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BMW Mini Supercharger in MGB - started up today

Moss Motors
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gray Graham Moore
CAMBRIDGE, CAMBRIDGE, UK   GBR
having spent best part of a year profiling a needle to give all the good driving features of BCG but with better cruising AFR numbers (than high 13's) for better economy, i can now announce the AFR/mpg results from about 3000 miles driving... (drum roll)......



MPG BCG needle 24

MPG CUSTOM needle 25

so having spent countless gallons / days running up and down flat roads, rubbing, changing, cutting (springs), adding weights, polishing, ......

I can contribute to saving our planet.

bah humbug.

G



Edited 1 time(s). Last edit at 2018-06-14 04:48 AM by gray.

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Mano864 Avatar
Mano864 George H
Rowville, Victoria, Australia   AUS
1972 MG MGB MkII "Sam (Samantha)"
G
Is that, BCG needle 24mpg and custom needle 25mpg?
G

pinkyponk Avatar
pinkyponk Gold Member Adrian Page
Berwick, NS, Canada   CAN
What afr are you seeing when cruising down the highway at 65mph?

Adrian



Home built Eaton M62 Supercharger with 9psi boost, "stock" high ratio rocker arms, 8:1 compression, Piper 270 cam, ported head, matched manifolds, CB Performance computerized ignition.

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gray Graham Moore
CAMBRIDGE, CAMBRIDGE, UK   GBR
correct George.

Adrian - with my custom needle, 15, with BCG high 13's.

i found that making a custom is always a compromise. if i profile it to get 16 at 60mph, then i just get too long of a spike as soon as on boost and it really takes the edge off the performance at that transition.

i suppose that it would be interesting to only cruise for a few full tanks - but then that isn't representetive of the driving i do.

G



Edited 1 time(s). Last edit at 2018-06-14 03:38 PM by gray.

Denis Avatar
Denis Denis Hill
Bearii, Nth Victoria, Australia   AUS
I used to get 32 with the HIF but our roads are more long and flat than yours, 16.5 :1 A/F was pretty normal on a flat road although even a strong headwind would knock it back a little. Are you worried about the mixture spike on the meter, or an actual performance flat spot. Denis



68 B roadster, Daffodil yellow, supercharger, Burgess SC head, SC cam, Mikuni HSR 48 carburetor and engine built for supercharging.

73 BGT V8 conversion starting with a bare shell. Built the engine "3.9L Rover" early in 2016 with high comp pistons and a few other nice bits, plus a T5 ford trans. Started on the body late 2016 and complete late 2017, Did all the work myself, mechanical, body. paint etc.
Finished and going well, great to drive and quick. Now has a nice 3.07 LSD.

gray Avatar
gray Graham Moore
CAMBRIDGE, CAMBRIDGE, UK   GBR
both really denis. if i have a needle where i can cruise at 16, if i am say driving at 40mph and stomp on the gas, i will see the AFR hit 16.5, perhaps higher, and it will take several seconds tumbling 15.5, 14.5, 13.5....until eventually getting to the low 12's.

i have never detected detonation under these circumstances (but that's not to say i haven't missed it) , but the performance saps. whilst the gauge says i am in boost, i may as well not be for a few seconds.

in contrast, the BCG drops AFR immediately to 13.5 and is hitting 12 in a second or less.

G



Edited 1 time(s). Last edit at 2018-06-15 03:48 PM by gray.

mgbsc72 Avatar
mgbsc72 Silver Member Ross Holt
Melbourne, victoria, Australia   AUS
1973 MG MGB
2004 MG ZT
Hi Graham

I have found with my hif that the yellow 8 oz spring gives a better cruse AFR with the same custom needle
Say 13.8 to 14 with red spring and 14.8 to 15.2 with yellow
My custom needle was a BCF richer from station 6 , it was Hans Peterson who told me to try the 8 oz spring
It also helped the lean spike a bit . the taller gearing in my car made the lean spike worse
I also had to loosen my damper piston a bit eg more free play it made the spike a little worse but much shorter
eg hit 17.5 but down to 12.5 much faster
On the whole the yellow spring some how makes the cruise leaner but full throttle a bit richer
If I had a stock BCF , all my ones have mods I would give it a go with the 8 oz spring

regards Ross ps still waiting for my mikuni

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Denis Avatar
Denis Denis Hill
Bearii, Nth Victoria, Australia   AUS
The main thing that controls a flat spot on application of throttle is the damper. The damper only affects mixture during that action. I used to run an oil called "fuel set " It was to stop fuel in storage going stale. Used it cause I had gallons of it but it was heavier than SAE 90 and worked a treat. No flat spot although the A/F meter would still show a very short lean spike. The OE for the early Moss HIF was a "4 oz spring with a BCA needle, small piston lift jets, 90 oil" and most were spot on with that. Mostly standard engines then, but I think I would still use it as a starting point, I was still running that up to the Mikuni. Denis



68 B roadster, Daffodil yellow, supercharger, Burgess SC head, SC cam, Mikuni HSR 48 carburetor and engine built for supercharging.

