38 Degrees Of Advance

DorianL

solid fixture here in the forum
Staff member
I just bought an installed a used, mid-performance 400 SBC. The seller claims that it has less than 1000 miles on it, and I only have evidence to believe him.

As I understand, the compression is 10:1. The cam is an unknown with a little lope to it. I am getting 14.5 inches of vacuum at 800 RPM idle.

The heads are aluminum with, presumably large valves.

I am running a Fi-Tech EFI.

The engine seems to have some power, but I really haven't put my foot into it yet.

Here's the thing - the engine manufacturer claims that, for best performance, the distributor needs to be set to 38 degrees and vacuum advance should be ported. I am a little uncomfortable with 38 degrees, especially on a higher compression engine. Is running 36 ok and actually preferable ?

Also, the note says specifically that it will make less power if I run direct manifold vacuum. I do admit the lower RPM range feels a little spongy... but what is the logic there ? I find running 8 degrees of vacuum advance improves my idle.
 
the only way you'll tell for sure is by running the engine on a precise dyno and having its properly tuned,
yes there are scan tools that can be used to detect detonation issues on the older cars,
and yes theres fuel/air ratio meters that are not limited to use on the newer OBDII cars.
Id stick with 36-38 degrees total advance for now,
and if you post clear color pictures of the spark plugs after a hard acceleration run you can tell a good deal from that alone


related info/links/threads etc.
https://www.summitracing.com/parts/cei-102001

https://sites.google.com/site/kensa.../articles-and-tips/tips---reading-spark-plugs

http://garage.grumpysperformance.com/index.php?threads/spark-plug-info.202/#post-13169

http://garage.grumpysperformance.co...uild-a-knock-detector.14441/page-2#post-74901

http://garage.grumpysperformance.com/index.php?threads/verifying-your-real-advance-curve.4683/
 
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With only 14.5" vacuum at idle, your camshaft is probably bigger than you think it is.
More initial advance will help, but you have to limit the centrifugal advance if you do so.
If you are going to mess with this, disable the vacuum advance for now to take it out of the equation.
What are you using for a distributor? Some brands, like MSD, make it easier than other brands to dial in the advance curve your engine requires.
 
don,t assume or guess take the time and effort required too, verify your ignition advance curve

this is what you need to start with
chart3e1.jpg


http://garage.grumpysperformance.com/index.php?threads/verifying-your-real-advance-curve.4683/

http://garage.grumpysperformance.co...ouble-shooting-rebuilding-hei-ignitions.2798/

http://garage.grumpysperformance.com/index.php?threads/timing-tabs-and-indicators.1015/

http://garage.grumpysperformance.com/index.php?threads/finding-top-dead-center.967/

http://garage.grumpysperformance.co...ter-1-for-timing-ignition-cam.966/#post-18999
 
With only 14.5" vacuum at idle, your camshaft is probably bigger than you think it is.
More initial advance will help, but you have to limit the centrifugal advance if you do so.
If you are going to mess with this, disable the vacuum advance for now to take it out of the equation.
What are you using for a distributor? Some brands, like MSD, make it easier than other brands to dial in the advance curve your engine requires.

Yes, the cam is probably quite big - I may well swap it out in a couple of years. If I do, I will be looking for some recommendations ;)

It is a Chinesium no-brand dizzy. It looks new. I modified the vacuum advance myself to pull 8 degrees.

Good point on checking the ignition curve.

I guess I am puzzled by recommending 38 degrees on a high compression engine and ported vacuum.
 
Conversationally you should read the links provided by Grumpy. They are a wealth of real detail.
That being said.. I do not feel 38° total advance is an excessive advance since
chamber design and piston dome will play a huge role in total required advance for an effective burn.
While not a SBC I run a FiTech MeanStreet on my 496 and use a vacuum advance controlled by manifold (not ported) vacuum.
I run 38° total mechanical advance and 10° more of vacuum advance.
Remember that properly configured vacuum advance only exists under light load and completely vanishes at WOT.
My last long drive resulted in 21.8 mpg and she still hands out an ass whoop’n of a stroked big block when I unleash the beast.
Oh.... 6” of idle vacuum at 1000rpm.
 
38 is a lot of total timing for a small block! I would say 36 is max, but some of them I have built run great at 32-35 total. I agree with Grumpy, do some testing, chart how it’s advancing, where it’s all in then do some adjusting. Just remember timing for torque, and jet the carb for horsepower!
 
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