Carburetor size ... why formula doesn't work

[@kylelove927]

This is REAL technical information. People tend to glaze up.when I start talking like this. Appreciate seeing an intelligent man share invaluable information.

[@luisbalderrama8145]

Scott, you are doing such amazing work with these videos. The 472 / 500s are not talked about nearly enough. Thank you for being such a solid source of information on these. Keep it up! -Luis

[@bryanst.martin7134]

Very nice presentation. I had a 4150 on my Mustang with a 302 and fairly hot cam. A few idle tweaks and bowl fuel level tweaks, and it was a reliable runner. The Vacuum secondaries was a key. Tune the vac side to what the engine demands. Several spring changes, and we're happy. You clearly point out the value of the vacuum level.

[@gordonborsboom7460]

Small point to make. Fuel is pushed into the venturi as the pressure in the fuel bowl is nearer to atmospheric than in the venturi. Anything called a vacumn is really only pressure that is less the local atmosperic pressure. Fluids, like gasoline and air, move from high to low pressures An intake vacumn is just a lower pressure than the location at which you are measuring it, and is relative to the pressure at that point. 28" of water column is equal to a pressure difference of 1 psi.

[@scotthatch4548]

That's really just semantics .... It's a pressure differential that causes it to move ... Atmospheric is kind of a constant and the variable is that change of pressure at the booster

[@trailerparkcryptoking5213]

The old carb formula is based on steady state flow, which an engine doesn’t experience!

[@TheRolfano]

Hreat video! So many people on YT know nothing but the wrong formula only with no info behind it!

[@goldsgarage8236]

Very informative Scott. Thanks for your comments on my channel. AG

[@lesliedickey2820]

Thank for your presentation. However I don't understand how converting from 28 to 20.4 results in 3.38. Seems to me it should be (20.4*4)/28 which is 2.91. I'm asking because I want to calculate the carb size for a 6 cylinder engine which I think is 3 times (runner flow rate) times the adjustment factor.

[@scotthatch4548]

@@lesliedickey2820 you have flow at 28 inch's to get flow at 20 inch's you multiply by .845 .... 4 cylinders times .845 is 3.38 ... In your case 3 cylinders times .845 is 2.535

[@talesfromthetarheelstate351]

I am loving this so much. I have a slightly built 20r Toyota with a 32/36 weber, I added a 1 inch spacer. Now I think it needs a 38/38. should I do that? I runs better now at high rpm and still didn't bog at the low end.

[@scotthatch4548]

I would assume your head flow is around 154 cfm ...with that you would need like 330 cfm of carb and the 32/36 flows like 320 ... I would say it's about right in size

[@talesfromthetarheelstate351]

@@scotthatch4548 Thanks for the info, I will not waste the money. Great video by the way. Thanks again.

[@talesfromthetarheelstate351]

@@scotthatch4548 I just saw them run a gauge on the intake and it flows at 233! I was surprised it was that high. I assume that's a pretty big increase on a 4 cylinder, yes or not that big?

[@scotthatch4548]

@@talesfromthetarheelstate351 intake alone or intake and head at max lift

[@talesfromthetarheelstate351]

@@scotthatch4548 they had the head off the motor. they had a gauge and computer read out of the air flow through the port. I guess air was being pulled through.

[@jamesford2942]

How does this apply when you have individual carbs with a butterfly for each port like a pair of Weber DCOE carbs on a 4 cylinder?

[@scotthatch4548]

In that case it needs to flow just under the runner flow .. because you have no plenum it has to support the entire pulse draw but you need to try and keep airspeed up for low end and to help with reversion especially if it's going to be a big camshaft

[@SeanOBryanZZ]

um how did you know i was gonna ask this? mines a V8 though!

[@TomSmith-cv8hk]

Yep, they prove plenum volume is BS, but they aren't big enough for big displacement motors.

[@scotthatch4548]

@@TomSmith-cv8hk plenum volume can help when the carb is undersized ... But on IR and even completely split dual plane you do need the carb to keep up with the runner flow ... And because of that total cfm on IR type for the whole engine can get big on a big engine you can increase the runner length a bit to help in runner volume ...not cross section as you need the airspeed but a bit in length can help

[@Waipen]

Now the next chapter would be: The difference between a square bore and a spread bore :)

[@claytonstoolbox]

3.38x280 is 946. I have an 850 carb. Do you think that's too small? Small town drag racing, big block Oldsmobile that revs to 6500.

[@scotthatch4548]

@@claytonstoolbox really depends on HP ... 850 is good to about 550 HP or so ... Are you topping out somewhere about there

[@claytonstoolbox]

@@scotthatch4548 I was shooting for 600 but 550 is probably more realistic. It’s in a light car. I hear that makes a difference as well.

[@jacquespoirier9071]

most auto manufacturers uses an undersized carb for an obvious reason : bigger a carb is for a given engine, more sensitive it is to atmospheric changes so when you go for an oversized carb, you buy a passport for continuous tuning sessions.

[@dondotterer24]

What is the best way to flow a Q-Jet?

[@scotthatch4548]

Why do you feel like you need to do that?

[@dondotterer24]

@@scotthatch4548 To see what any changes would do

[@scotthatch4548]

@@dondotterer24 it takes a big bench to flow a whole carb .. most of the time you test one bore at a time with a block plate on the other ones ... Not sure what change you can do on a quadrajet carb ....

[@dondotterer24]

@@scotthatch4548 Thank you

[@michaelgarrow3239]

Can you explain air bleeds? I understand a smaller air bleed gives you more fuel at high vacuum and at low vacuum not so much. But I am unclear about it.. It isn’t the same as changing jet sizes? 🤔 Also part throttle discussion- you know where we are 99% of the time. On the street.

[@scotthatch4548]

The air bleed question is really a video about how carbs work ... Not sure if there is already something out there but I will look into a video on just carb function.... The part throttle question is a good one and I should probably do a second video about that ... One of the functions of a carb is to reduce the air density and actually reduce the power an engine makes so you can drive it but that also changes airspeed and you get into carb response to throttle change .. that video I definitely need to do ..

[@scotthatch4548]

Just realized I didn't answer .... The difference between jetting and air bleed is curve ... Jetting changes the fuel across the whole curve low rpm to high ... Air bleed changes the mix mostly at higher rpm .... As you go up in air bleed you lean out top end mix as you go down it will richen it ... Basically tilting the fuel curve ... So between the two you can balance the fuel curve across the rpm of the engine and not have it rich at low and lean at high rpm or vice a versa

[@michaelgarrow3239]

@@scotthatch4548 - That is what I thought. But haven’t heard anyone explain it. Thanks!

[@jaywalker7617]

@@scotthatch4548 Best carb info period. Thank you Scott

[@TomSmith-cv8hk]

Yer the old carb formula is BS. But you need to do more homework on how a dual plane works. The 180 degree separation idea was patented in 1909, Henry used it on the 1934 Ford V8 after the poor performance of the 90 degree manifold 1932/33 V8.

[@JC-gw3yo]

interesting

[@OutlawCarson]

Stan Weiss

[@scotthatch4548]

users.erols.com/srweiss/tablehdc.htm

[@ronsmith7739]

Use a mike, use a mike, use a mike !!!!!

[@scotthatch4548]

Problem is the space it's open and echoes... On videos after this I put up sound shields to block some of it and they came out better

[@ThomasELeClair]

,,,,,,,,,,,,,,,,,,,,,,You got it correct,,,,,The audio is just fine,,,,,,,i put many points into my hand written journal,,,,,,,,,,@@scotthatch4548