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MG MGB Technical - Over-advanced but not pinging


Happy (almost) New Year to all Listers-

My recently rebuilt engine was set to 14 BTDC, vacuum unplugged, proper working centrifugal advance,
etc. Since I can't keep myself from playing around with the engine, I got out my vacuum gauge, advanced to
max vacuum, rpm, and smooth idle. Now I am at 22 BTDC-way beyond spec, but absolutely no pinging
whatsoever. Is there any disadvantage to a higher advance if you have no pinging? Isn't it best to go for
the max? She runs great, lots 'o power and cooler operating temp. Engine was completely redone with new
cam, slightly larger bore, SU's with K&N's, 25D distributor, detoxed.

Mike King
'69 BGT
Mike King

I've read the archives over and over and I've been afraid to do this but here goes:

I have an 1800 3main not bored-out, balanced with mga carbs and flywheel lightened by 6lbs and 9:1 compression. Head is diy ported w/3 angle grind. Have an mga dizzy. This is to be a street engine.

1. My local mg specialist who is a champion E prod scca mgb racer and mechanic said to grind a bit off my 10 degree advance plate to give about 16 degrees. He also said to not use the vacuum advance. I may have misunderstood him, any comments.

2. I've read about 7* btdc, 16*, 22* and more. I don't know what to do. With the advance plate at 16* as it is, do I set it static to 7*btdc to begin? With an additional 16* mech adv will I have 23*btdc?

3. On the dist vac unit, if I use it, what do I use for a start point re: the knurled adj nut. There are lines on the vac body, do I start in the middle or at one end of the scale?

4. I have a harmonic pulley, and a timing cover with the timing pointers on the bottom. Who knows if they match? Are different pulleys notched for the top pointers vs the bottom pointers?

5. Now that the head is on, I get all kinds of readings for tdc. How can I be more accurate other than putting a pencil down the plug hole and guessing tdc?

6. I'm not sure how to proceed

Funny thing is, on my current 1500(now out) with the cracked block and #3 very low on compression, I just turn the dizzy till it runs well!

Fred Horstmeyer

Hi Mike, I've been watching and hoping some of the experienced guys on this list would chime in on your question. I've played around a little with the timing on my on B and noticed some odd things. I tried the max vacuum thing too and noticed I ended up with timing quite a bit more advanced than I expected and was comfortable using, so I didn't. I also tried the pinging method: 30-50mph runs in top gear, keep advancingthe timing until pinging is just heard under these conditions. I could never get the engine to ping, even after advancing till the performance fell of noticably. I finally figured out it was the fuel here. If I used 91, 92 or 94 octane it would never ping no matter what I did to the advance. If I put in 89 octane the engine would ping and run on if the timing was too advanced, so I would use that to set my timing and then drive it with 92.

If your car is running well with the current setting, sounds like you're good to go. You might want to double check the vacuum advance is working, and I'd be tempted to try some time trials (30-50mph in top gear) to compare the stock timing against your current, just in case it is too advanced.

But the real point is to hear what the best tuners here do to set the optimum timing - hopefully they'll speak up soon :).
Mike Polan

Fred,

To find TDC, screw something long enough to interfere with engine rotation into the spark plug hole and mark the pulley at the point where the engine stops. Do this by hand, not on the starter. Rotate the engine in the opposite direction until it stops and mark that location on the pulley. Halfway between the marks is TDC.

HTH,
George B.
George B.

I'd also like to see some expert comments on this. My '78 with a DCOE is currently set at 20 BTDC at idle w/o vacuum advance connected. At 10 deg., low end power was non-existent. At 15, it was better. At 20, it runs great.
Joe Reed

Mike - The method you are using has been used for years on various cars. I have the write up on using it from a Hastings engine rebuild booklet and the following is the correct way to use this method:
Connect an electronic tachometer and a vacuum gauge to the engine. Set the engine speed to 1000 r.p.m. with the idle speed adjusting screw. Loosen the distributor mounting screws and slowly rotate the distributor back and forth until the highest vacuum reading is obtained on the gauge. If the engine speed changes, readjust to 1000 r.p.m. After the highest vacuum reading has been obtained, rotate the distributor in a retard direction until the vacuum gauge has dropped exactly one-half inch and then tighten the distributor locking screws.

I have used this method for some time on both my MGB and TD and have had great results with it. Bottom line, don't question success.

