Ali, looking at the diagram - if air leaks under the dome what harm is it going to do? There is already an airway to supply air to the bottom of the piston.
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SU carbs WILL work perfectly on my modified L28 and give DCOEs a run for their money!!!
- Thread starter AliK
- Start date
You know, in my hours of insomnia last night I was having the same thoughts. But all I can imagine is the vac venting out the engine side of the piston is somehow having an effect.
Edit: or maybe it’s controlled airflow v uncontrolled!? Otherwise why would they go through the trouble of machining such an elaborate mechanism? [emoji848]
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Farmer42
Club Member
It does make a difference. If air is being sucked in above the piston the pressure forces the piston back down a little making it run lean. The piston needs the full force of air through the front of the carb to drive the piston up and move the needle out of the jet thus allowing more fuel in to give it the right mixture. Any decrease in the airflow and the piston will not rise as much etc etc.
On a different point, speeding the airflow up at the front of the carb will allow you to use a different needle to richen the mixture at high revs or acceleration. This is why carb trumpets are used to act as a venturi to speed up the air. Made hell of a difference to mine.
On a different point, speeding the airflow up at the front of the carb will allow you to use a different needle to richen the mixture at high revs or acceleration. This is why carb trumpets are used to act as a venturi to speed up the air. Made hell of a difference to mine.
Farmer42
Club Member
I dont know. It was something I read on how su carbs work. Apparently air getting sucked into the dome gets above the top of the piston and stops it rising to its full capability as the air needs compressing. I don't know why it ends up above the piston but there is some truth in the theory and Ali seems to have proved it when he stopped his air leak and it improved the running.
He's just clutching at straws
Damn right!!! [emoji1787] Necessity is the mother of all clutched straws!! [emoji106][emoji106] I really was out of ideas at that point but can’t tell you how pleased I am that it solved the issue. Also the fact that the whistling is way way way and once more way louder than before proves that the carbs are sucking harder across the holes meant for the atmospheric pressure.
You’re both right - some of the air will of course end up above the piston as it’s not a perfect seal, what matters as Paul is alluding to without knowing [emoji12] is that the pressure differential is what matters. So if the vac can leak or the atmospheric pressure gets sucked down at the back of the piston then it “should” affect its operation. More straw right there!
Alphabettispaghetti
Club Member
Generally having air being drawn in anywhere isn't correct, so by fixing a minor leak its a step in the right direction.
240L31
Club Member
Ali,
I don't want to capture your thread - I did a dyno run today with my 3.1L stroker. I can now say that SUs are for sure very capable if properly tuned.
I achieved 210 Hp / 314 Nm with an awesome torque curve.
F54 89mm/N42/Delta Cam/Maxspeeding L24 Rods/KA24E pistons/LD28 crank/Zstory Race-Sport full exhaust line.
The mixture was pretty rock stable at 0,84-0,87 from 2000-6000rpm . So I'd say I could achieve another 5-10 Hp with a slightly leaner needle.
I don't want to capture your thread - I did a dyno run today with my 3.1L stroker. I can now say that SUs are for sure very capable if properly tuned.
I achieved 210 Hp / 314 Nm with an awesome torque curve.
F54 89mm/N42/Delta Cam/Maxspeeding L24 Rods/KA24E pistons/LD28 crank/Zstory Race-Sport full exhaust line.
The mixture was pretty rock stable at 0,84-0,87 from 2000-6000rpm . So I'd say I could achieve another 5-10 Hp with a slightly leaner needle.
Ali,
I don't want to capture your thread - I did a dyno run today with my 3.1L stroker. I can now say that SUs are for sure very capable if properly tuned.
I achieved 210 Hp / 314 Nm with an awesome torque curve.
F54 89mm/N42/Delta Cam/Maxspeeding L24 Rods/KA24E pistons/LD28 crank/Zstory Race-Sport full exhaust line.
The mixture was pretty rock stable at 0,84-0,87 from 2000-6000rpm . So I'd say I could achieve another 5-10 Hp with a slightly leaner needle.
Exactly what carbs please?
Hitachi 1.75, SU 1.75 or SU 2.0 ?
Ali,
I don't want to capture your thread - I did a dyno run today with my 3.1L stroker. I can now say that SUs are for sure very capable if properly tuned.
I achieved 210 Hp / 314 Nm with an awesome torque curve.
F54 89mm/N42/Delta Cam/Maxspeeding L24 Rods/KA24E pistons/LD28 crank/Zstory Race-Sport full exhaust line.
The mixture was pretty rock stable at 0,84-0,87 from 2000-6000rpm . So I'd say I could achieve another 5-10 Hp with a slightly leaner needle.
Wow! Congrats dude, that is truly awesome. Love that torque curve!!
Please feel free to post things like that here anytime, the purpose of this thread is to capture SU related info that will help others with modified engines, so the more body of knowledge the better as far as I’m concerned.
As a matter of interest do you know your cam’s duration, lift and overlap / lobe angle separation? Also do you know if your cam is advanced / retarded or installed neutrally?
I was out the other eve in mine (egg hunting) and was thinking how nice the torque on a stroker must be.
240L31
Club Member
All measured by myself at the valve retainer at 0,05" lift:
Intake duration: 236deg
Exhaust duration: 232deg
LSA 108,5deg
Overlap 17deg
Lift 0,460" - IRC
installed at cam sprocket position 1. Cam is advertised with 282 deg duration.
I think that shows that SUs work great until they are the thing limiting air flow into the engine.
On my L24, with 1.75 hitachis I think that happened somewhere around 5500 to 6000 RPM. On this L31 engine, it looks like it's happening 1000 rpm earlier.
It would be interesting to see how that L31 would do with 2" SUs or triple 45s. I think it would make a fair bit more (at the higher RPMs).
