Friday, 4 December 2015
Saturday, 3 October 2015
Monday, 28 September 2015
Quick Fuel 750DP Tuning Part 3
Continuation of this thread
Quick recap, I've done a fair bit of tuning with my Quick Fuel 750 mech secondary, annular booster carb.
Stock settings
Primary Jet: 72
Secondary Jet: 82
IFR: Unknown - suspect its .032 or .033
IAB: 72
MAB: 33
PVCR Unknown - estimate somewhere around 0.055" to 0.060"
Revised Settings
Primary Jet: 68
Secondary Jet: 82 - reduced to 78 after very short test drive
IFR: .030
IAB/MAB unchanged
PVCR 0.040"
I was hoping that the revised settings would be perfect however after a few miles I think I either went too aggressive on the IFR or need to look at the accelerator pump. On very light acceleration, there is a brief lean spike and a slight stumble. It sounds like classic accelerator pump but I'm not entirely sure as it certainly wasn't a problem when I first got the carb (before I started making changes).
I suspect that because the original settings were so incredibly rich it was covering the brief lean hole so I have gone back to 0.032 IFR to see if it makes a difference short term and have a larger accel pump shooter on order which will deliver a faster squirt on acceleration.
Below is a snippet of data - the yellow line is manifold vacuum (0 is full acceleration) and the white is AFR.
Just for interest, here's a similar plot of gear changes with my Edelbrock carb which had nasty lean hesitation on quick throttle openings that I could never tune out:
Anyway, back to the top graph. the dips in the yellow line are gear changes. The data shows me accelerating at about half throttle in 2nd to begin with (on the left) then foot flat in 3rd, 4th and 5th after that. I was exiting a roundabout hence part throttle in 2nd and rolled fairly gently into 3rd gear to be super safe in a straight line before nailing it properly. You can see from the later gear changes (shifting to 4th then 5th) that there's clearly nothing wrong with the accelerator pump, the pump volume is adequate and there is no lean spike on shifting. If anything the pump shot is a tad too much. You can also see the AFR leans out at high RPM. I haven't got RPM plotting working yet as I think I messed up the tacho feed wire but I was shifting at around 5000rpm. An adjustment to the high speed air bleed should help reduce the top end lean out.
The red highlighted area is a lean spike going into 3rd. It wasn't lean enough to feel a hesitation so I've never noticed it before until I looked in detail at the data. I'm a little confused as already discussed - the other gear changes the pump shot is definitely sufficient. More learning required!
Saturday, 29 August 2015
Cobra has a new friend
The final (hopefully) carb parts have arrived and I now need to find time to fit them and then an excuse to drive somewhere to test (as if I need an excuse!). Unfortunately my spare time has been taken over by servicing the newest arrival to the family, a Nissan Stagea:
If you don't know what one is, its very closely related to the Nissan Skyline, just an estate and slightly stretched floor plan. They are extremely rare in the UK, being import only (from Japan). Engine is the same RB25DET in the R32 Skyline, producing somewhere around 240-250bhp. The Stagea also has the same ATESSA AWD system which is permanent RWD then some fancy electronic controlled transfer box thingy that brings the front wheels in when you are about to die. Because of this it's a heavy old beast at around 1650kg but still manages 0-60 in 6.5 seconds.
A part of me (which I am ignoring at the moment) suspects that on a bumpy b road this car would probably be faster than the cobra.
The craziest bit - have a look in here:
No carburettor, its witchcraft!
Sunday, 16 August 2015
Quick Fuel 750DP Tuning Part 2
Continuation of this thread
Had a chance to go for a good 80 mile round trip which was an excellent test of the revised carb settings - here are the results.
