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Dash pot oil

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Old Jul 20th, 2018, 08:20   #11
ASt85
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Originally Posted by Ron Kwas View Post
AS;

Don't take this wrong, but where do I start...based on what did you change Metering Needles and Damper oil viscosity?............if for any reason less than full range dynamometer motor tests, where engine response and exhaust gases are checked under all load (steady-state and transient) conditions, I expect that explains half of the tuning problems......

I doesn't surprise me that you gave up in the end and went to Webers...I wonder how that went...

Cheers
The recommended "fix" at the time to get that increased the volume of fuel/litre of air on the induction stoke was to go the richer needles, by reducing the SAE, the piston rose more quickly so theoretically leaning the mixture to the point of not running overly rich; the real effect was as you state above - which compounded the nightmare of balancing the twin 1¼ SUs. The Webers held their tune much better but needed some modification to the bulkhead to make them fit, that said I always thought that although some of the improvement was due to the carbs ability to increase air flow much of the benefit was derived from the better air flowed manifold itself.

I eventually sold the Weber set-up and went back to SUs with 20/50SAE in the dash pots (Duckhams Q worked best) using twin 1½s on a flowed manifold which seemed to work better with the longer dwell Janspeed cam. In real terms it was over tuned for a road car, but was an absolute delight to drive...it was just waiting for the car in front at traffic lights to get 20 or so yards ahead before you let in the clutch that was a pain!
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Old Jul 20th, 2018, 13:27   #12
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Originally Posted by arcturus View Post
That was my understanding. Thicker oil slows the rise of the piston under heavy acceleration and momentarily causes a richer mixture.
I also thought the same, and used engine oil in my twin SUs on my Maxi.

On my stage 1 tuned Mini, I used richer needles and still got it to pass emissions.
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Old Jul 20th, 2018, 14:15   #13
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I also thought the same, and used engine oil in my twin SUs on my Maxi.

On my stage 1 tuned Mini, I used richer needles and still got it to pass emissions.
Emissions weren't a major factor when I broke my Cooper S up in the summer of 1975, there was far too much to repair in terms of sills/rear subframe/bootfloor, rear floor panel, front inner sills & etc. most of the non-fibreglass panels were riddle with the rot and at the time I had neither the time nor the money to replace the body............sold the engine for more than I'd paid for the car and all the "go faster" parts over the preceding 7 years, which paid for a four year old Ford Corsair V4.

The only car I've driven which even comes close to the handling of that 997S is my son's Japanese grey import '92 Eunos/MX5 - it produces the same grin factor when you start it up and slot it in gear...sadly, (nah ) it is sat on our drive and I am forced to drive it every week or so to stop the brakes sticking/seizing.....the C70 almost does it but can't quite get the hang of this new-fangled thing called aircon in a cabriolet, but then the Eunos also has it - still can't figure out why anyone would need air-con in a ragtop!
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Old Jul 21st, 2018, 22:28   #14
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I miss my custom mini pickup... No electronics, just clean pure fun..

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Old Jul 22nd, 2018, 22:34   #15
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I have used a special 'v8250' home brewed mix for many decades now, that is thinner than what SU/Burlen Fuels suggest. It allows, combined with correctly spec'd springs, much quicker take up, excellent idle and excellent stability throughout the rev' range.

A 50/50 blend of 10w/40[or 20w50] with 3in1 will do very well for most road applications. It's really worth adding this book to this years birthday or Christmas present list...

https://www.amazon.co.uk/Build-Power.../dp/1901295141

ps I use something altogether different and sometimes have the fastest off-the-line SU's...the recipe's tip-top secret but does include super thin sowing machine oil
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Old Jul 23rd, 2018, 11:04   #16
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I used air tool oil years ago because the garage had loads of it and it cost me nowt.
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Old Jul 23rd, 2018, 16:00   #17
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AS; "the richer needles" implies there are a limited number available...in fact, I have no less than 3 8 1/2" X 11" pages full of dimensional data for metering needles for 0.100" Jets, the sizes of which are given in ten thousands of an inch accuracy (!)...determining which is "the one" for your engine configuration takes some engineering, and trial and error, accompanied by solid research and practical load tests, not just a quick look at the list...

