LMAC

Put my motor back on the dyno today, mainly to check Air/fuel ratio and to test some carb spacers.
I built this Demon carburated 410 ci. SBC last year and dyno'd it last year with dyno headers: 496HP.@5400, 526 TQ@4500. A/F ratio 14.7 @WOT (yeah a bit lean) and the motor had some "run on" when we killed it. This "run on" would only last for about a second, but usually would result in backward engine spin. I went up on main jets 4 sizes from the 78 primary and 86 secondary, but did this off the Dyno. I regularly monitor plugs, and the motor has never had any "run on" in the boat.

So now to my results today, best of 487HP@5400, 517TQ@4400. A/F ratio 14.2 and the same old dyno "run on"
I went up 2 more sizes on jets, this helped the run on a little a bit and brought A/F down to the mid 13's.
I swapped my 1" four hole spacer (used in all previous tests) for a 1" open spacer. This lost about 5-8 HP and TQ across the board except at 5300-5400, gained 1hp and 1ftp, but no "run on". Next I tried no spacer, same result HP and TQ down, but no "run on". Put the 4-hole back on, power was back up and so was the "run on".
So am I still too lean? What's the deal with the spacers? I realize that my wet exhaust will fatten up the A/F, I'm just puzzled about this on the Dyno.

SB

iTrader: (1)

Run on can be from simple things like too high idle or shutting off at too high rpm , also.

List engine specs / parts used / timing / fuel used, etc.

RumRunner

Is this an actual A/F ratio from a WBO2 sensor, or a calculated number from the dyno?

Also, how much timing are you running at idle, and total?

racinfever

How big is your carb (cfm) Maybe to big , giving poor signal to the booster ? THats why it likes the carb spacer.

LMAC

Air flow calculated from dyno. Timing 21 degrees initial, 33 total.
750 Marine demon mechanical sec. After each pull, engine would idle down to 875 RPM and a/f would stabilize in the low 14's.
Like I said in the boat, even after a hard run, it will shut down smoothly.

4.00 stroke
4.04 bore
228/[email protected] 112 LSA setup on 110 ICL
10.3 static CR.
RPM Air Gap
91 octane

Without recurving the dist. I pulled total down to 30 which put initial @18 really no difference in run on, but a substantial difference in power.

RumRunner

Your Air Fuel Ratio numbers are WRONG! You may be able to idle properly at that kind A/F but you’re not going to run properly there. What are you running for an ignition system to get that kind of curve? 21 degrees would be extremely high for a combination like this; it should probably be closer to 15 or 16 degrees. If it doesn’t run on in the boat it may be due to the way your dyno operating is loading the engine. A 4-hole carburetor spacer usually is going to draw more fuel through the carburetor. As far as your tuning goes I would go with what runs the best in the boat, not on the pump.

SB

iTrader: (1)

I'm guessing that with your cylinder pressure, higher water temps of dyno, fuel octane, and possibility of some fuel still dripping out of boosters, is causing your run on.

What was water temp when this happened ?

The colder lake/ocean water and cyl head temps probably are keeping this from happening.

Can't guess if there is some other issue.

If no WB 02 the dyno's use air flow readings and fuel flow readings to determine A/F ratio's. Not accurate this way.

LMAC

162 water temp. Mallory Dist., sloted adjustable plate. I can set as little or as much advance as I want within it's parameters. This motor likes 20 degrees initial without stumbling when blipping the throttle. I realize the caculated a/f isn't exact, and I ran this motor all of last year without any problems and it ran STRONG. I believe that my setup is fine for this application.

So maybe the 4-hole, unlike the open spacer, is creating a stronger signal and trying to pull fuel out of the boosters?
This dyno session was mainly to see the difference in power after a year and to also test spacers to see if any difference.
It looks like power was down about 2% from last year.

Strip Poker 388

iTrader: (6)

At Idle....doesent a carb pull fuel from the idle transfer slots that not regulated by the main jets ????????

SB

iTrader: (1)

Yup.

But at close to 900rpm if just using primary throttle blades for idle speed they may be open just enough to uncover them.........Also, a little too high float would dribble some thru boosters also.

Having one spacer do it vs another spacer would lead to possible chance this is happening.

Speaking of which - that cid's, rpm, and hp the open spacer should have made some more power. A few possible reasons why it didn't is #1 intake manifold too much of a restriction and/or #2 carb calibration is off.

A dual plane intake on top of an engine like this is going to have a 'killer draw' on the boosters and thus pull mucho fuel from them. A 500hp 410cid SBC is moving quite some air.