Hitachi SU carbs - the David Vizard modifications

[AliK]

BACKGROUND

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I lie in bed at night thinking of this stuff but then comes a moment of madness when I decide to do something about it, so here goes. (Having said that, this could end up being a very long experiment happening in glacial timescales as it will be an as and when effort - so please bear with me).


David Vizard - a brilliant brilliant engineer whose automotive work and thinking I greatly admire - showed that with simple modifications to the SU carb, they can flow as much as the next size up, but without “over-carbing” the engine.


I could just fit 2” SUs - but where’s the fun in that!? Besides, I want to see just how much could be extracted from the Hitachis. So the purpose of this thread is to document my modifications on the Hitachi 44.5mm SU carbs and indulging my curiosity! Especially as I can't find anyone who's done them to their SUs in the Z world and laid out what they did.


In summary, looking at Mr V’s article (attached), he shows the following impressive gains from these mods.



ChatGPT tells me that 240z Hitachis were rated at 180-200CFM - but it steadfastly refuses to divulge it's reference! For comparison, HS6s have a measured max flow rate of 210CFM, so I used the mid CFM of the Hitachi for my theoretical column in grey. For a bit of fun I unscientifically derived the potential CFM improvements using the % improvement of the HS4 in the article. (Incidentally, no.7 is omitted as it was without the butterfly).




The big ticket items are obvious but what surprised me the most was the effect of cutting away the protruding screws! I suspect that's got more to do with fluid dynamics and turbulence than their cross sectional area. The other item that surprised me was how much flow was gained by rounding off the burrs and sharp edges in the piston area. We often read about turbulence reducing the effective flow but wow over 6%!


For anyone wondering what those mods can look like (except for the low res. photos in the article), here are some pics I stole of 'tinter.

Piston - looking at our carbs the piston really doesn't have a material protrusion, but this picture is extreme and interesting all the same.



Shaft mods



Bores - Smoothing off the bores where the casting has lots of burrs - it's important to not take this one too far otherwise the piston can't do it's job as well.





THE PLAN

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So what we are aiming for are low hanging fruits and some slightly more involved work. What I plan is:

1. De-burring the walls of the carb to minimise the effects of flow reducing turbulence
2. Reducing the effect of the butterfly screws
3. Reducing the effect of the shaft
4. Edging the butterfly
5. And a VERY bad idea I have which will involve some CNC accurate work, which I’ll divulge later...

I will leave the bridge well alone as I’ve read people have destroyed their carbs modifying this bit. There are all kinds of flow mechanics involved in creating vacuum across the jet / nozzle and required turbulence for fuel atomisation / accelerator pump function.




SOME MATHS ON THE HITACHIS

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Carb opening = 44.5mm

Shaft diameter =10mm

Butterfly height=2mm

Bridge height = 7mm

Butterfly bolts= M4 and sticking out by 4mm

For simplicity let’s ignore the butterfly screws and their presented cross section.

- Shaft and butterfly cross sectional area presented at WOT = 10 x 44.5 = 445mmsq

- Carb barrel cross sectional area: 44.5/2 squared x Pi =1555.5 mmsq


So in effect the shaft and butterfly constitute 445/1555.5 = 29% of the carb barrel!!


- Bridge area = 141.3sqmm (if you really want to know how to calculate that, see https://planetcalc.com/1421/)

- But if you deduct the bridge cross section to arrive at the actual available cross-sectional area through the carb =1555.5-141.3 =1414.13mmsq


- So the shaft and butterfly as a percentage = 445 / 1414 = 31% of the opening when looking through the carb!!

That is an ASTONISHING amount. No wonder Mr Vizard’s mods, however small they may seem, have a significant impact. And that’s before you take fluid dynamics and flow reducing turbulence effects into account!!

To satisfy myself, I made a to scale drawing:





So the numbers and theory seem promising on paper!


Next up, the guinea pig spare carbs and possible home made way of measuring improvements in flow - tune in again next time, same time, same channel.

 

[MCBladeRun]

I will come back and read from the top when time allows, I saw chatGPT so, assume someone has done their research

 

[MCBladeRun]

Very good Alik, look forward to seeing the next bit shame the easiest mod (the screws) doesn't gain much

 

[Rob Gaskin]

This will be interesting. Don't forget to also ensure that you are actually getting a wide open throttle. The linkages can get mal-adjusted.

