When you Port a head and pick up flow alot does it usually require more jetting?

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When you Port a head and pick up flow alot does it usually require more jetting?

Post by NXBOY » Sun Jun 03, 2018 6:03 am

I did a set of Dove ford bigblock heads with CJ valves. I would jet up and read plugs but it aint my car and trying to get him to try as car gets warm 230 on trips. Runs cool in traffic. It worse on long hills. I feel the combustion temps the problem.I feel More fan doesn't totally fix combustion temp much and don't want them to crack. Car pulls Harder , sounds great , just seems flat on upstair power above 5,000 rpms and sounded lean. Just looking for more Logic to help him get it up to atleast stock jetting. Feel with 10.5 plus compression on 93 octane it should be a little rich in front to help cool chambers. I know Fuel quality of port work, like bur finish will help mixture and not require more jet. But if all else is same but head flow is alot better how that effect jetting, if it does. I know exhaust flow does as far as ex. system. Car seems fine with new cam and pump gas except 3,000 rpm cruising hot days. Cams was installed at 105 ICL and went up 6 degrees on intake lobe to 250I@/050/260E 621 lift ,108 LSA from 110 . Old cam was a 292 magnum single pattern 244 on 110 .560 lift, installed 108 ICL. Glad I polished chambers lol. Its the 64 Galaxie you guys help me with. Car used about the same gas on trip. Needs to put restrictors in Power Valve Channel to only add 4 jets of fuel and jet way up possibly. Or maybe smaller airbleed?? Heavy car takes some power to pull at 3,000 lol I think its part of the problem, its in the lean spot between the circuits.

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Re: When you Port a head and pick up flow alot does it usually require more jetting?

Post by NXBOY » Sun Jun 03, 2018 6:30 am

Stock jetting on his 950 HP is 79 front, when first road tested he had 73 in. Talked him over phone to 76 so it wouldnt effect readjusting mixture screws for sure like 79 might. Cam making more Cylinder pressure/torque at 3,000 so that might be why its running hotter. Glad to hear sudgestions, Wish I still lived in the city, i would work on it more lol. Its a Nice Ride!

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Re: When you Port a head and pick up flow alot does it usually require more jetting?

Post by travis » Sun Jun 03, 2018 8:11 am

Did it run cool before the upgrades? Too lean will make it run hot as will not enough timing. Does it have vacuum advance?

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Re: When you Port a head and pick up flow alot does it usually require more jetting?

Post by NormS » Sun Jun 03, 2018 9:13 am

Rather than change the ratio between jet size and power valve channel restriction size, I suggest using a power valve with a higher vacuum rating on the primary end. This will bring the power enrichment in sooner as you get into more throttle, like pulling long up grades.
Another thing that help a lot on street driven cars is to insulate the fuel lines in the car anywhere that they subjected to heat. Keeping the fuel cooler keeps its density up, and that helps prevent a leaner and leaner mixture coming from the carb, as fuel gets hotter and hotter.
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Re: When you Port a head and pick up flow alot does it usually require more jetting?

Post by Walter R. Malik » Sun Jun 03, 2018 10:13 am

NXBOY wrote:
Sun Jun 03, 2018 6:03 am
I did a set of Dove ford bigblock heads with CJ valves. I would jet up and read plugs but it aint my car and trying to get him to try as car gets warm 230 on trips. Runs cool in traffic. It worse on long hills. I feel the combustion temps the problem.I feel More fan doesn't totally fix combustion temp much and don't want them to crack. Car pulls Harder , sounds great , just seems flat on upstair power above 5,000 rpms and sounded lean. Just looking for more Logic to help him get it up to atleast stock jetting. Feel with 10.5 plus compression on 93 octane it should be a little rich in front to help cool chambers. I know Fuel quality of port work, like bur finish will help mixture and not require more jet. But if all else is same but head flow is alot better how that effect jetting, if it does. I know exhaust flow does as far as ex. system. Car seems fine with new cam and pump gas except 3,000 rpm cruising hot days. Cams was installed at 105 ICL and went up 6 degrees on intake lobe to 250I@/050/260E 621 lift ,108 LSA from 110 . Old cam was a 292 magnum single pattern 244 on 110 .560 lift, installed 108 ICL. Glad I polished chambers lol. Its the 64 Galaxie you guys help me with. Car used about the same gas on trip. Needs to put restrictors in Power Valve Channel to only add 4 jets of fuel and jet way up possibly. Or maybe smaller airbleed?? Heavy car takes some power to pull at 3,000 lol I think its part of the problem, its in the lean spot between the circuits.
In my opinion ... it is to lean or not enough ignition timing or both at THAT continuous throttle opening and RPM used for steady highway cruise trips.
There should not be any lean spot between circuits ... they should overlap enough to smoothly transition from one to the other.

