+- Austinsevenfriends (https://www.austinsevenfriends.co.uk/forum)
+-- Forum: Austin Seven Friends Forum (https://www.austinsevenfriends.co.uk/forum/forumdisplay.php?fid=1)
+--- Forum: Forum chat... (https://www.austinsevenfriends.co.uk/forum/forumdisplay.php?fid=14)
+--- Thread: Query about gearing... (/showthread.php?tid=4913)
RE: Query about gearing... - Tony Betts - 06-09-2020
hi duncan,
the easiest thing is to start again.
the phoenix crank, other modern cranks are available. is purly a reliability thing. not power.
the high comp head is fine. but it does very little on its own.
i find on the seven its main advantage to a fast engine. is that the chamber flow over the valves is bigger. so it lets the gasses in and out quicker. but unless you do the rest of what the engine needs it gets a bit lost.
start from the bigining, the bill williams book is cheap and explains well for beginners. read the section on tuning engines.
so, to go faster. you want as much gas and air to move through the engine as quick as possible.
if you gas flow the engine to a high standard, thats lining up all the inlets and outlets to the block so there are no steps. you can take the tops of the guides. polish the ports. open the inlet and exhaust ports. it all helps gas flow. same as the high comp head, the chamber over the valves lets more gas in and out.
once you have that, you need to look at were you are going to use your revs.
i would suggest if a lack of speed is the problem, a modified trials cam to raise the revs is a must.
but again this wont work on its own.
its no good having a standard gearbox on a trials cam, when you change from first to second. the revs will fall of the cams range.
so one of your problems is likely to be a lack of the correct cam, and a close ratio gearbox.
the last thing, but very important, is a lightend flywheel. i find is a must to stay on a higher cam.
but buyer be ware.
my supersports is fast and all that. but you then have to live with the negative. it doesnt like slow speed. and is unhappy staying below a village speed limit.
im with the others, your carb may not be giving you the fuel supply the speed you want needs.
i find a good starting point for most people is.
5.25 rear axle.
close ratio gearbox.
trials cam.
a well ported block, matched to the exhaust.
a high comp head.
1 1/4 SU carb
a lightend flywheel.
is a good start for all, it should give 30+ BHP and an easy 65 mph.
but you need to do it all together, as each individual improvement helps each other. or are useless without each other.
after that you have to fine tune things.
good luck.
tony
RE: Query about gearing... - Chris Garner - 06-09-2020
As good a general precis as you'll get to set up a sporting Seven. Thanks Tony.
RE: Query about gearing... - Dave Mann - 06-09-2020
When I tried a Reliant carburetor on one of our RN saloons it was a noticeable improvement on the 22 FZB however without a pump I lost the reserve capability.
RE: Query about gearing... - Mike Costigan - 06-09-2020
My first thought here is: why are you changing into top gear at 27mph? To make progress I would expect to keep in third to at least 35mph, at which point the revs in top will be around 2,300rpm. Changing into top below 30mph is going to drop the revs below 2,000rpm which is not really fast enough.
RE: Query about gearing... - Alan - 06-09-2020
Tony is right in what he says. For a quick Austin 7 two things matter. Gearing and breathing. But, you have to consider them as a package. It’s no good having big valves if you are breathing through a 12mm carburettor choke. It’s no good having good breathing if the gearing means you can’t get it on the cam. The thing that’s not been mentioned is the importance of the exhaust manifold. If the cylinders can’t get rid of the burnt gases they can’t breathe in fresh. A standard manifold simply will not do.
RE: Query about gearing... - Stuart Giles - 06-09-2020
If the Reliant carb is off a sidevalve motor it should work at least as well as a standard A7 carb; the power output of the Reliant SV motor was about the same as the late Ruby IIRC. A light special should easily beat 50 MPH on the flat even with a bog standard motor. After doing the obvious stuff like checking the points gap hasn't shut down, I think it would be worth adjusting the timing and seeing what the result is from advancing it from the current setting.
RE: Query about gearing... - Duncan Grimmond - 06-09-2020
Tony's list is very helpful as while I've played quite a lot with other vehicles, Austin 7s are still bit of unknown territory.
I change to top at around 27mph because that sounds like the point at which the engine is at its max output and it will hardly reach 30 on the flat no matter how hard I push the gas pedal.
The carb is mounted on a tall(ish) vertical inlet tract which I would have expected to give a good flow. My previous car had a horizontal inlet which was changed to take the downdraught SU. I should have kept that carb/tract when I sold the car, doh!
I'll have a further play with the thing this afternoon...
RE: Query about gearing... - Steve Jones - 06-09-2020
(06-09-2020, 10:38 AM)Alan Wrote: Tony is right in what he says. For a quick Austin 7 two things matter. Gearing and breathing. But, you have to consider them as a package. It’s no good having big valves if you are breathing through a 12mm carburettor choke. It’s no good having good breathing if the gearing means you can’t get it on the cam. The thing that’s not been mentioned is the importance of the exhaust manifold. If the cylinders can’t get rid of the burnt gases they can’t breathe in fresh. A standard manifold simply will not do.
Endorse everything Alan says. As an example, my Trials car has a four branch under the bonnet exhaust manifold. On a Derbyshire Trial a few years ago we managed to break it and, as the Herefordshire Trial was the next weekend, substituted it with a standard exhaust manifold. With the exception of that and the log type inlet manifold, everything else stayed the same including the 1 1/8" SU that was mated to the standard manifold. The thing was hopeless indeed it was something of a revelation to us the negative difference it had made.
I don't have any four speed Sevens but I've been on holiday with my absolutely bog standard 3 speed SWB Saloon this last two weeks and have used it pretty much every day. The engine has no tuning enhancements whatsoever but will cruise on the flat at a happy 40 - 45 mph and, if you give it time to get there, 50 mph is available. I really do think your engine may be too far retarded, Duncan, as even as you've described its specification, it should be able to beat that handsomely . An easy thing to check. Advance it and see what happens.
Steve
RE: Query about gearing... - Duncan Grimmond - 06-09-2020
Ignition timing was the first thing I looked at and it was slightly retarded. I've advanced it as far as it will go within comfort range and I'm wondering about the A/R mechanism. I have an unused electronic dizzy which I have considered but I don't like changing more than one thing at a time as it's easy to mistake symptoms for problems you've created!
RE: Query about gearing... - Howard Wright - 06-09-2020
Hi Duncan
Unless the head has been doctored by skimming it looks like a bog standard low compression head to me.
I would echo what Steve has just posted. A four branch exhaust and decent inlet manifold will work wonders.
Cheers
Howard
PS I’d ditch the air filter.