CARB SIZE???
 

I was wondering what you carb people would put on my combo. I have a 306 Edelbrock performer RPM heads milled .030,1.6 rockers, Comp cams 565-575 lift with 234 int-244 exh @50 11.1-1 comp. Victor Jr. intake 15/8 long tubes/with 21/2 pro chamber and race magnums dumped. It has an ATI c-4 full manual/no brake. The converter is a 4200 8in. I also run a 4:10 rear. I ran the car before it was even tuned and it did the best of
12.401@108mph shifting at 5800rpms leaving at idle. I have a Holley 600DP 4150/with mechanical secondarys.The jetting is 66front/76rear. It seem to be fine, but above 6500 it starts to break-up a little bit. This engine was built to rev, and was estimated to have 500hp. Some people are telling me to put a 750 on some say 600 is fine some even say a 900. Come on now this is still a small motor. I think a 650-700DP would be good but maybe someone out there has a similar combo that could help. I just don't want to spend $400 on the wrong carb.
Also I do drive this car on the street occasionally
Any help would be great.

Thanks Guys/Gals

I'd say a properly jetted 650DP would work just fine. You could go as large as a 700 and probably wouldn't have to worry. But in my opinion, it's vac sec. all the way.

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1984 1/2 GT350 (#842 Hatchback w/ T-tops), 302HO, Comp cams Xtreme Energy cam, Carter 625cfm carb, Weiand Stealth intake, MSD distributor, MSD coil, FMS 9mm wires, 1 5/8" MAC unequal shorties, 2 1/2" MAC Prochamber H-pipe, Flowmaster 2 chambers, KYBs, 16"x8" 4 lug Cobra Rs, Falken 245/45ZR16s

Personally, i'd go with the 750 dp, you'll still need to jet it properly though but they do pretty good out of the box.i put together a brand x small block with ported stock heads, low compression pistons,performer intake,small tubes, way to big a cam, and the 650 dp worked great , but rejetting it worked way better. that motor did not put out anywhere near 500 hp.usually a 650 is rated to perform well under 500 hp.

get a 700 dp. if you plan on using nitrous you can go 750. a 306 will work good with a 750 dp if its a race motor and you rev it which you plan to do. but you also want to drive it so theres the problem. why not tune the carb you have? 66/76 jets are good for a mild street motor not for a race motor that revs. you will have to run it a little rich at low rpm due to the design of the motor but up top it will pull harder.

I would go with a 650DP - You can always rejet if you need to, but thats a stong carb for your setup.

Get a Holley 4779. You won't regret it.

Take care
~Chris

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MustangNet

My site: JimPorterRacing

RACECAR spelled backwards is RACECAR

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I'd look to see why it's breaking up 1st... carb shouldnt cause that unless the pump couldnt keep up...new carb wouldnt help that.. make it worse actuallt because it would need more fuel...

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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 ;-)

The magizine car craft sept2001 has a great article on the right size carb for the applications, maybe that could help you.

USE THIS TO FIGURE CARB SIZE.
CIDxRPM devided by 3456 then add 10% for good measure.
a 306 spinning @7000rpm = 681cfm using the formula
306 @ 6500 rpm = 633 cfm

another way to decide if you need a bigger carb is to run a vacum guage to the intake.. run the line to the inside of the car.. make a run.. and if the vacum goes higher then it is at idle.. you need a bigger carb...

Thanks for your opinions. I tried rejetting my 600cfm, but not better. Its still seem like it need more upstairs. As for the fuel pump I have a Mallory 250 with #10 line from the sump to the BG regulator.I have #8 going to the carb so I know i'm getting enough fuel. I'm going to try a 650 and go from there. I'm hoping to get a decent one on EBAY, not unless any of you got an extra one laying around? If you do let me know.

Thanks Again http://www.mustangworks.com/msgboard/biggrin.gif

I wouldn't go bigger than a 700dp, even if you do go to the sauce. Nitrous seems to like a smaller carb so the 700dp will work. I think a 650 will work fine, and Dark Knight has it right the vacuum guage is the best way to tell.

Keep the 600dp for the street, swapping a carb takes maybe 5 minutes so use a bigger badder one at the track and then switch back. That way you can ditch your choke.

