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Old 04-21-2009, 07:30 AM
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SBC: Summit dist, changing timing with a bushing?

Hello Guys,

You all have been great help while I am adding/changing bits & pieces to my daily driver rod project and answering any questions I might have.

Since this past weekends Cam & Heads project was a success, I need to address my ignition timing. I currently have a base timing of 14 degrees but since bumping my compression up a bit (10.34-1 to 10.6-1 with a thinner head gasket) the valves are beginning to clatter under a load. So all that means is I need to pull a few degrees out. Instead of taking away from the base/idle timing I was thinking of just changing the settings in the dist.

I've got the Summit 850055 dist in my SBC, it looks just like a MSD unit but with a black cap.



The Dist comes with all kinds of different centrifugal weight springs as well as advance bushings. The weight springs I understand, the bushings I don't.

The dist has a 21 degree bushing installed upon arrival but can be changed out for a 18, 25 & 28 degree bushing. Does this limit the amount of total ignition advance? ie:

14 (base) + 21(bushing) = 35 degrees of max timing

or does the bushing only affect one piece of the timing advance puzzle, the vacuum advance is hooked up to ported vacuum

I'd really like to shoot for a base timing of 16-18 degrees for my 224/230 cam and have a nice sweep up to a max timing around 2500-3000RPM that won't detonate or make my valves clatter

Engine Combo:

.040" over 4 bolt, one piece rear, roller cam block
H345NCP-40 pistons sit .004" in the hole @ TDC
ZZ4 bottom end
Patriot "Vortec" heads; 2.02"/1.6" valves .008" mill off each
Mr.Gasket 1134 head gaskets; 4.13" x .028"
Comp XR276HR-10 cam & 1.6 roller rockers
Comp 26918 springs & 787 retainers
Professional Products split dual plane, air gap manifold
Edelbrock 600cfm (soon to be a Speed Demon 650)
Summit dist, Accel coil, MSD 6a, Taylor wires, NGK TR6 plugs
91 pump gas
3200rpm stall lockup converter/4L60E
4:10 rear gears / 30.4" tall tires
4200lb chevy pickup



Cheers ~Mykk

Last edited by 04SilveradoMykk; 04-21-2009 at 08:54 AM.
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Old 04-21-2009, 11:01 AM
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I've been playing with the distributor & ignition timing all morning.

The bushing does just add from the base timing, as far as the centrifugal advance is concerned.

so the 14 degrees base + 21 bushing = 35 degrees total with the vacuum advance unplugged.

And I'm getting full mechanical advance right @ 2500rpm. However, plug the vacuum advance back in and I'm seeing timing jump up past 40-50 degrees. Yikes, but the engine sounds better and revs quicker with the vacuum advance kicked in.

Does this sound about normal to you guys?

Oh, and BTW. The reason my valves were clattering... my buddy timed my engine by ear, and had the base timing past 20 degrees for a max of 41+ and than some with the vacuum advance. No wonder!

Last edited by 04SilveradoMykk; 04-21-2009 at 11:07 AM.
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Old 04-21-2009, 11:15 AM
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You want more initial (18 to 22) and a slower/smoother curve (heavier springs) that tops/stops out around 3200rpm. (34 to 36deg)
Then you need to physically limit the vaccum advance travel to around 10-12 max limit.
The rate of vacuum advance in and out (diapram spring tension) will have to be determined by trial and error.
Your cr is too friggin high for 91 octane. If you want to run 10.6:1+cr put some decent gas in it.
If the piston really are only .004" below the deck at TDC this combined with a .028" gasket risks having the piston smack the head at rpm.
In this case the gain in compression is not worth the potential engine damage and you're getting engine knock issues to boot. Reinstall a .038" to .041" gasket.

rejet the edelbrock carb to the 600cfm eddy marine carb base calibration.
work form there.

Last edited by F-BIRD'88; 04-21-2009 at 11:23 AM.
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Old 04-21-2009, 03:08 PM
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Quote:
Originally Posted by 04SilveradoMykk
so the 14 degrees base + 21 bushing = 35 degrees total with the vacuum advance unplugged.
IMO, those heads might not need quite that much ignition advance- see if you can get by w/ 32 degrees total (before adding vacuum advance).

