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MG MGB Technical - dynamic timing for modified engines
|I have two engines in mind regarding this question. Both run on normal points systems|
Engine 1. A '72 car with a 1860cc engine, Fast road cam, ported head, larger valve head type of head(not racing valves), 25D4 dizzy
Engine 2. A 1968 car with a 1950cc engine, probably a normal cam rather than a fast road one. Head not ported but has the larger valve type 25D4 dizzy.
What should be the dynamic timing for these engines, following the dizzy curve specs for either of the dizzies doesn't seem to work too well.
|With today'd fuels, it's difficult to give an accurate answer to your questions. Also, depending on the age and condition of your distributors, figuring out at what point the mechanical timing advance begins and ends is almost impossible to sat. Most people, who set up a performance engine, send their distributors out to be reconditioned and re-curved to the needs of their particular engine. In the US, Jeff Schlemer has a huge following of people who have sent their distributors to him to be re-curved. Your distributors are going on 40+ years old and the advance springs have stretched and the distributor shaft bushings have worn down. This all adds up to a very unreliable ignition system. I had a custom distributor made up for my 1860 engine with a performance street cam, flowed head and a supercharger system. I gave the manufacturer the specifications of my engine and they made the distributor accordingly. I'm running 17 degrees of initial timing and a total of 28 degrees, all in by 3,000 rpms. Hope this can be of some help. RAY|
|In crude terms use as much advance as your engine will take without pinking. But as Ray said, there are many things, including compression ratio, affecting performance. Both these engines deserve, at least a re-built dizzy. Better still a re-curved one and ideally a programable one. Most tuners will aim to get max advance earlier to help acceleration but there are so many variables the only way to get the best is to set up on a rolling road.|
|See my page at http://www.mgparts.co.nz/advice/technical-notes/setting-up-ignition-timing-for-modified-engines-2/|
Thanks to you all for your comments. Very good stuff you have there Paul, will go down that route to start with the 1860cc - saving up my pennies now to buy a new dizzy for that car.
The other has a new dizzy but needs sorting, re springing etc.?? Any recommendations?
Ta once again.
|apart from replace it with a 123 or 123Tune (would normally put from Peter Burgess but it doesn't look like he sells them now?)|
rebuild - Distributor Doctor - http://www.distributordoctor.com/
recons usually last about 3 years before wear sets in again but I'd imagine the Dissy doc ones last longer but you'd have to confirm that with him
|Nice write up, Paul.|
|C R Huff|
A good starting point for intermediate advance with modified engines is 20 degrees at 2000 RPM. This is usually very close to the mark for 10:1 compression on a standard camshaft or 10.5:1 on a fast road camshaft, and for other combinations is close enough that it minimises the chance of damage.
If you want to go further than what I have outlined and adjust the springs yourself as an interim measure, you could close up the hook of the heavier spring to take the (designed in) slack out of it. That should allow you to get the right amount of full advance without the engine also pinking around 2000RPM, and so won't be too far off the mark until you can afford to get the job done properly. The notes on my page about compensating standard distributors for high mileage wear will be a help in explaining aspects of this process even if some of the actual settings not directly relevant: http://www.mgparts.co.nz/advice/technical-notes/distributor-advance-rate-adjustments-compensating-for-high-mileage-wear/
On distributor rebuilds:
The major cause of erratic ignition timing is not (as is very commonly assumed) a stuffed distributor but is in fact far more commonly backlash in betwwen the camshaft gear and the spindle gear. In the B series, the camshaft gear also drives the oil pump and so is significantly loaded - especially when the oil pressure has been uprated. This causes significant wear, sometimes substantial. (A series can suffer but nowhere near to the same extent.)
This backlash can be so bad that it can result in up to 30 deg of timing scatter. 10 degrees is quite common. Many a distributor has been rebuilt in vain trying to eliminate this. The only cure is a new camshaft, and if necessary a new spindle (or unworn second hand one) which results in the timing mark becoming very steady when viewed with the strobe. It also results in better power and torque not to mention better engine life!
|Just done a B on the rollers with a 123Tune.|
Engine +60, Fast Road head 9.75:1 CR, HR270 cam 1 1/2" SUs with K&N filters. Moss Tourist Trophy ex system.
1000 rpm 18 degrees
2000 rpm 26
3000 rpm 29
4000 rpm 31
5000 rpm 32
5500 rpm 32.5
6000 rpm 33
NO VAC ADVANCE
1 degree less everywhere lost 1.5 bhp everywhere. 1 degree more lost 1 bhp everywhere.
Max Bhp 95 at wheels, 25 losses gives 120 bhp at flywheel.
|Peter Burgess Tuning|
Which Carb needles and did you need to "touch" them?
|sorry to go O/T now but I have to do these things whilst I think about them or I forget (like I already have) |
are you now longer selling 123 as I tried to link to the page on your site and it wasn't there ?
|Hi MG Mike|
This is a car to go to Switzerland so I left it on the weak side with ABD needles, I am guessing he will be above the 400ft level of Alfreton most of the time.
I have stopped stocking the 123s as I cannot afford to stock the whole range. I am happy to set em up for folks though. I am down to a couple of side exit capped 123Tunes suitable for midgets, a couple of +ve earth Bs and a couple of MGB replacement ones.
|Peter Burgess Tuning|
I thought it might be along those lines, I'll think about recommending you to dry up your present stock, as you know elsewhere many treat my every word as golden threads of information and advice and follow without question - be prepared to be mobbed (just for knowing of me)
|I kneel before thee oh stuff what legends that are rough hewn from rock are made of.|
Failing that, Carol still does those yummy bacon cobs and I will buy you one next visit.
|Peter Burgess Tuning|
|since when did you start to think you could bribe me through my belly!|
I've been looking and I can't find the post the info about the last remaining 123 would be relevant to
(btw - it might be dough rather than rock)
off I go searching again, not like me to forget
What fuel was it using?
|cheapest leadfree. whatever that is in terms of octane and ethanol %|
|Peter Burgess Tuning|
This thread was discussed between 23/07/2013 and 26/07/2013
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