Carb selection help for new engine

[6T9Mach1 12]

The biggest mistake it seems most people make is to over carb for the street. Again IMO, rarely does TRUE street engine need over 650-750 cfm.

 

Overcarbing with poor valve timing events is more like it. If you dont know how to tune a carburetor and are picking a random OTS camshaft then I agree with you about over carbing. It is probably best for someone like this to stick with a slightly smaller carb. A 750 Mighty Demon is not too large of a carb for the OP however. Once again overcarbing only happens when there is a lack of signal to the boosters. Throwing out a random 650-750 cfm number doesnt mean much. Is it wet flowed? Dry flowed? What is the venturi size? What is the throttle blade size? What depression was the carb flowed at? What depression does my motor pull at? Will it flow those numbers on my motor? Lots and lots of variables to take into consideration.

[Moteaux 10]

Overcarbing with poor valve timing events is more like it. If you dont know how to tune a carburetor and are picking a random OTS camshaft then I agree with you about over carbing. It is probably best for someone like this to stick with a slightly smaller carb. A 750 Mighty Demon is not too large of a carb for the OP however. Once again overcarbing only happens when there is a lack of signal to the boosters. Throwing out a random 650-750 cfm number doesnt mean much. Is it wet flowed? Dry flowed? What is the venturi size? What is the throttle blade size? What depression was the carb flowed at? What depression does my motor pull at? Will it flow those numbers on my motor? Lots and lots of variables to take into consideration.

 

And from my earlier post this in much more in depth of what I was saying. Average Joe putting in a cam and hi-rise is not going to be going through most of the carb tuning and actually matching components, although it is getting easier these days with the top-end kits available. In most cases for the average person thinking that just putting a larger carb on will automatically give better results often can deter performance for all the reasons you mentioned. Where if staying at a relatively lower (and yes, arbitrary number) that I suggested will not exacerbate the issues that could come from this.

 

Then there is the whole set of opinions on what is really a streetable engine and that whole can of worms....

Edited March 13, 2010 by Moteaux

[Moteaux 10]

Overcarbing with poor valve timing events is more like it. If you dont know how to tune a carburetor and are picking a random OTS camshaft then I agree with you about over carbing. It is probably best for someone like this to stick with a slightly smaller carb. A 750 Mighty Demon is not too large of a carb for the OP however. Once again overcarbing only happens when there is a lack of signal to the boosters. Throwing out a random 650-750 cfm number doesnt mean much. Is it wet flowed? Dry flowed? What is the venturi size? What is the throttle blade size? What depression was the carb flowed at? What depression does my motor pull at? Will it flow those numbers on my motor? Lots and lots of variables to take into consideration.

 

And from my earlier post this in much more in depth of what I was saying. Average Joe putting in a cam and hi-rise is not going to be going through most of the carb tuning and actually matching components, although it is getting easier these days with the top-end kits available. In most cases for the average person thinking that just putting a larger carb on will automatically give better results often can deter performance for all the reasons you mentioned. Where if staying at a relatively lower (and yes, arbitrary number) that I suggested will not exacerbate the issues that could come from this.

 

Then there is the whole set of opinions on what is really a streetable engine and that whole can of worms....

Edited March 13, 2010 by Moteaux

[Burn 14]

Thanks for all the great info. It seems that I cant go wrong with either mechanical or vac secondaries, but I'll probably go with the Summit racing 750 mechanical sec w/ annual boosters. I like the fact that it's a Holley design, it's similar to the one piece Autolite 4100. It's also in my price range, brand new and comes with a tuning DVD. My rookie butt will need that video. Thanks again for all the info.

 

Jim "Burn"

[Burn 14]

Thanks for all the great info. It seems that I cant go wrong with either mechanical or vac secondaries, but I'll probably go with the Summit racing 750 mechanical sec w/ annual boosters. I like the fact that it's a Holley design, it's similar to the one piece Autolite 4100. It's also in my price range, brand new and comes with a tuning DVD. My rookie butt will need that video. Thanks again for all the info.

 

Jim "Burn"

[Max Power 74]

Rev up a motor with a vacuum carb and watch how long it takes the secondaries to open. You have to drop manifold vacuum and wait for the spring to push the secondaries open. Sometimes that's 1-2 seconds.

 

That's because when you are revving a motor, it's usually not under load. You can get to zero vacuum darn quick under load. Heck, half the time you can't even get the secondaries to open if there is no load.

[Max Power 74]

Rev up a motor with a vacuum carb and watch how long it takes the secondaries to open. You have to drop manifold vacuum and wait for the spring to push the secondaries open. Sometimes that's 1-2 seconds.

 

That's because when you are revving a motor, it's usually not under load. You can get to zero vacuum darn quick under load. Heck, half the time you can't even get the secondaries to open if there is no load.

[maxum96 32]

That's because when you are revving a motor, it's usually not under load. You can get to zero vacuum darn quick under load. Heck, half the time you can't even get the secondaries to open if there is no load.

 

 

I'd agree with that. But even under a load it's nowhere as quick as a dp on the secondaries opening. That's why vacuum secondary carbs are better on automatics and heavy cars.

[maxum96 32]

That's because when you are revving a motor, it's usually not under load. You can get to zero vacuum darn quick under load. Heck, half the time you can't even get the secondaries to open if there is no load.

 

 

I'd agree with that. But even under a load it's nowhere as quick as a dp on the secondaries opening. That's why vacuum secondary carbs are better on automatics and heavy cars.

[6T9Mach1 12]

There is a reason why guys at the track run mechanical secondaries. If vacuum secondary carbs turned the times that mechanical secondary carbs do then you would probably see more of them at the track.

[6T9Mach1 12]

There is a reason why guys at the track run mechanical secondaries. If vacuum secondary carbs turned the times that mechanical secondary carbs do then you would probably see more of them at the track.

[PaleRider 12]

I bought a 650 DP for my 347 build - has 3.5 gears and c4 with shift kit. Have not dropped new motor in yet. I did not go vacuum this time - cause my previous experience was could not get the secondaries to open up on 302 I was running - even after changing springs. Probably some other issue.

 

But curious on difference in gas mileage? What should you expect between vacuum and mechanical?

[PaleRider 12]

I bought a 650 DP for my 347 build - has 3.5 gears and c4 with shift kit. Have not dropped new motor in yet. I did not go vacuum this time - cause my previous experience was could not get the secondaries to open up on 302 I was running - even after changing springs. Probably some other issue.

 

But curious on difference in gas mileage? What should you expect between vacuum and mechanical?