To recap, the car was originally extremely rich in all circuits:
Stock settings
Primary Jet: 72
Secondary Jet: 82
IFR: Unknown - suspect its .032 or .033
IAB: 72
MAB: 33
PVCR Unknown - estimate somewhere around 0.055" to 0.060"
Revised Settings
Primary Jet: 68
Secondary Jet: 82 - reduced to 78 after very short test drive
IFR: .030
IAB/MAB unchanged
PVCR 0.040"
I was hoping that the reduced IFR, primary jet and PVCR would see more reasonable AFR in light throttle cruise at low rpm, medium rpm and medium acceleration. As per the previous post, I only had chance for a very short test drive and on very light throttle it was very lean and misfiring. Heavier throttle it would clear up.
Before setting off on the 80 mile round trip, I drove a few miles locally armed with carb tweaking tools. WOT was still incredibly rich so I changed from 82 secondary jet to 78 before we set off on our longer journey. I also very quickly got to the bottom of the light throttle lean misfire - its the dreaded Holley lean tip in that is well documented. This is because I went too small on the IFR. I need a 0.031" IFR but don't have one, so as a temporary fix I compensated for this by sticking a bit of guitar wire (B string 0.016") in the primary idle air bleeds (0.071" stock) and setting all four idle mixture screws a quarter turn richer than they needed to be. This got rid of the lean stumble at light throttle but obviously is not a permanent solution as it causes the engine to load up rich at idle and almost die, and on the overrun there's way to much fuel and it was popping and banging.
The solution should be straightforward - I went too lean on the IFRs so I need to go to 0.031", so they are now on order. Good news though is that with my wire and idle screw bodge I was getting consistent light throttle cruise AFR of about 14:1. The reduced primary jet means that high rpm cruise (above 2500rpm) is now almost sensible, hovering around 13.3 or 13.4:1. Medium acceleration staying on primaries only, but getting the vacuum low enough to bring the PVCR in was showing about 12.5:1 which was an incredible improvement on the 11:1 I was seeing with the stock carb settings. This shows that the PVCR change was the right thing to do. Acceleration was now nice and crisp, not blubbery and sluggish. One jet leaner on primary will hopefully nudge the high rpm cruise towards the 13.5:1 to 14:1 range and maybe nudge the medium acceleration figure up a notch, which shouldn't be detrimental to performance. To be honest, I don't cruise much above 2500rpm so the car won't spend much time here so its not so critical.
Similar good news is to be found on heavy acceleration when getting into the secondaries. Hard acceleration ( between 3/4 throttle and WOT) amazingly showed 12.4:1 flooring it from 2000rpm, leaning out to 12.7:1 by 5000rpm which I was very pleased with. The plan here is to go one step leaner on the secondary jet which should raise that 12.4 up to around 12.6 or 12.7: 1, then I can trim the fuel curve with the high speed air bleeds to keep it flat and reduce the high rpm leanout. No lean spike on fast gear changes.
So in summary:
From stock settings
Lean out primary jets 4 jet sizes
Lean out secondary jets 4 sizes
PVCR reduced from unkown size (somewhere between 0.055 and 0.060") to 0.040"
IFR reduced from unknown size (somewhere close to 0.033 or 0.034" to 0.030"
Low rpm cruise - improved from 12:1 to 14:1 but needs slightly larger IFR to cure lean tip in.
Medium rpm cruise (2500rpm and above) imrpoved from 12:1 (or even lower!) to approx 13.4:1
Light accel (primaries but above power valve vacuum) improved from around 12:1 to 12.5:1
Medium accel (primaries only but power valve activated) improved from 11:1 to 12.5:1
WOT improved from around 12:1 or even richer, to between 12.5 and 12.7:1
MPG with my old Edelbrock Thunder 650 was about 18mpg. With the stock calibration on the Quick Fuel, that dived to around 10mpg. With the current revised setup I am now getting around 15mpg but I was enjoying the car so much that quite a lot of our 80 mile journey was with my foot buried. I think perhaps 16 or even approaching 17 is possible with one more step lean on primary and secondary jet and a lighter right foot.
Had a chance to go for a good 80 mile round trip which was an excellent test of the revised carb settings - here are the results.
