Who modifiys carbs?

Any one modify carbs? I'd like to compare notes on some of the changes shown below (thinned shafts, button heads, choke horn removal, booster modification). What were your results?

http://tinypic.com/wh0682

http://tinypic.com/wh06fc

http://tinypic.com/wh06qd

Looks like you went to the extreme with the choke horn removal. I normally just remove the horn itself, not mill then entire top of the carb. But it may be an improvement, I don't know....

Ryan

I doubt it also. Originally, I hacksawed the horn off then leveled it with a hand grinder. That's how I ran the carbs last year; there are two. This winter I went ahead and threw it up on the mill, only to make it flat and more attractive. I should have used a bigger cutter, would have looked even better.

Anyway, do you have any before and after information (jetting, dyno, or drag strip mph)?

Did you change the boosters?

Yes, I machined the bars out.

I had some information somewhere on a carb I did awhile back.... Can't find it now though, it was from a flow bench, I believe it picked up 30 cfm over stock with just the choke horn removed and another 20 when I blended the top in better. You have to be careful doing that though. You can also ream the boosters out which gives a bit better response. What all did you do to the carb other than milling off the top? Did you precision lap the mating surfaces?

Ryan

I thinned the shafts and installed button heads, removed the choke horn, and removed the divider in the boosters.

That'd be great if you could find the flow bench information.

As to the boosters, doglegs would be my next move. The carb shop sells a super booster that looks like it would get the job done.:cool:

What's the claim to fame with the super booster?

I looked and I can't seem to find the flow bench info..... Do you have any detail shots of the thinned shafts? I haven't really heard of doing this. Have you thought about blending the top of the venturis?

Ryan

Yes, I machined the bars out.

Do you know why the bars were there? What was the reasoning for removal?

You can look in summit or Jegs, they sell thinned shafts that come with button heads; They are made by Quick Fuel Technologies. I don' thave any better pictures than the one above, sorry. I put the carbs back on the motor. I run a stub stack of sorts that provides a smooth path for airflow into the venturi. Thinned shafts are also purported to increase booster signal.

The bars provide a restriction in airflow which increases the velocity of the air for better atomization of the fuel and increased signal. I removed them for better airflow at the cost of reduced atomization.

Did you remember to put extensions on the tubes in the secondary bowl?

since you machined the "bars" from the boosters out this will greatly affect the signal to the boosters.

with out those bars on the boosters the signal will decrease greatly and you will have to put in smaller main air bleeds to have the have the main circuit come in at the proper time as well as run more main jet/PVCR area to keep the engine from running lean. Also this will affect atomization of the fuel.

boosters without the bars are specifically positioned in the venturi to make up for this loss in signal.

You will probably have to have the boosters replaced but, try it for the hell of it to see how it works out

I'm not real familiar with extension tubes, are they for foreward mounted carbs, these are mounted sideways.

I've already ran these carbs, with tuning, I went from 10.60 to 10.13. I'm confident they work.

I did add a great deal more jet to make them work. As the AFR came into line the car just keep getting faster and faster. Perhaps I have band-aided the potential air bleed problem. I'll have to look into the air bleeds, but you say they provide main circuit timing not necessarily fuel delivery quantity.

air bleeds do have a little to do with fuel "quantity" but there primary purpose is to time the main system.

too small of an air bleed= nozzle drip and it coming in to early
too big of an air bleed = delaying of main too much so a lean spot shows up between the intermediate circuit and the main

and of course they contribute to emulsion

when you say "bars" do you mean the truck type boosters, thats what they look like anyway?

what did the car run before you did these modifications to the carb(s)?

have you done an test with the boosters being the only variable changed (except for jetting to compensate)?

How much jet did you have to add? I would guess 5-10 square

I agree with the mods you have done except for the booster, but if it works for you great.

I do quite a bit of carb work but for blowthrough turbo/supercharged stuff so thats why I am obsessed with the booster signal.

I had a steady AFR at the track last year. It took 10-15 jet sizes to get there.

On the street the AFR does vary, but I have not spent any time tuning this years setup. Last year's street setup and tuning resulted in a steady AFR. I changed the air filters and the fuel pressure this year.

