tuning fresh air impco & sizing

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Posts: 359
Joined: Wed May 16, 2007 11:46 pm
Location: New South Wales, Oz

tuning fresh air impco & sizing

Post by jono » Sat Apr 12, 2014 12:29 am

Got a bit long winded, but sets my thoughts out for those who can comment :)

OK, it is almost too late to be asking this as I have been running it for six years as was, but started to question efficiency

1800 cc flat four Subaru, with Impco L and 200 mixer - was returning 15 litres per hundred km until some decent tuning (2 leaning rings and adjustments using tail pipe analyser) to get it down to about 12 litres.

The max boost has been wound up to about 12psi, mainly to get over a pulse of the turbo sort of making boost gauge needle jump about. I can expec it must use more fuel the more boost used.

Previous tank I did 300 km, 100 of which was trailer towing - used 45 litres so back to 15 litres per hundred.

Then , after next fill, the drive was very sedate along country lanes getting some volumetric efficiency going on a damp day, keeping slow pace to stop for sheep, kangaroos, pot holes etc.

Instead of the sort of not accurate gauge reading 45 litres at empty mark of 72 litre useable, it read just below half at same 300km !
Now got it to 450km and at the empty mark of approx 45 litres. I have not checked it and not likely as car may be scrapped for future project very soon

My reckoning is that with real sedate driving I could get this thing down to near 10 litres per 100 !

I use an LED O2 reader in dash - set in the factory to 15.5:1 AFR for the middle LED

With perfect tail pipe mixtures, the LED unit ran a few LD to the rich side, so I adjusted it as far as I could so it would reflect tail pipe readings.

Told the manufacturer I did this - was not happy with me as it was now inaccurate and has his name on it !

I also tried the tail pipe O2 unit screwed in pre cat to find it read very oddly, all over the shop - maybe due to fact it was a more advanced 4 wire type O2 sensor instead of my single wire LED unit ?

Figuring the engines true mixtures would show the health of things from an O2 sensor mounted pre cat as the tail pipe analysis is for emissions targets and if a cat is doing an exceptional job could be covering up for a rich propane fuel mix leading to more fuel used than required, and the turbo could likely do without the extra heat generated by a richer mixture.

So, in a 15 min job I went in to add a second leaning ring as I thought there was only one. Uh oh, two already !! Is it correct - no more than two leaning rings ?? OK , so I was in there, so added a third leaning ring and reassembled things.

Bugger to start - too lean of course, due to the leaning ring added. Got it running and found would stall if left too long at idle.LED mixture meter indicating at max lean, so after a drive I was able to increase idle mixture to the middle of the still inaccurate due to my actions gauge. Idles nice, no stalling.

My aim by installing the extra leaning ring was to bring the cruise mixture down a notch i a fuel saving effort, so long as boost mixtures are not harmed.

I know, leaning rings are for idle mixture tuning, but I have found previously a comment by someone here, that cruise mixtures can be reduced a small amount by a leaning ring and tried it in another vehicle , now saving about five litres out of a what was 70 litre fill at a certain distance - regularly been about 65 litres instead - even after an engine swap !

Another question relating to subject heading
: is a 200 too big for my 1800 cc turbo as a fresh air? To lean out cruise mixtures further, can you get leaner than standard fresh air diaphragms as you can for the feedback diaphragms being richer ?

Wondering if I should try stepping down to a 100 or 125 mixer - said to be good for up to three litre Nissan I6 engines (though every one of them I have seen have run 200 series mixers !!)

Posts: 359
Joined: Wed May 16, 2007 11:46 pm
Location: New South Wales, Oz

Re: tuning fresh air impco & sizing

Post by jono » Sun Apr 13, 2014 9:41 pm

well, not sure if it is my head, the new timing belts or, the extra leaning ring ...I'd swear I have crisper feel to the accelerator pedal feel in light cruise.

The mixture meter reading off pre cat is a little less flitty about it LED display and closer to stoic - tail pipe emissions still on stoich !

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Re: tuning fresh air impco & sizing

Post by C3H8 » Mon Apr 14, 2014 1:21 pm

First comment you made: Leaning rings for idle only! Not true. leaning rings will affect idle, cruise and moderate acceleration. Leaning rings lower the gas valve further into the vapour outlet opening. Considering the gas valve is tapered this means the mixtures should be affected throughout the full range with the smallest affect at WOT. They were originally designed for forklifts to lower the emissions (CO) in warehouses.

Second comment. Mixer size. Your engine is 108 CI. At 6000 RPM it will need approximately 150 CFM normally aspirated. Yours is turbo charged at 12 PSI or a 90% increase. This equates to 285 CFM if my calculations (charts) are correct. The 200 is a good pick. The 100/125 is only capable 197/235 CFM. Too small for your application unless you stay under about 4000 RPM.

Posts: 359
Joined: Wed May 16, 2007 11:46 pm
Location: New South Wales, Oz

Re: tuning fresh air impco & sizing

Post by jono » Wed Apr 16, 2014 7:10 pm

I think someone in here or the impco book, or a mate who not know any better put the only changes idle mixture comment in my head :)

I found out otherwise when i tried it myself.
I thought I sized the CFM quite well :)
Important thing is tailpipe emissions for the authorites,
important for me is less fuel use for same power delivery , and indicated by my pre cat O2 sensor, could still be reduced somewhat. A hand flowed and graded diapgram on a new mixer might be my best shot

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Re: tuning fresh air impco & sizing

Post by storm » Wed Apr 16, 2014 8:42 pm

jono wrote:Important thing is tailpipe emissions for the authorites,
If you have a car that must use an authorised kit then yes the authorities think this is important. The fact you don't have a car that requires an authorised kit means they don't care.
Every job is a self-portrait of the person who did it. Autograph your work with excellence.

Fuel flow requirements calculations viewtopic.php?f=5&t=1638

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