Correct air fuel ratio and tuning help while I have dyno time availabl - Chevelle Tech
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post #1 of 14 (permalink) Old Jul 12th, 19, 6:30 PM Thread Starter
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Correct air fuel ratio and tuning help while I have dyno time availabl

Looking for some help and or suggestions for tuning this engine in. This is the first pass on dyno NA after the initial break in. Wet kit 200 shot on a plate will be going on this weekend and then more actual dyno tuning next week. Have 2 days scheduled on the dyno. What is the ideal A/F ratio to have this engine at. Dyno shop is not very current on carb tuning. They are more efi. Carb is a holly 4150 950hp “ boost referenced” because that is the carb I had in the garage so I used it. Any carb and or set up recommendations from those more experienced is greatly appreciated. Street strip iron 026 head big block. Dyno is linked below. Hoping to break into the 10’s? with my 275 street 71. Thanks guys just trying to take full advantage of the dyno time before it goes in the car. Will likely be getting a converter from mark. 28” tire 4:10 gear th400.
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post #2 of 14 (permalink) Old Jul 13th, 19, 11:50 AM
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The way we do our cars is read the plugs when spraying a motor as far as setting up the air/fuel ratio. But the way we do it is we run ngk with heat range of 10 that way it will foil the plug before it messes anything up. Idk what brand nitrous kit you are running but all of them run different numbers on jets we run the nos wet system. If you use the numbers they send with the kit it will be set fat on the fuel side. Then we make a pass and as soon as we cross the line we shut it down have someone at the end ready we pull the easiest to get to plug out and replace with a new plug then come back and check how the plug looks then adjust from there on the fuel side. If you are worried about melting a piston down as long as you have the fuel on the fat side you will foil the plug out and save the motor. When you hear about these racers melting pistons it's bc they have got the car right on the verge of running to lean and are running hotter plugs. I spray 175 on mine and get about 10 passes out of a set of plugs. Yea it cost more to do it this way as far as plugs but we have never melted a piston either so 30 on plugs vs10,000-20,000 on a motor. I'll keep buying plugs
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post #3 of 14 (permalink) Old Jul 15th, 19, 3:14 AM
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If you are going to run a carb that is set up with a boost referenced power valve, but use on a NA engine, you need to remove and block the power valve, or modify the power valve to read vacuum properly again.

Best would likely be to put a different carb on and sell the boost referenced one you have.
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post #4 of 14 (permalink) Old Jul 15th, 19, 7:55 AM Thread Starter
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Would hooking the boost reference port up to the plate under the carb be adequate? If I simply plug it, will I just be running fat at idle/low rpm? What would be an ideal carb to run, do you believe I’m leaving anything on the table by running this holly 80577s. Thank you I will be reading the plugs after the dyno pulls to verify.
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post #5 of 14 (permalink) Old Jul 15th, 19, 4:04 PM
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Re: Correct air fuel ratio and tuning help while I have dyno time avai

Around 13 is a good number. You're a tad rich but have a little safety margin. Is that pull without spray?

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post #6 of 14 (permalink) Old Jul 15th, 19, 6:12 PM Thread Starter
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Thank you. Yes that was the first pull after the break in sequence. All motor. Spray is going on Thursday.
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post #7 of 14 (permalink) Old Jul 15th, 19, 7:10 PM
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Re: Correct air fuel ratio and tuning help while I have dyno time avai

Quote:
Originally Posted by 71malibuu View Post
Would hooking the boost reference port up to the plate under the carb be adequate? If I simply plug it, will I just be running fat at idle/low rpm? What would be an ideal carb to run, do you believe Iím leaving anything on the table by running this holly 80577s. Thank you I will be reading the plugs after the dyno pulls to verify.
Yes, connect it to the full manifold vacuum port on the carb baseplate. Not the ported vacuum port.

That carb is a good choice IMO. It is a true 950 with 750 sized venturi and 850 size baseplate. A Q950 quick fuel may give you a gain at the track, as it flows more cfm having larger venturi. But what you have will rev and respond quicker as it has more velocity through the venturi.
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post #8 of 14 (permalink) Old Jul 15th, 19, 7:12 PM
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Re: Correct air fuel ratio and tuning help while I have dyno time avai

I would confirm that the IFR is located low in the metering blocks, and that the emulsion only has 2 open per side, not the new 3, 4 or 5 open emulsion packages. The metering is more linear with the old school emulsion.
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post #9 of 14 (permalink) Old Jul 15th, 19, 8:05 PM Thread Starter
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Pockets thank you. So to clarify I should take the hose from the boost reference port on the carb and attach it to the vacuum port on my nitrous plate? Or should I drill and tap my Vic jr and attach it there? Would you mind going a little in detail about the ifr and emulsion, my carb is 2 years old, model number is posted above.
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post #10 of 14 (permalink) Old Jul 15th, 19, 8:09 PM Thread Starter
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Also would I have to change the diaphragm/spring to account for the lack of vac/boost, to run the boost reference on a nA set up? I’m not very familiar with this carb, I’m sure it will clean up the idle a bit with the port plugged in. But will it perform a function at wot? Being na?
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post #11 of 14 (permalink) Old Jul 15th, 19, 9:15 PM
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Re: Correct air fuel ratio and tuning help while I have dyno time avai

N/A I would be looking to be .83 to .81 Lambda (12.2 to 11.90:1 on gas scale)

that's what I always tuned my N/A stuff to, and that's where it made the best/most consistent power.



by nature, a properly prepped blowthrough carburetor with properly sized main well, e-hole configuration, HSAB'S and power valve restrictors is just going to work poorly N/A. What we do to a carburetor to work good as a blowthrough pretty much screws it up for ever being a good N/A gas carb again.

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post #12 of 14 (permalink) Old Jul 16th, 19, 4:01 PM Thread Starter
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Thanks for the responses guys. The carb was set up on my roots blower. I’m going to install this poly carb spacer with 2 vac ports above my nitrous plate. 1 so I can hook up vac brakes, and 2 so I can hopefully figure out a decent pcv set up so I can stop looking at wet valve covers. Also was going to hook up the boost reference port to this plate to give the carb a vac signal. Any thoughts or is their a better way.
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post #13 of 14 (permalink) Old Jul 17th, 19, 6:38 AM
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Re: Correct air fuel ratio and tuning help while I have dyno time avai

sorry, saw boost ref power valve and figured it was a blowthrough.

yes hooking the boost ref line up to a carb spacer will work.

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post #14 of 14 (permalink) Old Aug 5th, 19, 8:01 AM Thread Starter
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Odd dyno run? Does this run seem a little odd? Compared to previous dyno graph. What could have changed same dyno same engine. New dyno operator. Was doing a baseline for before adding spacer and wet plate. How do I add a video? Apparently it’s only showing my photos
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