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Lagging under load

1777 Views 9 Replies 4 Participants Last post by  tenforty_2
The Engine:
466, c8 heads (mild porting), dual plane stealth w/ 2" spacer, 850 demon (4150), Holley Blue electric pump (-8 lines no return), MSD dizzy, 6al box, AR 32 plugs (68 heads limit spark plug options), 40 deg timing. That should get us started......

The Truck:
1979 F350 4x4, 4 sp, single disc ram clutch (sintered iron), 4.10 gears, used for truck pulling.

The issue:
Falling on its face under load! Only happens briefly and then she comes to life very good. From Idel to WOT without any load I do not notice any lagging at all. I was working with it to day and took it out in a wide open field and just let it idel in 2nd gear, smashed it to WOT and it has a noticable lag. When I hook to the sled, bring the RPMS up to around 5000, start releasing the clutch it just continues to fall way down to the point that you will think it is going to die and then it gradually starts to come back to life.

I was blaming this on my driving technique (not bringing the R's up enough) and the clutch engagement (very aggressive) until I tested it in the field without being hooked to anything. Here is a link to one of my pulls: You will hear the engine lag way down.

Any thoughts of what I should check first?
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When does the total advance come in? What cam? My 512 with the C8 heads liked 34 total at 2900 rpm's. I don't think your tune is quite right.
The Cam:
Comp, Solid Flat Tappet, Gross Lift = .589 .615, @20 = 294 304, @50 = 256 257, 108 separation.

The timing:
My light is broken, but the engine builder tuned it as part of the package. I was at his shop the day it was tuned and I recall him saying 40 deg total timing. I will check it as soon as I get another light. What should it be?

Also it has one "Heavy Silver" & one "Light Silver" spring in the dizzy (MSD). According to the MSD chart for the dizzy that should make it begin to advance at around 1800 and peak out at 5000. The MSD chart stops at 30 deg.

Gonna change to a lighter spring in the dizzy and see what happens. Any other opinions from you experts is welcome!
Is the 2 inch spacer open or a 4 hole? That carb is going to be big and if the spacer is open it may be losing signal as well.
Is the 2 inch spacer open or a 4 hole? That carb is going to be big and if the spacer is open it may be losing signal as well.
It is open.
It is open.

What kind of rpm does this motor see on the top?
What kind of rpm does this motor see on the top?
Not as much as I would like LOL! I ve got it chipped at 7000.......depending on the track, but usually around 6500-6800.
Are you running a big dome on that piston?? It sounds like your are running to much timing. My puller stays at 33 deg. It dosent like more,I am running dove-c heads. 800 CFM carb single open spacer,weiand stealth, 33" tires wth a mild cam (must carry 15 hg vaccum at 850rpm) and it goes down the track at 6700. You may try locking out your timing, buying a 40.00 timing light and setting your timing about 33-34 degrees.

I changed the plugs from a AR 33 to a AR 32, put the lightest spring set in the dizzy and it did much better. Brought it out of the hole at about 5000, no lag and ran about 66-6700 all the way to the end. I think I will go ahead and lock the dizzy out anyway.

Thanks for the help!
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