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MG MGB Technical - cam timing
|Hi all, I've been away from the forum for a while but am now back into getting best performance from my renovated roadster. I rebuilt the engine with a Peter Burgess fast road head and HR270 cam. I was never particularly happy with how it ran. Having an incorrect distributor probably didn't help. However I have now fitted an Aldon Amethyst unit with not much improvement! So I'm back to checking basics. Cam timing I believe for this cam is 108 degrees ATDC. Checking back through my posts on here I achieved between 106 and 106.5 degrees. I have confirmed this today over several measurements including checking on no.1 and 4 cyclinders. I fitted a 4 degree offset key when I originally set up the cam. As I have the radiator out at the moment will I gain any benefit in trying to get the cam timing spot on or even a bit advanced as was suggested at the time I fitted the cam. Should I go for say a 6 or 7 degree key. I need to get to Peter's for a RR session (something I've been talking about for years!) but want to make sure I have got the basics right. After spending a lot of time and not inconsiderable amount of money on this engine (rebore, new pistons, new crank, fast road head, hotter cam) I am not that happy with how it goes! I was questioning whether I had been measuring correctly but it doesn't seem to be rocket science (put degree wheel on pulley, zero it at TDC, rotate engine clockwise until no.1 intake valve fully open, read off degrees. I did use a dial guage to confirm max lift position). Most other things on the engine can be adjusted by Peter but basic cam timing has to be right.|
|Steve. How did you determine when the valve is fully open? That, too, requires the use of an accurate dial indicator. If you did use a dial indicator, measuring from .001" before fully open until the indicator again shows .001" less than the fully open position, your "full open" position would be half way between the two degree readings. |
I would be interested in your expansion of your remarks about why you are not satisfied with this engine and what problems you are having. Cam timing is important, but may not be the correct, or the sole, diagnosis.
Nice to hear from you again. Les
|Steve, the cam is advanced at 106 degrees so it is fine. We have seen these run from 105-109 and they run fine as they are so mild. You really need to get it on the rollers or at least fit an afr lambda sensor so you can sort the fuelling. As long as you have checked the valve timing as Les describes you are fine.|
|Peter Burgess Tuning|
Yes I did just that but used 5 thou before and after as my reference point. I was turning the engine clockwise for all measurement to remove the effects of any play. I did measurement on no.1 inlet valve then on no.4 as a check.
Thanks for that. I'll button it all up again (rechecked the valve clearances as well). I just wanted to get the "difficult to sort" bit out of the way before coming to see you. Probably in the spring. I believe you said you were happy to tinker with an Aldon Amethyst unit.
As long as you bring your laptop up Steve and we will work with you to'mess around' and see what happens. What sort of 'curve' have you installed and is you base 0 degrees? I guess I am trying to say, what is the 'actual' curve you are running if taking readings from TDC and measuring timing offset with adjustable timing light, rpm only not vac advance.
|Peter Burgess Tuning|
yep will have laptop with me. I put in a curve based on some figures you quoted to another poster with a similar spec engine. I've not had time to play around with any adjustments yet. I'll do some specific timings later. I have a new distributor and have strapped down the weights as per Aldon's setup proceedure.
I have just discovered a leaking brake master cylinder so need to sort that out first. Modern day rubber isn't what it used to be!!
I have similar setup but with Peter's Econotune. I set the cam at 108 and my max torque is at about 2750, max power around 5000 which then plateauxs. My understanding is that timing the cam over or above the recommended figure is unlikely to affect the magnitude of output, other than to shift where it occurs. This is what I have read but Peter is the man to confirm this or otherwise.
I also have been running with amethyst and Peter's rollers did a great job on mine. I was running with far too much timing - everywhere.
What curve are you using? When did you get the amethyst? I have recently spent some time with Kevin, the guy who developed the amethyst - he came around to my house and put the oscilloscope on the car to do some checking.
If you want a chat about it, my email is firstname.lastname@example.org
Thanks for the post. I'll contact you off site later.
This thread was discussed between 04/12/2015 and 07/12/2015
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