DutchEdwin Posted October 16, 2009 Report Share Posted October 16, 2009 I'm about to set my engine timing (a P27 form 1955) I know the octane number of gas in 1955 was 68. Now it is 96 or 98. Does this, or anything else because of difference between old and modern gas, effect the timing an engine has to be set to. The service manual specifies 4 degrees BTDC (V8 engine). I looked in the search engine about how to set the timing optimal. What I understand is tune timing until the vacuum on the inlet manifold is max (with vacuum advance disconnected). Did I understand this right? Thanks. Quote Link to comment Share on other sites More sharing options...
Plymouthy Adams Posted October 16, 2009 Report Share Posted October 16, 2009 only after a good power balance of the carb...sensitive tach, vacuum meter is necessary....go to a Chilton manual or online to get a chart on reading the vacuum meter for existing conditions..shows you what to look for..if possible do check your max vacuum advance for accuracy..the specs are in the book...the regular grade octane of today should be in your favor..if your timing is a bit off it will help in "ping control" Quote Link to comment Share on other sites More sharing options...
greg g Posted October 16, 2009 Report Share Posted October 16, 2009 Edwin the major difference in modern fuel over old fuel is that additives have been mixed in to reduce volitility (the tendency of a fluid to evaoprate) this has changed the speed of the flame front of the combustion from bang to whooomph. I guess this is actually benieficial as you get a longer push agains the piston instead of a short explosion. Probably easier on the pistons also. But modern engines also have sensors and electronic controls that get feed back from the process and adjust the spark to take advantage of that. However our older systems aren't as smart and or as agile as the new ones. So the theory is you want to have the sprark ignite the fuel mix before TDC to get it lit off in anticipation of the piston reaching the top so the push can work on the down stroke. But not so soon as to cause preignition. My factory spec is TDC, I am currently running 4 BTDC which has been fine, good dtarting, good power, and decent fuel milage also. If you are checking with the vacuum gauge and the advance tube disconnected you should block that port so it not sucking air. But I do not think you need to worry about disconnecting the vacuum cause it doesn't have any effect at idle. Quote Link to comment Share on other sites More sharing options...
Jim Yergin Posted October 16, 2009 Report Share Posted October 16, 2009 Greg, While reading your post it dawned on me that we have sort of come full circle from the old manual control on the spark advance used on the really old cars. Then you could tune the car to match the driving conditions and the gas you were using while driving the car. The only difference today is that the computer does it instead of the driver. Puts a new light on the concept of "progress." Jim Yergin Quote Link to comment Share on other sites More sharing options...
Norm's Coupe Posted October 16, 2009 Report Share Posted October 16, 2009 Actually, some newer cars have to be adjusted too due to the gas. When my wife bought her new 02 Kia back in 02, it wasn't but a few weeks and we got a recall notice in the mail. The recall was to either replace the sensor or reset it due to a change in the gas. It had to be changed, otherwise it would not meet emission testing according to the notice. After that was done, I really couldn't tell the difference in the way it ran. So........it does change something with the new gas. Quote Link to comment Share on other sites More sharing options...
Plymouthy Adams Posted October 16, 2009 Report Share Posted October 16, 2009 Norm..the recall was probably due to the fact that to introduce the car as sellable they had to detune it to pass EPA..now that it is in place and sold..the car will be tuned to run...this was a serious Ford tactic in the mid 70's as well. Quote Link to comment Share on other sites More sharing options...
P-12 Tommy Posted October 16, 2009 Report Share Posted October 16, 2009 Norm..the recall was probably due to the fact that to introduce the car as sellable they had to detune it to pass EPA..now that it is in place and sold..the car will be tuned to run...this was a serious Ford tactic in the mid 70's as well. Good call Tim! I agree.Tom Quote Link to comment Share on other sites More sharing options...
DutchEdwin Posted October 17, 2009 Author Report Share Posted October 17, 2009 the theory is you want to have the sprark ignite the fuel mix before TDC to get it lit off in anticipation of the piston reaching the top so the push can work on the down stroke. But not so soon as to cause preignition. My factory spec is TDC, I am currently running 4 BTDC which has been fine, good dtarting, good power, and decent fuel milage also. Greg, Thank you for the answer, I am familiar with the theory. But it does not tell me on how to set the timing in that way the burn front has reached it's full capacity just at TDC so the full force of the burn, explosion, can be used when the piston is going down. So how did you discover you timing should be 4 BTDC? Keep in mind, my engine is overhauled completely, I did adjusted the carb to the service manual spec's but did not clean it (winter job). The adjustments of the 4BBL carb was far-of spec so the mixture screws will be far-of as well I assume. So setting the mixture and setting timing both at the right points can/will:confused: influence each other????? Quote Link to comment Share on other sites More sharing options...
greg g Posted October 17, 2009 Report Share Posted October 17, 2009 Edwin, it was just a trial and error deal. I used a vacuum gauge, and rotated the distributor till I got the highest reading at idle. then I snugged the dist down and hit the road. There was a bit of pinging climbing hills, so I retarded the timing till the ping went away. Then I retarded it a bit more as it was having a hot start problem. After that I checked with the light, and it was at 4 degrees before. So maybe set your at 4 and go from there. Quote Link to comment Share on other sites More sharing options...
DutchEdwin Posted October 17, 2009 Author Report Share Posted October 17, 2009 Greg, Seams like rocket science what you did. Me being a rocket scientist...no big deal. Just the way I thought I had to go. Thank you very much for your help. I'm almost ready to start my engine. Building up the fenders and radiator frame now. Next is hooking up wires and hose, fill oil. Then........fingers crossed at the first ignition. Quote Link to comment Share on other sites More sharing options...
jim leman Posted October 17, 2009 Report Share Posted October 17, 2009 I'll add my two cents to what has already been said about this topic. The advice about setting the advance early on flathead 6 mopar engines is good. I am not sure what the factory timing spec is for a '55 motor -- check your manual and set 4 degrees or so advance from that spec. For a P15, which I have, and which has a TDC timing at factory spec, this means a setting of 4 degrees BTDC. A well-studied old flathhead V-8 guy told me that as today's gasoline burns faster (basic fuel quality and ethanol) you want the timing set so the ignition starts just before TDC to give the fuel in the cylinder time do its job...if set at TDC you miss out on the full combustion...the piston will be on its way back down before full combustion is achieved (probably have this a little off as it has been some time since he told me this; did a complete story on this theory and practice in the P15-P24 DRIVER newsletter I used to published; can send you a copy of the article if you send me your email. Proof is in how much better the engines run at 4 degrees or so before TDC. The other advice about being sure to have vac advances disconnected and idle as low as possible while setting the timing is crucial. May also likey have to adjust the points as well, as timing affects the points. Quote Link to comment Share on other sites More sharing options...
DutchEdwin Posted October 17, 2009 Author Report Share Posted October 17, 2009 Jim, My factory specification is 4 deg. BTDC. I would like to have this article. You can send it by just clicking on my name. A window will appear in which you can send an email to my personal mail address. Quote Link to comment Share on other sites More sharing options...
Jim Saraceno Posted October 18, 2009 Report Share Posted October 18, 2009 Point gap change, changes timing Shell, I think you have it correct. If you change the points it will change the timing (as it will change the dwell). If the timing changed the points then AND the points changed the timing it would be and endless procedure to tune your car. Quote Link to comment Share on other sites More sharing options...
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