I'll look into air bleeds as an additional tuning tool.

The carbs are 3310s, so that's the booster style. You think doglegs will replenish some signal, I agree. They would also provide additional restriction.

These carbs sit atop a 6-71.

What modifications have you made booster wise. Any data?

for a strong signal I would go with annualar boosters, and that is what I plan to do for my blow through set up

for air bleeds just drill out the old ones and tap the holes for 8-32 threads

then buy some brass set screws and a numbered drill set and you can make any size airbleed you want.

I have just modified the contours of my down leg boosters on the inside that the casting process couldnt. on my engine @ WOT is was worth about .1-.2 AFR richer at the same jetting

I like the annular boosters also.

Another way to increase signal strength is to machine a step (or steps) diameter just below the fuel outlet on the underside of the booster. I have not done this but will if signal bocomes a problem.

I like the brass set screw idea. I use brass screws for jets in my nitrous kit.

Any pictures of the booster mod? No other data than AFR?

Umm, if this is on a roots blower and you have two of these why are you doing modifications to kill signal? You say increase flow, but why? The carburetors are going to flow more than their nominal 750 cfm sitting on top of a roots. I think if you measure vacuum at the baseplate of the carburetors you will find more than 1.5" of mercury if you're really driving the blower hard. If you find more than 1.5" then you can assume you are pulling more air through the carburetor (1.5" mercury being the depression holley used for 4 barrels). How much overall boost are you running?

I once just touched up the casting flash on the inside of a 0-4781's boosters before I sent the carburetor off to C&S. They sent them back in a baggie with a note that said something like "don't touch!" and replaced them (without charging me). They have Rochester's old wet flow bench and are informed in this area. Boosters are an area that really shouldn't be dicked with unless you have a way to quanitify your change. If removing the center bar required an increase in 10-15 jet sizes because of that change alone then your carbs signal is screwed.

I am not trying to be a jerk and I like to experiment with carburetors too, but I don't understand the reasoning behind looking for 60 cfm and destroying the signal and less importantly on your roots, distribution in the process.

I dont have any other data than AFR because the car has not been to the track with the blower, I was simply looking for a way to increase signal strength at the booster for being blowthrough.

I do not have a picture of my boosters (mainly because i suck with computers) but I took a inline grinder and lightly worked my way around where the inner and outer parts meet (both top and bottom) and blended it(down leg boosters).

the step method does the same time but I dont have the tools to do the machiniing.

if you have jetted to compensate your only down fall will be some lean flat spots while transitioning to WOT

I'm not real familiar with extension tubes, are they for foreward mounted carbs, these are mounted sideways.


LOL, never mind.

markinkc69z. "Umm, if this is on a roots blower and you have two of these why are you doing modifications to kill signal? You say increase flow, but why?

Roots type superchargers are sensitive to inlet restrictions. Less restriction is more power. And, max power is the name of the game. As long as it's still tunable why do i need super signal strength? Atomization would be the only other concern. Otherwise, I'll take 60 free CFM any day of the week.

As a testament to the tuneability and atomization, I drove 26 miles to the drag strip, laid down a half a dozen low 10 second passes, and drove 26 miles home, on pump gas (fuel atomization problems should show up here as detonation). It makes for a long day in the 85 degree heat, but no problems. This setup runs five pounds boost (10% underdriven).

I have provided you with some before and after data, but certainly solid overall performance data (It's hard to change just one thing at a time and then go and evaluate, time constraints). I do agree booster modification is not for everyone.

All you have provided here is tell us how you fouled up your boosters. You should condem yourself. I've provided some information here on at least what I've done and my results. You're sounding a lot like the guy who complained to me about my propane injection system, check the signature, it worked out well.

lamerandomname...LOL, never mind.
Wow that's insightful, my guess is you are scouring the net trying to define extension tube function. I still don't see what extension tubes have to do with booster signals. Help me out here.

master of a-bodies,

hey have you been changing your own boosters? Did you buy or make a tool. Looks like they are pretty hard to come by.

What size engine is your supercharger on?

454

no I have not been changing mine own boosters, I can not find the tool to do it either. It was I huge bitch to get on the under side of them while still in the main body.

