CARB SIZE??? - Page 2

juiceman · 08-08-2001, 08:58 PM

i must say, you guys are pretty good at reading. and its good to read and use formulas but there is no formula that will tell you exactly how an engine will react to anything. be it carb, cam, heads whatever. you assume that motor will make 100% or better v.e. but thats all theoretical. in the real world that doesnt work. too many variables. winston cup motors run small carb and small cubic inch yet make good power at high rpm. drag cars run big carbs on small motors and also make good power at high rpm. we run 306's with 900 cfm carbs and they like it. the best way is to experiment.

PKRWUD · 08-09-2001, 01:20 AM

Making something fast go faster has been my hobby and my passion for over 20 years, and has put food on my table for over 15 years. When you make a living doing something, you'd be a fool not to try and learn as much as you can. You do read books, and you do read formulas. And they aren't all bad, but there isn't a single one that applies to everyone, so what's the point, right? The point is that it's a guide for those who don't make a living at it, or who love it so much they bleed oil. They already know what works for them in their environment. I agree that trial and error, and ultimately experimentation is the only way to ever learn, but if you don't know much about it, experimenting won't get you anywhere either. Everyone should read the formulas, and about two thirds should abide by them.

Take care
~Chris

------------------
Retired Moderator
MustangNet

My site: JimPorterRacing

RACECAR spelled backwards is RACECAR

HEY !!! Are you ASE Certified ??? If you are, ask me about iATN. The best tool you'll ever have, and it's free !!!

88PONY · 08-09-2001, 11:29 AM

PKRWUD is right!!! This is a hobby and a passion for most of us. It can't hurt to experiment a little. Only 2 things could happen. Either it gets better of it gets worse, and you can always go back to the way it was. Even if you do the formulas you may relize your car may like a bigger or smaller carb than what it mathematically equated to.

Hey you could build 2 motors the same, but they will always be different.

PKRWUD · 08-09-2001, 05:42 PM

I agree. You remind me of one very important thing that needs to be remembered when experimenting: Only make one modification at a time! I myself am horrible at that. When I tear into something I try and do as much as I can, but when I'm all done, and the car is faster, I can't say for certain which item made the greatest impact. Over time I've gotten pretty good at knowing what does what, but for someone less experienced, it really is a good idea to try and only make one modification at a time. Especially if you're trying to learn from it.

Take care
~Chris

------------------
Retired Moderator
MustangNet

My site: JimPorterRacing

RACECAR spelled backwards is RACECAR

HEY !!! Are you ASE Certified ??? If you are, ask me about iATN. The best tool you'll ever have, and it's free !!!

juiceman · 08-09-2001, 06:04 PM

pkrwud, that was a good point that you brought up. most people dont just do one thing at a time. it is hard. but it is the right way. sometimes if you do more than one thing at a time they cancel themselfs out. in other words one part may add 5 hp and the other part may lose 5 hp so in the end you get nothing. believe it or not it happens alot. pkrwud, we do this for a living so its easier for us and i do forget that at times. formulas are good for certain things like air flow and such but alot of things dont really apply to the average street guy. best advice, dont be afraid to try things it how we all got experience.

MiracleMax · 08-09-2001, 08:48 PM

Emperical testing will win out over theory any day, but I like to play with the formulas a bit to see what direction I might want to go in. Do I do this, do I do that? Lets scribble it down on paper before I go and blow my cash.

I wish manufactures woud release more technical info about thier products so you could fiddle a bit more mathematically with the parts before you buy them.

Carbs and headers are not to bad, known size and generally lengths there, Intake manifolds are kinda fuzzy along with cylinder heads. Does the intake runner have a taper? How long is it? What does it flow? Ditto for cylinder heads, how much crossection at the bowl? how much flow around the valves vs. the port? ect. Don't get me started on my peev, throttle bodies! How much does it cost to print the flow numbers along side the bore size?

65mm, how much does it flow?
70mm, same
75mm, ect

While not as sensetive as carbs to velocity, just going to the biggest t-body brings you to a point of diminishing returns and in some cases I feel even detrimental, especially with bigger cams where reversion and lazy airflow could casue a hiccup or two.

