPS's plunger. Using the wire carrying the 5.0VDC reference signal that goes to the stock TPS, we could feed a linear 0 to 5 VDC signal back to the ECU so it would advance the ignition timing correctly. This is the only trick you'd need to accomplish if you were to do this with a different engine.

4. Throttle response was greatly improved, since the SUs are "constant depression" units which keep the carb venturis' air velocities very high at part-throttle operation and prevent the "open door" issue a butterfly throttle's opening alone will do to an intake tract's flow velocity (throttle blade-to-intake valve "air colum velocity" drops terribly immediately following a rapid throttle opening, thus killing any inertial benefit of a high-velocity air stream with regards to cylinder filling). 

5. Power felt about the same (never dyno'd it other than my calibrated tush) and the sound was fantastic. 

There, much more than you probably wanted to know.

If you're wanting to do this on a Toyota 20R, you might want to go with the HS6 carbs to be closer to the right CFM flowrate. Tuning is easy - there are reams of information online on jetting SUs. 

A photo from my book:

Lo-res detail of the TPS: 

 

Please let me know if you have any comments or questions on what you're trying to accomplish.

Norman G

 

David S. Wallens
David S. Wallens Editorial Director
8/2/19 11:58 a.m.

Thank you, Norman!

Robbie
Robbie UltimaDork
8/2/19 12:01 p.m.

Best thread of the week. This is why I love this place.

Thank you all!!

twentyover
twentyover Dork
8/2/19 12:10 p.m.

Thanks for the feedbaack, Norman, looks like I need to buy a book as a starting place. Sorry, I'm a cheap SOB so I'm going to buy used....

 

Unfortunately, the manifolds build will be crappily MIG welded steel plate and exhaust tubing. Good info on the balance tube

 

I know that the HS6 is a better fit, but I have a rack of HIF4's hangin' from the roof of the garage. Drop in a fixed needle adapter and OI should be set. If I decide latter I need more carb, I can make the change to HS6 and try a needle comparison because of the .100 jet used on the 6's, while the 4's use an .090 jet. The 20R is more a draft horse than a thoroughbred, so not certain I need the 6  since it won't spin 7200 (at least this one won't.)

Needle selection is my biggest worry. Figure I'll start w/ the B needle, maybe drop the jet a couple flats.

 

May I contact you via PM thru the GRM forum if I have any questions?

 

LanEvo

 

I understand the national attraction Mikunis or even Solex, but remember Nissan ran Hitachi licensed SU's on 1600, 2000, and 240Z cars, so I don't think I'm TOO far off base

 

 

 

David-

I read about the swap somewhere, & I agree i seem to recall it was not without issue, but I've never to the best of my seriously fading memory) read a Miata magazine. Anyway,  I think I may have accidently retired a couple months ago, so have nothing but time to fettle something like this....

 

Side note- conversation between me and a best friend I've known, literally, since kindergarten-

Me-  They want to get rid of me, they offered me a little money to leave. Think I should take the buyout?

Him- The offered a golden parachute?

Me- Yes, but they're throwin' me off the fookin' airplane, and we all know how well gold floats in air

 


 

David S. Wallens
David S. Wallens Editorial Director
8/2/19 12:14 p.m.

You know, a few issues back we did feature a Porsche 924 running an SU. 

twentyover
twentyover Dork
8/2/19 12:15 p.m.
David S. Wallens said:

You know, a few issues back we did feature a Porsche 924 running an SU. 

remember which one? I can dig through my back issues....

David S. Wallens
David S. Wallens Editorial Director
8/2/19 12:20 p.m.

In reply to twentyover :

April 2019.

David S. Wallens
David S. Wallens Editorial Director
8/2/19 12:22 p.m.

But Norman provided much more useful feedback. The Porsche item just describe a quick fix at a Lemons race, and it sounds like the car ran poorly. 

twentyover
twentyover Dork
8/2/19 12:30 p.m.

Thanks David- won't even need to dig too deep into the pile. And Norman's book will be on order this afternoon

 

And if anyone wants to korrekt speling in the title...

David S. Wallens
David S. Wallens Editorial Director
8/2/19 12:31 p.m.

And maybe there's a future GRM article in this subject.... smiley

Docmiata
Docmiata New Reader
8/2/19 2:12 p.m.

Better photo found on an old 5.25" flopppy: Balance tube and TPS set up shown more clearly

2002maniac
2002maniac Dork
8/2/19 2:56 p.m.

Very cool!

_
_ HalfDork
8/4/19 10:25 a.m.
David S. Wallens said:

And maybe there's a future GRM article in this subject.... smiley

Yes please! An article showing popular carb conversions on modern ish efi engines, and how to obtain the highly coveted manifold adapters! For sit a certain, they make a kit for the ka24 engine, it was used in Japanese off-road truck racing. There is also one in Australia for the 1uzfe v8, it is for the circle track guys (their cars go fast and turn RIGHT! Lol)

oldeskewltoy
oldeskewltoy UberDork
8/7/19 9:27 a.m.

eye candy.... BIG SU's  HS8 on a TR4 manifold...  might even still be available for sale...???   (I'd need to check)

 

 

Keith Tanner
Keith Tanner MegaDork
8/7/19 9:48 a.m.
_ said:
Yes please! An article showing popular carb conversions on modern ish efi engines, and how to obtain the highly coveted manifold adapters! For sit a certain, they make a kit for the ka24 engine, it was used in Japanese off-road truck racing. There is also one in Australia for the 1uzfe v8, it is for the circle track guys (their cars go fast and turn RIGHT! Lol)

Source for adapters: https://www.piercemanifolds.com/

Norm, that picture shows something that looks suspiciously like a supercharger on the exhaust side. I'd love to see that plumbing ;)

Docmiata
Docmiata New Reader
8/16/19 12:32 p.m.

Good eye!

 

That was prototype #1 for the Eaton installation for what would become the Sebring/Jackson Racing kit.

 

Apparently, I was throwing anything I had in the garage on that engine those days...

bobzilla
bobzilla MegaDork
8/16/19 1:54 p.m.

I still feel carbs are the devils work and require knowledge in black magic and voodoo to make work correctly.... but that is still cool. 

Robbie
Robbie UltimaDork
8/21/19 1:22 p.m.

What about fuel pump? Do you change the efi one out completely or just massively regulate it down to carb pressure?

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