[TCSGTO]

The same set of old style 72cc heads needed different timing on different shortblocks. On a 464 with Ross dished pistons and .040 Cometic gaskets they made best power at 43! I never ran it that high in the car and usually kept it around 39.
On a 517 also with Ross dished pistons and .040 Cometics they made best power at 38, 36 was close so that’s what it’s set to in the car.

[steve25]

Another big player in the timing needs of any motor is the size of the Intake port itself and how much swirl it makes, and there are a lot of detail factors that go into that.

Some big port heads have good low flow rate swirl that drops off like a rock above .300" lift, other heads have little at low lift and pick up to a usable level by by .470" lift, and then there's other heads that have good rates at low lift with swirl in one direction, then the swirl drops off to nothing but at lifts above .550" the swirl come back into play but spinning in the other direction .

The best way to combat low swirl is with a tight Deck clearance and enough Deck surface area in the head to agitate the air and fuel into a better and faster to burn state of mixture.

In general regardless of chamber shape swirl rates go down as the ports get into the size range needed to make over 650 hp, and as that swirl rate and inturn mixture motion goes down the motors need for more timing will go up.

[Kedavra]

I have a 468, 4.185 Mahle flattops, ZD, .040 Cometic, old pre-02 87cc EHeads 2.19/1.77 lifted to 320/230, 10.35 SCR, HR 242/250-112LSA@108, portmatched Performer RPM, now running a Sniper but had a BG850 when it was engine dyno’d - 608HP@5600/640TQ@4500. Did not have time to really tune it. All dyno runs at 38 timing. Since then I have been gun shy about running it over 36 @WOT. . It will be a long time before I can get this on a chassis dyno, and it’s in my 67 FB conv that I only drive with the top down so I can’t trust my ability to hear any pinging. Appreciate any thoughts as to whether this is safe to run at 38 max (non-cruise) adv, and what would be a safe cruise adv. because of the throttle body and DP intake, I have not taken cruise as lean as I would like because I am worried about cylinder/cylinder differences in fuel distribution. Any input appreciated!

[Skip Fix]

One I had liked 40 the other 37. Both old style chambers

[steve25]

The chemical composition of the fuel your running besides it Octane rating will determine the jetting you need and also how much timing you can use.

If your bracket racing and going nuts to eek out very last scrap of hp your motor can produce, then your likely missing the greater need and the bigger picture that the car first and foremost needs to be deadly consistent and pure hp is not going to get you there.

If your going to a drag strip purely for ET and mph bragging rights then that's a different story.

[grandville455]

Originally Posted by Skip Fix

One I had liked 40 the other 37. Both old style chambers

what size engine?

[Skip Fix]

455 and 500" 455 liked more and 60 ft'd better