OEM Vortecs have worked well w/32 degrees total advance. The Patriot "Vortec" heads might respond similarly, unless the cam demands more ignition lead.

At least worth a look.

Last edited by cobalt327; 04-21-2009 at 03:24 PM.
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Old 04-21-2009, 10:45 PM
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By the way, those aren't your valves clattering, it's your pistons being beat to H***
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Old 04-22-2009, 08:55 AM
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Quote:
Originally Posted by F-BIRD'88
Your cr is too friggin high for 91 octane. If you want to run 10.6:1+cr put some decent gas in it.
If the piston really are only .004" below the deck at TDC this combined with a .028" gasket risks having the piston smack the head at rpm.
I've read a few articles where a successful quench height of .032" was used. My pistons are flat top 4 valve reliefe

This last weekend I did a new cam & heads, it did have a LT4 hotcam & vortec 906 heads


With the 10.6-1 static compression and the 64 degree ABDC closing of the intake valve makes for 8.536 dynamic compression

While the previous 10.34-1 static & intake closing @ 38.8 degrees ABDC makes for 9.593 dynamic... wich I ran as my daily driver using 91 octane at a max of 36 degrees timing without any detonation.

The reason I went with a higher SCR because I knew there was going to be a lower DCR with the new cam

But today, I will try your recommendation and shoot for a base of 16 - 18 degrees timing. Slower sweep to a max of 36 degrees @ 3200-3500 rpm and I will see if I can adjust some timing out of the advance

Quote:
Originally Posted by crownver
By the way, those aren't your valves clattering, it's your pistons being beat to H***
I suppose it's a good thing I backed off the timing when I did, no more clattering

Cheers ~Mykk

Last edited by 04SilveradoMykk; 04-22-2009 at 09:01 AM.
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Old 04-22-2009, 11:20 AM
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Useing the .050" closing point method to calculate the (flawed and oversimplified) DCR, the LT4 hot cam and your new comp cam have exactly the same intake closing point 38BTDC.

You do not know the actual running seat to seat valve open close duration so there is no way to accuratly pinpoint the actual intake closing point on the running engine (with the limited info you have) so any DCR calc is a rough guess estimate at best.

Looks like you are going to learn the hard way the harsh realities of real world engine operation build limits and practical cr/octane limitations somewhere out on the road on a blazing hot Arizona day. Hopfully it does not leave you stranded when the engine fails on you.

With 10.6:1 you will most likely need to compromise the spark timing to avoid pinging under real world load and varying, less than idea conditions.

Last edited by F-BIRD'88; 04-22-2009 at 11:38 AM.
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Old 04-22-2009, 08:13 PM
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Quote:
Originally Posted by 04SilveradoMykk
Hello Guys,

You all have been great help while I am adding/changing bits & pieces to my daily driver rod project and answering any questions I might have.

Since this past weekends Cam & Heads project was a success, I need to address my ignition timing. I currently have a base timing of 14 degrees but since bumping my compression up a bit (10.34-1 to 10.6-1 with a thinner head gasket) the valves are beginning to clatter under a load. So all that means is I need to pull a few degrees out. Instead of taking away from the base/idle timing I was thinking of just changing the settings in the dist.

I've got the Summit 850055 dist in my SBC, it looks just like a MSD unit but with a black cap.



The Dist comes with all kinds of different centrifugal weight springs as well as advance bushings. The weight springs I understand, the bushings I don't.

The dist has a 21 degree bushing installed upon arrival but can be changed out for a 18, 25 & 28 degree bushing. Does this limit the amount of total ignition advance? ie:

14 (base) + 21(bushing) = 35 degrees of max timing

or does the bushing only affect one piece of the timing advance puzzle, the vacuum advance is hooked up to ported vacuum

I'd really like to shoot for a base timing of 16-18 degrees for my 224/230 cam and have a nice sweep up to a max timing around 2500-3000RPM that won't detonate or make my valves clatter

Engine Combo:

.040" over 4 bolt, one piece rear, roller cam block
H345NCP-40 pistons sit .004" in the hole @ TDC
ZZ4 bottom end
Patriot "Vortec" heads; 2.02"/1.6" valves .008" mill off each
Mr.Gasket 1134 head gaskets; 4.13" x .028"
Comp XR276HR-10 cam & 1.6 roller rockers
Comp 26918 springs & 787 retainers
Professional Products split dual plane, air gap manifold
Edelbrock 600cfm (soon to be a Speed Demon 650)
Summit dist, Accel coil, MSD 6a, Taylor wires, NGK TR6 plugs
91 pump gas
3200rpm stall lockup converter/4L60E
4:10 rear gears / 30.4" tall tires
4200lb chevy pickup


Cheers ~Mykk
You're starting at the wrong end of the question. You don't decide on the advance nor the relationships of base versus vacuum and/or mechanical. These are decisions the engine makes. You pick what hopefully is a reasonably close starting point and tune from there. But every change you make affects everything else.

