Who modifiys carbs?
Thread Starter
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Joined: Aug 2002
Posts: 413
Re: Who modifiys carbs?
Blower motors run over 100% VE. My engine's simulation shows over 125% VE. Ignoring that, the Blower Drive Service (BDS) carb formula, a very basic starting point formula, calls for about 1250 CFM (454 @ 7000 rpm). Wa La you are right; the stock 750s have it covered.
Now, if you had bothered to read any information on blowers you'd find pretty quickly that everyone from B&M to Dutweiller to Enginemaster's use either 850s or 950s on a big block, and, oh my gosh, the engines make more power. This is 20 year old information, nothing even new. The formulas are intended to give you a safe starting point.
The only uninformed person who grabed a die grinder and started grinding was you, by your own admission. I've invested 20-30 hours researching boosters and booster modifications. As a side note David Vizzard has good information on booster modification.
As reported from a variety of prominent sources, large droplets of fuel increase an engine's tendency to detonate. I've been very careful to check the plugs on the street and at the track. I also use an oxygen sensor to monitor AFR.
Shame on you for wasting my time. May the fleas of a thousand camels infest your armpits. Now that's funny!
Now, if you had bothered to read any information on blowers you'd find pretty quickly that everyone from B&M to Dutweiller to Enginemaster's use either 850s or 950s on a big block, and, oh my gosh, the engines make more power. This is 20 year old information, nothing even new. The formulas are intended to give you a safe starting point.
The only uninformed person who grabed a die grinder and started grinding was you, by your own admission. I've invested 20-30 hours researching boosters and booster modifications. As a side note David Vizzard has good information on booster modification.
As reported from a variety of prominent sources, large droplets of fuel increase an engine's tendency to detonate. I've been very careful to check the plugs on the street and at the track. I also use an oxygen sensor to monitor AFR.
Shame on you for wasting my time. May the fleas of a thousand camels infest your armpits. Now that's funny!
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Joined: Mar 2005
Posts: 849
From: Shawnee Kansas
Re: Who modifiys carbs?
First off you started this thread looking for feedback and to compare notes on what you and others have done. You did not preface the post by saying you only wanted to hear from people who agree with what you have done. Maybe you should have?
Put me under the "I don't agree with some of the modifications you have made and I don't think you understand why" column.
You continue to point out that I mildly cleaned some boosters once and had an excellent shop steer me away from my mod as I didn't fully understand the ramifications at the time. I am trying to help you understand that other people have made less radical changes in that area that were wrong. Hello??? My ego can handle sharing an experience for the sake of helping someone else.
Your 6 psi combination doesn't need the carburetors modified in many of the ways you have. They'll happily supply the airflow for at least 10 psi on your 454 in stock form. Unless you've measured a post modification boost increase you haven't done anything to improve air density.
Understand that it takes airflow to make pressure in a boosted engine. You're only asking for 6 psi above atmospheric pressure. By my guess that's about 350 cfm over your naturally aspirated airflow or 1150 so cfm. A single dominator could handle it. Certainly 2 750 cfm carburetors are enough as is.
You had to make radical jet changes to maintain an appropriate AFR. If that doesn't tell you metering is hosed now I don't know what will. By the way, an O2 sensor doesn't know anything about raw poorly atomized fuel that may be running our of your exhaust. O2 sensors react to oxygen only.
Knowing why you want to modify something is as important as knowing how to go about it. In your case you're focusing on removing a perceived inlet restriction that isn't really there and destroying the metering performance in the process. This is my point.
Okay, if you're still with me then I will share a formula that may prove helpful at some point when trying to determine cfm needs of a boosted engine.
Factor naturally asprirated cfm requirements. Use your simulation program if you want.
(((EngineCID * MaxRPM) /3456) * VE% estimation NA) = CFM
(((454 * 7000) / 3456) *.87) = 800 cfm
CFM / AtmosphericPressure = CFM per psi absolute
800 / 14.5 (good average pressure) = 55 CFM per psi
((AtmosphericPressure + BoostPressure)* CFM per psi) = CFM requirements
((14.5 + 6) * 55) = 1128 CFM
Witrh a roots your actual cfm requirements may be lower due to the heating of the air during compression in the manifold.
Put me under the "I don't agree with some of the modifications you have made and I don't think you understand why" column.
