Jetting with a Blower
02-02-2014, 12:18 PM
#32
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Basically what Icdedppl said. On a N/A aspirated engine, the carb is mounted on the intake manifold, and the power valve reads this intake manifold vacuum. When throttle is opened, manifold vacuum drops, power valve opens.
With roots supercharger, the carb sits on top of the blower. It no longer can read intake manifold vacuum. It reads whatever the vacuum is at the top of the supercharger. In certain scenerios, your intake manifold will see boost psi, but your power valve is still seeing vacuum.
Boost referencing is simply modifying the carb so that you can hook an external vacuum line from the carbs PV circuit, to the intake manifold. This way the power valve only sees manifold vacuum/boost.
With roots supercharger, the carb sits on top of the blower. It no longer can read intake manifold vacuum. It reads whatever the vacuum is at the top of the supercharger. In certain scenerios, your intake manifold will see boost psi, but your power valve is still seeing vacuum.
Boost referencing is simply modifying the carb so that you can hook an external vacuum line from the carbs PV circuit, to the intake manifold. This way the power valve only sees manifold vacuum/boost.
I have to share an experience with you on the boost ref, carb set up. back in the mid 80s I put together a non intercooled blower motor, with water/meth injection at 2lbs of boost. Never having any experience with superchargers I called B&M tech for tuning tips long story short I drilled a port through the throttle plate to intersect with the power valve port, next took a bigger bit and went in 1/4 inch so I could pound in a slosh tube (with Loctite) for a vacuum hose hook up to Manifold. Next I tapped the port in the bottom of the TP and inserted a allen set screw (Loctite) to seal of the manifold vacuum. proceed to tune the carb via jetting. the motor ran great and for it's time was quite a sleeper. Here's the real story, when I sold it the guy took it to (his) mechanic and was told " who in the hell F!!d up this carb? and what is this water tank on here for? The mechanic removed both and installed a stock carb with who knows what jetting, and within a week the motor burned down!! buyer came to me pissed off, I seen what was done and told him to pound sand!! He never did find anyone to get it to run right again! Asked me twice to help but with his bad attitude I just answered "don't think I know how to do that technical ****" LOL Besides that I was busy helping my boat dealer friend put together a set of twin turbo intercooled motors. for his 30 ft Cat. That's my Sunday afternoon contribution.
02-02-2014, 04:27 PM
#33
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I have to share an experience with you on the boost ref, carb set up. back in the mid 80s I put together a non intercooled blower motor, with water/meth injection at 2lbs of boost. Never having any experience with superchargers I called B&M tech for tuning tips long story short I drilled a port through the throttle plate to intersect with the power valve port, next took a bigger bit and went in 1/4 inch so I could pound in a slosh tube (with Loctite) for a vacuum hose hook up to Manifold. Next I tapped the port in the bottom of the TP and inserted a allen set screw (Loctite) to seal of the manifold vacuum. proceed to tune the carb via jetting. the motor ran great and for it's time was quite a sleeper. Here's the real story, when I sold it the guy took it to (his) mechanic and was told " who in the hell F!!d up this carb? and what is this water tank on here for? The mechanic removed both and installed a stock carb with who knows what jetting, and within a week the motor burned down!! buyer came to me pissed off, I seen what was done and told him to pound sand!! He never did find anyone to get it to run right again! Asked me twice to help but with his bad attitude I just answered "don't think I know how to do that technical ****" LOL Besides that I was busy helping my boat dealer friend put together a set of twin turbo intercooled motors. for his 30 ft Cat. That's my Sunday afternoon contribution.
02-02-2014, 06:27 PM
#34
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Maybe related, but couldn't be pslonaker because this guy actually had a shop registered with the state, and worked on bread and butter motors, not one off custom beasty mills. I don't think he had a state of the art "Magic Room" filled with specialty tools and climate controlled as our "YouTube video instructor" pslonaker !!
02-02-2014, 06:58 PM
#35
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Maybe related, but couldn't be pslonaker because this guy actually had a shop registered with the state, and worked on bread and butter motors, not one off custom beasty mills. I don't think he had a state of the art "Magic Room" filled with specialty tools and climate controlled as our "YouTube video instructor" pslonaker !!