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GT-40 heads (ported, polished, + milled), B303 cam, 1.7rr's, JE pistons, Offy intake, Carter AFB 625cfm carb, Flowtech 1 5/8" shortys, Flowtech X-pipe, MAC Flowpath exhaust, MAC pulleys, 373's, subframes, Eibachs+Tokicos, B&M ripper, FMS Clutch, Zoom Quadrant+cable, 17" CSA Ultra rims, 235/45ZR17 Yokohama AVS S4's, MSD 6A ignition+coil, FMS 9mm wires, Carbed, Naturally Aspirated, and Nasty!

[This message has been edited by red82gt (edited 08-07-2001).]

Dark Knight-

I never ask questions here, and this is my second one in 20 minutes! What a great day for learning! Anyway, I am a huge advocate of the underused vacuum gauge, and install them in every vehicle I ever own, but I am unaware of your advice. I am having trouble understanding your instructions, and was hoping you would elaborate on them. I love knowledge, so please share this with me!

Thanks!

Take care
~Chris

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Retired Moderator
MustangNet

My site: JimPorterRacing

RACECAR spelled backwards is RACECAR

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if the vacum is higher at full throttle than idle... it means the engine is trying to pull more air than the carb can give it.. makes sense too...idle is a high vacum situation...
if you are pulling more vacum at full throttle than you are at idle... there's a restriction... of course you cant tell how MUCH bigger ;-)

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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 ;-)

That's where I'm confused. Manifold vacuum will always be lower at full throttle than it will at idle, no matter what the induction is like. You could put a one barrel on a 460, and have 18 inches at idle, but as soon as you crack the throttle, vacuum will drop, eventually to zero if the throttle is opened all the way. I understand what you're saying, I think, but I don't see how that's possible. At idle, the throttle is completely closed, minus a few thousanths, which will create the highest vacuum, other than when you close the throttle at high rpms, in which case it will exceed your idle vacuum by 4 to 6 inches, provided your rings are good. I really hope I'm missing something here, because trial and error, and now experience, have always been my guides for carb selection, and if you can figure it out with a vacuum gauge, I want to know how! Tell me what I'm missing. At which point during your run are you supposed to check the vacuum? And are you comparing manifold vacuum (WOT) to manifold vacuum (idle), or is ported vacuum involved here?

Very confused, but eager to learn!

Take care
~Chris

------------------
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MustangNet

My site: JimPorterRacing

RACECAR spelled backwards is RACECAR

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I'm not gonna guess on carb size, because the best way to find out is with the vac method that has already been mentioned, or on a dyno. I found out that my 600 cfm was a good size once I put the right jets in it. I think the dyno is the best way to tune it, because you can check the air/fuel ratio for the whole run and at idle. But I had a problem with my car breaking up too and it turned out to be my ignition system, and my shift light. My total timing was too high so it was breaking up, but I also found out that when my shift light kicked on it pulled enough current you make it miss for a split second, but that pulled out 25 horses and ft/lbs for a few hundred rpm. They are well worth it.

Phantom

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ultimately the dyno or constant track tuning is the best. however since neither is practical the vac method will work. although the vac drops to sero when you hit the throttle it will start to climb as you go.
a 650 dp will probably be the best choice for you if you want to drive it. there are alot of choices and advice. generally a 600 is good for a mostly stock motor reving to 5500 rpm. a mildly modded motor with juice likes a 650 dp. when you mod a motor alot and factor in the high rpm range in which you plan on reving then the 700 comes into play. vac. will not tell you what carb makes more power. and the math theory will only give you a carb guideline for a stock motor. if you are lucky enough to have some friends who use carbs, convince them to go to the track with you and try their carbs. when you find a carb that seems to be best you will know what is good for you.

C.I.D x MAXRPM/3456 = Carb size @ 100% volumetric efficiency.

If you are shooting for a known amount of power then the formula is

1.6 x HP = Airflow needed

Or another way of saying is 1 hp requires 1.6 cubic feet of air to generate.

My not so handy HP sim says your in the neighborhood of 440 to 450 hp with your current setup with a peak VE of 101%

the closest models I have are a set of Windsor SRs and A Xtreme Energy cam (35-522-8).

Is your cam a hydraulic roller, solid roller, hydraulic flat tappet, solid lifter?

