Query about gearing...

["Slack Alice" Simon]

I'm very pleased with my cheap electronic rev counter someone on here recommended.

It seems to be accurate, or at least, consistent.

The only connection is a bit of wire wrapped around a plug lead.

My top revs, seldom acheived whilst in top, are 3,300  

[Howard Wright]

Hi All

Some initial feedback on iPhone app rev counters....

“Engine rpm” available on the AppStore (£9) was downloaded this afternoon.  It’s not that intuitive to use and the racket from my engine seemed to confuse it to start.  However placing the phone a little way away from the engine and using a long sensing period gave realistic results (800rpm tick over).  I need to repeat the tests but it seems positive.

Cheers

Howard

[Ian Williams]

Of course Andrew, forgive me for not knowing what I am talking about, I will leave the matter in your more than capable hands.

[Alan]

Andrew you are mistaken. At the speeds we are talking about rolling resistance is more significant then aerodynamic resistance. Rolling resistance is very dependant on weight so I fail to see how you can say top speed is determined only by engine power and wind resistance.

[Colin Reed]

HI Duncan,
A Couple things you might like to try have someone sit in the car and depress the accelerator and check that the butterfly valve is fully open,
second do a test for compression on all cylinders with the butterfly valve fully open this will give you a very good idea on the condition of the Engine.
At the very least it will eliminate two things

good luck

Colin 
NZ

[Bob Culver]

Presumably rolling resistance includes all driveline and tyre and idle axle drag losses. With a streamlined heavy modern with wide tyres at 50 mph, rolling resistance presumably dominates. An early fwd Ford Escort at 50 mph was reckoned to need 8 hp "'at wheels." I suppose this means as a rolling road and not at hub so most of would be to counter wind. I presume a laden unstreamlined Seven saloon would need at least as much, so with about 12 flywheel bhp at 50 mph wind loss possibly about matches or somewhat exceeds rolling losses, and increases dramatically with more speed.
One passenger adds about 10% weight absorbing about 10% of rolling resistance power. But a 10% loss of speed would release 30% of power formerly absorbed by the wind, As the two commencing powers around 50 mph are probably vaguely equal speed reduces far less than 7%. I am too senile to solve the exact reduction.

Nevertheless it is surprising how little degradation will reduce a stock lc saloon to about 45 mph as per many original road tests. My RP when running the lc head used to recover about 4 mph from just removing carbon, a considerable gain in bhp.

Back to the original question, a fair idea of engine freedom is available from the crank handle, esp with plugs out.

Altered to ereflect weight of modern passenger

[Tod]

Hi Duncan, 
I had a very similar problem with my special, not reaching the higher rev band just shy of 3k absolute max. really struggling to get 40mph.

Some time ago I had an issue with the car not idling properly, I re set the ignition timing (electronic) and the SU carb, and it idled fine. After changing the rear axle for a late Ruby axle and having run the engine in for a period I found the car really quite sluggish and not achieving the higher revs, as above.

My initial thought was carbonation, tried various needles, settings etc. to no avail. Changed to the standard  Zenith. I ignored the distributor as it was almost new.

However on checking the distributor yesterday I found that a weight anchor pin had broke, but more importantly there was a pin from somewhere holding the weights out (hence the previous poor idling). Therefore whilst the timing was set correctly for idling there was very little or no Advance at higher revs.

I changed the distributor this morning and took the car out for a run. It now revs freely to 4k. I chickened out at 55mph owing to a strong cross wind and a poor road surface.

I hope you resolve your issue soon,
Tod

[Bob Culver]

An article in Autocar 30.7.65 gave the Cd for an RP as .608 (cf .35 for typical modern) The cross section of my compact Javelin (less wide than a Minor!) is 22 sq ft. The Austin is taller and 7 inches more narrow but does not narrow at the topr. Assuming 18 sq ft the air resistance at 50 mph is over 9hp! So air resistance certainly predominates. Would also seem to dispel the oft quoted 10 bhp as max power for engines of those years, a notion which caused a near riot here years ago.

[Alan]

Bob, that’s interesting. I just measured the approx cross section of my RL saloon at 15sqft. That equates to 7.5hp at 50 mph according to your figures. If we assume at that speed wind resistance is half of the total (well it has been of that order on everything’s else I’ve worked on), then we have about 15hp at 3500 revs. Altogether more believable than 10.5.

[Mike Costigan]

If I remember correctly that 'official' 10.5bhp was a figure quoted at 2,000rpm, which probably makes sense.