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Old 06-08-2004, 12:34 AM
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VE and HP, I have some questions

I have been comparing some Dyno results from my engine last year to my engine this year and wanted some feedback. 347 SBF stroker. #1 had Victor Jr. heads (out of the box) and a Victor Jr. Intake gasket match only. With the cam (fairly aggressive) and 12.4 C/R the motor made 605 HP at 7200 RPM. At peak torque at 5800 VE was 102.6 %. A/F ratio showed at 12.8/1 average.

347 SBF stroker #2: Same bottom end, only changes were that the heads CNC'd and a new Super Victor intake, also CNC'd. Engine made 590 at 7800 (expected with the intake that RPM would be higher) but VE dropped to 90.9 %. A/F ratio was 10.6/1.
Any ideas? Here is the path I am going down...since the Dyno is stationary, and no air is moving accross the carb, the new setup requires a whole bunch more air to reach peak VE and power, so a hood scoop should be the next order of business?

Other thoughts?

Ron

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Old 06-08-2004, 01:05 AM
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3 points in A/F ratio will do it. Dynos never repeat within 5% anyway, don't sweat it. Work on the carbs fuel delivery, 3 points is a lot to be off.

12.5:1 is perfect for power, 10.6:1 is pretty rich and would explain the power difference. Lets not forget we are only talking 16 HP here, not even noticeable from the drivers seat.
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Old 06-08-2004, 07:49 AM
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Ditto with Chuck. You need to get AFR to ~12.5. I would be willing to bet that you will be able to gain back that 16HP and stack a few on top of it.

Chris
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Old 06-08-2004, 08:00 AM
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I agree that correcting the A/F will do the trick. I guess the question is, how can it be achieved. From what I am thinking it can be done 2 ways. Decrease the fuel by jetting down, or ad air. I'm thinking it needs more air, not less fuel. Does that make sense? With the increased port sizes, I would also assume that the velocity of the air going into the motor decreased, so the cylinders were not completely filling, bringing down the VE. Although in both cases the air was fed in the same way, would port size not have an afect? Like doubling the size of the hose on your vacuum cleaner...would it work just as well?

Ron
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Old 06-08-2004, 08:20 AM
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I think you are right about port velocity changing, but I dont think that a ram air effect is going to fix it. Looks to me like you need to go down a jet size and see how it likes it. When intake velocity changes, it certainly affects the carb. If your carb has adjustable air bleeds, you may be able to tweak them to get AFR back where it was. I would try and pay close attention to where AFR is when the engine is accelerating. You might find that it is 12ish in lower RPM and 10ish higher up. It would take digital loging to really see it since gauge reaction might not be fast enough if the engine revs quickly. If that is the case, the air bleed adjustment could help. I have never personally run a carb with the adjustable bleeds, but you are suppose to be able to fine tune the fuel curve with them.

VE will be lower with the rich mixture since you are getting a less efficient burn. I have protested in the past that increased velocity through the carb would result in a richer mixture due to a better signal in the venturi. With your setup, the new intake may be allowing a little better breathing which would increase velocity in the carb.


Chris

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Old 06-08-2004, 10:02 AM
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On the Dyno the AFR average accross the RPM range was 10.6/1.
At the lower RPM's it ran as low 8.36/1 to a high of 11.49 at 8000 RPM.
When I changed to the Super Victor intake, because of the manifold design being taller, the top of the carb is now an inch and a half closer to the hood. Not a lot of space under there.
I did not mention it earlier because it should not have affected anything on the Dyno, but it would have a major impact on airflow in the car.

Ron
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Old 06-08-2004, 10:09 AM
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Well, let us know what you do and how it works out. I am curious about how the tune changed that much with just an intake swap.
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Old 06-08-2004, 12:52 PM
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thoughts on the vacuum hose analogy.........
a hose twice the diameter can move more air, but at a loss of vaccum power....... so while your able to overall move more air, it's not at the same velocity it was in the smaller hose......

i'm justa schnook,but i'd think it would make some difference.....

i can't understand why, after increasing the overall displacement of the intake, why it would need to be jetted down. i'd think that the opposite would be the case....
but af ratios don't lie.....
like i said, i'm justa schnook.
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Old 06-08-2004, 01:02 PM
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The thing to remember is that the carb size did not change. If, with that high RPM operation and head size, the intake was a restrictor, the engine may be flowing more air now than it was before. Since all the air has to now go through the same carb, velocity at the carb venturi is increased. This will produce a stronger signal to pull more fuel for the increased air volume. What I am proposing is that the increase in fuel flow is not linear with increase velocity and therefore the AFR will change with velocity changes. If you think about it this makes good sense. If the fuel flow was linear, there would seldom ever be a need to change jetting since AFR of 12.5 is pretty much always desirable for NA engine at full power.

Just my theory.....fill it full of holes

Chris
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Old 06-08-2004, 01:09 PM
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so.

increased cfm capacity of said carb would increase HP?

theoretically?

or maybe bigger venturi to decrease the amount of fuel being pulled?
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Old 06-08-2004, 01:37 PM
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Larry,

HP on goes up if the AFR is right. A bad AFR will kill power and performance; hence the lower VE that he was seeing on the dyno.

Chris
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Old 06-08-2004, 01:37 PM
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I am curious, whats your rod ratio and length of rod. A;so what was the cross-sectional dia. of the intake ports measured before and after the CNC?
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Old 06-08-2004, 10:30 PM
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Turbo S10...everyone who has looked at the problem agrees with you. The lower VE is actually a good sign, because it indicates (in my thinking) better utilization of the inlet path. Its definitely pulling on the carb harder, and earlier too.

brainsboy...4.035 bore by 3.400 inch stroke. Rod lenght is 5.400. I am not sure I can answer the question on what the cross sectional dia was before and after the CNC. I can however provide the flow numbers after CNC:

0.1 0.2 0.3 0.4 0.5 0.6 0.7
Intake 76 143 204 260 298 314 320
Exhaust 66 125 174 215 244 261 270

Ron
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Old 06-08-2004, 10:43 PM
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Hey Ron! Let me know if you decide to change hoods & the cowl becomes available.

I know a good spot for it.
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Old 06-08-2004, 10:46 PM
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LOL...I have like 3 guys here locally that have had their eyes on that hodd for quite a while.

How are things with you?

Ron
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