Light throttle manners and a Dyson
Discussion
Shunting and lumpiness at light throttle have a bit of an obsession over the last few years. The car can be very hard work to drive at around 30/40 in 4th or 5th and on the overrun it was always horrible...I invariably end up de-clutching to avoid a nasty transmission shunt as the engine stumbles slightly.
Good news is...its now (finally) very good (imo). I can run up slowly to to say 40 in 2nd, it accelerates nice and smoothly and I can then overrun in gear all the way back down to idle revs with almost no shunting and fussing. Here was the recipe:
o Refurbished throttle bodies with plain oilite bronze bushes and *no* shaft play at all has proved to be vital (ball races also work by all accounts). The shafts on mine each have a small o-ring seal to their outer ends against the face of the bush, pushed up with a retaining disc. Shaft play was a good chunk of the issue, even a few thou is a killer. You *can * see it in the adaptives...they really do fluctuate more than normal (I mean peak to peak changes of perhaps 5 or 6 % over seconds rather than perhaps 2 or 3% over 30 secs to a minute)
o I got a remap by Joolz...that helped too (can't recommend him enough). Even he had to admit it was a bit weird.
o Later I found a step improvement by taking the plug out of the purge canister...seriously! Have now blocked the purge airways in each body using a tapered screw where the casting-drill seal screws would normally be. I can only assume there is a largely uncontrolled amount of air entering via this path and its causes body-to-body imbalance that is almost impossible to see when metering the air into the body mouth.
o I relentlessly tweaked the air balance for each body...but still couldn't quite nail it until today! Moment of clarity: do the balance with the engine cold using a Dyson up the exhaust pipe! I know! Shove it up with a bit of foam wrapped around the nozzle to seal it...then rotate engine by hand until that body sucks (obvs have to do this for each pipe/bank). Set the flow. Soooo easy. No nasty heat, gases, vibrations or spitting to deal with. I actually finally matched them with a 10thou feeler under the throttle stop to simulate around 1300rpm (1300 rpm needs just 0.25mm of movement, no wonder this is so bloomin' hard to make good!). Got them all reading 4.5cfm to within a spit. Interestingly going back and checking them at idle they are not that well matched...between 3.5 and 4 cfm (with my model Dyson of course...the numbers don't matter so much as the matching). But it idles beautifully at 700 to 800rpm when hot despite that, AND now when cruising at 1200-2100 rpm with steady throttle in 4th its nice and stable.
Yay! Now I can do something else with my life! Oh, and get less annoyed when driving it around town...
Hope this helps someone.
Good news is...its now (finally) very good (imo). I can run up slowly to to say 40 in 2nd, it accelerates nice and smoothly and I can then overrun in gear all the way back down to idle revs with almost no shunting and fussing. Here was the recipe:
o Refurbished throttle bodies with plain oilite bronze bushes and *no* shaft play at all has proved to be vital (ball races also work by all accounts). The shafts on mine each have a small o-ring seal to their outer ends against the face of the bush, pushed up with a retaining disc. Shaft play was a good chunk of the issue, even a few thou is a killer. You *can * see it in the adaptives...they really do fluctuate more than normal (I mean peak to peak changes of perhaps 5 or 6 % over seconds rather than perhaps 2 or 3% over 30 secs to a minute)
o I got a remap by Joolz...that helped too (can't recommend him enough). Even he had to admit it was a bit weird.
o Later I found a step improvement by taking the plug out of the purge canister...seriously! Have now blocked the purge airways in each body using a tapered screw where the casting-drill seal screws would normally be. I can only assume there is a largely uncontrolled amount of air entering via this path and its causes body-to-body imbalance that is almost impossible to see when metering the air into the body mouth.
o I relentlessly tweaked the air balance for each body...but still couldn't quite nail it until today! Moment of clarity: do the balance with the engine cold using a Dyson up the exhaust pipe! I know! Shove it up with a bit of foam wrapped around the nozzle to seal it...then rotate engine by hand until that body sucks (obvs have to do this for each pipe/bank). Set the flow. Soooo easy. No nasty heat, gases, vibrations or spitting to deal with. I actually finally matched them with a 10thou feeler under the throttle stop to simulate around 1300rpm (1300 rpm needs just 0.25mm of movement, no wonder this is so bloomin' hard to make good!). Got them all reading 4.5cfm to within a spit. Interestingly going back and checking them at idle they are not that well matched...between 3.5 and 4 cfm (with my model Dyson of course...the numbers don't matter so much as the matching). But it idles beautifully at 700 to 800rpm when hot despite that, AND now when cruising at 1200-2100 rpm with steady throttle in 4th its nice and stable.
Yay! Now I can do something else with my life! Oh, and get less annoyed when driving it around town...
Hope this helps someone.
You dont care about absolutes dont forget, just want to get the same flow in each body...and you do that with the flow meter. As you say, you need to get the valves at overlap but then you get full suck from the vacuum...it works a treat. But I'm not forcing you do it this way 
Just offering an interesting (and tested) alternative way that avoids the heat and variability of doing with the engine running.
Just offering an interesting (and tested) alternative way that avoids the heat and variability of doing with the engine running.I think the reason it works well is the valves are plenty open enough for the tiny air flow needed...hence it's consistent. As you turn the engine to open the valves you can see that the flow meter reaches a peak and stays there over quite a wide range of rotation, then slowly drops back.
Edited by PetrolHeadPete on Thursday 9th July 07:53
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