SU carbs WILL work perfectly on my modified L28 and give DCOEs a run for their money!!!

richiep

Club Member
I remember reading something standing waves, harmonics and reflections off the inside of the airbox vs inner wing. And of course, its cooler air.
This. Eliminates the detrimental impact of reflected pulses off the inner wing, alongside cooler charge.

They work great - perceptibly more power, better delivery, cooler air eliminating run-on issues in my red car's case.

And yes, they are designed to run trumpets inside (50mm length in my car).
 

Rob Gaskin

Treasurer
Staff member
Site Administrator
Glad that's cleared up, I read what Rob G wrote as it was designed to work without.
Any idea what aspect of the design made the gain, as I think there is room for improvement (for triples/SU's anyway)?
You misunderstood my comment. The original post was questioning whether the improvement in performance was due to the stacks or airbox. I asked why the assumption was the stacks because I know the airbox design was finalsed after thorough testing. So I wasn't surprised that there was a measurable improvement.

The poster had assumed that the improvement was due to the stacks, not the airbox. I asked 'why the stacks'?

That has been interpreted into why use stacks with the airbox! I never said that.
 
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240L31

Club Member
I'll do a dyno side-by-side comparison stock box vs. Zclub airbox in a couple of weeks.

What I don't like so far is that the fuel bowl overflow hoses are not connected to the airbox anymore. There's significantly more fuel smell in my garage now.
 

jonbills

Membership Secretary
Site Administrator
I'll do a dyno side-by-side comparison stock box vs. Zclub airbox in a couple of weeks.

What I don't like so far is that the fuel bowl overflow hoses are not connected to the airbox anymore. There's significantly more fuel smell in my garage now.
I think you could fix that with a couple of these?
 

240L31

Club Member
No the connector at the fuel bowl is still there. It is the open hose end which was previously routed into the stock air cleaner.
 

richiep

Club Member
No the connector at the fuel bowl is still there. It is the open hose end which was previously routed into the stock air cleaner.
I think Jon means install a couple of those into the airbox to allow you to connect the hoses from the float bowls.

The downside of those vent hoses - as I once found out a long time ago, when I still used SUs - is that if, for any reason, a float decides to stick, the fuel overflows into the standard airbox...
 
There’s a few threads out there where people fitted a std airbox onto triple carbs, where it was on a Dyno on the same day against open trumpets. It made more power on the standard airbox each use
 

AliK

Vehicle Dating Officer
Staff member
Club Member
I think Jon means install a couple of those into the airbox to allow you to connect the hoses from the float bowls.

The downside of those vent hoses - as I once found out a long time ago, when I still used SUs - is that if, for any reason, a float decides to stick, the fuel overflows into the standard airbox...

I did exactly that with 90 degree ones! The back plate on mine was more custom though ;)

Also important for pressure equalisation in the float bowls from what I understand.

File 23-08-2022, 22 09 49.png
 

AliK

Vehicle Dating Officer
Staff member
Club Member
They are from the uk actually: https://www.med-engineering.co.uk/

Where did you get yours from? Did you cut the backing plate yourself or is it laser cut?
Thanks for the link 👍.

I templated it and mocked it up using plywood, printed out a very accurate overlay onto aluminium sheet, cut it with a jig saw (and metal cutting blade) then spent a bit of time filing it down. I used a hole cutter for the intake holes and they are within 0.5mm of the carb mouth diameter.

I also swapped over to DCOE trumpets that are fully radiused and with the backplate, have the same height as the stock air horns. It also makes it way way easier to put the airbox on. Not sure if you saw, but I have butterfly bolts on the back of my backplate which secure the airbox on.

I’m using an S2000 AP2 filter with a heat shield to protect it from road spray and high speed debris.

Oh and I made my own aluminium surround for it too to finish off the rough edges of the box - pretty is just as important as power 🤣🤣

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SeanDezart

Well-Known Forum User
Beautiful work, two questions please :

1) why collect air at the centre and not on the immediate LHside ?
2) why not bring the heatshield higher up to better (?) seperate the intake from exhaust manifold ?
 

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240L31

Club Member
Thanks for the link 👍.

I templated it and mocked it up using plywood, printed out a very accurate overlay onto aluminium sheet, cut it with a jig saw (and metal cutting blade) then spent a bit of time filing it down. I used a hole cutter for the intake holes and they are within 0.5mm of the carb mouth diameter.

I also swapped over to DCOE trumpets that are fully radiused and with the backplate, have the same height as the stock air horns. It also makes it way way easier to put the airbox on. Not sure if you saw, but I have butterfly bolts on the back of my backplate which secure the airbox on.