 

[SeanDezart]

BACKGROUND

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I lie in bed at night thinking of this stuff


I could just fit 2” SUs - but where’s the fun in that!? Besides, I want to see just how much could be extracted from the Hitachis. So the purpose of this thread is to document my modifications on the Hitachi 44.5mm SU carbs and indulging my curiosity!

Respect what you're doing however to answer your question on 2'' SUs, the 'fun' is they look beefier than those MGMetro-sized carbys on such a long engine ! Mine back in '91.

I'm also a sad Z-being going to sleep thinking of Zed-ness.

 

[Ian Patmore]

Nice.
Be careful knife edging the butterfly not to take any material off the radius, how the butterfly closes/at idle makes for smooth running.

For reference, rebuilding so Mikuni's, one had a slight nick out of 1, I changed it. All the screws on the Mikuni are inset (the head) and not proud (threaded end).

Article very much points to getting flow through the SU is critical, but also as critical is getting the air there.

I am looking at a mod after the SU carb, in the insulator, to aid flow, thus power and fuel economy.

 

[MCBladeRun]

I would have thought a vortex shape to twist the air as it enters would increase the volume of air, except you'd have a butterfly in the middle of it.

Shame a camera shutter couldn't work in combination. Alas my imagination only takes me so far, I don't know how the needle / jet would work then

 

[AliK]

Respect what you're doing however to answer your question on 2'' SUs, the 'fun' is they look beefier than those MGMetro-sized carbys on such a long engine ! Mine back in '91.

I'm also a sad Z-being going to sleep thinking of Zed-ness.

We have a winner! I was expecting Mr. Gaskin to tell me that first! Nice green Z that and indeed those carbs look great.

Nice.
Be careful knife edging the butterfly not to take any material off the radius, how the butterfly closes/at idle makes for smooth running.

For reference, rebuilding so Mikuni's, one had a slight nick out of 1, I changed it. All the screws on the Mikuni are inset (the head) and not proud (threaded end).

Article very much points to getting flow through the SU is critical, but also as critical is getting the air there.

I am looking at a mod after the SU carb, in the insulator, to aid flow, thus power and fuel economy.

Great advice, thank you - very much looking forward to seeing what you do to the insulators.

I would have thought a vortex shape to twist the air as it enters would increase the volume of air, except you'd have a butterfly in the middle of it.

Shame a camera shutter couldn't work in combination. Alas my imagination only takes me so far, I don't know how the needle / jet would work then

Agreed. I tried this a while back with a web cam at the carb mouth. The fuel standoff at the mouth was impressive but there was just too much vibration, mainly from the shockwaves from the intake / carb hitting the camera. My make do rev counter in the picture (timing light) really didn't like the electrical noise where it was. I did a better video with the bonnet shut and a light shinning into the mouth. I must power up that old laptop and see if I can dig it out.

 

[Rob Gaskin]

Yes the HS8 or HD8 look better on the L6 but I've also seen 3xHS6 which look good but strange because it's not normal like it is on a Jag. However Webers look better still but when the bonnet is down noone can see them anyway.

 

[AliK]

THE CARBS

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I’ll be using a set of SU carbs I bought from our brethren on the US forum. He assured me they were in perfect working order. They came off a race car, so I should have known to translate “perfect working order” to mean “perfect at WOT but otherwise worn to hell for any other condition”. No matter, in every problem there is an opportunity - so in this case, I have a pair of carbs that I don’t really worry about messing with.


The throttle shafts have impressive play in them and there is 1mm wear gap between shaft and bush at the open end of the rear carb. Measuring it, there is tiny wear on the shaft and tiny wear on the bushes but collectively they add up.



When I finish messing with the carbs, they will go to Birmingham carbs for some new bushes / possibly shafts. But I have a new design for the shaft but more of that anon.


First up, taking the carbs apart to measure things.

The butterfly bolts refused to budge - so I brought out the farming solution!! But that failed! And frankly I was nervous about melting things with MAP gas.




Why oh why did I (or many of you reading) not invest in a decent set of JIS screw drivers when i bought the car!? Made life so much easier and I wouldn’t have had to drill out one of the butterfly screws after rounding off the head!