To recap, the car was originally extremely rich in all circuits:
Stock settings
Primary Jet: 72
Secondary Jet: 82
IFR: Unknown - suspect its .032 or .033
IAB: 72
MAB: 33
PVCR Unknown - estimate somewhere around 0.055" to 0.060"
Revised Settings
Primary Jet: 68
Secondary Jet: 82 - reduced to 78 after very short test drive
IFR: .030
IAB/MAB unchanged
PVCR 0.040"
I was hoping that the reduced IFR, primary jet and PVCR would see more reasonable AFR in light throttle cruise at low rpm, medium rpm and medium acceleration. As per the previous post, I only had chance for a very short test drive and on very light throttle it was very lean and misfiring. Heavier throttle it would clear up.
Before setting off on the 80 mile round trip, I drove a few miles locally armed with carb tweaking tools. WOT was still incredibly rich so I changed from 82 secondary jet to 78 before we set off on our longer journey. I also very quickly got to the bottom of the light throttle lean misfire - its the dreaded Holley lean tip in that is well documented. This is because I went too small on the IFR. I need a 0.031" IFR but don't have one, so as a temporary fix I compensated for this by sticking a bit of guitar wire (B string 0.016") in the primary idle air bleeds (0.071" stock) and setting all four idle mixture screws a quarter turn richer than they needed to be. This got rid of the lean stumble at light throttle but obviously is not a permanent solution as it causes the engine to load up rich at idle and almost die, and on the overrun there's way to much fuel and it was popping and banging.
The solution should be straightforward - I went too lean on the IFRs so I need to go to 0.031", so they are now on order. Good news though is that with my wire and idle screw bodge I was getting consistent light throttle cruise AFR of about 14:1. The reduced primary jet means that high rpm cruise (above 2500rpm) is now almost sensible, hovering around 13.3 or 13.4:1. Medium acceleration staying on primaries only, but getting the vacuum low enough to bring the PVCR in was showing about 12.5:1 which was an incredible improvement on the 11:1 I was seeing with the stock carb settings. This shows that the PVCR change was the right thing to do. Acceleration was now nice and crisp, not blubbery and sluggish. One jet leaner on primary will hopefully nudge the high rpm cruise towards the 13.5:1 to 14:1 range and maybe nudge the medium acceleration figure up a notch, which shouldn't be detrimental to performance. To be honest, I don't cruise much above 2500rpm so the car won't spend much time here so its not so critical.
Similar good news is to be found on heavy acceleration when getting into the secondaries. Hard acceleration ( between 3/4 throttle and WOT) amazingly showed 12.4:1 flooring it from 2000rpm, leaning out to 12.7:1 by 5000rpm which I was very pleased with. The plan here is to go one step leaner on the secondary jet which should raise that 12.4 up to around 12.6 or 12.7: 1, then I can trim the fuel curve with the high speed air bleeds to keep it flat and reduce the high rpm leanout. No lean spike on fast gear changes.
So in summary:
From stock settings
Lean out primary jets 4 jet sizes
Lean out secondary jets 4 sizes
PVCR reduced from unkown size (somewhere between 0.055 and 0.060") to 0.040"
IFR reduced from unknown size (somewhere close to 0.033 or 0.034" to 0.030"
Low rpm cruise - improved from 12:1 to 14:1 but needs slightly larger IFR to cure lean tip in.
Medium rpm cruise (2500rpm and above) imrpoved from 12:1 (or even lower!) to approx 13.4:1
Light accel (primaries but above power valve vacuum) improved from around 12:1 to 12.5:1
Medium accel (primaries only but power valve activated) improved from 11:1 to 12.5:1
WOT improved from around 12:1 or even richer, to between 12.5 and 12.7:1
MPG with my old Edelbrock Thunder 650 was about 18mpg. With the stock calibration on the Quick Fuel, that dived to around 10mpg. With the current revised setup I am now getting around 15mpg but I was enjoying the car so much that quite a lot of our 80 mile journey was with my foot buried. I think perhaps 16 or even approaching 17 is possible with one more step lean on primary and secondary jet and a lighter right foot.
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