I’m using an S2000 AP2 filter with a heat shield to protect it from road spray and high speed debris.

Oh and I made my own aluminium surround for it too to finish off the rough edges of the box - pretty is just as important as power 🤣🤣
Wow, nicely done. Do you have any more pictures how the air box is attached to the backing plate?

I'd also cut down the intake hose so that the filter sits at the left hand side. At least in theory, it would minimize pressure drop.
 

AliK

Vehicle Dating Officer
Staff member
Club Member
Thanks gents.

Some pics of the mechanism - I appear to have deleted some good photos! The insets I made for the butterfly bolts are themselves bolted up from the outside, as I wanted NOTHING inside the box that can possibly get sucked into the intake. The aluminium bar was meant to straighten the box and it worked well, but there is so little clearance to get the box on with the trumpets in place that I had to lose it. It transpired I didn’t need it after all!

The filter is only that far away because on a late 260z you have two captive nuts (where the red tape is on my rad panel on one of the photos) and they sit PERFECTLY either side of the filter. I assume these were in place for the injection model’s airbox and filter. I made my own brackets to fit the filter mounting points and to clamp it to the car. It was purely an aesthetic, convenience decision - nothing to do with best practice or power ;)

As for the heat shield, I wanted it close to the exhaust manifold: 1. For ease of access to things under the carbs and 2. The shield also gets hot and radiates heat so the further away from the carbs the better.

Really like what you’ve done with yours Sean! Especially with the flexible material that allows for engine movement while nearly fully blocking off the air flow upwards. 👍

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Ian Patmore

Well-Known Forum User
This. Eliminates the detrimental impact of reflected pulses off the inner wing, alongside cooler charge.

They work great - perceptibly more power, better delivery, cooler air eliminating run-on issues in my red car's case.
And there was another post saying this was a load of blah blah, the reflected pulses bit.

All good, but its not saying about how the airbox itself, what design aspects is that bringing (for carbs, as they work slightly different)? Apart from cooler air. All I see with this airbox is a box with an inlet, anything else special? What could maybe be done to give even more power?
 

jonbills

Membership Secretary
Site Administrator
Ian Patmore said:
And there was another post saying this was a load of blah blah, the reflected pulses bit.

All good, but its not saying about how the airbox itself, what design aspects is that bringing (for carbs, as they work slightly different)? Apart from cooler air. All I see with this airbox is a box with an inlet, anything else special? What could maybe be done to give even more power?
So, no design aspects to this airbox to aid performance then, apart from cooler air?
The people who designed it aren't here to have the debate with you Ian.🤷‍♂️
One of the great things about science rather than religion is that it works even if you don't understand it or believe in it. :)

From my own experience with carbs:

and with EFI on my current engine, here's the fuelling table with and without airbox from an experiment I did last year.
(the point being, that with the airbox, all other things equal, it uses more fuel at peak torque for the same AFR).

This is without :
Screenshot 2022-09-07 at 14.52.52.png
This with:
Screenshot 2022-09-07 at 15.28.02.png
 

Ian Patmore

Well-Known Forum User
The people who designed it aren't here to have the debate with you Ian.🤷‍♂️
One of the great things about science rather than religion is that it works even if you don't understand it or believe in it. :)
Thanks Jon. But, it's not particularly difficult to figure out how it works, as it is science, but a large part of how it works is logic (I have). Just because it works, doesn't mean it works as good as it can. Plenty here are tweeking their S30, but somehow the airbox isn't included. So, you haven't thought about how your airbox works (or any owner has)? Ever thought more improvement was left on the table? Gladly follow without thinking, hang on...?

Development doesn't stand still. I shouldn't have to chat with the designers, there are plenty here who are bright enough to get a handle on their bit of fibreglass box.
 

jonbills

Membership Secretary
Site Administrator
Thanks Jon. But, it's not particularly difficult to figure out how it works, as it is science, but a large part of how it works is logic (I have). Just because it works, doesn't mean it works as good as it can. Plenty here are tweeking their S30, but somehow the airbox isn't included. So, you haven't thought about how your airbox works (or any owner has)? Ever thought more improvement was left on the table? Gladly follow without thinking, hang on...?

Development doesn't stand still. I shouldn't have to chat with the designers, there are plenty here who are bright enough to get a handle on their bit of fibreglass box.
Ian, I think we've gone quite a way off topic: this is a thread about SU carbs giving DCOEs a run for their money - feel free to start another thread about airbox designs.
 
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