You need to understand the basic reasons for timing advance both in amount and rate of change to those amounts.

People get into a rut thinking that a big cam needs lots of spark advance, ergo, lots of spark advance makes a fast engine. The answer to that is NO, at least not for the reasons most people think. This is a cause and effect relationship, the cause at idle with a cam that has a lot of overlap and a late closing intake valve is that a lot of the incoming mixture is leaking out the exhaust valve during overlap and being pumped back into the intake when the piston is rising against the low speed incoming mixture flow with the late closing intake valve. The result at idle and off idle is a low density mixture which burns slowly, if at all. The two keys to fixing this is more compression and more initial spark lead. Another big help is a multi strike spark box which gives a repetitive chance to fire these thin mixtures in-case the first spark doesn't light it, this avoids, or at least reduces miss-fires till the dynamic mixture density get to a point where it will usually fire with one strike of the match.

The other end is how much total advance engine will tolerate at WOT with the RPMs at red line or peaked for the load and the engine is fully loaded (working hard against all the dragging forces of the body, suspension, and tires). You want the spark advance at this point to be just under the detonation limit if you're hunting for the most power it will deliver without going up in smoke.

In between these two spots is where the most tuning happens. Unfortunately vacuum and mechanical advance systems are pretty dumb because they don't make changes for engine load, which is something you can do with a computer. The load on the engine changes with the total gear ratio. The more freedom the engine has from load ,the more advance it will take without the need to replace toasted pistons. This means for a given RPM in low gear, the engine will accept its maximum tolerable spark lead faster and sooner than it will in high gear. As the gear ratios diminish from low to high gear, the load on the engine goes up and it's tolerance for advance without detonation goes down. This is where you need to learn what it wants, this is tuning, or at least part of what tuning is about.

Other factors such as the brand of fuel, its real resistance to detonation (all brands of 91 octane don't react the same, some have better resistance to pressure than temperature, others the other way and a few are good (damn few) with both. Inlet air temp, engine operating temp, with the SBC and other engines that use adjacent exhaust in the middle of the head are better with additional cooling delivered directly here from an external source, fuel mixture, richer is more tolerant; things like this establish how much spark advance the engine needs in any given situation. Oh yeah, don't forget the atmosphere, pressure, temperature, and humidity change the density and that affects everything from the work the engine needs to do to push the body to how much power it will generate to do that work.

You have to remember that in all things you can tune for, you're looking for the best solution. Taking everything to its maximum is not the ultimate solution, these things are more complicated than that.

Bogie
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Old 04-25-2009, 10:30 PM
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I took your guys advise and changed the timing bushing & springs.

I now have a base timing of 16 degrees for a max of 46 @ 3,350rpm (without vacuum advance) and although my vacuum advance isn't adjustable, summit tech says it's a fixed 8-10 degree.

The engine loves the new timing curve, thanks for the advise

A buddy & I will be going on a 200+ mile round trip tomorrow to a much lower elevation and back. We'll see how she does, I'm bringing tools & carb tuning pieces along for the ride.

Cheers ~Mykk

Last edited by 04SilveradoMykk; 04-28-2009 at 08:30 AM.
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Old 04-28-2009, 08:42 AM
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The truck & engine made it from Prescott (5368ft) to Chandler (1214ft) and back without any issues. 117 miles one way. I am so pleased there were no issues or malfunctions, 24mpg on the way down and 17mpg back. We had a blast at Firebird raceway. Although the track wouldn't let me race at this particular event they did let us park in the show next to the U.A.S. booth with the skimpily dressed models
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Old 04-28-2009, 09:28 AM
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how much of a difference was there between the patriot vortecs and your 906's?
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