73 BGT V8 conversion starting with a bare shell. Built the engine "3.9L Rover" early in 2016 with high comp pistons and a few other nice bits, plus a T5 ford trans. Started on the body late 2016 and complete late 2017, Did all the work myself, mechanical, body. paint etc.
Finished and going well, great to drive and quick. Now has a nice 3.07 LSD.

pinkyponk Avatar
pinkyponk Gold Member Adrian Page
Berwick, NS, Canada   CAN
In reply to # 3755817 by mgbsc72 Hi Graham

I have found with my hif that the yellow 8 oz spring gives a better cruse AFR with the same custom needle
Say 13.8 to 14 with red spring and 14.8 to 15.2 with yellow.
My custom needle was a BCF richer from station 6 , it was Hans Peterson who told me to try the 8 oz spring
It also helped the lean spike a bit . the taller gearing in my car made the lean spike worse
I also had to loosen my damper piston a bit eg more free play it made the spike a little worse but much shorter
eg hit 17.5 but down to 12.5 much faster
On the whole the yellow spring some how makes the cruise leaner but full throttle a bit richer
If I had a stock BCF , all my ones have mods I would give it a go with the 8 oz spring

regards Ross ps still waiting for my mikuni

Something odd going on there. A stronger spring will make the mixture richer at all stations on the needle. It does this by restricting the flow though the venturi. The spring(and piston's weight) sets the level of "constant depression" over the jet.

Adrian



Home built Eaton M62 Supercharger with 9psi boost, "stock" high ratio rocker arms, 8:1 compression, Piper 270 cam, ported head, matched manifolds, CB Performance computerized ignition.

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gray Graham Moore
CAMBRIDGE, CAMBRIDGE, UK   GBR
I have found heavier springs (i am currently using a chopped 4oz which gives about 2.5, but i have red, yellow, green) do 2 things. cruise AFR is richer and acceleration is very limp. perhaps smooth? there is no hoof in the backside when flooring the pedal with the heaviest ones. I have tried many combinations.

one thing i did find weird was BCA with green. BCA with red or yellow was way too rich on cruise (~12.5), but the green made the AFR leaner (~14.5) - but acceleration felt uninspiring. I tried this as Peter Burgess found this was the only combo of stock needle/spring on another car which had lean issues low in the rev range. it did not help my flat spot ~2k RPM. I could not help wondering if the 12oz spring actually limits the travel of the piston to less than 100% and therefore changes the fuelling quite considerably.

Corrected typos in previous post. not sure what happened there.

G

pinkyponk Avatar
pinkyponk Gold Member Adrian Page
Berwick, NS, Canada   CAN
The SU attempts to maintain the vacuum above the jet at a constant level. The SU is designed to reach equilibrium above and below the piston... and thus maintain the constant vacuum level. That is why the piston does not slam to the top of its bore when you start the car.

A stronger spring requires more vacuum to lift the "heavier" piston. If there is more vacuum above the piston(to defeat the stronger spring) there is more below. More vacuum below the piston sucks more fuel out of the jet. Physics in action.

I just got back from testing a HIF44 piston/dashpot assembly on my HIF6 carb body. I was delighted to discover that the HIF44 ball bearing piston/spring combo appears to be the correct "weight" for my engine setup. I only had to turn my jet screw out 3/4 of a turn to get everything spot on again. 12:1 idle, 12:1 flat out under boost and a nice lean 16:1 highway cruise in between. (in OD)

Adrian



Home built Eaton M62 Supercharger with 9psi boost, "stock" high ratio rocker arms, 8:1 compression, Piper 270 cam, ported head, matched manifolds, CB Performance computerized ignition.

gray Avatar
gray Graham Moore
CAMBRIDGE, CAMBRIDGE, UK   GBR
adrian
if you are cruising about 40 in top then hammer the throttle, what kind of behaviour do you see in AFR then? spike-wise.

the piston you have now - has 'stock' ~6/7mm vents?
G

Mano864 Avatar
Mano864 George H
Rowville, Victoria, Australia   AUS
1972 MG MGB MkII "Sam (Samantha)"
Adrian
Re spronger spring question.

I was of the impression (and my expereance with my car's configuration was) that a stronger spring when fitted would reduce the fueling over the range because it takes more vac to lift the spring therefore reducing the amount of needle exposed.

However a stronger spring (and different dampers) do help when you plant the right foot countering the effect of sudden increase in vac thereby slowing the piston lift with a subsequent increase in fueling as more vac (airflow) over the jet draws more fuel out.

Where a weaker spring increases fueling along the range and if you get the damper right you dont have huge lean spike.

George

mgbsc72 Avatar
mgbsc72 Silver Member Ross Holt
Melbourne, victoria, Australia   AUS
1973 MG MGB
2004 MG ZT
On my car when I change from the 4oz red spring to the 8oz yellow I have to lean of the idle as it goes richer by a fair bit
This must change the running stations on the needle . The car is much sharper with the yellow spring which makes no sense at all
To give you all an idea with the red spring the lean spike is a flat spot and the boost is a bit lower
The only thing that maybe happening is the faster air flow due to the lower piston height helps
Graham after your test I will make my needle a bit richer at cruise say 13.8-14.5 current is 14.5-15.5
This will make the lean spot better
Have any of you had the drop off then rise again on boost in 1st and 2nd The only way to remove it was to drill piston to 3.5 mm up from 1.8mm
I did that in stages . 3 rd and top never did it .

regards Ross

pinkyponk Avatar
pinkyponk Gold Member Adrian Page
Berwick, NS, Canada   CAN
In reply to # 3756350 by gray adrian
if you are cruising about 40 in top then hammer the throttle, what kind of behaviour do you see in AFR then? spike-wise.

the piston you have now - has 'stock' ~6/7mm vents?
G

I try to never drive 40 mph in top. That is a very awkward speed for an MGB. I run it in 3rd OD at that speed. To answer the question... it will go about 3 points lean and then recover. It would be more prudent to drop a gear if you are at 40 mph in top and want to get underway. I never drive my car with the revs below 2000 unless I am in first gear, in traffic.

I have no idea what size the piston vent holes are in a HIF44. 1/4" at a guess?

Adrian



Home built Eaton M62 Supercharger with 9psi boost, "stock" high ratio rocker arms, 8:1 compression, Piper 270 cam, ported head, matched manifolds, CB Performance computerized ignition.

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