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.
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.
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.
Why are you saying air is being sucked in ABOVE the piston?
............... and Ali seems to have proved it when he stopped his air leak and it improved the running.
He's just clutching at straws
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.
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.
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 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).
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:
View attachment 38101
45 Dellortos:
View attachment 38102
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.