Alot of timing to make it run...48

[SonnyJames]

This is a post I put up on another site in the beginning of last summer:

Ok, I have been tuning with plugs and timing for 2 weeks now after I realized my .030 over 427 with .250 long rod, 110cc RaceRites, 10.4-1,custom piston, rpm airgap,248/260@50 custom grind BBC responded to more timing than 38 deg. I have settled on 48 deg timing and a 7373 NGK plug. The timimng mark on the plug strap appears to be right where its supposed to be and the fuel ring actually reads a little wide on the porcelain. Its light chocolate color too which is saying a tad rich but because im shell shocked at the amount of timing that its taking im hesitant to drop the jet. Its got a 750 Holley ultra aluminum on it with 74 in front with pv and 81 in rear no pv. Thoughts on this anyone? And no I have not errored on setting the pointer! BTW this is a pumpgas street car. Also the amount of drivability/performance gain from 38 to 50 is super cool!

3 MO ago I removed the engine to fix a rear main seal leak. One thing led to another and I dug into the engine to re-verify some things/change a couple things. I found TDC to be spot on like I knew it would be. Bearings looked good along with everything else. One thing I did find is that it was not 10.4-1 compression so I removed the felpro gasket and installed .027 cometic plus advanced cam 2 deg. It is now 10.3-1. Cam is 248/260@50 with 580/600 lift. Also installed 2" primary hustler headers with 3" exhaust. Fired engine up the other day and all is well. I figured that 38-40 deg timing would be the max with the new compression and cam advance. I was wrong. It stumbled in neutral so I put it up to 42 and it got better....finally back at 48 deg and this thing revs better but certainly acts like it wants even more! why does thing take so much timing to wake it up?? Also when the timing was at 38 and I drove it the valve train was real loud. the more timing I added, the better it ran and quieter! Already on first fire up at 40 deg timing the valvetrain was quieter. I'm attributing that to advancing the cam. I know that's bizarre but that is whats happening. At 48 the thing at idle sounds so damn good both under the hood and out the exhaust. I have not been able to drive it yet.

[SonnyJames]

Ohh yea forgot. The cam was in on a 108.75 icl now it's 106.75

[Tuner]

Is it a closed chamber piston dome with open chamber heads?

[SonnyJames]

The pistons do have a dome. 29cc. And the heads are Brodix racerites. The chambers are open

[Tuner]

I’ve seen open chamber GM head on closed chamber piston act this way. I think the edge of the dome comes too close to the chamber wall above the plug and cuts off the flame travel away from the sparkplug. When open chamber heads first came out the magazine hoopla was “big power” so some people put them on without changing pistons, just bolted on the heads expecting big gains. The engines ran like junk and needed a ton of timing to run as good as they would, which wasn’t good.

If you have some pictures or measure the clearance at TDC between the dome and the chamber, not the deck clearance, the dome clearance.

[dirtracr5]

double check your timing marks before spending any more time on it.

[66Vette]

Sonny:

This sounds like a street motor. You haven't mentioned if you are using any advance mechanisms (centrifugal or vacuum). Idle speed is also a factor.

There are three components to advance, initial, centrifugal, and vacuum. The number that is most often kicked around is the "total advance" and this is initial plus centrifugal. For most street applications, total advance numbers are around 34 to 36 degrees. The centrifugal advance kicks in at idle and is typically all in at around 3000 RPM.

So at 3000 RPM you are sitting at 36 degrees, in a cruise situation you will have full manifold vacuum. Now add a typical OEM vacuum advance of 15 degrees and you get to around 50 degrees of what I would call "gross" advance. Many people are not aware that a street engine with OEM type advance mechanisms sees gross advances around 50 degrees. 50 degrees (ish) gives smooth cruise performance.

Under hard acceleration, manifold vacuum goes away and you are left with total advance. The reduction in advance at WOT keeps the engine away from detonation and operating at maximum power.

So smooth cruising conditions at 50 degrees is not unusual, but I certainly would not run it at WOT with that much advance.

[SonnyJames]

66Vette, thanks for the input. I been through all of that and ended up locking the distributor out. This combo works best this way. I think Tuner may be on to something as I do remember without a head gasket the dome came very close to hitting the chamber on the sparkplug side. Caused me to clay it and come to the conclusion that it would work with head gasket. The dome does fill the chamber up. I can see how it could possibly do what Tuner says. Makes me contemplate hooking a 7al digital up to create a timing curve...say 50 deg to 3000 then pull it out ending up at 40 by 7300. How else could I get best of both worlds, killer response with power under load and be sure it won't detonate? Vacume advance would take it out too quick. I already been through it. Welded my weights and filed them to give me 29 at idle, set screw on canister rod for 50 with vacume and 38 total.

[SonnyJames]

This is a damn convertible 67 stick Camaro I take the family for beach rides and ice cream! Gotta make it run tho!!

[dirtdemon59]

So 38 is full advance(or all in since weights are locked). And with vacuum advance kicked in its at 50, that's what Im reading. If so congrats you got your timing curve is right. You may even get more Total timing in there.

[SonnyJames]

Nahh. Read it wrong. It WAS like that but didn't run. Locked out now

[F-BIRD'88]

Clarification:Does this distributor have vacuum advance?

[SonnyJames]

Yes it does.

[F-BIRD'88]

so it seems to like 48 deg +- @ idle and cruise (part throttle)) and 38 ± @ wide open throttle. ,?

[SonnyJames]

It likes 48 all the way around. I've had it to 52 but with wife and kids in car it pinged with light load on it in 3rd gear so I backed it back to 48 with no ill effects. Very weird I know