Timing Questions/Opinions???

[horseballz]

Hi Folks,
First, here's the combo:

>Late Explorer 5.0
>GT40-P heads
>B303 cam
>1.7-1 ratio roller rockers
>Mallory 57 series magnetic breakerless vacuum advance distributor
>Performer RPM intake
>Autolite 4100-1.12 carburetor
>Tri-Y headers
>T5 transmission

Here's the timing story/questions:

>At 800 RPM idle, vacuum disconnected/plugged 12 deg BTDC
>Cent. advance starts at 1000-1100 RPM "all in" +24 deg at 3100-3200 RPM giving 36 deg BTDC before adding vacuum advance.
>Vacuum advance adds 12 deg giving total "all in" timing 48 deg BTDC at 3100-3200 RPM.

Engine seems to run quite well like this, but 48 deg BTDC seems like a dramatic amount of advance. No spark knock/ ping or kickback on the starter cold or hot. Vacuum for advance is from manifold source (14-15 in. at 800 RPM) as the vacuum advance operation seems erratic when connected to ported source. I followed some advice to set at 34-36 deg "all in" (initial+cent+vacuum) which gives 0 deg initial and car runs like poop. It seems like engine could even handle a bit more timing, but that seems excessive. What do you all think? I don't want to cause any damage, but I also want the engine to run at it's optimum. Any and all thoughts, comments and opinions are appreciated.
Thanks,
Gene

 

[RebStew]

At idle you should pull a reading of 15 degrees. This should give you 34 totle. Or pull the advance, hold the gas open to 3K and set at 34. after 34 totle they loose power on a SB Ford.
If this seems like it's not working right maybe check your balancer and make sure your timing marks are true.

 

[horseballz]

RebStew,
Thanks for the reply, but check the math. 15 degrees initial + 24 degrees centrifugal = 39 degrees total before adding the vacuum advance of 12 degrees which would add up to 51 degrees total at 3100 RPM. I've checked for TDC and it's right on the 0/TC mark. I guess what I'm trying to find out is if I should limit my amount of total centrifugal advance or the total amount of vacuum advance or a little bit of both to end up with 34-40 degrees total BTDC.
Thanks again,
Gene

 

[D.Hearne]

RebStew,
Thanks for the reply, but check the math. 15 degrees initial + 24 degrees centrifugal = 39 degrees total before adding the vacuum advance of 12 degrees which would add up to 51 degrees total at 3100 RPM. I've checked for TDC and it's right on the 0/TC mark. I guess what I'm trying to find out is if I should limit my amount of total centrifugal advance or the total amount of vacuum advance or a little bit of both to end up with 34-40 degrees total BTDC.
Thanks again,
Gene

You don't add the vacuum advance in the total. When you're at or near WOT, the vacuum signal decreases, thus whatever advance was added disappears.

 

[horseballz]

So, if I'm reading you guys correctly, 34-36 degrees for total including initial and centrifugal/mechanical advance is in an acceptable range and possibly as much as 40 degrees as I've seen posted several times? If this is the case, how much vacuum advance should I be looking to put on top of that for mileage purposes? This is my first experience with a semi-performance oriented engine and I just want to confirm that my "old school" knowledge, experience and techniques can still apply. For example, on almost any stock engine, I generally set the timing by advancing until I get a kickback on starting or a slight ping/spark knock under load and then retard timing just enough to eliminate the ping or kickback. Is this a generally acceptable way to set timing for a modified engine? I appreciate your willingness to help inform the woefully uninformed.
Thanks Again,
Gene

 

[brianj5600]

Distributor advance

 

[D.Hearne]

Run as much timing as the fuel you're using will allow.

 

[horseballz]

brianj5600,
Thanks for the link! Very clear and concise information. My big mistake has been trying to figure the vacuum advance into the TOTAL! I'm on the right track now and all should be good. Thanks to all for the thoughts and replies.
Happy Motoring,
Gene