If there is too much Advance built into a timing curve can MPG go down Especially in city Driving where you are revving through much of the curve?



Greg

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I have noticed with the E254 cam if you go over 34 degrees it feels flat. 30 to 32 degrees is allot punchier and you feel more power, average vacuum is higher resulting in better mileage because you end up getting out of the throttle quicker. I always tune the vacuum pod for full effect all the way then unscrew it 3 turns so it doesn't come on too quick which gives the engine a punchier feel.

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

Yes. Over-advancing the timing through base timing, light springs, or too aggressive of vacuum advance can really kill mileage and performance. I have run into this re-curving distributors for my van and for my brother's van. Too much advance and the motor falls flat on its face, pings, and gets crappy gas mileage.

Yes,

I. as Ted and ohthers have found 32 to be about the limit. Just as surely as with too much retard and for the same physical reason.The energy released from the explosion in the cylinder has to be transferred to the crank at a time when minimum power loss through valves, rotational mass, and volume will occur. It is of course related to flame travel across the piston top so the appropriate timing is almost a one to one relationship to any given engine, because even stockers all vary. That is why each build has to be adjusted in, and compounding the problem of adjusting in is the richness or leanness. A fine summary was recently given a link on the forum, something like timing 101, as I recall. Ya have to think in terms of a point in a graph with 3 axes.

rock
'64d100

Okay Good. This corroborates my findings on my 79 Volare EFI.. Well I don't have to worry about advance Springs and weights and vacuum pods there, I have been dialing back the timing map ad trying to get all of the transitions smooth on the map. My max timing is 32* and sometimes depending on load / rpm I even run up to 2.0* retarded to minimize Ping on 87 Octane.


I have still been playing with the timing as I have had the EFI conversion done for about 6 months. Tuning tuning, tuning is the key.. I had the distributor in a goofy spot for triggering / timing and it was burning up caps and rotors until I figured out the problem.. Then I had to redo my timing map as I had moved the trigger point around which changes timing a whole lot!


Thank you to all contributors!

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One other thing to consider is advancing excessively can result in high CHT's.
Depending on the condition and effectiveness of the cooling system, you may experience high speed cruise overheating when carrying too much advance.

CJ

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Part of Tyrde-Browne Racing

ok, conversely can lack of ignition timing also cause high temps when highway driving ?? Specifically, i discovered my vaccuum advance on the distributor is inoperative and the car gets quite warm after 30 minutes on the highway. and that is with 15-25 degree ambient air temp. Car is a 74 satellite how bad is my actual timing curve ??

anyone ??

Slanty,

Regardless of timing a car interior can get quite warm from a slant if the heater control valve never lets heat go "off", or if the thermal barrier from under the carpet was lost by a previous owner and a replacement carpet put in.

I am not being a wise ass like it may sound. If the problem is excessive water temp, what temp is the gage showing, and how do you know the gage is correct? Also, what is the timing with no vacuum advance? You can actually run with no vac advance, it is just not efficient and doesn't cause overheating. What temp are you shooting for as coolant temp? I like to click along at 175 to 180 degrees, but have seen 210 pulling a load uphill. Retarded timing after top dead center shouldn't cause overheating, mostly because you will eventually get so little power you can't run on flat ground. The engine in such a case is revving higher to get power and then overheating, rather lots of heat is lost in exhaust from delayed firing.

rock
'64d100

Too much spark absent detonation will not raise coolant or cylinder head temperature, but will result in power loss. Late spark, on the other hand, will raise both coolant and cylinder head temp rather quickly.

Mitch

Slanty,

"Warm" is not specific enough. Do you have a proper temp gauge on the car that is known to be good? The factory guage can read "hot" when the engine is not actually hot. You can easily run 200-210 deg on a Slant and not hurt anything. 195-200 is probably a good normal operating temp to shoot for.

Greg and all,

32 deg should be the max advance AT WOT. HOWEVER, under part load cruise you may want as much as 45-55 deg for best economy. I have run mine 45-50 (48 is my default now) on a 225 and done well with economy there with no overheating issues. This a$$umes everything else is in good working order and you are not running too lean.

I can't see why you would need to go down to -2 anywhere on a stock compression motor and 87 octane to avoid pinging, unless you are not keeping it cool or some other fault like excessive carbon in the cylinders, lean mix, or too hot a plug.

Lou

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Lou,

This is not exactly a stock motor It has .100" off the head With 1.70 and 1.44 valves.. I think this head had a small chamber 'cause I cannot always get away with 87 espically in the warmer months.. Oh yeah cam is still stock. I also Have some cold plugs in there now too. NGK 9's.. 5 Is the stock heat range for a slant.


Greg

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Greg-

Last year I was playing around with distributor curves when I was setting up my brother's van. I mapped out stock distributor curves in a 76 Aspen wagon distributor and a 74 Valiant sedan. Here they are:

76 Aspen distributor mechanical advance curve:
Factory base timing = 2BTDC

500 RPM = 2BTDC
750 RPM = 8 BTDC
1000 RPM = 15 BTDC
1400 RPM = 20 BTDC
1850 RPM = 20 BTDC starting to get twitchy, +/- 1 degree
2100 RPM = same as 1850
2400 RPM = same as 1850

My under-hood tach runs out at 2500 RPM on the 6 cylinder scale.

Looks to me like this distributor relies on vacuum advance to get the final ten degrees or so of advance.

73 Valiant mechanical distributor curve test
Factory base timing = TDC

600 RPM = TDC
750 RPM = TDC
800 RPM = 2BTDC
900 RPM = 8BTDC
1100 RPM = 11BTDC
1350 RPM = 20BTDC
1500 RPM = 22BTDC
1550RPM+ = 22BTDC

Maybe those will help you rough out a spark map for your project. These number are with the van in park, if that makes a difference. Those are the stock springs, governor, etc. with no vacuum advance. I know there aren't many data poiunts to plot on a graph, but that's all I got for you. Hope it helps.

And then add water injection to the mix!

Is that a stock cam from 70-74?

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'79 Maxivan 360 Offy Qjet Comp RV cam/rusting in the driveway.
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2005 Golden Couch Buick

Well in my case the gauge is a fairly accurate(?) Auto Meter temp gauge. And my definition of "warm" should actually be "hot" because I am talking pegged at 250 degrees after 30 minutes on the highway and then not cooling down for about 20 minutes after getting off the highway(lower speeds). Last night it occurred to me: what if I have a missing or broken centrifugal advance spring ? This would surely cause over-advance at highway speeds correct ? dave