327_TPI_77_Maro

iTrader: (8)

Hi guys, you probably saw my post regarding how I was able to run 38-40 degrees total timing on my stealth rammed 327 w/ no knock retard. Surprise, the 76cc heads I had gave me just under 8:1 compression w/ the forged flat top pistons in my engine. I was running a 98.5 mph average 1/4 mile trap speed when this engine was carbed in my heavy camaro -- I'm amazed the little 327 ran that well at really low squeeze. Anyhow I grabbed a clean set of 305 1.84/1.5 58cc heads and installed new valve springs and valve seals and have them on the 327 now -- this gives a hair under true 10:1 compression. I am only able to run 32 degrees total before I start getting sigificant knock retard atbove 4500 rpms going wide open. The car runs very strong, but is this enough total timing for an sbc w/ 10:1 compression? I was thinking I might be going lean up top and hence not being able to run a lot of timing, but the narrow band O2 was way up in the 900s during the time I had KR, so I don't think I was anywhere near lean.

Here is my current timing map. I have no PE spark add above 3200, and only a 1.7 degrees at 1200 and 2000.

RBob

iTrader: (1)

That's a small chamber, probably with a rapid burn too. I have a '85 LG4 with those heads (stock Camaro engine) and it too doesn't like much timing. I'm running 24 deg max at WOT from 3600 RPM up. This is 9.5:1 engine on 89 octane.

If it's knocking you don't really have a choice but to pull timing.

RBob.

327_TPI_77_Maro

iTrader: (8)

Rbob, is this 24 degrees max you are referring to relative to TDC or relative to the base spark (24+6 = 30 relative to TDC)?

RBob

iTrader: (1)

24 degrees BTDC at the crank.

RBob.

Mark 89Formula

There's a pretty good chance your heads have a similar chamber to the ones in my stock heads which yield around a CR of around 9.3:1. I found another 0.3-0.5 MPH by dropping the total timing at WOT 2 degrees from a max of 30 degs to 28 degs. I wasn't getting any knock counts at 30 degs but I consistently picked up MPH on every pass when I pulled the timing back. I would have never known this if I hadn't measured the results at the strip and only relied on being knock "free".

327_TPI_77_Maro

iTrader: (8)

The chamber size and shape of my 416 heads are identical to the 081 305 heads (the 416s had perimeter bolt valve covers, that was the only real difference). The 083 350 TPI heads had a larger but same shaped chamber as the 416/081 heads, and the same basic ports. I don't understand why the factory bins for engines w/ these heads runs up to 36 degrees max WOT timing if 28-30 is better for performance (AND would require less octane!) I guess my only real way to find out what this engine likes is to burn some chips at the track and see how my mphs do. By the way, are you guys taking into account added PE spark when you say "30 degrees total timing". The factory bins often have very aggressive PE spark added, up to 8-9 degrees. I'm sure you know this but I am trying to justify running so little timing when the factory ran a lot more (set up for 89 octane too)

Mark 89Formula

Yep, PE spark is zeroed out. Initial timing is at 6 degs in the BIN file and on the balancer. I don't understand the factory fuel and timing strategy in PE either but I know it slows my car down if I rund the stock parameters. Maybe they throw a bunch of fuel in at the higher RPMs as a margin of safety since there's no closed loop feedback in PE then crank in additional timing because the mixture is so rich?