Some measurements

Bush OD = 12mm

Bush ID = 10.06 mm open end

Bush end to inside carb = 16mm

Edit: Bush length =10.5mm

Shaft, open end = 9.52mm

Shaft, linkages side =9.97mm

Shaft, unused part 10.00mm

Shaft length = 110mm long

Nissan hard chromed the shafts which is probably why they lasted 40+ years so well. Below the surface they appear to be brass or some alloy of it.

The shaft wear is 180° on each end but 90° rotationally offset!



It appears - and logically so now that I think about it - that the butterfly is not circular. It is actually an oblong shape; which makes perfect sense given that when closed it’s at an angle






Edit: incidentally, the holes are also off round vertically. I suspect to allow for butterfly alignment when mounting into the carb.

Why this is interesting will make more sense as I reveal my more dastardly plans later.

For now, I need to petrol bath the carb to get rid of the black stuff.

In the meanwhile I need to put together a homemade flow bench. Why? Because “I gots to know man”! (You have to be of a certain vintage for that to make sense). I’m not sure if I would or would not be able to adequately measure the effects of changes made but I will try at least.

The plan is (and partially tested) to use half the inlet manifold, a DIY vacuum cleaner, some clear tubing and coloured water. The vac can only lift the piston 1/3 up. More of that to come.

Some interesting reading uncle @Huw sent me: https://www.engineeringtoolbox.com/pitot-tubes-d_612.html

THE GEEKY BIT

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Looking at fluid dynamics, the cylindrical shape is possibly one of the worst for aero and turbulence. But not much better is a flat shaft. This is why Mr V in his article states that the aerofoil shaped shaft worked best.



Even at part throttle, it has an effect



You can eliminate half the shaft and reduce its impact but at WOT you still have the other half in the airflow.

All good reasons to hate the shaft!

Then I got thinking - why do we even need the shaft? What if the butterfly itself becomes a structural element and the shaft was almost completely eliminated?

 

[AliK]

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ELIMINATING THE SHAFT (AKA GOING COMMANDO)

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Clearly we don’t want the engine to ingest any of the components in the carb or this will become a very expensive experiment!! So with that in mind let’s kill the shaft - safely!

For now, I’m going to probably cut away the top half of the shaft, drill new counter sank bolt holes 5mm away from carb body and countersink m3 bolts into it, but at a 2 mm shaft height. Then I can use the original holes as locating holes with grub screws flush with the top of the butterfly and cut away the rest of the shaft.

Meanwhile, I would take the back side of the shaft down to 2mm. This will give me something to play with - if I can get the flow bench to work while I finalise my intended new shaft and butterfly design.

THE MADNESS BEGINS HERE

Down with the shaft!! The shaft must go! Viva la Liberty!!!

I have two designs in mind - neither of which are finished yet and unlikely to see the light of day for some time. But it’s the kind of thing that I think about before going to sleep, so it helps to get it down on paper (or in this case the PC).

Imagine shaft and butterfly made of stainless steel for the sake of strength and rust resistance. Sure the SS will be harder than the bushing, but I doubt new bushes will be killed in my lifetime with the carbs.

DESIGN 1 - Screw in shafts

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I like the idea of a precision butterfly with lumps at each corner where a 2 piece shaft screws into from each end.

The lumps will be double symmetrical aerofoil shapes to minimise turbulence. Something like this:



The leading / lagging edges will also be machined to a sensible point to minimise turbulence.

The open side shaft can be loctited in and I plan some way of plugging the open end of the bush (similar to Z therapy but more robust) to stop it “falling out”.

The challenge is making sure the linkage side aligns correctly and doesn’t suddenly accidentally let go and result in an uncontrollable WOT condition. For that reason, I have a small grub screw to be loctited in (probably forever with the green stuff.


DESIGN 2 - Slotted butterfly

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The idea here is much the same as numero uno with the split shafts but this time an interference fit. This avoids the need for bolts and screws but I have yet to figure how to full secure the open side - which still comes down to plugging the open end of the carb bush securely.

The challenge is some seriously accurate machining is required for this to work.



MORE GEEKINESS

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So the difference between the designs above and the stock shaft set up is quite clear.


This has got to be worth quite a few CFM. I can’t find a definitive answer to the relationship between intake CFM and BHP, but it would be interesting to see if the butt dyno can tell. Assuming of course we can make any of this work safely!

 

[Rob Gaskin]

I daren't comment.