Avon tyre sizes

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Dave Woodward
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Re: Avon tyre sizes

Post by Dave Woodward » Sun Apr 10, 2016 6:12 pm

Roger. Rear wheel enter is c. 328mm (just shy of 13") from the ground at 26 psi.

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Roger King
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Re: Avon tyre sizes

Post by Roger King » Sun Apr 10, 2016 7:37 pm

Dave Woodward wrote:Not sure Roger. How would I check.

Feels 'full on' well before that :D
Check with a timing light with revcounter, and a vacuum gauge is useful too. Engine should be warmed up first. Switch off, and if you have a vacuum advance distributor, disconnect the vacuum advance and plug the connection on the carb it was connected to (or maybe intake spacer if there isn't one on the carb). Distributor end doesn't matter, leave the hose loose. Connect the light to no.1 plug lead (front right on the car, that is front pot on the driver's side for RHD). Check your timing marks on the crank damper are easy to read for 0, 10, 20 etc. degrees BTDC (before top dead centre). Start the engine and point the timing light at the marks on the damper. A typical starting point for a 289 at idle would be around 12-14° advance, but this is relatively unimportant and presumably you are close to this as the engine is running fine. Rev the engine via the carb linkage and you should see the marks begin to move under the light almost immediately - for a 289 hipo they should certainly start moving at around 12-1500rpm. They should move smoothly forward until you reach about 25-3000rpm, at which point they should remain static and stop advancing. This is now ignition timing all-in. Note the rpm at which the marks stop moving. Note the amount of advance the light is now showing on the damper, which might typically be around 32-34°. I would strongly advise against any more than 34° total advance for a 289, depending on what pistons and CR you have. Hypereutectic pistons in particular have been known to fracture at the crown with advance over 36° or so, but it depends who you believe. Best not take the chance.
The ideal advance for a particular engine is determined by carburation, cam profile, CR and so on. You really want as much advance throughout the rev range as you can get without the engine pinking under load. Pinking (detonation) will cause damage to the piston crowns, such as pitting and even burning holes through the top. I have a funny story about that in a Humber Super Snipe, but I digress...
The above will set the distributor up as best you can without a curving machine, or multiple rolling road runs at different rpm. It will give a decent compromise but won't wring every last horse out of the motor. If setting this on a rolling road, the distributor can be turned until peak power is produced with no pinking and that's where it should stay. The idle advance may be higher than the manuals suggest but that doesn't matter, idle advance is pretty much irrelevant. Fuels etc. have all changed since the manuals were written for these engines.
For carb tuning, you really do need to go the rolling road route. Suck-it-and-see out of the box (I think you have an Edelbrock? Or hipo Autolite?) will run OK but you have no real way of knowing if the mixture's correct right up the rev range under load and coasting. If you have a 'bung' in the exhaust you could check with a wide-band O2 sensor, but they're not cheap and take a bit of learning to use.
I don't know who fitted the engine for you, but hopefully they did a rolling road session to set it up. However it would be worth repeating now it is run in.
The above checks will enable you to know when the timing is all-in, and when the carb is comfortably running on the mains circuits. This is the dynamic condition the engine is designed to run in, where it will be happiest, probably most economical and most responsive. A decent rolling road should give you a print-out that will show where peak power is developed, usually somewhere around 5250-5500rpm for a healthy mildly-tweaked 289. A good 289 build in good condition should be able to cope with revs up to 7000 when asked, which is the kind of thing you will hear at Goodwood. For a yank V8 this is a very short-stroke little buzzmotor, which is why it was and is so successful in motorsport.

Sorry that's a bit long-winded.

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clive
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Re: Avon tyre sizes

Post by clive » Sun Apr 10, 2016 9:52 pm

Roger King wrote:
Dave Woodward wrote:Will sort some measurements later today Roger.

The slight increase in leg length will be welcome on those long cruises tbh. Previously 70 = c. 3000 rpm.

Just looking at compact tyre inflators now!!
Don't get too ambitious - for reasons given elsewhere, I wouldn't really want to drop much below 3000rpm at 70, if at all. Do you know at what rpm your timing is all in? You really want full advance and the carb(s) comfortably on main circuits (assuming they're fixed-jet carbs) while you're cruising.

It would be interesting to compare MPG on a long motorway cruise at over 3,000 rpm. If someone would care to cover my fuel costs I could be happy to try it on the 800+ mile round trip to Silverstone.
Cheers, Clive.

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Roger King
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Re: Avon tyre sizes

Post by Roger King » Sun Apr 10, 2016 10:01 pm

Hmm... if you run a 4.7 V8 engined car for fuel economy you may be disappointed...

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Re: Avon tyre sizes

Post by Dave Woodward » Sun Apr 10, 2016 10:31 pm

Clive.

Currently I cruise at 3000 or slightly above. I navigate by petrol station names rather than pubs :(
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Re: Avon tyre sizes

Post by Huws » Fri Apr 15, 2016 1:40 pm

Good tyres in the dry. Not much tread, 5.2mm and soon wear out!! but good grip and look the part. :D

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