Tuning a blow through carb with boost

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thatblue_67stang
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Tuning a blow through carb with boost

Post #1 by thatblue_67stang » Mon May 21, 2018 12:31 am

I need help tuning or getting my carb to run not so rich at idle but rich enough at wot. I have electric fuel pump. Holley boost reference fuel regulator, holley 2300 with boost referenced bowl and power valve. Is there anything else I can do to help it not run so rich at idle and run good at wot with boost. I'm wondering if drilling the powe valve holes with help. Any help is appreciated
Brayton
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pmuller9
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Re: Tuning a blow through carb with boost

Post #2 by pmuller9 » Mon May 21, 2018 10:02 am

Drilling the power valve hole is one of the important modifications to a blow-thru carb.

You should have been able to adjust the idle mixture with the 2 idle adjustment screws.
Where you able to adjust the idle for maximum idle vacuum?

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Re: Tuning a blow through carb with boost

Post #3 by powerband » Mon May 21, 2018 1:59 pm

I'm new to Forced Induction and want to learn more, here's a few comments:

Which Holley 2300 are you using 7448-350CFM or 4412 500CFM carb?. Do you use an AFR gauge?.

My supercharged 250 blow-thru setup started with a 500 cfm 4412 and I could not get near a good AFR. The AFR was way too rich with the 4412 and typical 'H18' mods. I had no success with enlarged PV ports . Now AFR's are getting closer to 'Stoichiometric' levels with a 350-7448 and incrementally smaller main jets. currently running '62' main jets, 7448 has blow out protector sealed for bowl-pressure boost-ref Power Valve and some 'Hangar18' type sealing mods. My cruise and WOT AFR's are very good @ 14:1 typically but still idle/low speed is a little fat @ 12:1 . I have Ignition locked @ 28 degrees @ full , Elec fuel pump and carb hat boost ref' fuel pressure reg.'.

I made no progress tuning until I added a fuel bowl vent extension @ 4" into charger tube. Without extension carb would run OK until boost then swing from lean to rich, load up and sputter.

I'm running @ 5-8lbs boost max with the Vortech SC, previous 250 with T3 turbo didn't like 15-20 lbs boost capability - the piston lands blew off with a plume of oily blue smoke across my windshield ...

Have( boosted) Fun



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thatblue_67stang
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Re: Tuning a blow through carb with boost

Post #4 by thatblue_67stang » Mon May 21, 2018 11:12 pm

pmuller9 wrote:Drilling the power valve hole is one of the important modifications to a blow-thru carb.

You should have been able to adjust the idle mixture with the 2 idle adjustment screws.
Where you able to adjust the idle for maximum idle vacuum?



yes i have done that. i just am not sure how big to drill the holes for the powervalve
Brayton
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Re: Tuning a blow through carb with boost

Post #5 by thatblue_67stang » Mon May 21, 2018 11:14 pm

powerband wrote:I'm new to Forced Induction and want to learn more, here's a few comments:

Which Holley 2300 are you using 7448-350CFM or 4412 500CFM carb?. Do you use an AFR gauge?.

My supercharged 250 blow-thru setup started with a 500 cfm 4412 and I could not get near a good AFR. The AFR was way too rich with the 4412 and typical 'H18' mods. I had no success with enlarged PV ports . Now AFR's are getting closer to 'Stoichiometric' levels with a 350-7448 and incrementally smaller main jets. currently running '62' main jets, 7448 has blow out protector sealed for bowl-pressure boost-ref Power Valve and some 'Hangar18' type sealing mods. My cruise and WOT AFR's are very good @ 14:1 typically but still idle/low speed is a little fat @ 12:1 . I have Ignition locked @ 28 degrees @ full , Elec fuel pump and carb hat boost ref' fuel pressure reg.'.

I made no progress tuning until I added a fuel bowl vent extension @ 4" into charger tube. Without extension carb would run OK until boost then swing from lean to rich, load up and sputter.

I'm running @ 5-8lbs boost max with the Vortech SC, previous 250 with T3 turbo didn't like 15-20 lbs boost capability - the piston lands blew off with a plume of oily blue smoke across my windshield ...

Have( boosted) Fun



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i would agree that the 500 cfm is to big with the stock log intake but i have cut mine off and made a big intake which seems to love the 500 cfm 2300. i just need to tune it more.
Brayton
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Re: Tuning a blow through carb with boost

Post #6 by thatblue_67stang » Mon May 21, 2018 11:17 pm

yes i run a afr gauge aswell. i have been repairing it so havent driven it in a while.

trying to insert photo but it will not let me
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Re: Tuning a blow through carb with boost

Post #7 by thatblue_67stang » Mon May 21, 2018 11:23 pm

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you may have to click on it but that is my setup, i belive i can use a 500cfm because of the intake i have

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Re: Tuning a blow through carb with boost

Post #8 by pmuller9 » Mon May 21, 2018 11:27 pm

What happens if you simply install larger main jets?