You continue to point out that I mildly cleaned some boosters once and had an excellent shop steer me away from my mod as I didn't fully understand the ramifications at the time. I am trying to help you understand that other people have made less radical changes in that area that were wrong. Hello??? My ego can handle sharing an experience for the sake of helping someone else.
Your 6 psi combination doesn't need the carburetors modified in many of the ways you have. They'll happily supply the airflow for at least 10 psi on your 454 in stock form. Unless you've measured a post modification boost increase you haven't done anything to improve air density.
Understand that it takes airflow to make pressure in a boosted engine. You're only asking for 6 psi above atmospheric pressure. By my guess that's about 350 cfm over your naturally aspirated airflow or 1150 so cfm. A single dominator could handle it. Certainly 2 750 cfm carburetors are enough as is.
You had to make radical jet changes to maintain an appropriate AFR. If that doesn't tell you metering is hosed now I don't know what will. By the way, an O2 sensor doesn't know anything about raw poorly atomized fuel that may be running our of your exhaust. O2 sensors react to oxygen only.
Knowing why you want to modify something is as important as knowing how to go about it. In your case you're focusing on removing a perceived inlet restriction that isn't really there and destroying the metering performance in the process. This is my point.
Okay, if you're still with me then I will share a formula that may prove helpful at some point when trying to determine cfm needs of a boosted engine.
Factor naturally asprirated cfm requirements. Use your simulation program if you want.
(((EngineCID * MaxRPM) /3456) * VE% estimation NA) = CFM
(((454 * 7000) / 3456) *.87) = 800 cfm
CFM / AtmosphericPressure = CFM per psi absolute
800 / 14.5 (good average pressure) = 55 CFM per psi
((AtmosphericPressure + BoostPressure)* CFM per psi) = CFM requirements
((14.5 + 6) * 55) = 1128 CFM
Witrh a roots your actual cfm requirements may be lower due to the heating of the air during compression in the manifold.
Thread Starter
Registered User
Joined: Aug 2002
Posts: 413
Re: Who modifiys carbs?
I would recomend any of the changes shown above to anyone wishing to run a little faster for cheap.
The choke horn is quickly removed with a hack saw then cleaned up with a die grinder or the mill (which leaves a nice appearance). Milling is actually the only "change" I made this year.
Be very careful when removing the bar from the boosters, assuming you do it with the boosters in the carb.
I'm very pleased with the results, O2 readings, plug readings, overall driveability, and drag strip performance is excellent.
As a side note, without the O2 sensor these changes would be difficult to tune. I really wish I had the capability of the LT1 OBDI computer to fine tune this combo with the aid of data aquisition. That would be killer.
I have a post on my local boards offering to modify someones carb. Hopefully, it will be a naturally aspirated engine. I'll report the results.
The choke horn is quickly removed with a hack saw then cleaned up with a die grinder or the mill (which leaves a nice appearance). Milling is actually the only "change" I made this year.
Be very careful when removing the bar from the boosters, assuming you do it with the boosters in the carb.
I'm very pleased with the results, O2 readings, plug readings, overall driveability, and drag strip performance is excellent.
As a side note, without the O2 sensor these changes would be difficult to tune. I really wish I had the capability of the LT1 OBDI computer to fine tune this combo with the aid of data aquisition. That would be killer.
I have a post on my local boards offering to modify someones carb. Hopefully, it will be a naturally aspirated engine. I'll report the results.
Thread Starter
Registered User
Joined: Aug 2002
Posts: 413
Re: Who modifiys carbs?
I thought you guys might like to see these carbs in action. Like I said these mods were done last year. All I did this year was mill the air filter mating surface, so it looks better.
http://www.zippyvideos.com/4539926155106076/andy/
http://www.zippyvideos.com/4539926155106076/andy/
Thread Starter
Registered User
Joined: Aug 2002
Posts: 413
Re: Who modifiys carbs?
I finally got a chance to take the car out last night, between rains. I was barely able to build any heat in the motor as it's around 50 degrees outside. Anyway, I managed to get a semi decent reading on the used plugs in the motor. Attached are the pictures of plugs from cylinders 1,3,2,and 6. I tried to get a closer shot but the pics just came out blury. They look pretty good considering they are used plugs and no heat in the motor. I've done a lot of idling in the garage looking for an oil leak (rear main seal).
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Thread Starter
Registered User
Joined: Aug 2002
Posts: 413
Re: Who modifiys carbs?