Anywhosit, I see the media doesn't help. Over at Fordmuscle.com (?), they had an article on cylinder heads, Turns out Canfield turned them down because someone else did a shotout using ported stockers and did an unfair (in there eyes) comparison so no more Canfields vs. whoever unless somebody forks over some cash to buy a set (then again magazines could have yearly cylinder head give aways

) Oh boy, just what I wanted to see! A set of mystery cylinder heads with the only released info being a gasket size, valve dia, and combustion chamber size. Real helpful

::rant off::

Whew...slipped into the mode there for a bit

blue oval 50h · 08-10-2001, 08:30 AM

I have a similar combo and have asked an ex-nitrous express user(hint)mustang shop what carb would be good and they said 750-820. They said a 650 is good for throttle response, say for street.

blue oval 50h · 08-10-2001, 08:31 AM

I have a similar combo and have asked an ex-nitrous express user(hint)mustang shop what carb would be good and they said 750-820. They said a 650 is good for throttle response, say for street.

Dark Knight · 08-12-2001, 10:01 AM

found this... http://www.ondoperformance.com/tech.html

kinda interesting... ;-)
United States, most carburetors are rated in flow capacity at a test pressure of 20.4" of water, (1.5" of mercury, 51.8 cm of water). An 850 cfm (4001/s) carburetor is one
that passes 850 cfm of air at 8 test pressure of 20.4" (51.8 cm) of water. However, if you observe a manifold vacuum gauge on a racing engine at full throttle, you will see
that it only reads about 0.5" of mercury (6.8" of or 17 cm water). At a test pressure of 6.8" of water, the same carburetor would only pass 490 cfm (2301/s) of air. This is
why carburetor ratings appear to be all out of proportion to engine requirements.

------------------
84 convt,'95 302,AFR's, performer
3.55's, underdrives BBK shorties
stock cam, 1.7's
13.85@102.5
and a '68 stang that WAS nasty ;-)

88PONY · 08-13-2001, 09:02 AM

That was very cool. There are a lot of different formulas, but what I think works best is physical experimenting. I wish I could get about 5 carbs got to the track and see what it likes best. Easier said than done.

Thanks for the info!!!

Dark Knight · 08-13-2001, 01:50 PM

save the money and get a demon with changable venturis.. hehe
PKRWUD, when you run at full throttle, the vacum will climb back up.. it's like putting your hand over a vacum hose..the smaller the opening, the more suction you have because of the restriction, and the harder the motor has to work to try and pull the air thru..
think of it like this... you put a 100cfm one barrel on a 460... it's probably not gonna rev over 3000rpm, because it cant pull enough air into the engine..high vacum..

------------------
84 convt,'95 302,AFR's, performer
3.55's, underdrives BBK shorties
stock cam, 1.7's
13.85@102.5
and a '68 stang that WAS nasty ;-)

PKRWUD · 08-13-2001, 10:28 PM

Thanks. I eventually understood, I just have never seen the vacuum even get close to idle vacuum when racing, or never noticed. Honestly, I observe my vacuum gauge mostly when i'm checking on something, like a funny vibration when I stop at the light, or to verify a miss, etc. i'll have to go for a spin in the truck and see what happens.

Thanks!!

Take care
~Chris

------------------
Retired Moderator
MustangNet

My site: JimPorterRacing

RACECAR spelled backwards is RACECAR

HEY !!! Are you ASE Certified ??? If you are, ask me about iATN. The best tool you'll ever have, and it's free !!!

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08-08-2001, 08:58 PM	#21
juiceman Registered Member Join Date: Jan 2001 Location: ny Posts: 197	i must say, you guys are pretty good at reading. and its good to read and use formulas but there is no formula that will tell you exactly how an engine will react to anything. be it carb, cam, heads whatever. you assume that motor will make 100% or better v.e. but thats all theoretical. in the real world that doesnt work. too many variables. winston cup motors run small carb and small cubic inch yet make good power at high rpm. drag cars run big carbs on small motors and also make good power at high rpm. we run 306's with 900 cfm carbs and they like it. the best way is to experiment.