On my L24, with 1.75 hitachis I think that happened somewhere around 5500 to 6000 RPM. On this L31 engine, it looks like it's happening 1000 rpm earlier.
It would be interesting to see how that L31 would do with 2" SUs or triple 45s. I think it would make a fair bit more (at the higher RPMs).
240L31
Club Member
The real question to me is: How would the torque curve change? Right now I got almost full torque (>250 Nm) from 2000 rpm on, >300 from 3500 rpm onwards. I would never trade my torque curve for more high rpm HP
Honestly, I am more than happy right now. I doubt I am going to change a thing this season at least
Here's a couple of graphs from my L24. Same engine, difference is SUs & Stock airbox vs Dellortos and Skiddell/Pmac airbox.
1.75 Hitachis:
45 Dellortos:
Its hard to tell from the graphs, but the peak torque for the Hitachis is 157 at 5300, and falls away quite quickly whereas the Dellortos its a little bit more but at around 5700. At 5300, the Dellortos are also about 157.
1.75 Hitachis:
45 Dellortos:
Its hard to tell from the graphs, but the peak torque for the Hitachis is 157 at 5300, and falls away quite quickly whereas the Dellortos its a little bit more but at around 5700. At 5300, the Dellortos are also about 157.
Not fully convinced by that flow argument Jon as by contrast on mine, the engine power (top line) was still climbing at 6400rpm and the torque started flattening / dropping as the carbs went too lean (see the CO2 square wave heading south). So there was more air than fuel at the top end, hence my needle work. (I’m too embarrassed to share the rest of the curve due to being plagued by reversion!!) Blue line is at the wheels, green is the drivetrain losses.
I acknowledge that the 3.1 stroker will suck harder than my 2.8 but given my experiences I feel that the peak and drop off have more to do with the combination of the cam duration, lift and LAS.
Ztherapy run their SCCA L24 engined car to 8K rpm without flow issues on stock sized (not bored out) SUs with highly modified needles.
The triples should of course flow better at the top end as per your example and their design brief for racing - no disagreement there. BTW, do you have the AFR or CO2 plots to overlay on those curves? Are you sure your SUs were able to fuel well enough at those revs and that air flow was really the limiting factor? I ask because after the base tune for running in, my engine was totally done by 5200rpm because it was tuned up to 4K and was very lean at about 5000. The top end only came to life after the final tune and needle profiling. At 7K she’s still pulling the rev counter needle round but I daren’t go there very often (hence rev limiter at 6.5k) as I’m on stock flattop pistons and rods.
In my case air flow was far from the issue, it was lack of fuelling at that rate of air flow; which I have to say, I am most disappointed with the professionals not sorting it while being paid to profile the needles. Maybe they had more exciting / money making projects to get to on with on the day and properly profiling the last stages of my needles wasn’t in their interest. No matter, the KDs with 4oz springs have pretty much sorted it and MCs according to my spreadsheet may do even better once I’m allowed out again.
Here’s another interesting one for further discussion though. After fixing my dome leaks, the AFR was lovely and as expected. Changing out the more restricted Turbo muffler for the free flowing straight through while doing the bearings, caused the AFR to go way lean everywhere; e.g. from 15 at idle to 18+!!!!. With the exception of the reversion points where it got richer - going from 10.8ish down to sub 9.5. Nothing changed, except rear bearings and back box! [emoji848] This whole air flow / AFR thing is quite interesting and fascinating how everything affects each other.
Must have been the bearings for sure [emoji1787]!! More of that anon.
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Obvs lots of things can affect an engines performance - like the cam spec, the head flow, valve size etc. Restrictions at the start of the air ingest (the throttle) is only one of them. That's why I thought it interesting to show a couple of plots from the same engine with just the throttle sizes being different, because it negates all those other variables. (although it is also true that the two plots were done on different days a couple of years apart on different dynos!)
Thinking about your engine - clearly the fact that yours is running lean at the top end is not evidence of how well a good SU tune works vs how well a good triple 45s tune can work. It is merely evidence that you haven't got your needles right yet
Likewise, observing that your engine's power continues to build to 6400 RPM is only telling you that at lower revs, your throttle opening isn't the main limiting factor of your engine at that RPM. Maybe something else is limiting it so much below 6400 that you can't see in the graph that the SUs are the limiting factor from 6000 RPM (or whatever)Ztherapy running SUs at 8000 RPM again is only evidence that the L engine can run at 8000 RPM. I could put 1.0" SUs on my engine and tune the needles to get a good AFR and it would still rev beautifully to 8500. It would just make a lot less power above say 4000 RPM than it does with 6x 45mm throttles.
I don't have the AFR plots to go with those graphs, but yes, I'm confident that they were well setup with each carb type. Both were the end result of a full day of tuning on the Northampton Motorsport Dyno.
I'm not saying that SU/Hitachi carbs are rubbish. I'm just saying that throttle bore sizes can be a limit, and the stock 1.75" carbs were probably chosen to be the cheapest possible option that DIDNT limit the power below what Nissan specced for the stock engine. So shouldn't be a surprise that it will be a limitation for modified engines.
Why else would SU make a 2" carb if a 1.75" carb could flow as much?
Anyway - I've shown this pic before right? it's my VE table. I think it shows that below 4000 rpm or so, the head/cam dynamics mean that % throttle opening is the limiting factor up to about 50% open, after which opening the throttle more doesn't do much, because the engine can't consume the air - the throttle isn't the limiting factor. at 6000 RPM, there's a linear(ish) growth in fuel consumption up until about 90% and then it tails of just a little to 100%. which I think is showing that the engine is consuming all the air the 45mm ITBs can pass. It could probably use more fuel/produce more power with 50mm ITBs.
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