timing curve problem
Le Mans Master
Joined: Jul 2006
Posts: 8,117
Likes: 104
From: charlotte north carolina
Set all timing with the vacuum advance disconnected. The surging that you are experiencing may be an excessive amount of vacuum advance. Modern fuels (ethanol) may cause this. Set your initial to about 20* and then use bushings on the centripetal advance pin to limit total (no vacuum added) to 36*. Then use bushings on the vacuum advance arm to limit vacuum advance to 12*. The total (initial+partial centripetal+vacuum ) at idle will then be in the 40* range(there will be a few degrees of centripetal at idle). Original stock engines ran totals in the high 40s or even low 50s of advance with old style fuels.
Former Vendor
Joined: Jun 2012
Posts: 145
Likes: 13
I bought a curve kit today and changed my springs. My mechanical is coming in later now.
From my research it seems like these heads like 29°or so. It's just been tricky for me with the cam and cr and all coming into play. I'll try to do away with the vacuum advance and up my initial timing. I ran it today and it seems to be fluttery @ normal acceleration. Is that the knocking/pinging you were talking about?
Thanks for your help
From my research it seems like these heads like 29°or so. It's just been tricky for me with the cam and cr and all coming into play. I'll try to do away with the vacuum advance and up my initial timing. I ran it today and it seems to be fluttery @ normal acceleration. Is that the knocking/pinging you were talking about?
Thanks for your help
The heart shaped combustion chambers on Vortec heads are far more efficient than the old smogger heads of the '70's. They do like less timing. The more efficient the combustion chamber, the less timing advance that is needed. On the crate engines we build with GM's Vortecs, we run a total timing at 34*, all in at 3500 rpm. We also don't recommend the use of the vacuum advance at all. The vacuum advance was there to help with emissions and increase gas mileage. But, on an engine with a big cam like yours, it won't improve either.
Your fluttery condition could be just a lean condition in the primary side of your carburetor. That may be a different tuning issue. Let's concentrate on the on timing issue for right now, then see how if the carb needs some adjustment later.
Last edited by BluePrint Engines; Dec 29, 2015 at 09:17 AM.
Former Vendor
Joined: Jun 2012
Posts: 145
Likes: 13
Set all timing with the vacuum advance disconnected. The surging that you are experiencing may be an excessive amount of vacuum advance. Modern fuels (ethanol) may cause this. Set your initial to about 20* and then use bushings on the centripetal advance pin to limit total (no vacuum added) to 36*. Then use bushings on the vacuum advance arm to limit vacuum advance to 12*. The total (initial+partial centripetal+vacuum ) at idle will then be in the 40* range(there will be a few degrees of centripetal at idle). Original stock engines ran totals in the high 40s or even low 50s of advance with old style fuels.
Scott Liggett
BluePrint Engines
1(800)483-4263
info@blueprintengines.com
Last edited by BluePrint Engines; Dec 29, 2015 at 09:19 AM.
Thread Starter
Instructor
Joined: Apr 2012
Posts: 135
Likes: 1
From: Kirbyville Texas
I will try some of the suggestions today. I've read more about centrifugal timing and vaccum advance till my head is spinning. Lol
I've learned alot. I think I have a good idea what to do now.
Thanks for your help. I will update my progress or problems
I've learned alot. I think I have a good idea what to do now.
Thanks for your help. I will update my progress or problems
Thread Starter
Instructor
Joined: Apr 2012
Posts: 135
Likes: 1
From: Kirbyville Texas
I advanced my initial timing at 19° then played with the springs till I got a total timing of 29° @ 3000 rpm and deleted my vaccum advance.
She came alive!.... Nice idle and quick throttle response. I just run her up the road to open it up for a 1/8 mile thrill. Hahaha. I might bump it up a few degrees another day, but it is fine for now.
19° initial
29° mechanical @ 3000
Deleted the vacuum advance.
She came alive!.... Nice idle and quick throttle response. I just run her up the road to open it up for a 1/8 mile thrill. Hahaha. I might bump it up a few degrees another day, but it is fine for now.
19° initial
29° mechanical @ 3000
Deleted the vacuum advance.
Pro
Joined: Oct 2009
Posts: 664
Likes: 89
From: Marysville Ohio
kcat - I have run the MSD Streetfire before and think you should check your mechanical advance beyond 3000 RPM.
As mentioned, the streetfire typically has around 22-24 degrees advance regardless of the rate of advance - so check at 4000 RPM and maybe even higher - I would bet you will see more advance coming in. At 19 initial, you may still have a little too much advance at higher RPM.