Update - I took the old heap out last week with a little more jet and a larger squirter. The 60 foot improved from 1.49 to 1.44 and the car went .07 quicker and over 1.5 mph faster.
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Thread Starter
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Joined: Aug 2002
Posts: 413
It's a good time for a little recap. Here's a collection of information, not a complete collection, but a small sampling of tuning data I used to perform the previously mentioned carb modifications.
First off the universally accepted blower carb formula as defined here from BDS (blower drive service) also used by every other blower manufacturer is http://www.blowerdriveservice.com/faq.php:
{(CID x RPM) ÷ 3456} x {Boost ÷ 14.7) + 1} = CFM required. I don't need two 750 carbs rather 1294 CFM or two 650s. This same formula shows I'm way to lean now at 16 pounds of boost and 7500 RPM (I need to 1050s, yet the O2 sensor and the plugs still read good). It's ok to change the carb tune to maintain a flat AFR of your choice.
As previously mentioned roots blowers are sensitive to inlet restrictions as outlined here is Ken Duttweiler's 700 hp 454 build-up where he states:
"This engine was equipped with dual 850-cfm Holley double pumpers. Is this overcarburated? Duttweiler, whose dyno was used to generate our horsepower and torque figures, doesn’t think so. His experience has taught him that you can’t overcarb an 8-71 Roots-style supercharger, because he has found that both the 6-71 and 8-71 blowers need an unrestricted inlet side to really work well. To make this happen, larger than normal carburetors do the job. Could you get away with a single 850-cfm or even dual 750-cfm carburetors? Maybe, but Duttweiler knew that in this case, bigger is better. He has experimented with different carburetor sizes and determined this one to be the best and most powerful for the given application."
You can read the full text here http://www.chevyhiperformance.com/te..._engine_build/
Concerning me using some "phantom" volumetric efficiency I use the one in desktop dyno which agrees with every other one I've ever seen. Blower motors just plain attain over 100% volumetric efficiency. Do a google search or read any text on blowers. Here's a screen dump of desktop dyno's output page (more reflective of my 9.5x passes).
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Concerning the function of an oxygen sensor, it's pretty simple it measures the remaining oxygen in the exhaust: rich = excess fuel, lean = insufficient fuel. Search for any one of thousands of o2 sensor internet references, here's wikipedia's definition http://en.wikipedia.org/wiki/Oxygen_sensor. As stated in any text, O2 sensors are designed to indicate AFR: excessive or insufficient fuel. The modern ECU depends upon this function. If unburnt fuel were pouring out of my exhaust the sensor would show it and so would the spark plugs (see pics above).
And lastly, here's a very good read on boosters, it's written by David Vizard. You can read about the many types of boosters, booster gain, and the importance of atomization. I've chosen to remove the booster bar as outlined in Vizard's books on how to build horsepower Volume 2 & 3 (not outlined in this article). These same changes are outlined in some of Brad Urban's writings. You might recognize his name from "The Carb Shop." In the event I felt my signal were "hosed" as stated above I'd add the step on the bottom side of the booster and smooth the entry as outlined in this text. You can read the full text here http://www.popularhotrodding.com/tec...boosters_tech/. I'd suggest you read Vizard's books and Urban's text before you grab a die grinder and "dick up" your carbs as outlined above. I've done my homework; I know what I want to change and why. The result speaks for itself, 9.5x's with vacuum secondary carbs, who knew?
Again, most of the information presented here is at least 20 years old, nothing new and nothing original. Additional references include B&M's supercharger technical manual, enginemaster's articles, and street supercharging by Pat Ganahl.
Stay tuned, as soon as the snow's gone I'm adding three more pounds of boost!
First off the universally accepted blower carb formula as defined here from BDS (blower drive service) also used by every other blower manufacturer is http://www.blowerdriveservice.com/faq.php:
{(CID x RPM) ÷ 3456} x {Boost ÷ 14.7) + 1} = CFM required. I don't need two 750 carbs rather 1294 CFM or two 650s. This same formula shows I'm way to lean now at 16 pounds of boost and 7500 RPM (I need to 1050s, yet the O2 sensor and the plugs still read good). It's ok to change the carb tune to maintain a flat AFR of your choice.
