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Most aluminum heads have a Modern combustion shape that doesn't require as much timing … A great combustion shape will be happy with 30 to 32 degrees total timing and you can't do that with a old Iron factory head
I agree with Eric on this, it's just not what I am running. I am running the old L88 style aluminum heads with old style open chambers. I want to keep it as close to "day 2" stock as possible. That leaves out the programmable ignition for me as well. I won't be making any other major component changes, so it is a set-it-up-once-and-done type of deal. I'll be setting up the rough tune before the Dyno, fine tuning it on the dyno, and then another fine-tune will be during early driving, but it should be pretty close by then anyway. I'll probably use an A/F meter on the driving tune-up.
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Originally Posted by Eric P
Most aluminum heads have a Modern combustion shape that doesn't require as much timing … A great combustion shape will be happy with 30 to 32 degrees total timing and you can't do that with a old Iron factory head
Jebby beat me to the punch here, but the previous discussion involved head material (iron or aluminum), not new design versus old, or new aluminum versus old cast iron.
So turns out my distributor has a brand new main shaft, but it has this really odd parallelogram shaped timing cam on the top of it. Must be a bad overseas knockoff. I know this controls the timing curve and it should be very smooth, and rounded. The normally smooth football shape is what controls the shape of the curve, and how quickly the advance comes in. I'm pretty sure this would be way too sudden of a timing change. I'm gonna order a better one. This is a high performance engine, and the part is not.
On the other hand I rigged up a degree wheel much like you would use for a cam. And double checked my mechanical advance with the reduced slot from the 722 cam, and the vacuum advance from the new vac can using Lars timing limiter. Worked well, and came out right where I wanted. 22 degrees mechanical advance. The B26 vac cam is all-in at 9.5 inches and has only 11 degrees advance with the limiter in place. That may work just fine, but will be helpful to know come tuning day.
Homemade degree wheel of cardboard and copper wire pointer with 4 inch protractor. Easily read to 1/2 degree.
Here you can see how the slot is reduced in size with Lars limiter in place. 16 degrees vac advance turned into 11.
Last edited by leigh1322; Oct 23, 2019 at 10:53 PM.
Hey, I like the degree wheel setup you have there. Very clever.
Thanks. I think I like figuring out ways to fix stuff, at home, as much as fixing it. I'd much rather figure out how to do it myself than just pay someone else to just do it for me. Here's how I built my degree wheel. Downloaded a protractor picture, and blew it up to 3.5 inch diameter. That size gave me 2 mm for every single degree, which seemed to be plenty.
I think your Sun Distributor had babies and machine grew a few wires!
Including an oscilloscope.
I love it!
BTW I figured out the difference in the weight cam.
One gives a slower standard curve, and one gives a faster factory high performance curve.
New main shaft and cam on the way!
Smoother curve shape gives a faster and smoother curve, like the explanation of factory timing curves below, from T.I. Specialties:
Curve 1 is the Hi-Po cam curve.
Last edited by leigh1322; Oct 24, 2019 at 10:16 PM.
My existing shaft looks like the two on the right, So that part is OK I guess. With the cam perfectly horizontal, the oil pump tang is turned about 10 degrees to the left. (11:00 o'clock angle)
I can measure more closely if needed.
Well the new high-performance mainshaft cam is back-ordered at Paragon and won't be here for a week.
Being a curious kind of guy, I want to see if I can plot the advance curve from this cam. As well as the new one when it comes in.
Stay tuned!
Yes. That's the widely recommended overall performance engine advance curve, basically "all-in by 3500".
I am interested in plotting the "shape" of the curve as delivered by the two "advance cams" that are readily available today, the standard one and the high perf one. According to T.I. specialty Chevy made 38 different "shaped cams" and now we have only two to pick from. This is a graph from their info, the fastest available chevy curve and the slowest curve, the shape is changed strictly by changing the distributor cam. I'll wind up with both styles, so why not test them? "Inquiring minds want to know."
Mechanical advance curve of distributor only: The beginning and ending point here, as determined by the springs, is exactly the same in this graph. But the in-between values are determined by the shape of the distributor weight cam, and vary widely. Notice how quickly curve 1 gets to 10 degrees vs curve 2. It's about half the rpm. But both have peak advance at the same rpm.
Last edited by leigh1322; Oct 27, 2019 at 11:52 AM.
Sunday Morning UPDATE:
I tested the advance of the Std Cam on the Distributor Mainshaft that I have and it almost perfectly matches curve 2 from TI Specialties above.
My HIgh Perf cam/mainshaft should arrive shortly, and will also be "plotted"
I understand how to set the high rpm advance for best WOT on the dyno. (basically 36 degrees +/-)
I know how to set the idle initial advance for best vacuum & emissions. (from 12-18)
So lets assume I set the springs to bring the above mechanical advance curve all-in by 3000rpm, (as recommended by Chevy and many others)
Then the mechanical advance curve above only controls the WOT timing between 1000 & 3000 rpm (crank)
The two cams give drastically different advance in the middle at say 2000rpm (crank).
One cam curve would add 4 degrees advance at 2000rpm crank, the other one 13 degrees.
If I start with initial at 10, then one cam would give 14 degrees advance at 2000rpm crank, the other one 23 degrees.
Question:
How do you go about determining which one is best? The one the engine "wants"?
Is there a recommended one?
Can WOT timing be tested on the dyno at 2000 rpm?
Or is this testing best done in the car on the road?
Basically I would prefer to get the WOT mechanical advance curve fully fine-tuned and set on the dyno, where there is ready access to data such as A/F ratios, EGT temps, and HP.
Distrib update:
I received the main shaft with the high perfromance cam last week, and finally had time to test them:
High performance mainshaft on top, standard on bottom
Here is the mechanical advance that I measured using the above two cams,
Double the distributor degrees to get crankshaft degrees.
The High Performance one looks very linear, like the Chevy Power Book. The standard one is retarded as much as 5 crankshaft degrees at the mid-point.
What was very interesting was the curved advance cam in combination with the curved advance weights, creates a linear advance rate curve.
I will run the linear increasing one, the High Performance mainshaft. I see nor reason to run the standard one that delays the advance by 5 degrees or 800 rpm.
