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J A Harriman
Unregistered guest
Posted From: 203.54.32.95
Posted on Saturday, 03 June, 2006 - 19:34:   Edit PostDelete PostView Post/Check IP

1950 Silver Dawn SBA 74
Have any members had experience in regrinding the camshaft to improve performance without sacrificing the smooth running quality of this type of engine. Why is there no valve overlap in this camshaft design. Any imput to this question would be greatly appreciated.
John A. Harriman.
Melbourne

(Message approved by david_gore)
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Arthur Francis
Unregistered guest
Posted From: 202.67.65.166
Posted on Sunday, 11 June, 2006 - 09:24:   Edit PostDelete PostView Post/Check IP

John,
What is the result you are seeking?
Is it more bottom end,more mid range or more top end power?
I went through all of these deliberations with a six cylinder Holden and in the end changed the cylinder head (larger valves and modified porting)and fitted extractors and a "sports" exhaust.I had a drive of a car with similar mods plus a 25/65 cam rather than the 20/60 fitted to mine and it was a pig to drive in traffic.
The Silver Dawn is a very sweet running,smooth and reliable car.Whilst a 4 1/2 Mk VI is more powerful there just isn't a huge power increase to be gained by modifying the Dawn.
Mk VI,R Type and Silver Dawn are slow cars by todays standards.
The late Silver Cloud with the 2"SU carbys and 8:1 compression comes closest to todays expectations of performance.
My advice-leave it alone.



(Message approved by david_gore)
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Richard Treacy
Grand Master
Username: richard_treacy

Post Number: 1011
Registered: 4-2003
Posted on Sunday, 11 June, 2006 - 22:40:   Edit PostDelete PostView Post/Check IP

Look no further than to this RROC(A) forum, searching for Mild Engine Tuning, and you will see all !

http://au.rrforums.net/forum/messages/17/2395.html?1109009980
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John A Harriman
Unregistered guest
Posted From: 203.54.36.174
Posted on Tuesday, 13 June, 2006 - 12:52:   Edit PostDelete PostView Post/Check IP

Dear Arthur and Richard thankyou both for your imput, Richard you have provided me with a wealth of interesting information Regards John A Harriman

(Message approved by david_gore)
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Norman Geeson
Unregistered guest
Posted From: 80.3.128.7
Posted on Tuesday, 20 June, 2006 - 00:07:   Edit PostDelete PostView Post/Check IP

John
There is valve overlap in this camshaft design, in fact there is overlap on all the post war six cylinder camshafts.I presume you have made the common mistake of reading a static valve timing diagram.
On a RE 3672 Dawn Camshaft, give or take the odd degree, Inlet opens 22 BTDC. Inlet closes 84 ATDC, Exhaust opens 65 BBDC, Exhaust closes 27 ATDC. Valve overlap 49 degrees compared with a later S1 4.9 Engine of 50 degrees.These assume the crankshaft damper is working and it is not stuck off centre.

A camshaft change on 4.25ltr Dawn is going to make barely any difference, although the later Bentley or Silver Cloud camshafts would help.In addition in the case of a RE 3672 Dawn camshaft forget about trying to reprofile one, the base circles are already too small and the contact speed of the cam and tappet increases.

For comparison purposes the 4.25 ltr Dawn is actually more powerful up to 2800 rpm than the original small carburettor Bentleys. R-R actual BHP figures not mine!

The problem with the 4.25ltr lies with the cylinder head and is one reason why R-R never increased the compression ratio.The most powerful increase would be gained by fitting a Continental exhaust.Incidently what might work on Richards late R type will not necessarily work on your engine, in fact you can put money on it.
The cylinder head combustion chamber is different, the inlet tracts are different, the compression ratio is different, the bore is different, the carbs are different and the exhaust is different.

Having said all the above I suggest leaving it alone and going out and swapping it for a R type after chassis number B2TN. You will then have a car, which will perform if that is what you require. Should that not be sufficient raise its compression ratio to 8.0:1 fit a R type Continental rear end and say goodbye to Clouds of any specification including Continentals.

Best Regards

Norman Geeson







(Message approved by david_gore)
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Richard Treacy
Grand Master
Username: richard_treacy

Post Number: 1020
Registered: 4-2003
Posted on Tuesday, 20 June, 2006 - 18:33:   Edit PostDelete PostView Post/Check IP

I must endorse Norman's comments thoroughly.

I stress that the work I did, mainly increasing the overlap at the camshaft to 57 degrees and increasing the lift modestly, was purely experimental, and definitely not in any way supposed to be a recommendation to anyone. You could call it all frivolous tinkering, but to be fair it was using my second spare camshaft, spare tappets contour ground to match, and third spare cylinder head, is completely invisible, and is completely reversible in a few hours. More than anything, I was curious at why there had been so many valve timing/overlap/lift variations implemented on these motors, and wished to explore the sensitivities a little on the computer models and in practice. I have only left it that way because the result, probably mainly by luck, has been rather advantageous.

I probably would have achieved far more by working the exhausts further, but a wholesale change there can be a bit too in-your-face. I settled for a fully austenitic system which has greatly improved flow but looks completely original, apart from a subtle flexible bellows on each downpipe to eliminate any possible manifold stresses. It sounds right too, albeit a little quieter. It even has the original external wadding and covers for the four mufflers.

RT.

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