Forum; Dashpot oil is indeed crucially important to be present, but viscosity 20W +/- is just not that critical for anything but where its necessary to get the absolute best out of the engine (and this applies only to transitions ie throttle response!), consequently a lot of people play with viscosity and swear they get a noticeable result...I have to question that...now if I had a lightened Flywheel and Intake and Exhaust flow which allowed a much faster throttle response, I might be tempted to try varying the viscosity (if I had a dyne or a lot of track time to check it).

Cheers

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Old Jul 23rd, 2018, 19:06   #18
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Originally Posted by Ron Kwas View Post
AS; "the richer needles" implies there are a limited number available...in fact, I have no less than 3 8 1/2" X 11" pages full of dimensional data for metering needles for 0.100" Jets, the sizes of which are given in ten thousands of an inch accuracy (!)...determining which is "the one" for your engine configuration takes some engineering, and trial and error, accompanied by solid research and practical load tests, not just a quick look at the list...

Forum; Dashpot oil is indeed crucially important to be present, but viscosity 20W +/- is just not that critical for anything but where its necessary to get the absolute best out of the engine (and this applies only to transitions ie throttle response!), consequently a lot of people play with viscosity and swear they get a noticeable result...I have to question that...now if I had a lightened Flywheel and Intake and Exhaust flow which allowed a much faster throttle response, I might be tempted to try varying the viscosity (if I had a dyne or a lot of track time to check it).

Cheers
Thank you for your informative post; the last time I played with SU needles was the summer of '68

Access to the data to which you refer was rarely available in those times, most BMC/Leyland dealers only carried standard needles, indeed even locating richer/weaker needles was a question of "ringing around" the county (Kent, Surrey, & Sussex at the time) to find alternatives. Those I found came from a now defunct small engineering/racing engine builder some 60 miles away.

Much of what was achieved was by trial and error and I assure you there were many errors; in stage 3 tune it was running a tuftrided racing cam (Piper's seemed less prone to wear than Janspeed's at the time), triple valve springs, coupled with a Lotus derived duplex timing chain with the associated gear mods, tuftrided fully balanced crank & lightened flywheel, plus a competition clutch - the engine was redlined @ 10,200rpm on 5* fuel meaning only one gear change was necessary to hit 60mph in a shade under 6.1 seconds - not bad on the 10inch wheels - it literally was a very hot little motor and would blow away most sports cars from a standing start, leaving many an Elan in its wake. Various heads were used over time, the best coming from a frogeye sprite which had a little more metal to play with when it came to skimming & smoothing again compression ratios of 10.6 to 13.1 were used over time - the best set up was achieved using 12.3:1, anything more than that just didn't work as road car + five star fuel aka the old leaded 101 octane became increasingly difficult to find at the roadside hence the sale of that wonderful engine in the summer of '75.

The twin 1.5" SUs on "normal" needles and Duckhams Q 20/50 made for a much more tame drive, but still enabled a very fast take off without the thirst of the Webers.

The point being that the SUs were designed for an SAE20 oil.
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Last edited by ASt85; Jul 23rd, 2018 at 19:37.
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Old Jul 25th, 2018, 03:08   #19
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Ron, as usual I agree with most all you say, except the part about the need for a dynomometer to get it exactly right when f**king around with different needles and other stuff e.g. oil, ignition, flow alterations, etc...

I recently traded my 16' B20-powered speedboat for, among other things, an old working top-of-the-line Snap-On Exhaust Gas Analyzer (tailpipe "sniffer") and an Ignition/Fuel Injection Oscilloscope (way overkill for our simple cars).

Looking at CO readings from tailpipes of modified cars is something I've always wanted to do, and is a definite gas.

Best, from the chilly North
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Old Jul 25th, 2018, 13:35   #20
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Originally Posted by Ron Kwas View Post
AS; "the richer needles" implies there are a limited number available...in fact, I have no less than 3 8 1/2" X 11" pages full of dimensional data for metering needles for 0.100" Jets, the sizes of which are given in ten thousands of an inch accuracy (!)...determining which is "the one" for your engine configuration takes some engineering, and trial and error, accompanied by solid research and practical load tests, not just a quick look at the list...
Which is why I bought mine from a well respected tuning company who made the inlet and exhaust manifolds also, and had already done the hard work selecting the right needle for my camshaft etc.
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