Timing Question
Thread Starter
Melting Slicks
Joined: Mar 2006
Posts: 3,342
Likes: 7
From: Connecticut
Timing Question
Hey all
Since I had the top end of my engine redone a few weeks ago I have not had much of a chance to drive it let alone check/change settings due to a busy schedule
I just came back from a ride and checked the timing
12* initial and 36* @ 3,000 RPM rather than the suggested 2,500-2,700 RPM range
Is this critical?
It starts to break up and pops some (I believe through the carb, ) at about 53-5,500 RPM
I also have not had the time to re-Jet the carb... I am running a Qj with 73/44 and DA secondary rods
I had Emailed Lars my new engine specs and he recommended 74/43 -DA which I will try and get done tomorrow
My question is about the curve since thats where Lars always starts his tuning.
How critical is 36* @ 3,000 compared to the recommended 36* @ 2,500 to 2,700 RPM ?
If it is a problem would it require a lighter spring setup?
Thats what I'm thinking... I am simply looking for a starting point
Thanks
Bob
Since I had the top end of my engine redone a few weeks ago I have not had much of a chance to drive it let alone check/change settings due to a busy schedule
I just came back from a ride and checked the timing
12* initial and 36* @ 3,000 RPM rather than the suggested 2,500-2,700 RPM range
Is this critical?
It starts to break up and pops some (I believe through the carb, ) at about 53-5,500 RPM
I also have not had the time to re-Jet the carb... I am running a Qj with 73/44 and DA secondary rods
I had Emailed Lars my new engine specs and he recommended 74/43 -DA which I will try and get done tomorrow
My question is about the curve since thats where Lars always starts his tuning.
How critical is 36* @ 3,000 compared to the recommended 36* @ 2,500 to 2,700 RPM ?
If it is a problem would it require a lighter spring setup?
Thats what I'm thinking... I am simply looking for a starting point
Thanks
Bob
Drifting
Joined: Sep 2006
Posts: 1,986
Likes: 21
From: Columbus MN
Similar problem - solved
I had similar symptoms with my GMPP 350 HO with a Holley 80457 600 cfm carb, non-adjustable vacuum advance on a SHO HEI distributor. It popped and stuttered every time with medium to hard throttle until it seemed to "catch up". I didn't have these problems until I put full length headers on. I think the extra efficiency on the back end put new demands on the front end.
After 4 advance curve experiments, I found 2 light mechanical advance springs work best. I also increased the initial advance to 12 degrees first, then 16, and now it's at 18 deg btdc. The 38 degs of mechanical advance are all in at about 2500 rpm and another 20 deg of vacuum after that with absolutely no ping.
I also changed the accelerator pump cam to be more aggressive and longer squirt. The total difference is AMAZING. Smooth and responsive, but it just doesn't seem to be at peak yet.
I am going to increase the primary jets from 64 to 70. I also have a larger power nozzle (accelerator pump squirter), I don't think I will need that.
12 deg btdc initial and 36 at 3000 both seem low to me.
After 4 advance curve experiments, I found 2 light mechanical advance springs work best. I also increased the initial advance to 12 degrees first, then 16, and now it's at 18 deg btdc. The 38 degs of mechanical advance are all in at about 2500 rpm and another 20 deg of vacuum after that with absolutely no ping.
I also changed the accelerator pump cam to be more aggressive and longer squirt. The total difference is AMAZING. Smooth and responsive, but it just doesn't seem to be at peak yet.
I am going to increase the primary jets from 64 to 70. I also have a larger power nozzle (accelerator pump squirter), I don't think I will need that.
12 deg btdc initial and 36 at 3000 both seem low to me.
Le Mans Master
Joined: Apr 2004
Posts: 7,106
Likes: 38
From: Newark DE
Bob
your 36º all in at 3,000 rpm is fine. That's how I have my '65 set up.
Bringing it in a little sooner such as 2500-2800 rpm may give you a little bit of extra performance but bringing it in slightly later as yours is is just fine and is giving you an extra degree of safety from possible detonation - the sooner you bring the timing all in the more performance you will usually see but you also increase risks of detonation.
I prefer to trade off ragged-edge performance to a point that gives perforamnce and safety for my motor.
