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BBC Timing Question

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srfdude

Member
I've been trying to find a definitive answer, perhaps there isn't one. My Crusader manual has curves for the Mk IV engine, and it lumps it in with the small block, running up to as high as 35 at around 4000 rpm. On forums folks seem to be saying the big block doesn't want that much advance. Is there an solution for this? I'm ordering a pair of Davis Unified distributors, and probably a spring kit as they are somewhat random out of the factory according to Summit.
 
A friendly FYI.......

TA numbers (I.E., total spark advance) without an associated RPM are rather meaningless.
The RPM at which the engineers want xx degrees of “full in” spark lead is very important!


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I run DUI distributors in my Crusader 454s. Have the total advance set at 32 degrees at 3200 rpms. The run strong and the platinum plugs look great. This season 3 since complete overhauls and think I'll do compression checks this year just out of curiosity? I know the DUIs have the total advance come in a little sooner than 3200 rpm but when cruising I run at 3100 TO 3200 RPM anyway.
 
Great information! I'm just getting my 34C engines broken in, it looks like around 3100-3200 will be the sweet spot to keep it on step, running around 18 kts. How many hours do you have on yours now? Do you idle all the time when drifting/stopped? I was having some slight heating problems when first going, but that seems to have gone away with some hours. I called Performance and asked them how they set the DUIs for Chevys, they said they run at 34 degrees/3200 rpms for either BBC or SBC. I may cut the idle advance back a little, dropping that 34 to 32 degrees. Very happy so far, I changed the worn out Rochesters to new Edelbrocks/Performer manifolds and my fuel usage has dropped considerably. Also installed Comp Cams marine torque cams, but it still idles as low as 200. I'm like a kid in a toy box!
 
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srfdude said:
Great information! I'm just getting my 34C engines broken in, it looks like around 3100-3200 will be the sweet spot to keep it on step, running around 18 kts.
Your best performance for most any planing hull is when the engine(s) are able to get the hull up and out of the water, and then some.
The "then some" is very important!

I called Performance and asked them how they set the DUIs for Chevys, they said they run at 34 degrees/3200 rpm for either BBC or SBC.
While that may work for the BBC, the Marine SBC is typically built using the GM Full Dished pistons.
FYI..... 34* @ 3,200 RPM is risky for this build!
Click to expand...
 
Thanks for your prompt reply, Rick. I am interested in your comment re: out of the water "and then some". Could you amplify on that, please? I tend to agree you with the comment about 34º being too much, which is why I will have to back it off at idle. 8º will work, as I don't plan on idling that much. I'll just have to watch the temp.
 
Ok...... once we reach the speed at which the hull is now planing, we have effectively reduced the amount of power required to push the hull through the water. However, at this speed we are running at an RPM that just keeps the hull up on step.
If we take it up just a tad bit more, we will find that “sweet spot” that is even better and more economical.
That is what I was referring to as the “then some”!


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Two observations: 1) A 34C is likely to present a significant load, even to a pair of big blocks so I'd encourage you to be conservative as you dialin the timing on your engines & 2) The factory spec set was determined in a different time frame and set of circumstances - you want to adjust accordingly....fuel today is significantly different from a few decades ago....
 
Thanks for the tip. Never thought about the gas nowadays, not sure we even have 89. Think I'll call Performance and get some advice, its possible there are parts available without sending it back.
 
srfdude said:

I've been trying to find a definitive answer, perhaps there isn't one. My Crusader manual has curves for the Mk IV engine, and it lumps it in with the small block, running up to as high as 35 at around 4000 rpm. On forums folks seem to be saying the big block doesn't want that much advance. Is there an solution for this? I'm ordering a pair of Davis Unified distributors,
Click to expand...




Is this the ignition distributor that you are ordering?

Davis Unified Ignition Marine Performance D.U.I. Distributors M12720BL

dui-127212bl_w.jpg

If so, these are HEI with magnetic triggering and mechanical advance.


Here is a Q and A from their website:


Q: What advance curve do they put into the distributor or is this something I can request? I know they put what I wanted for the one in my race car when ordered direct from DUI.

Asked by MARK on February 21, 2017

A:Thank you for your question. Davis Unified Ignition M12720BL can be ordered with a requested advance curve.

Please call one of our sales agents at 800-230-3030 for pricing and to order.
from-sre.jpg
Summit Racing Answer - March 02, 2017
srfdude said:
I'm ordering a pair of Davis Unified distributors, and probably a spring kit as they are somewhat random out of the factory according to Summit.
Click to expand...

Be very careful when changing flyweight return spring values.
One small change to the spring value can have a drastic affect on the progressive advance.
Excessive ignition advance, and/or ignition advance that comes on too early may cause severe engine damage!

My suggestion:
If you want to/need to change the advance curve (by changing flyweight return springs) do this while the unit is on a Sun, Allen or King distributor machine, and with your OEM ignition advance curve in hand.
Change the springs as needed as to obtain the curve that is desired.

If you are exceptionally well experienced, this can be done "on engine" without the need of the distributor machine.
I would still suggest using the machine!


NOTES:

A change to the springs themselves will not change the degrees of advance. This will change the rate of advance as per RPM only.

With mechanical advance, any change to BASE/Initial will cause an equal change to the TA.


Good luck with it.



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