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MG MGA - Vacuum advance causing overheating?
|Does the MGA with its “ported” vacuum advance take off cause some of the overheating issues at idle, would it be better if the take off point was “direct” into the manifold?|
I raise this issue after reading the following article.
|Depending on the dizzy you have the max vacuum advance should be no more than 13 or 14 degrees. Assuming you static or dynamic timing is correct, that shouldn't give you issues. Vacuum take off at the manifold gives more idle advance. The later R/B B dizzy actually had more advance at idle and a manifold take off.
If you have over-heating issues check the mixture isn't too weak, but if you have a mechanical fan, convert to electrical. Mechanical, engine driven fans, move the least volume of air when you need it most; i.e., when crawling or stationary.
|Too much advance can cause overheating through pre-detonation (aka pinking)
If your distributor springs are worn out, then you will have unpredictable timing and all the issues that will create.
I find most overheating issues are down to one of the following
a) gunked up cooling passages,
b) using book settings on timing - which have no relevance to modern fuels c) running the mixture too lean,
d) worn out distributors and resulting timing issues
e) wrongly specified radiator cores
A correctly set up MGA with all the parts above being as they left the factory doesn't overheat - and I don't mean original but worn-out and full of sixty years worth of gunk
|I don’t have any overheating issues. I raised this topic as maybe a way of reducing overheating for those who have trouble when idling in heavy traffic where pinking isn’t issue (no load). Does the MGA design give you the maximum advance at idle, but non-pinking advance at other loads/speeds? Has anyone ever changed the vac advance takeoff to the manifold and did it make a difference?|
|MGA with ported vacuum gives zero advance at idle, large advance as soon as you touch the throttle (good when the traffic light turns green), zero advance again when you floor it, appreciably good advance when cruising at half throttle (good for fuel economy and least heat), zero advance when you floor it again, and zero advance off throttle on overrun.|
I can see your question and probably think that yes the engine would possibly run a tiddle cooler with the advance up a bit from std. at idle but if there is an overheating problem I don't think it would be enough to make much difference really
"Mostly" it was automatic cars that were hooked up this way to give them a slightly stronger idle
Personally, I don't like it direct to the manifold
With a conventional setup, there is an increase in advance just off idle as Barney pointed out and this helps that initial increase in zoom as you get underway, but with it hooked to the manifold direct you are already on max. vac. advance and as you push the throttle to move off the opposite happens and you loose a bit of vac/timing instead
I sorted a flatspot with a mate's Corvette that he had coverted from auto to manual and that was what it ended up being-As soon as we went back to normal ported vac. supply and reset the idle speed like that it went fantastic without the dive when taking off
I'm a bit of a hater of aftermarket electric fans as well and prefer a steel flexi fan or a viscous coupling if there's room
|So Barney wouldn’t a direct connection give you the same results but with the added advantage of more advance at idle (and less heat)?|
|Thanks Willie - I had posted my query to Barney before I saw your reply - I think you have answered my query.|
|This is going to get somewhat contorted. On late model MGB when they used manifold vacuum, the distributor had a vacuum retard mechanism. That is, vacuum was applied on the opposite side of the diaphragm. High manifold vacuum at idle gave maximum retard, which is where you set the appropriate idle timing.
When you hit the throttle the manifold vacuum dropped, and the retard was reduced, giving you spark advance moving off the line. That much was okay. But cruising at half throttle brought back some of the retard, so spark timing was not so much advanced at cruising speed, not so good for fuel economy. When you would floor it for power, manifold vacuum would drop, and spark advance would increase, which is not what you want for full throttle operation. To make that work I think the the mechanical advance had a rather short limit. There was also a TCSA switch (Transmisson controlled Spark Advance, or something like that) that would disable vacuum to the distributor unless it was in 4th gear (or 4th and overdrive). That would also cancel out most of the effects noted in the first half of this paragraph.
Take this with a grain of salt, but I don't think you want to go anywhere near that sort of setup for an MGA.
I've always believed that the later cars with vac. retard at idle used that setup so they ran retarded at idle so that the throttle had to be further open at idle to keep the exhaust flowing faster/hotter to keep the cat. converter working
Is that how you see it or am I up the creek
|I dunno. I wasn't the guy who did it. I just know it had to do with emissions, and not the best for performance.|
This thread was discussed between 01/06/2018 and 02/06/2018
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