As the title suggests the gauntlet is down!!!
Don’t get me wrong, I salivate like the next man at the mere sight of the DCOE carbs but this is something I want to crack!
This will be a long post / thread as it will be the musings of a madman refusing to give into conventional wisdom that “them SUs just won’t flow”.
There is a wealth of information out there about how to modify your L series engine, but very little on AFR, needles and carb set up for modified engines using SU carbs.
So I set about working it out from first principles and I’m doing this write up in case it is helpful to anyone else. I will keep updating it as I experiment; I humbly ask for kindness in your commentary and feedback however dumb my attempts appear.
Having had the car RR tuned and the SM needles re-profiled professionally, the car is producing great power at the top and a has very bumpy power curve in the low to mid range.
This is primarily a road car and needs to be more than bragging rights on headline figures so the carb setup has to work for more than just the top end power.
The AFR on the modified needles is rich everywhere minus the very top end (over 6000 rpm in 4th) where it goes lean (13.5ish); I was told I need triple DCOEs to get a richer top end with less compromise on being rich low down.
The challenge:
I will ABSOLUTELY keep my SU carbs. After-all, there are people with them who are racing and making more power than I do + some running 12 second quarter miles on SUs. So it is very possible, it just needs patience, data and analysis + a rather sizeable learning curve!
The goal:
To maintain the top end WOT AFR of between 11.5-13.0 across the rev range and between 14.0 - 16.0 cruising.
The engine is an L28 with compression of 10.5:1, 270 degree cam on an E88 head with large valves, sucking through Z therapy SU carbs, blowing through a Z Story Race/Sport full system and producing over 232BHP.
I wrote to Z therapy to see if I need bigger nozzles and this is what came back, I love those guys ....
Onto the carbs.
Firstly, I needed to understand the principles of SU carb operation.
Lots of info on the links below and the internet on the topic.
https://www.mgexp.com/phile/1/492119/SU_idiot_guide.PDF
https://sucarb.co.uk/technical-su-carburetters
https://zparts.com/index.php/resources/su-carburetors-explained/
In short for those new to this: the SU is a variable choke carb where the choke size increases with engine load and raises a tapered needle out of the jet allowing more fuel flow.
The greater the load, the greater the suck and the higher the piston / needle raises, opening more area in the jet. The other key factor to consider is that the air flow velocity across the "bridge" / jet affects fuel drawn, so a piston high up with the needle pulled up may still cause a lean condition as air flow velocity (read vacuum) across the bridge is reduced compared with the piston at a lower position.
There are very few things you can adjust:
Float level
· Height of fuel within the jet tube and float bowls. Most important and the first thing to set at around 10mm below bridge as a rule of thumb.
Needle profile
· Over 700 needles and profiles available, note: the taper is not always linear
· 14 stations, each station is 3.2mm: stations 1&2 = idle, 3-6 = Cruise and Acceleration, 7-9 = top speed - last 3-4 stations not used
Jet Height
· Each full turn = 6 flats; 1mm
Spring Rates
· Controls the height of piston hence the Venturi choke size and therefore the air velocity across the bridge / jet. Stronger springs = Faster air velocity = more fuel but more choked airflow and richer operation in general.
Damper oil:
Thin = sluggish throttle response,
Thick = snappier response.
Note the damper oil only affects mix between piston height transitions but not the ultimate AFR once the piston has fully risen/dropped. SAE 20-30 oil is recommended depending on what you read; I use SU official oil.
Still with me and reading?
Then onto the needle work next. I will write it up the on my commute tomorrow ...
Don’t get me wrong, I salivate like the next man at the mere sight of the DCOE carbs but this is something I want to crack!
This will be a long post / thread as it will be the musings of a madman refusing to give into conventional wisdom that “them SUs just won’t flow”.
There is a wealth of information out there about how to modify your L series engine, but very little on AFR, needles and carb set up for modified engines using SU carbs.
So I set about working it out from first principles and I’m doing this write up in case it is helpful to anyone else. I will keep updating it as I experiment; I humbly ask for kindness in your commentary and feedback however dumb my attempts appear.
Having had the car RR tuned and the SM needles re-profiled professionally, the car is producing great power at the top and a has very bumpy power curve in the low to mid range.
This is primarily a road car and needs to be more than bragging rights on headline figures so the carb setup has to work for more than just the top end power.
The AFR on the modified needles is rich everywhere minus the very top end (over 6000 rpm in 4th) where it goes lean (13.5ish); I was told I need triple DCOEs to get a richer top end with less compromise on being rich low down.
The challenge:
I will ABSOLUTELY keep my SU carbs. After-all, there are people with them who are racing and making more power than I do + some running 12 second quarter miles on SUs. So it is very possible, it just needs patience, data and analysis + a rather sizeable learning curve!
The goal:
To maintain the top end WOT AFR of between 11.5-13.0 across the rev range and between 14.0 - 16.0 cruising.
The engine is an L28 with compression of 10.5:1, 270 degree cam on an E88 head with large valves, sucking through Z therapy SU carbs, blowing through a Z Story Race/Sport full system and producing over 232BHP.
I wrote to Z therapy to see if I need bigger nozzles and this is what came back, I love those guys ....
Onto the carbs.
Firstly, I needed to understand the principles of SU carb operation.
Lots of info on the links below and the internet on the topic.
https://www.mgexp.com/phile/1/492119/SU_idiot_guide.PDF
https://sucarb.co.uk/technical-su-carburetters
https://zparts.com/index.php/resources/su-carburetors-explained/
In short for those new to this: the SU is a variable choke carb where the choke size increases with engine load and raises a tapered needle out of the jet allowing more fuel flow.
The greater the load, the greater the suck and the higher the piston / needle raises, opening more area in the jet. The other key factor to consider is that the air flow velocity across the "bridge" / jet affects fuel drawn, so a piston high up with the needle pulled up may still cause a lean condition as air flow velocity (read vacuum) across the bridge is reduced compared with the piston at a lower position.
There are very few things you can adjust:
Float level
· Height of fuel within the jet tube and float bowls. Most important and the first thing to set at around 10mm below bridge as a rule of thumb.
Needle profile
· Over 700 needles and profiles available, note: the taper is not always linear
· 14 stations, each station is 3.2mm: stations 1&2 = idle, 3-6 = Cruise and Acceleration, 7-9 = top speed - last 3-4 stations not used
Jet Height
· Each full turn = 6 flats; 1mm
Spring Rates
· Controls the height of piston hence the Venturi choke size and therefore the air velocity across the bridge / jet. Stronger springs = Faster air velocity = more fuel but more choked airflow and richer operation in general.
Damper oil:
Thin = sluggish throttle response,
Thick = snappier response.
Note the damper oil only affects mix between piston height transitions but not the ultimate AFR once the piston has fully risen/dropped. SAE 20-30 oil is recommended depending on what you read; I use SU official oil.
Still with me and reading?
Then onto the needle work next. I will write it up the on my commute tomorrow ...
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