Using a correctly tuned vacuum advance usually band-aids that type problem.
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Re: When you Port a head and pick up flow alot does it usually require more jetting?

Post by PackardV8 » Sun Jun 03, 2018 11:42 am

FWIW, After sixty years of turning carb screws, I finally learned how much I still didn't know. I no longer tackle carb mixture without a wideband O2 senor and readout.

The other reason for O2 sensor readout is today's fuels make spark plug reading much more difficult. Back in the bad old days of leaded fuel, plug insulator color and tailpipe color would tell pretty close to A/F at cruise. Not so much today.
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Re: When you Port a head and pick up flow alot does it usually require more jetting?

Post by groberts101 » Sun Jun 03, 2018 11:44 am

Walter R. Malik wrote:
Sun Jun 03, 2018 10:13 am
NXBOY wrote:
Sun Jun 03, 2018 6:03 am
I did a set of Dove ford bigblock heads with CJ valves. I would jet up and read plugs but it aint my car and trying to get him to try as car gets warm 230 on trips. Runs cool in traffic. It worse on long hills. I feel the combustion temps the problem.I feel More fan doesn't totally fix combustion temp much and don't want them to crack. Car pulls Harder , sounds great , just seems flat on upstair power above 5,000 rpms and sounded lean. Just looking for more Logic to help him get it up to atleast stock jetting. Feel with 10.5 plus compression on 93 octane it should be a little rich in front to help cool chambers. I know Fuel quality of port work, like bur finish will help mixture and not require more jet. But if all else is same but head flow is alot better how that effect jetting, if it does. I know exhaust flow does as far as ex. system. Car seems fine with new cam and pump gas except 3,000 rpm cruising hot days. Cams was installed at 105 ICL and went up 6 degrees on intake lobe to 250I@/050/260E 621 lift ,108 LSA from 110 . Old cam was a 292 magnum single pattern 244 on 110 .560 lift, installed 108 ICL. Glad I polished chambers lol. Its the 64 Galaxie you guys help me with. Car used about the same gas on trip. Needs to put restrictors in Power Valve Channel to only add 4 jets of fuel and jet way up possibly. Or maybe smaller airbleed?? Heavy car takes some power to pull at 3,000 lol I think its part of the problem, its in the lean spot between the circuits.
In my opinion ... it is to lean or not enough ignition timing or both at THAT continuous throttle opening and RPM used for steady highway cruise trips.
There should not be any lean spot between circuits ... they should overlap enough to smoothly transition from one to the other.

Using a correctly tuned vacuum advance usually band-aids that type problem.
I am also of that same opinion as well. Seen it happen and have corrected similar situations many times. From my experiences, nearly 8 times out of 10 it's more ignition advance related than fueling because everyone gets so caught up on WOT spark requirements which are typically far different than the combustion dynamics relating to lighter load/part throttle operation. Less densely packed fuel and air molecules along with far slower mixture speeds under light load part throttle scenarios require much more spark lead to maximize cylinder pressures/BMEP.

What many don't realize is that the ignition timing needs to be calibrated to a tighter tolerance/more idealized before the fueling can be fully optimized. Too little timing and they end up rich out the tailpipe, so many incorrectly work towards leaning it out. Too much ignition spark lead(sometimes they will not always ping but power flattens and falls over) and they may begin to run lean out the tailpipe and then some tuners are mislead towards enrichening it to compensate. Then power falls over even further, more throttle is applied to help compensate and fuel mileage and efficiency goes to hell in a handbasket. Oil often gets dirty faster and carbon accumulations build up far quicker as well. Because each one(fuel and timing curves) acts with or against the other it usually takes a bit of back and forth to get it all dialed in correctly for each applications throttle angle over the various loads.