I remember when I was about 16 years old. A friend of mine had a 75 Buick Lesabre with a 455. I was young and still experimenting with carbs and tuning them. His car wouldn't run right, so I got in there and did some adjusting, modifying, etc. It ran worse. We brought it to his family's mechanic, and old timer who was a great mechanic, but grumpy and cranky as all hell. He took the air cleaner lid off, started poking around the carb. He looked at my buddy who owned the car, and said "did you have someone mess with this carb", my buddy said "ya, a friend of mine tried adjusting it". Not knowing I was the friend who was standing right there, the grumpy old mechanic said "well, whoever it was, needs to have his fukin hands chopped off so he never touches another carb again". You could def hear crickets after that, and I went back to reading up on carburetors. lol.
02-02-2014, 07:26 PM
#36
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Very true on the custom engines!
As far as the carb on the Buick, that is how we learned the back in the day, just jump in and see how it works turn this bend that, and then research afterwards, I would hang out as a kid at the corner Sinclair gas station and work for free sweeping, cleaning and all the time absorbing as much knowledge as I could. They finally hired me at 15 years old and started to teach me hands on stuff.. What a great way to learn the basic stuff. I made a lot of mistakes but learned on every one!! Not much of that going on these days.
I do hire one intern every summer and try to teach them the basics and encourage them in automotive restoration field.
As far as the carb on the Buick, that is how we learned the back in the day, just jump in and see how it works turn this bend that, and then research afterwards, I would hang out as a kid at the corner Sinclair gas station and work for free sweeping, cleaning and all the time absorbing as much knowledge as I could. They finally hired me at 15 years old and started to teach me hands on stuff.. What a great way to learn the basic stuff. I made a lot of mistakes but learned on every one!! Not much of that going on these days.
I do hire one intern every summer and try to teach them the basics and encourage them in automotive restoration field.
05-05-2014, 03:36 PM
#37
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Definitely some great info in here.. Mild Thunder great posts....
This is where I am right now, major major tuning issues... 516ci BBC, entirely re-done from top to bottom with all quality stuff. B&M 420 sent out for a full retro update to 3 Lobe Helix rotors. Scrapped the old Holley 750's and got brand new QFT 750 Boost Reference Carbs... That was the first mistake... Family friend's brother in law is a big wig over at QFT, so we were promised the world with these carbs to be angels from heaven... Yea... NO....
Started with 8.2psi boost on the dyno, spend 3/4ths of the day chasing the carbs and a tune... They were doing crazy things on the dyno after air bleed changes and jetting, but not really following suit as to what they should have been doing with the changes... after the first pull, made an air bleed change, picked up 26hp, after that, air bleeds didn't do any, no response on the dyno with additional changes. Called it a day, and overnighted a smaller pulley...
Running 10.2psi boost....Same dyno reults... Gained nothing after adding 2 PSI... Still chasing the carbs for a tune... I cant even begin to tell you how many times we had the bowls off... Finally throw our hands up and call QFT... Explain what is going on, and from the looks of the data sheets, we have a Flow issue and a metering block issue... So we are told that the jets in the Metering Blocks should be .028, Blank, .028, .028 on each side... They tell us to swap the blank to the 3 spot, and still the 4th spot to .033... Scratch our head and say, OK... Start measuring the jets in the metering blocks, and 1 spot is .029, 2 spot is a blank, 3 spot is .030, 4th spot is .031... Look at the opposite side and the Blank is in the 3 spot instead of the 2 spot... Now we are getting really ticked.. Nevertheless, we put them the way they want, though we didn't have jets to make them .028 like they should have been, but get the blanks in their 3 spot and the bottom jet to .033... Put them back on, and it makes it worse....
Go back to 8.2psi and try dialing in some distributor advance, get a little better... Start measuring vacuum under the carbs... 1.2" @ 6500rpm.. Well, they don't flow that's for sure.... Finally decide on the 10.2psi setup cause the motor liked the early boost, took a reading with 10.2psi and its 2" @ 6500rpm... Shoot me now!!
With no time left, 2 weeks before DS, and 2 days on the dyno, 20 pulls later, we try to get a somewhat usable tune for DS... Find a rich jetting set to be safe, air bleeds weren't responding to anything.... so we went back through all the pulls and came up with a tune...
Even did Back to Back pulls and both pulls weren't even remotely close to each other...
So don't know if both carbs are the issue or 1 carbs has major issues, but they definitely didn't flow for crap....
End up heading out to DS.... not a ton of running around or pushing the motor, but for whatever reason the left side of the transom was 2x as black as the right... Obviously plugs were black... Only thing we could think of is the progressive throttle linkage opens up the secondary's at cruising speed and is dumping way too much fuel in....