Cheers - Dave
David DuBois

I used the vacuum gauge method as well and it works great. I'll have to do it again real soon since its emission test time. The trap I think some people fall into is that they look at what the factory books say when their engines are far from stock. My engine is 1950cc, big valves, lots'o'port work, HS6s, full mechanical dizzy, etc., etc. Is it going to run right on factory settings? Hell no! You don't have to worry about holing pistons and so forth until you're around 35-40 degrees of advance. I'm not at all sure about those figures so anyone is welcome to elaborate/correct.
Wade Keene

What I am doing to set the timming on motors is set the timming for total advance, this is were you get the best power & gas mileage. I went to sears & got there timming light that has a dial on the back that you ajust after you run the motor up to say 3,000 to 3,500 r.p.m. were the adv is maxed out & set the timming to 36-38 deg. adv. with the vac. hooked up (make sure it is working). With setting the motor this way you will not swell a piston or burn a hole on the top of the piston (triumph cars & M/C.s) If it has 10 or 20 deg. adv. at idel I do not care, I don't want a motor comming apart! I see a lot of motors with a lot of miles on them & the springs are getting tired & thay have 20 deg. timming at 900 -1,000rpm. I got my timming light 6 years ago & thay were $60-$70???? A lot cheaper than a motor! I have seen were the late dezzy has had 60 deg. adv. with no vac. hooked up!!!! & I will take it apart & pull the adv. cam out that is marked 18-19 & put in a 13 deg. (or weld 1/8" or more on the adv. stop) to get it to stop at 38 deg at3,500 rpm & 10 to 15 deg at 1,000 rpm. The vac adv. is your fule mileage!!! It is 5 to 8 deg. when you are at a steady speed, when you mash the gas the manifold vac. falls off & so the vac. adv. drops off & the timming back off were it will not ping. I have seen were no vac. adv. has cost 4 to 5 m.p.g.
Glenn Towery

To find the exact tdc with the head on I crimped/soldered a thin probe to and alligator clip and then clipped it to the tip of my dial indicator. Got the proper angle through the spark plug hole and voila.

Thanks for the help guys!

Fred
Fred Horstmeyer

I always used to adjust my A-series timing and mixture to the vacuum gauge and got noticeably better power and economy than when adjusted to factory specs. However doing the same on by standard B ended up with it being almost to factory specs anyway, so I don't both now. The greatest advance you can get without pinking/pinging, minus a little bit more, the better. The centrifugal and vacuum advance mechanisms attempt to do this but very crudely and have to err on the side of caution i.e. retarded, so you may only get an ideal timing at one or two points on the curve. With modern electronic systems if you suddenly tramp on the loud pedal you can often hear it pink quite loudly, gradually reducing as the computer backs off the timing until the knock sensor stops triggering.
Paul Hunt

'lo Mike,

I run a standard 25D disi with about 22 degrees advance at 800rpm idle. The curve's not quite right so it'll knock like a mofo if I plant the throttle below 2000 rpm, but above that it just flies and I get great fuel economy. When idling in traffic even on a hot day, the engine actually cools down!

I'm sure if I could be bothered I'd build electronic ignition with a knock sensor for it, but I can't be bothered so I won't :o)

ttfn & good luck,
--
Olly S
Oliver Stephenson

The amount of advance by which an engine can use is limited mainly by the CR and the fuel used. EG in a race engine that has optimum CR to the fuel octane it is using one would expect to see a maximum advance of 28/30 degrees at revs above 4K. This is demonstrated by an earlier post regarding pinking using 89 octane fuel. At all other points when the throttle may not be wide open then the timing of an engine can be advanced considerably in an attempt to decrease fuel consumption. It follows therefore that if the CR of an engine is such that it allows advancing the timing beyond that stated above then things should be fine to a point that performance would suffer, I have to admit at not knowing what that figure may be.
The problems with CR and overall timing can be compounded by the selection of the cam used, effectively the actual CR will change dependent upon RPM and the cam used and generally any cam will have its highest effective CR at about 2-4K rpm and naturally that is the point under full throttle and full load that pinking will be heard.
I suppose a rule of thumb would be that an engine with a high CR should generally use a dissy with less mechanical advance and be set so that the maximum advance generally measured at about 4K should be no more than 30 degrees and a "normal" engine would at my best guess be happy with a figure of about 36/38 degrees assuming no detonation can be heard at lower revs and full throttles.
On a rolling road an engine can be run at various RPMs and the distributor advanced or retarded to find the optimum performance at that point, thus a distributor of correct curve could be found. that without doubt is the best way to go with any engine that has been modified.
Bob

This thread was discussed between 30/12/2002 and 02/01/2003

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