An adjustable vac pot that has been dialed in correctly for that specific application is well worth its weight in gold when it comes to the added ignition lead required during those types of combustion dynamics. Many think that 40+ degrees is far too much but they are mistaken and many computer based timing maps will exceed 50° during low throttle/low load situations. Get it closer to right and you start getting into lean burn territory and everything works better and stays cleaner overall.

PS. forgot to mention.. one can also use a handheld vac pump while in the drivers seat connected to a long hose running to the vac advance pot. That way you can bleed off or add vacuum to fine tune the correct part throttle cruise advance needed for a specific application. I've experienced cars pulling to higher MPH under exact same throttle angle when vacuum is applied(producing more power through increased cylinder pressures).. and vice versa. Some will even begin to surge or misfire as they go lean from the added advance which points towards LSB air bleed and/or IFR tuning requirements. Without an adjustable vac pot you'll just end up robbing peter to pay paul. You can also dial in the wide open throttle timing curve with that same method as well but need to make an rpm based ignition lead plot and then figure out what and how each specific vac reading on the hand pumps gauge relates to xx amount of additional spark lead. Also tried it prepumped to a specific timing mark and then released near full rpm peak to see if spark retard may be in order as well. Handheld vac pumps can be a very versatile tool for timing advance calibration if used correctly. And used in conjunction with a wideband AFR gauge will be very enlightening once you fully realize that many/maybe most motors don't like some arbitrary or theoretical "idealized AFR number" to make their maximum cylinder pressures.

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Re: When you Port a head and pick up flow alot does it usually require more jetting?

Post by NXBOY » Sun Jun 03, 2018 6:25 pm

NormS wrote:
Sun Jun 03, 2018 9:13 am
Rather than change the ratio between jet size and power valve channel restriction size, I suggest using a power valve with a higher vacuum rating on the primary end. This will bring the power enrichment in sooner as you get into more throttle, like pulling long up grades.
Another thing that help a lot on street driven cars is to insulate the fuel lines in the car anywhere that they subjected to heat. Keeping the fuel cooler keeps its density up, and that helps prevent a leaner and leaner mixture coming from the carb, as fuel gets hotter and hotter.
Car Idles with 9# of vacuum then in gear about 7.5. I would be afraid to run less then a 6.5 to keep from loading up and dipping effect lol. I would think hes a long way at 3,000 with 3.73 gears to drop vacuum to 6.5 pounds at cruise. No power valve I could work with, but gas milage should suffer some. Timing is locked out @ 38 , should he try more first? I was thinking maybe richen through low side air bleeds, maybe even emulsion circuit. Plugs burn good, might run mixtures on the slightly rich side to see if it helps. He needs to drill the 1 inch spacer for plenium vacuum source. Dont like PVC only coming from one cylinder. He had a Oxygen sensor hooked to his computer someway but it said Idle was lean when the power valve was open at idle(not enough vacuum for 10.5) and fumes burnt my eyes so its not working right. I will think about modifying one of my durasparks with vacuum cannister if he wants. I would weld up one side to about 10 dgrees and put a screw in to limit vacuum advance.

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Re: When you Port a head and pick up flow alot does it usually require more jetting?

Post by Walter R. Malik » Sun Jun 03, 2018 7:15 pm

ANY power valve number, even if open, will not have an effect on the idle mixture unless some fuel mist is coming out of the booster; though the power valve should be closed during idle speed.
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Re: When you Port a head and pick up flow alot does it usually require more jetting?

Post by NXBOY » Sun Jun 03, 2018 8:48 pm

Maybe so, but the back butterflys where closed off completely and the idle mixture screws weren't so the A/F meter was definately wrong. Sure see a ton of cars die when vacuum gets below the vacuum of the power valve after a short time because its starts loading up then. Step up one number PV Bingo Idles fixed. I know Books say jetting up doesnt effect idle mixture also, but after about 6 over stock it sure does. Different circuit I know. But most carb Gurus say the same I've talked too it does bleed over somehow.

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Re: When you Port a head and pick up flow alot does it usually require more jetting?

Post by cjperformance » Sun Jun 03, 2018 8:54 pm

Walter R. Malik wrote:
Sun Jun 03, 2018 7:15 pm
]ANY power valve number, even if open, will not have an effect on the idle mixture unless some fuel mist is coming out of the booster though the power valve should be closed during idle speed.
Exactly. X2
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Re: When you Port a head and pick up flow alot does it usually require more jetting?