My observation is that you may be able to easily slow the advance curve, but if you run high initial, you will probably need to limit the length of the curve, not just the rate. Actually limiting to a short curve requires physical modification of the distributor (adding a stop of some sort to limit the advance travel mechanism).
If you see this is the case, and want to keep the 19 initial, I would check into stopping mech advance curve length at 32-34 degrees (a 13 -15 degree curve "length" vs the stock 22).
It's worth a check to see, just for a little CYA if nothing else - I just don't want to see you potentially damage a new build.
As mentioned, the streetfire typically has around 22-24 degrees advance regardless of the rate of advance - so check at 4000 RPM and maybe even higher - I would bet you will see more advance coming in. At 19 initial, you may still have a little too much advance at higher RPM.
My observation is that you may be able to easily slow the advance curve, but if you run high initial, you will probably need to limit the length of the curve, not just the rate. Actually limiting to a short curve requires physical modification of the distributor (adding a stop of some sort to limit the advance travel mechanism).
If you see this is the case, and want to keep the 19 initial, I would check into stopping mech advance curve length at 32-34 degrees (a 13 -15 degree curve "length" vs the stock 22).
It's worth a check to see, just for a little CYA if nothing else - I just don't want to see you potentially damage a new build.
Last edited by cooper9811; Dec 30, 2015 at 07:04 AM.
Thread Starter
Instructor
Joined: Apr 2012
Posts: 135
Likes: 1
From: Kirbyville Texas
kcat - I have run the MSD Streetfire before and think you should check your mechanical advance beyond 3000 RPM.
As mentioned, the streetfire typically has around 22-24 degrees advance regardless of the rate of advance - so check at 4000 RPM and maybe even higher - I would bet you will see more advance coming in. At 19 initial, you may still have a little too much advance at higher RPM.
My observation is that you may be able to easily slow the advance curve, but if you run high initial, you will probably need to limit the length of the curve, not just the rate. Actually limiting to a short curve requires physical modification of the distributor (adding a stop of some sort to limit the advance travel mechanism).
If you see this is the case, and want to keep the 19 initial, I would check into stopping mech advance curve length at 32-34 degrees (a 13 -15 degree curve "length" vs the stock 22).
It's worth a check to see, just for a little CYA if nothing else - I just don't want to see you potentially damage a new build.
As mentioned, the streetfire typically has around 22-24 degrees advance regardless of the rate of advance - so check at 4000 RPM and maybe even higher - I would bet you will see more advance coming in. At 19 initial, you may still have a little too much advance at higher RPM.
My observation is that you may be able to easily slow the advance curve, but if you run high initial, you will probably need to limit the length of the curve, not just the rate. Actually limiting to a short curve requires physical modification of the distributor (adding a stop of some sort to limit the advance travel mechanism).
If you see this is the case, and want to keep the 19 initial, I would check into stopping mech advance curve length at 32-34 degrees (a 13 -15 degree curve "length" vs the stock 22).
It's worth a check to see, just for a little CYA if nothing else - I just don't want to see you potentially damage a new build.
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Former Vendor
Joined: Jun 2012
Posts: 145
Likes: 13
I advanced my initial timing at 19° then played with the springs till I got a total timing of 29° @ 3000 rpm and deleted my vaccum advance.
She came alive!.... Nice idle and quick throttle response. I just run her up the road to open it up for a 1/8 mile thrill. Hahaha. I might bump it up a few degrees another day, but it is fine for now.
19° initial
29° mechanical @ 3000
Deleted the vacuum advance.
She came alive!.... Nice idle and quick throttle response. I just run her up the road to open it up for a 1/8 mile thrill. Hahaha. I might bump it up a few degrees another day, but it is fine for now.
19° initial
29° mechanical @ 3000
Deleted the vacuum advance.
I once put these curves to the test on a chassis dyno using my '65 Impala's old and abused 383. I first tested the car with a stock, out of the box distributor. It had 24* mechanical advance. Then I recurved it to limit mechanical timing to 18* and added 18* initial. Both had springs that had full advance at 3000 rpm and both runs were done with total timing at 36*. This only allowed 12* initial with the uncurved distributor. Both were being fired by the MSD 6AL box and used the same cap, coil, rotor, wires and plugs. The recurved distributor picked up 26 hp at the wheels and greatly improved drivability and idle quality. I never looked back.
Have fun with your Corvette.
Scott Liggett
BluePrint Engines
1(800)483-4263
info@blueprintengines.com