08-09-2001, 01:20 AM	#22
PKRWUD Junior Member Join Date: Mar 1999 Location: Ventura, California Posts: 8,981	Making something fast go faster has been my hobby and my passion for over 20 years, and has put food on my table for over 15 years. When you make a living doing something, you'd be a fool not to try and learn as much as you can. You do read books, and you do read formulas. And they aren't all bad, but there isn't a single one that applies to everyone, so what's the point, right? The point is that it's a guide for those who don't make a living at it, or who love it so much they bleed oil. They already know what works for them in their environment. I agree that trial and error, and ultimately experimentation is the only way to ever learn, but if you don't know much about it, experimenting won't get you anywhere either. Everyone should read the formulas, and about two thirds should abide by them. Take care ~Chris ------------------ Retired Moderator MustangNet My site: JimPorterRacing RACECAR spelled backwards is RACECAR HEY !!! Are you ASE Certified ??? If you are, ask me about iATN. The best tool you'll ever have, and it's free !!!

08-09-2001, 11:29 AM	#23
88PONY Registered Member Join Date: Nov 2000 Location: INDIO CA USA Posts: 141	PKRWUD is right!!! This is a hobby and a passion for most of us. It can't hurt to experiment a little. Only 2 things could happen. Either it gets better of it gets worse, and you can always go back to the way it was. Even if you do the formulas you may relize your car may like a bigger or smaller carb than what it mathematically equated to. Hey you could build 2 motors the same, but they will always be different.

08-09-2001, 05:42 PM	#24
PKRWUD Junior Member Join Date: Mar 1999 Location: Ventura, California Posts: 8,981	I agree. You remind me of one very important thing that needs to be remembered when experimenting: Only make one modification at a time! I myself am horrible at that. When I tear into something I try and do as much as I can, but when I'm all done, and the car is faster, I can't say for certain which item made the greatest impact. Over time I've gotten pretty good at knowing what does what, but for someone less experienced, it really is a good idea to try and only make one modification at a time. Especially if you're trying to learn from it. Take care ~Chris ------------------ Retired Moderator MustangNet My site: JimPorterRacing RACECAR spelled backwards is RACECAR HEY !!! Are you ASE Certified ??? If you are, ask me about iATN. The best tool you'll ever have, and it's free !!!

08-09-2001, 06:04 PM	#25
juiceman Registered Member Join Date: Jan 2001 Location: ny Posts: 197	pkrwud, that was a good point that you brought up. most people dont just do one thing at a time. it is hard. but it is the right way. sometimes if you do more than one thing at a time they cancel themselfs out. in other words one part may add 5 hp and the other part may lose 5 hp so in the end you get nothing. believe it or not it happens alot. pkrwud, we do this for a living so its easier for us and i do forget that at times. formulas are good for certain things like air flow and such but alot of things dont really apply to the average street guy. best advice, dont be afraid to try things it how we all got experience.

08-09-2001, 08:48 PM	#26
MiracleMax Registered Member Join Date: Apr 1999 Location: Hayes, Va, USA Posts: 798	Emperical testing will win out over theory any day, but I like to play with the formulas a bit to see what direction I might want to go in. Do I do this, do I do that? Lets scribble it down on paper before I go and blow my cash. I wish manufactures woud release more technical info about thier products so you could fiddle a bit more mathematically with the parts before you buy them. Carbs and headers are not to bad, known size and generally lengths there, Intake manifolds are kinda fuzzy along with cylinder heads. Does the intake runner have a taper? How long is it? What does it flow? Ditto for cylinder heads, how much crossection at the bowl? how much flow around the valves vs. the port? ect. Don't get me started on my peev, throttle bodies! How much does it cost to print the flow numbers along side the bore size? 65mm, how much does it flow? 70mm, same 75mm, ect While not as sensetive as carbs to velocity, just going to the biggest t-body brings you to a point of diminishing returns and in some cases I feel even detrimental, especially with bigger cams where reversion and lazy airflow could casue a hiccup or two. Anywhosit, I see the media doesn't help. Over at Fordmuscle.com (?), they had an article on cylinder heads, Turns out Canfield turned them down because someone else did a shotout using ported stockers and did an unfair (in there eyes) comparison so no more Canfields vs. whoever unless somebody forks over some cash to buy a set (then again magazines could have yearly cylinder head give aways ) Oh boy, just what I wanted to see! A set of mystery cylinder heads with the only released info being a gasket size, valve dia, and combustion chamber size. Real helpful ::rant off:: Whew...slipped into the mode there for a bit