Post by cjperformance » Sun Jun 03, 2018 8:58 pm

If PV affects idle you have another problem. Leaking PV diapram, butterflies open too far, PV cracked, leaky PV gasket etc
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Re: When you Port a head and pick up flow alot does it usually require more jetting?

Post by groberts101 » Sun Jun 03, 2018 9:10 pm

I would seriously look at getting a distributor vac pot installed and limited to about 8-10 degrees assist. Or.. if you want to do a quick-n-dirty test to show what that bigger cam likes for advance?.. lock it at 40-42 and watch the idle come up. Could also go higher to see what the idle advance number could be.. but I'd be extremely cautious of anything more than doing just a quick bunny hop or two to check the newfound transient throttle response created by the higher manifold vac/booster tesponse. Readjust mixture screws and take it for a VERY light throttle cruise only test drive. Don't hammer on it or lug it.. nice and easy cruise testing only. I bet small cash manifold vacuum rises with the extra igmition lead, cruises with a tad less throttle angle, and stays a bit cooler during extended cruise too. If it leans out too much and starts to break up or even surge.. use the old wire double or triple wrapped around the vent tube and inserted into one or both LSB's to enrichen it back up for light cruise.

All this does is show you what initial timing(or in this case, base + vac pot assist) the engine wants and prefers, fully reversible, and you can tackle the vac advance pot from there. If the motor really likes it and he wants to avoid the vac pot altogether.. another option would be one of MSD's programmable 6 or 7 series boxes which is MAP based for a have your cake and eat it too scenario. I've run both boxes and will never go back to an old vac pot on anything I own and drive on the street.

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Re: When you Port a head and pick up flow alot does it usually require more jetting?

Post by NXBOY » Mon Jun 04, 2018 2:06 am

Most I remember was on single Circuit Idle Carbs. I've seen dripping on carbs booster at Idle before. On street car sure seems to clean them up at idle when P/V is above Idle. I have to test this now lol. Thats why I said maybe so, but i'm testing one on a single circuit so it should pull a little more on boosters and with a decent cam. So ya'll going to tell me jetting carb up like 10 numbers won't effect Idle too? Always wondered why on that one :). :oops:

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Re: When you Port a head and pick up flow alot does it usually require more jetting?

Post by NXBOY » Mon Jun 04, 2018 2:33 am

groberts101 wrote:
Sun Jun 03, 2018 9:10 pm
I would seriously look at getting a distributor vac pot installed and limited to about 8-10 degrees assist. Or.. if you want to do a quick-n-dirty test to show what that bigger cam likes for advance?.. lock it at 40-42 and watch the idle come up. Could also go higher to see what the idle advance number could be.. but I'd be extremely cautious of anything more than doing just a quick bunny hop or two to check the newfound transient throttle response created by the higher manifold vac/booster tesponse. Readjust mixture screws and take it for a VERY light throttle cruise only test drive. Don't hammer on it or lug it.. nice and easy cruise testing only. I bet small cash manifold vacuum rises with the extra igmition lead, cruises with a tad less throttle angle, and stays a bit cooler during extended cruise too. If it leans out too much and starts to break up or even surge.. use the old wire double or triple wrapped around the vent tube and inserted into one or both LSB's to enrichen it back up for light cruise.

All this does is show you what initial timing(or in this case, base + vac pot assist) the engine wants and prefers, fully reversible, and you can tackle the vac advance pot from there. If the motor really likes it and he wants to avoid the vac pot altogether.. another option would be one of MSD's programmable 6 or 7 series boxes which is MAP based for a have your cake and eat it too scenario. I've run both boxes and will never go back to an old vac pot on anything I own and drive on the street.
I got a factory Duraspark we can mess with. I'll give it to him if he wants. I like helping friends with the hotrods but most importantly didn't like the underhood temps after cruise when he got on it and by the way it sounded upstairs. It didnt really want above 5200. Might need to have him put a gallon or two of race gas in and check upstairs power. First thing I did after he got it running was put a vacuum gauge on it. After I clean Idle up and black plugs started to clean I played with timing. Idle wasn't effected at all or vacuum with more or less advance, but that is at 1000 rpms where its best with the tight converter that knocks Idle down alot when put in gear. Thanks guys got alot of good Ideas, Really don't want him running Hot even on long hills. Car didn't get hot before. He's adding more fan I think.

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