Needless to say, these carbs are going back, getting my money and coming up with a better solution....
As far as the AFR meter and water headers, I have a bung spot where the 4 goes into 1 on the Lightening Headers, is there anything special I have to do or worry about running the Sensor with Water through the exhaust, or will that not affect it?
This is where I am right now, major major tuning issues... 516ci BBC, entirely re-done from top to bottom with all quality stuff. B&M 420 sent out for a full retro update to 3 Lobe Helix rotors. Scrapped the old Holley 750's and got brand new QFT 750 Boost Reference Carbs... That was the first mistake... Family friend's brother in law is a big wig over at QFT, so we were promised the world with these carbs to be angels from heaven... Yea... NO....
Started with 8.2psi boost on the dyno, spend 3/4ths of the day chasing the carbs and a tune... They were doing crazy things on the dyno after air bleed changes and jetting, but not really following suit as to what they should have been doing with the changes... after the first pull, made an air bleed change, picked up 26hp, after that, air bleeds didn't do any, no response on the dyno with additional changes. Called it a day, and overnighted a smaller pulley...
Running 10.2psi boost....Same dyno reults... Gained nothing after adding 2 PSI... Still chasing the carbs for a tune... I cant even begin to tell you how many times we had the bowls off... Finally throw our hands up and call QFT... Explain what is going on, and from the looks of the data sheets, we have a Flow issue and a metering block issue... So we are told that the jets in the Metering Blocks should be .028, Blank, .028, .028 on each side... They tell us to swap the blank to the 3 spot, and still the 4th spot to .033... Scratch our head and say, OK... Start measuring the jets in the metering blocks, and 1 spot is .029, 2 spot is a blank, 3 spot is .030, 4th spot is .031... Look at the opposite side and the Blank is in the 3 spot instead of the 2 spot... Now we are getting really ticked.. Nevertheless, we put them the way they want, though we didn't have jets to make them .028 like they should have been, but get the blanks in their 3 spot and the bottom jet to .033... Put them back on, and it makes it worse....
Go back to 8.2psi and try dialing in some distributor advance, get a little better... Start measuring vacuum under the carbs... 1.2" @ 6500rpm.. Well, they don't flow that's for sure.... Finally decide on the 10.2psi setup cause the motor liked the early boost, took a reading with 10.2psi and its 2" @ 6500rpm... Shoot me now!!
With no time left, 2 weeks before DS, and 2 days on the dyno, 20 pulls later, we try to get a somewhat usable tune for DS... Find a rich jetting set to be safe, air bleeds weren't responding to anything.... so we went back through all the pulls and came up with a tune...
Even did Back to Back pulls and both pulls weren't even remotely close to each other...
So don't know if both carbs are the issue or 1 carbs has major issues, but they definitely didn't flow for crap....
End up heading out to DS.... not a ton of running around or pushing the motor, but for whatever reason the left side of the transom was 2x as black as the right... Obviously plugs were black... Only thing we could think of is the progressive throttle linkage opens up the secondary's at cruising speed and is dumping way too much fuel in....
Needless to say, these carbs are going back, getting my money and coming up with a better solution....
As far as the AFR meter and water headers, I have a bung spot where the 4 goes into 1 on the Lightening Headers, is there anything special I have to do or worry about running the Sensor with Water through the exhaust, or will that not affect it?
05-05-2014, 06:18 PM
#38
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According to the carb CFM calculator from BDS website, a 516CI spinning 6500 RPM, with 10lbs of boost, would require 1630CFM. A pair of 750's would be undersized, and not the size carbs I would have chosen for that combo. BDS says you can go up to 30% larger, which would be 2120CFM. My little blown engines in my boat require 1150 CFM according to that calculator, however, I have a pair of 850's on top of my B&M 420 blower. They made great power for what they are, and there is no lack of response, or any negatives I've seen from the size of the carbs. I think a pair of 950 or 1050 carbs would have been my choice on your setup. Not only from a fuel curve standpoint, but it has been proven, that going to larger, properly sized carbs, will also lower IAT temps. When you start seeing 2" of vaccum at WOT under the carbs, and basically see they are undersized, the metering goes out the window on them. I've seen this on a blow thru setup also. Carbs worked great at low boost. Crank the boost up past their comfort level, and the metering goes to $hit. They cant handle the turbulence.
