Timing Debate, 8 Deg. verses 10 Deg.

[grewvin1]

Have 1987 twin 5.7L carb, 4 bbl, thunder Bolt IV engines, spec's call for timing to be 10 deg. BTDC, I have also found information saying 8 deg. BTDC.
Question is what would be the effect to have 8 deg. if calls for 10 or vice versa? I have been told the change in degrees will alter the power band range. One setting starting the range sooner than the other with no real effect on the engine performance overall other than to provide higher performance sooner than later on acceleration or later depending on which setting has been selected. Would be very interested in some thoughts on this or is the entire theory wrong?

 

[achris]

Re: Timing Debate, 8 Deg. verses 10 Deg.

As far as performance goes, no difference, that you would notice. Where the difference would become significant is when the engine is being pushed to the margin. If the fuel quality was low, the load on the engine (over-propped) then the earlier timing (10BTDC) would increase the risk of detonation...

Chris.......

P.S. The Merc service manual calls for 8 degs BTDC...

 

[HT32BSX115]

Re: Timing Debate, 8 Deg. verses 10 Deg.

As far as performance goes, no difference, that you would notice. Where the difference would become significant is when the engine is being pushed to the margin. If the fuel quality was low, the load on the engine (over-propped) then the earlier timing (10BTDC) would increase the risk of detonation...

Chris.......

P.S. The Merc service manual calls for 8 degs BTDC...

All TRUE.

OMC, for some engines actually had 2 timing settings. 8 AND 10 degrees BTDC. 8 for use with 87 octane fuel and 10 for higher octane (91 I believe.)

You should set the timing where the manual specifies.

Regards,


Rick

 

[grewvin1]

Re: Timing Debate, 8 Deg. verses 10 Deg.

P.S. The Merc service manual calls for 8 degs BTDC...

Thanks for the input, makes me feel better, I'm using common 87 octane and have set the timing to 8 deg. BTDC.

 

[hottoddie]

Re: Timing Debate, 8 Deg. verses 10 Deg.

Initial timing is not as important as total timing advance. You need to determine the total timing advance and set using a digital advance timing light for optimum performance. The Thunderbolt IV has elctronic advance and will be more reliable than mechanical advance in some of the older motors.

 

[wca_tim]

Re: Timing Debate, 8 Deg. verses 10 Deg.

both advance and total timing are important, however total timing impacts the broadest range of performance and arguably the most important range. with a given set up if you're maximizing performance, once you get jets right (maybe a touch rich) and are propped right, advance timing 2 degrees at a time until you either don't pick up any more top end at wide open throttle (you're really just using the boat pushing through the water as a dyno... and are increasing total timing until horsepower doesn't increase with increasing timing) or until you get detonation. Back off 2 degrees for a margin of safety... That's running the crappiest fuel you would plan to run just to be sure.

For general use, go with the manufacturing specs and unless you're really good about keeping things carefully in proper tune and making sure you run high quality fuel all the time, I would err on the side of caution and go with the lower number. The decrease in performance is not going to be noticeable under the conditions most operate their boats most of the time...

 

[Silvertip]

Re: Timing Debate, 8 Deg. verses 10 Deg.

Full advance is not the issue here. A boat engine is under extreme load at hole shot time which is wrll before full advance occurs. Detonation occurs at low rpm high load just as much as light loading at high rpm. Set the darnef thing to spec. If two specs based on octane then chose which you prefer. No need to run anything other than 87 octane unless the manufacturer calls for it.