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Re: Tuning a blow through carb with boost

Post #9 by thatblue_67stang » Mon May 21, 2018 11:35 pm

pmuller9 wrote:What happens if you simply install larger main jets?



i dont remember what my cruising afr is, i blew it up bc i put shitty octane in it. so rn i think i have 83 jets in it which seems high but at wot it wanted more gas. i cant remember cruising. tomorrow i should have it back together and driving again.
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Re: Tuning a blow through carb with boost

Post #10 by pmuller9 » Mon May 21, 2018 11:44 pm

If you can't jet any higher without making the midrange A/F too rich then you will need to drill the PV passages.
I would just drill a few thousands at a time and check the A/F ratio at WOT each time.

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Re: Tuning a blow through carb with boost

Post #11 by thatblue_67stang » Tue May 22, 2018 12:06 am

pmuller9 wrote:If you can't jet any higher without making the midrange A/F too rich then you will need to drill the PV passages.
I would just drill a few thousands at a time and check the A/F ratio at WOT each time.

Do you think the holley 2300 500cfm is to much for my setup. I think it is good for my intake
Brayton
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Re: Tuning a blow through carb with boost

Post #12 by pmuller9 » Tue May 22, 2018 9:41 am

thatblue_67stang wrote:Do you think the holley 2300 500cfm is to much for my setup. I think it is good for my intake

What cam do you have in the engine?

What is the maximum RPM you will run the engine at?
Last edited by pmuller9 on Tue May 22, 2018 10:09 am, edited 4 times in total.

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Re: Tuning a blow through carb with boost

Post #13 by thatblue_67stang » Tue May 22, 2018 10:01 am

pmuller9 wrote:
thatblue_67stang wrote:Do you think the holley 2300 500cfm is to much for my setup. I think it is good for my intake

What cam do you have in the engine?

What is the maximum RPM you will run the engine at?


Stock cam for now and just 5k rpm max is what my rev limiter is at right now
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Re: Tuning a blow through carb with boost

Post #14 by pmuller9 » Tue May 22, 2018 10:07 am

Since your intake manifold has a large volume a 2300 350 would be a better choice.
The manifold volume supplies the peak cylinder flows while the carburetor supplies the average flow.

5000 rpm and a VE of 80% and an engine displacement or 200 cu inches.

5000 x 200 x 80% divided by 3456 = 231 cfm average flow.

A 2300 350 actual flow rating at a depression of 1.5 inches is 350 x .707 = 247 cfm.

Note: 2 barrel carbs are rated at 3 inches of mercury while 4 barrels carbs are rated at half the vacuum or 1.5 inches.

The least expensive forged pistons are supplied by Autotec.

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Re: Tuning a blow through carb with boost

Post #15 by drag-200stang » Tue May 22, 2018 10:53 am

pmuller9 wrote:Since your intake manifold has a large volume a 2300 350 would be a better choice.
The manifold volume supplies the peak cylinder flows while the carburetor supplies the average flow.

5000 rpm and a VE of 80% and an engine displacement or 200 cu inches.

5000 x 200 x 80% divided by 3456 = 231 cfm average flow.

A 2300 350 actual flow rating at a depression of 1.5 inches is 350 x .707 = 247 cfm.

Note: 2 barrel carbs are rated at 3 inches of mercury while 4 barrels carbs are rated at half the vacuum or 1.5 inches.

The least expensive forged pistons are supplied by Autotec.

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Re: Tuning a blow through carb with boost

Post #16 by thatblue_67stang » Tue May 22, 2018 11:03 am

pmuller9 wrote:Since your intake manifold has a large volume a 2300 350 would be a better choice.
The manifold volume supplies the peak cylinder flows while the carburetor supplies the average flow.

5000 rpm and a VE of 80% and an engine displacement or 200 cu inches.

5000 x 200 x 80% divided by 3456 = 231 cfm average flow.

A 2300 350 actual flow rating at a depression of 1.5 inches is 350 x .707 = 247 cfm.

Note: 2 barrel carbs are rated at 3 inches of mercury while 4 barrels carbs are rated at half the vacuum or 1.5 inches.

The least expensive forged pistons are supplied by Autotec.



I'm going to see if I can get thr 500 to work just because I have it, if not I'll get a 350. Thank you for your help.
Brayton
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