08-10-2001, 08:30 AM	#27
blue oval 50h Registered Member Join Date: Dec 1998 Location: NJ Posts: 219	I have a similar combo and have asked an ex-nitrous express user(hint)mustang shop what carb would be good and they said 750-820. They said a 650 is good for throttle response, say for street.

08-10-2001, 08:31 AM	#28
blue oval 50h Registered Member Join Date: Dec 1998 Location: NJ Posts: 219	I have a similar combo and have asked an ex-nitrous express user(hint)mustang shop what carb would be good and they said 750-820. They said a 650 is good for throttle response, say for street.

08-12-2001, 10:01 AM	#29
Dark Knight Registered Member Join Date: Oct 1998 Location: Az Posts: 854	found this... http://www.ondoperformance.com/tech.html kinda interesting... ;-) United States, most carburetors are rated in flow capacity at a test pressure of 20.4" of water, (1.5" of mercury, 51.8 cm of water). An 850 cfm (4001/s) carburetor is one that passes 850 cfm of air at 8 test pressure of 20.4" (51.8 cm) of water. However, if you observe a manifold vacuum gauge on a racing engine at full throttle, you will see that it only reads about 0.5" of mercury (6.8" of or 17 cm water). At a test pressure of 6.8" of water, the same carburetor would only pass 490 cfm (2301/s) of air. This is why carburetor ratings appear to be all out of proportion to engine requirements. ------------------ 84 convt,'95 302,AFR's, performer 3.55's, underdrives BBK shorties stock cam, 1.7's 13.85@102.5 and a '68 stang that WAS nasty ;-)

08-13-2001, 09:02 AM	#30
88PONY Registered Member Join Date: Nov 2000 Location: INDIO CA USA Posts: 141	That was very cool. There are a lot of different formulas, but what I think works best is physical experimenting. I wish I could get about 5 carbs got to the track and see what it likes best. Easier said than done. Thanks for the info!!!

08-13-2001, 01:50 PM	#31
Dark Knight Registered Member Join Date: Oct 1998 Location: Az Posts: 854	save the money and get a demon with changable venturis.. hehe PKRWUD, when you run at full throttle, the vacum will climb back up.. it's like putting your hand over a vacum hose..the smaller the opening, the more suction you have because of the restriction, and the harder the motor has to work to try and pull the air thru.. think of it like this... you put a 100cfm one barrel on a 460... it's probably not gonna rev over 3000rpm, because it cant pull enough air into the engine..high vacum.. ------------------ 84 convt,'95 302,AFR's, performer 3.55's, underdrives BBK shorties stock cam, 1.7's 13.85@102.5 and a '68 stang that WAS nasty ;-)

08-13-2001, 10:28 PM	#32
PKRWUD Junior Member Join Date: Mar 1999 Location: Ventura, California Posts: 8,981	Thanks. I eventually understood, I just have never seen the vacuum even get close to idle vacuum when racing, or never noticed. Honestly, I observe my vacuum gauge mostly when i'm checking on something, like a funny vibration when I stop at the light, or to verify a miss, etc. i'll have to go for a spin in the truck and see what happens. Thanks!! Take care ~Chris ------------------ Retired Moderator MustangNet My site: JimPorterRacing RACECAR spelled backwards is RACECAR HEY !!! Are you ASE Certified ??? If you are, ask me about iATN. The best tool you'll ever have, and it's free !!!