If Lars comes on and disagrees than listen to him, but the way I learned it is that the sooner your timing comes in the more performance, but also the more you increase detonation risks, therefore I chose a middle ground setting on my car.
Besides, think about it - between 2700 and 3000 rpm you are only talking 300 rpm difference anyway.
I will add though that another factor that made me choose 3,000 rpm setting on my '65 for safety against detonation was the sidepipes. They are loud enough that it makes hearing faint pinging or knocking pretty impossible so to avoid the risk of there being detonation I wasn't able to hear I played it more on the safe side on my timing settings.
This is just my opinion and others may vary
your 36º all in at 3,000 rpm is fine. That's how I have my '65 set up.
Bringing it in a little sooner such as 2500-2800 rpm may give you a little bit of extra performance but bringing it in slightly later as yours is is just fine and is giving you an extra degree of safety from possible detonation - the sooner you bring the timing all in the more performance you will usually see but you also increase risks of detonation.
I prefer to trade off ragged-edge performance to a point that gives perforamnce and safety for my motor.
If Lars comes on and disagrees than listen to him, but the way I learned it is that the sooner your timing comes in the more performance, but also the more you increase detonation risks, therefore I chose a middle ground setting on my car.
Besides, think about it - between 2700 and 3000 rpm you are only talking 300 rpm difference anyway.
I will add though that another factor that made me choose 3,000 rpm setting on my '65 for safety against detonation was the sidepipes. They are loud enough that it makes hearing faint pinging or knocking pretty impossible so to avoid the risk of there being detonation I wasn't able to hear I played it more on the safe side on my timing settings.
This is just my opinion and others may vary
Race Director
Joined: Dec 1999
Posts: 19,610
Likes: 778
From: Forked River NJ
Originally Posted by BarryK
Bob
your 36º all in at 3,000 rpm is fine. That's how I have my '65 set up.
Bringing it in a little sooner such as 2500-2800 rpm may give you a little bit of extra performance but bringing it in slightly later as yours is is just fine and is giving you an extra degree of safety from possible detonation - the sooner you bring the timing all in the more performance you will usually see but you also increase risks of detonation.
I prefer to trade off ragged-edge performance to a point that gives perforamnce and safety for my motor.
If Lars comes on and disagrees than listen to him, but the way I learned it is that the sooner your timing comes in the more performance, but also the more you increase detonation risks, therefore I chose a middle ground setting on my car.
Besides, think about it - between 2700 and 3000 rpm you are only talking 300 rpm difference anyway.
I will add though that another factor that made me choose 3,000 rpm setting on my '65 for safety against detonation was the sidepipes. They are loud enough that it makes hearing faint pinging or knocking pretty impossible so to avoid the risk of there being detonation I wasn't able to hear I played it more on the safe side on my timing settings.
This is just my opinion and others may vary
your 36º all in at 3,000 rpm is fine. That's how I have my '65 set up.
Bringing it in a little sooner such as 2500-2800 rpm may give you a little bit of extra performance but bringing it in slightly later as yours is is just fine and is giving you an extra degree of safety from possible detonation - the sooner you bring the timing all in the more performance you will usually see but you also increase risks of detonation.
I prefer to trade off ragged-edge performance to a point that gives perforamnce and safety for my motor.
If Lars comes on and disagrees than listen to him, but the way I learned it is that the sooner your timing comes in the more performance, but also the more you increase detonation risks, therefore I chose a middle ground setting on my car.
Besides, think about it - between 2700 and 3000 rpm you are only talking 300 rpm difference anyway.
I will add though that another factor that made me choose 3,000 rpm setting on my '65 for safety against detonation was the sidepipes. They are loud enough that it makes hearing faint pinging or knocking pretty impossible so to avoid the risk of there being detonation I wasn't able to hear I played it more on the safe side on my timing settings.
This is just my opinion and others may vary
I know this is only about the 3rd time we agree on something. 
Instructor
Joined: Oct 2005
Posts: 143
Likes: 0
From: Independence MO
If it is popping at high RPM you are LEAN!! Jet it up 2 sizes and see what happens. You need more GAS!! Keep going up 2 sizes at a time till it don't pop.
Last edited by Gary's 1977 Vette; Nov 18, 2006 at 07:13 PM.
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