As previously mentioned roots blowers are sensitive to inlet restrictions as outlined here is Ken Duttweiler's 700 hp 454 build-up where he states:
"This engine was equipped with dual 850-cfm Holley double pumpers. Is this overcarburated? Duttweiler, whose dyno was used to generate our horsepower and torque figures, doesn’t think so. His experience has taught him that you can’t overcarb an 8-71 Roots-style supercharger, because he has found that both the 6-71 and 8-71 blowers need an unrestricted inlet side to really work well. To make this happen, larger than normal carburetors do the job. Could you get away with a single 850-cfm or even dual 750-cfm carburetors? Maybe, but Duttweiler knew that in this case, bigger is better. He has experimented with different carburetor sizes and determined this one to be the best and most powerful for the given application."
You can read the full text here http://www.chevyhiperformance.com/te..._engine_build/
Concerning me using some "phantom" volumetric efficiency I use the one in desktop dyno which agrees with every other one I've ever seen. Blower motors just plain attain over 100% volumetric efficiency. Do a google search or read any text on blowers. Here's a screen dump of desktop dyno's output page (more reflective of my 9.5x passes).
[IMG]
[/IMG]Concerning the function of an oxygen sensor, it's pretty simple it measures the remaining oxygen in the exhaust: rich = excess fuel, lean = insufficient fuel. Search for any one of thousands of o2 sensor internet references, here's wikipedia's definition http://en.wikipedia.org/wiki/Oxygen_sensor. As stated in any text, O2 sensors are designed to indicate AFR: excessive or insufficient fuel. The modern ECU depends upon this function. If unburnt fuel were pouring out of my exhaust the sensor would show it and so would the spark plugs (see pics above).
And lastly, here's a very good read on boosters, it's written by David Vizard. You can read about the many types of boosters, booster gain, and the importance of atomization. I've chosen to remove the booster bar as outlined in Vizard's books on how to build horsepower Volume 2 & 3 (not outlined in this article). These same changes are outlined in some of Brad Urban's writings. You might recognize his name from "The Carb Shop." In the event I felt my signal were "hosed" as stated above I'd add the step on the bottom side of the booster and smooth the entry as outlined in this text. You can read the full text here http://www.popularhotrodding.com/tec...boosters_tech/. I'd suggest you read Vizard's books and Urban's text before you grab a die grinder and "dick up" your carbs as outlined above. I've done my homework; I know what I want to change and why. The result speaks for itself, 9.5x's with vacuum secondary carbs, who knew?
Again, most of the information presented here is at least 20 years old, nothing new and nothing original. Additional references include B&M's supercharger technical manual, enginemaster's articles, and street supercharging by Pat Ganahl.
Stay tuned, as soon as the snow's gone I'm adding three more pounds of boost!
Thread Starter
Registered User
Joined: Aug 2002
Posts: 413
Speak of the devil, I just got my new Super Chevy magazine and there is an article on roots blowers vs centrifugal blowers. In the article they built a 496 CID Chevy making 7 lbs boost at 6500 rpm. A quick look at the numbers reveals the motor needs only 1377 cfm or two 700 cfm carbs. They put two HP950s on the motor. Whoa?
They installed two HP 950s, and cited the following: "We stepped up to the large HP Holley carbs to eliminate any inlet restrictions to our supercharger."
Later in the article they quoted, "Luckily for us, the timing was the only tuning required, as the blower specific Holley carbs produced a safe and powerful afr/fuel curve, and required no jet changes on the 8-71."
It doesn't look like your formula for max power works.
It looks like you are the only one who doesn't understand minimum inlet restrictions produces maximum power.
The 496 motor made 890 hp "WAY" over carbed with the two HP 950s. Makes me wish I had the 496 and the two HP950s.
I'll post the link as soon as Super Chevy updates their website.
They installed two HP 950s, and cited the following: "We stepped up to the large HP Holley carbs to eliminate any inlet restrictions to our supercharger."
Later in the article they quoted, "Luckily for us, the timing was the only tuning required, as the blower specific Holley carbs produced a safe and powerful afr/fuel curve, and required no jet changes on the 8-71."
markinkc69z, Okay, if you're still with me then I will share a formula that may prove helpful at some point when trying to determine cfm needs of a boosted engine.
markinkc69z, Put me under the "I don't agree with some of the modifications you have made and I don't think you understand why" column.
The 496 motor made 890 hp "WAY" over carbed with the two HP 950s. Makes me wish I had the 496 and the two HP950s.
I'll post the link as soon as Super Chevy updates their website.