I've seen the quick fuel 1050-B2 blower carbs work very well first hand, almost right out of the box, with minor jetting changes. I am no carb expert or guru by any means, but my recommendation, would be lose the 750's, and go bigger. Do NOT get the standard 950 holleys that are based on the 750 body. They dont flow anywhere near 950CFM. The Holley ULTRA HP 950 is a much better carb. To sum it up, I recommend error'ing to the large size on carbs with roots blowers.
Who converted those B&M's to the 3 lobe setup if you dont mind me asking? I'd be curious to see what , if any, was gained here from getting rid of the stripped 2 lobe deal. Did you notice any change in overdrive/boost level? The theory behind a two lobe, wasnt less air, it was more air per revolution. When the blower turns and takes a gulp of air, having more room inside the case from not having the added displacement of a third rotor taking up airspace, was the idea. The downside to the two lobe, was the issue of "carryback" at low speed, which really never became much of an issue, unless extremely low drive ratios.
As far as the fuel distribution, from seeing 02's in each collector on various blown engines, the left bank/right bank difference seemed to be worse, with a 3 lobe twisted rotor, vs my 2 lobe straight rotor. With an 02 in each bank on my engines,, the AFR readings were almost identical to each other, at most .1-.2 difference. With the GMC style twist, I've seen them as much as .5 or more difference left to right.
I've seen the quick fuel 1050-B2 blower carbs work very well first hand, almost right out of the box, with minor jetting changes. I am no carb expert or guru by any means, but my recommendation, would be lose the 750's, and go bigger. Do NOT get the standard 950 holleys that are based on the 750 body. They dont flow anywhere near 950CFM. The Holley ULTRA HP 950 is a much better carb. To sum it up, I recommend error'ing to the large size on carbs with roots blowers.
Who converted those B&M's to the 3 lobe setup if you dont mind me asking? I'd be curious to see what , if any, was gained here from getting rid of the stripped 2 lobe deal. Did you notice any change in overdrive/boost level? The theory behind a two lobe, wasnt less air, it was more air per revolution. When the blower turns and takes a gulp of air, having more room inside the case from not having the added displacement of a third rotor taking up airspace, was the idea. The downside to the two lobe, was the issue of "carryback" at low speed, which really never became much of an issue, unless extremely low drive ratios.
As far as the fuel distribution, from seeing 02's in each collector on various blown engines, the left bank/right bank difference seemed to be worse, with a 3 lobe twisted rotor, vs my 2 lobe straight rotor. With an 02 in each bank on my engines,, the AFR readings were almost identical to each other, at most .1-.2 difference. With the GMC style twist, I've seen them as much as .5 or more difference left to right.
Last edited by MILD THUNDER; 05-05-2014 at 06:29 PM.
05-05-2014, 09:51 PM
#40
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According to the carb CFM calculator from BDS website, a 516CI spinning 6500 RPM, with 10lbs of boost, would require 1630CFM. A pair of 750's would be undersized, and not the size carbs I would have chosen for that combo. BDS says you can go up to 30% larger, which would be 2120CFM. My little blown engines in my boat require 1150 CFM according to that calculator, however, I have a pair of 850's on top of my B&M 420 blower. They made great power for what they are, and there is no lack of response, or any negatives I've seen from the size of the carbs. I think a pair of 950 or 1050 carbs would have been my choice on your setup. Not only from a fuel curve standpoint, but it has been proven, that going to larger, properly sized carbs, will also lower IAT temps. When you start seeing 2" of vaccum at WOT under the carbs, and basically see they are undersized, the metering goes out the window on them. I've seen this on a blow thru setup also. Carbs worked great at low boost. Crank the boost up past their comfort level, and the metering goes to $hit. They cant handle the turbulence.
I've seen the quick fuel 1050-B2 blower carbs work very well first hand, almost right out of the box, with minor jetting changes. I am no carb expert or guru by any means, but my recommendation, would be lose the 750's, and go bigger. Do NOT get the standard 950 holleys that are based on the 750 body. They dont flow anywhere near 950CFM. The Holley ULTRA HP 950 is a much better carb. To sum it up, I recommend error'ing to the large size on carbs with roots blowers.
Who converted those B&M's to the 3 lobe setup if you dont mind me asking? I'd be curious to see what , if any, was gained here from getting rid of the stripped 2 lobe deal. Did you notice any change in overdrive/boost level? The theory behind a two lobe, wasnt less air, it was more air per revolution. When the blower turns and takes a gulp of air, having more room inside the case from not having the added displacement of a third rotor taking up airspace, was the idea. The downside to the two lobe, was the issue of "carryback" at low speed, which really never became much of an issue, unless extremely low drive ratios.
