Let's talk "Tuning"
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Laughing Gas
Joined: Jul 1999
Posts: 1,047
Likes: 22
From: Lacey, WA
Let's talk "Tuning"
What procedure do you use for tuning? This would be after cam break in on a new engine. Specifically, setting initial timing, idle speed, idle mixture, etc.
Here's what I have always done with engine warm, vac advance plugged.
Set initial timing to about 10*
Then I adjust mixture screws for highest vacuum reading while also adjusting idle speed to keep it around 650rpm ( I have a manual tranny).
I end up with the mixture screws out about 3/4 on a turn from closed. This is on a Holley 80508 750cfm Vac secondary carb on my bored 390. Engine specs are in my signature.
The best I can get ~13" manifold vacuum.
I have been told that I should adjust the initial timing based on strongest vacuum reading. I tried this but ended up around 18* initial at strongest vac. This is way to much initial.
So, what are your step-by-step procedures for tuning?
Here's what I have always done with engine warm, vac advance plugged.
Set initial timing to about 10*
Then I adjust mixture screws for highest vacuum reading while also adjusting idle speed to keep it around 650rpm ( I have a manual tranny).
I end up with the mixture screws out about 3/4 on a turn from closed. This is on a Holley 80508 750cfm Vac secondary carb on my bored 390. Engine specs are in my signature.
The best I can get ~13" manifold vacuum.
I have been told that I should adjust the initial timing based on strongest vacuum reading. I tried this but ended up around 18* initial at strongest vac. This is way to much initial.
So, what are your step-by-step procedures for tuning?
FTE Leadership Emeritus
Joined: Jul 2002
Posts: 42,561
Likes: 420
From: Long Island USA
How big of a cam is that? 13 inches of vacuum might be all you can expect from it.
The 390 in my highboy got 11 at 900RPM. I had to mess around with manifold vacuum for vacuum advance to get the idling timing up high enough to get any decent vacuum, like above 15, but still get it to crank over to start it
If you don't have a big lopey cam, maybe something else is going on.
The 390 in my highboy got 11 at 900RPM. I had to mess around with manifold vacuum for vacuum advance to get the idling timing up high enough to get any decent vacuum, like above 15, but still get it to crank over to start it
If you don't have a big lopey cam, maybe something else is going on.
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
Tuning with a vacuum gauge - first adjust for highest vacuum at idle but then always back off about 1" on the gauge from the highest reading. This may work for you on pump gas, unless a high compression engine. Lumpy cams will have problems idling and poor vacuum too. At sea level a healthy stock V8 will pull 19 to 21 inches.
This method of tuning results in quite a lot of initial timing, but unless the distributor has already been curved to limit the total mechanical advance from stock specification these higher values of initial or crank timing are not practical , as you've discovered. If you've a lumpy cam installed 13" or even less may be the best it will pull, and PV selection becomes important. Here is where connecting vacuum advance to manifold source becomes important.
This method of tuning results in quite a lot of initial timing, but unless the distributor has already been curved to limit the total mechanical advance from stock specification these higher values of initial or crank timing are not practical , as you've discovered. If you've a lumpy cam installed 13" or even less may be the best it will pull, and PV selection becomes important. Here is where connecting vacuum advance to manifold source becomes important.
Thread Starter
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Laughing Gas
Joined: Jul 1999
Posts: 1,047
Likes: 22
From: Lacey, WA
Thanks for the reply's!!!!
Cam specs are as follows:
Duration degrees @ .050 Int. 206 Ext .221
Valve lift Int. .481 Ext .490
Not radical by any stretch. Not sure why the vac is as low as it is. I'm not sure how the cam was setup by the builder..... straight up I believe.
Engine has around 35,000 miles on this rebuild. It has been a reliable motor but, ... last year I bought a new Travel Trailer that comes in around 7000lbs loaded. I regularly take the trailer over a 6000ft pass. The truck pulls ok but I feel it should pull alittle better than it does thus me tinkering with the tuning to get every last bit I can out of it.
I curved the dissy years ago to have the 26* mechanical all in at 3000rpm.
BTW, PV is a 6.5
Cam specs are as follows:
Duration degrees @ .050 Int. 206 Ext .221
Valve lift Int. .481 Ext .490
Not radical by any stretch. Not sure why the vac is as low as it is. I'm not sure how the cam was setup by the builder..... straight up I believe.
Engine has around 35,000 miles on this rebuild. It has been a reliable motor but, ... last year I bought a new Travel Trailer that comes in around 7000lbs loaded. I regularly take the trailer over a 6000ft pass. The truck pulls ok but I feel it should pull alittle better than it does thus me tinkering with the tuning to get every last bit I can out of it.
I curved the dissy years ago to have the 26* mechanical all in at 3000rpm.
BTW, PV is a 6.5
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