If I did have the tool I most likely would have went to a annualar booster from CSU, I might end up sending my carb out eventually for boosters if I start running alot more boost but I will do all the air bleed and emulsion tuning my self, but for right now it is doing pretty well,

If you google enough, you'll find one. Eventually I found the tool, but thought it was cost prohibitive. There is also a safety issue. Appearantly not that many carb shops change boosters either. As I remember it was a "C" clamp looking tool.

I may, like I said, machine a step on the underside of the booster for better signal/atomization at some point, but not just yet.

The other side of the coin, at least with my carbs, is cost. Thus far it's all been free. Changing the boosters is going to cost money. I have 750s with vacuum secondaries. I believe the 850 carb uses a larger venturi diameter, which I would like to have, and I would also get away from the vacuum secondaries. Anyway, I think I'd just step up to a better carb rather than change boosters.

Let me throw one more thought out there for ya:

The less restriction, assuming you can tune it and have acceptable driveability, what ever your definition of driveability, the more power potential?

I'm riding a big block w/blower, low speed power is not an issue, detonation is not an issue, throttle response is unbelievable. Why change?

Hey here's the tool.

www.aedperformance.com/Tools.htm

454

Not knowing how high you rev this engine I'll take the higher rpm route. I'll say 7000, though I'd bet its closer to 6000. At 7000 rpm if your engine actually had 100% VE and then you added 9 psi of boost you wouldn't be exceeding the cfm rating of 2 stock 750 carburetors. This doesn't take into account the loss of air density from the heated supercharged air.

I don't know anything about the propane injection conversation you had. These threads remain here permanently and I would hate for somebody to look at this as a how-to and think that removing the bar in the booster was a good idea. That is all.

I'm glad you feel its working appropriately for you. By the way poor atomization would not lead to detonation. Detonation is caused by heat of compression combined with inlet air temp. Poorly atomized fuel will not provide as much heat (power) during combustion as finely atomized fuel does as less of it will burn and more will run out of the exhaust wasted.

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!

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.

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.

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/

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).

http://tinypic.com/zwz384

http://tinypic.com/zwz33n

I finally figured out how to make a vid with sound. I think this is a 10.4x pass.

http://www.zippyvideos.com/9872422635161276/andy_2/

Appearantly, now I forgot how to approprately post links. This should work. Again 10.4x with sound. Sorry.


http://i2.zvhost.com/2/b/bye9y154.jpg (http://www.zippyvideos.com/9872422635161276/andy_2/)

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.

http://i2.zvhost.com/2/u/ut2bsurh.jpg (http://www.zippyvideos.com/2512548065375716/10_oh_six/)

Update:

New cam and 8 pounds boost - 1.39 60 footers w/9.6x ETs @ 138 MPH.

Same carbs.

Update:

9.57 & 9.52 @ 140 & 141 MPH.

More boost.

Same carbs.

Update: Here's a pic of the "old heap" leaving on a 9.6x pass.

http://i12.tinypic.com/2ds4o07.jpg

Squirter tuning really paid off.

I'm still working on video.

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/techarticles/48499_cheap_horsepower_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).

http://i19.tinypic.com/4711150.jpg

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/tech/0511phr_carburetor_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!

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."

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. It doesn't look like your formula for max power works.

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.
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.

Video update - 9.6x passes: first pass was a lifting 10.50 (nice launch w/good 60'), second pass is a full on 9.64, third pass is 9.68 (not posted). Same carbs.

http://View My Video (http://video.tinypic.com/player.php?v=2jebkup)

http://View My Video (http://video.tinypic.com/player.php?v=4bhsaqu)

Update: the old heap just went 9.42. Same carbs.

http://i17.tinypic.com/67fjjif.jpg

so your sayin to over carb a blower motor?
We had a 540 BBC NA and it only had a 1050 dom on it and made 850+. It was going high 8's

I don't understand carb tuning too well...........

I'm not sure I'd call it "over carbed", rather providing a minimal inlet restriction with the correct afr. Nothing I've done or read about ever implied a over rich condition. I've also never heard of any roots blower combination running a single carb. Many years ago I looked into trying to run a single carb, people thought I was crazy.