Hydraulic rollers are heavy and limit the RPM potential of the engine (my bet) I have a brand X with a 650 mech DP and solid roller with VERY HEAVY springs (to heavy in fact), the car doesn't break up, it just revs to 8500 rpm and stays there until you lift off of the gas! I had a Vic Jr on it 3 inch exhaust and a 750 mech dp so it used to rev higher (seen 9000 on the tach), but the cam was a mild 278 degrees adv and I used 1.6 rockers with an .025 lash so it had a net lift of .567 and was probably 278 or 280 deg adv on duration

Anywhosit by changing to a 1-3/4 header your can get into the 450-460 hp range and your VE jumps to 102%

306 x 7000/3456 x 102% = 632 cfm by Holley's math or 460 x 1.6 = 736 cfm based on hp

WOW...This sh!t is harder than I thought. There are so many ways to figure this out. Some times I think F/I is easier. I'm going to try this vac. idea this weekend. I'll let you all know how it worked. I would like to thank all of you for you help and advice. All my friends are Chevy guys and say just put a 750-850 carb on you will be fine. Maybe thats why they all smoke!!! http://www.mustangworks.com/msgboard/tongue.gif


Thanks Again http://www.mustangworks.com/msgboard/biggrin.gif

You are just going to have to experiment, and see what works best.

Max- That was some really good reading. I had to re-read it a few times, but that was pretty good. I used to think it was funny that anyone believed they could ever achieve 100% VE , but I've gotta admit your closer than anyone I've seen. Your formulas assume you will have 100% VE. Volumetric efficiency theoretically highest at an engines peak torque. It's air capacity is highest at it's peak rpm, and it's real VE is found be dividing the actual cfm at peak torque by the theoretical cfm at peak torque. I gave up on all the formulas about 10 years ago. I was really into them, and piston speed was always one of my favorites, but when it came to carb size, the formulas just didn't wash. I worked in 1998 for Bill Elliott, and was really surprised to find out the Winston Cup cars are averaging 1hp to 1cfm, and they're able to sustain 8000 rpm for a lot longer than 10 seconds, and are closer to 100% VE. Apply that proven logic, and a 460 hp Mustang should be fine with a 500cfm carb. If you apply Holley's formula, which assumes you will never achieve more than 85% VE, his engine would require 527cfm for a daily driver, and 682cfm as an all out racer (the primary differences here being the rest of the drivetrain and vehicle weight). Meanwhile, the true theoretical cfm would be 620. So the same engine would require anywhere from 500cfm to almost 700cfm, depending on which respected formula you applied. I have built 302's for years, and the best average intake combo for the ones my customers request, is usually an RPM intake with a 4777 Holley. There have been several that performed better with a 4778, and even a few that hit 7500rpm with a 4779. I feel pretty confident in saying that I could put together a 4779 that would result in a faster 1/4 mile time than anyone elses smaller carb on this kids engine, but that's just me.

I decided the formulas were meaningless when i did the math for a nitrous injected 403 I used to have, and the most generous formula I could find said I needed an 800cfm carb. I followed those guidelines, and made every adjustment I could to get the most out of it. I was happy with it, but then a buddy of mine who owned the local speed shop let me "borrow" his 1100 cfm variable venturi Predator, and the difference was night and day. That thing went a full 900rpm higher, and was still pulling hard. I couldn't believe the difference. Since then I've run into many situations where similar engines ran better with different carbs,

After all that, I really was just trying to say that your formulas are quite good, and your right about the cams. You're a smart man, Max.

Take care
~Chris

Bottom line is most situations are different. Trial and error is usually the best bet.

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MustangNet

My site: JimPorterRacing

RACECAR spelled backwards is RACECAR

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[This message has been edited by PKRWUD (edited 08-08-2001).]

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.

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

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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 !!!

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.

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 !!!

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.

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 http://www.mustangworks.com/msgboard/biggrin.gif) 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 http://www.mustangworks.com/msgboard/rolleyes.gif

::rant off::

Whew...slipped into the mode there for a bit http://www.mustangworks.com/msgboard/smile.gif

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.

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.

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.

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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 ;-)

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. http://www.mustangworks.com/msgboard/rolleyes.gif

Thanks for the info!!! http://www.mustangworks.com/msgboard/smile.gif

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..

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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 ;-)

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

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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 !!!