As far as the fuel distribution, from seeing 02's in each collector on various blown engines, the left bank/right bank difference seemed to be worse, with a 3 lobe twisted rotor, vs my 2 lobe straight rotor. With an 02 in each bank on my engines,, the AFR readings were almost identical to each other, at most .1-.2 difference. With the GMC style twist, I've seen them as much as .5 or more difference left to right.
I've seen the quick fuel 1050-B2 blower carbs work very well first hand, almost right out of the box, with minor jetting changes. I am no carb expert or guru by any means, but my recommendation, would be lose the 750's, and go bigger. Do NOT get the standard 950 holleys that are based on the 750 body. They dont flow anywhere near 950CFM. The Holley ULTRA HP 950 is a much better carb. To sum it up, I recommend error'ing to the large size on carbs with roots blowers.
Who converted those B&M's to the 3 lobe setup if you dont mind me asking? I'd be curious to see what , if any, was gained here from getting rid of the stripped 2 lobe deal. Did you notice any change in overdrive/boost level? The theory behind a two lobe, wasnt less air, it was more air per revolution. When the blower turns and takes a gulp of air, having more room inside the case from not having the added displacement of a third rotor taking up airspace, was the idea. The downside to the two lobe, was the issue of "carryback" at low speed, which really never became much of an issue, unless extremely low drive ratios.
As far as the fuel distribution, from seeing 02's in each collector on various blown engines, the left bank/right bank difference seemed to be worse, with a 3 lobe twisted rotor, vs my 2 lobe straight rotor. With an 02 in each bank on my engines,, the AFR readings were almost identical to each other, at most .1-.2 difference. With the GMC style twist, I've seen them as much as .5 or more difference left to right.
New build got coated bearings, line honed correctly, Dragon Slayer crank, crane springs, inconel valves, new H bean rods and pistons went from an 8.53:1 to 7.5:1 setup to run higher boost, total seal rings, stud girdles for merlin heads, guess there only 1 mfg that makes them, studded mains and heads, MLS for the heads, cleaned up intake and exhaust ports, new distributor and oil pump, blah blah... LOL
Supercharger USA did the conversion, there was some major differences that we took into account, 1 was the pre-heat packing from the air packing in and escaping out, and being packed again which ultimately resulted in higher EGTs, so the 3 lobe helix would reduce this air surge and ultimately lower the EGTs cause the intake air would be cooler, I forget what the formula was for intake temp > Exhaust temp, but any help to reduce EGT was a plus, the other was efficiency, the efficiency went up 20 points, which obviously is a plus, lastly was a more sustained boost... These were our selling points on just fixing vs converting, granted it cost more for the conversion, but we also did the heat treated gears and input shaft and billet bearing retainers too.. Might as well do it right....
On the Dyno with 8psi, we laid down after a few adjustments just under 800hp, with just under 800 ft lbs... definitely missed our mark... So we overnighted a smaller pulley the 5" cause we just didnt have the pulleys to get 10psi... Running the 16 rib 6.3" Lower and 5" upper for 10psi.. 8psi setup was 6.75" lower and 5.8" upper...
Literally saw little to nothing on the dyno... 800hp and just over 800 ftlbs with 10psi... this was at 27 degrees timing... dyno was only plotting to 6400, but if you watch the dyno it ran to 6600 and it was still building HP... Has a 7k rev chip, dont know if I would wanna see 7k too much, but its a solid build and literally sounds better spinning 6400 than it does at 3500 LOL, it just sings...
Never the less we went back to 8psi and started dialing in some timing, bumped timing to 30, and literally laid down almost the same numbers with the 10psi setup.. again shocking... At the end of the day, we left the timing at 30, went back to the 10psi setup, and found a happy medium in the various tunes we tried.... One of the biggest things aside from flow and normal tuning issue we saw with the carbs is that it kept getting fat on the top end, and not matter what we tried we couldnt get the top end lean, so it was really hurting the HP....
I was going to go the Holley Ultra HP route until I was persuaded by our long time family friend to go with QFT... We are currently in talks with Brasswell since they are local here in AZ, still waiting for a build and price, they said they know exactly what is happening...
Either way, When you say 1050's I see my debit card melting at the gas pumps... LOL Although, going through all this unburnt fuel with these crappy carbs prob isnt helping the cause either...
