CARB SIZE???

88PONY · 08-03-2001, 11:50 AM

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

84_GT350 · 08-03-2001, 12:13 PM

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.

------------------
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

BlackFly · 08-03-2001, 12:38 PM

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.

juiceman · 08-03-2001, 03:24 PM

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.

79Stang · 08-03-2001, 03:30 PM

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

PKRWUD · 08-03-2001, 09:53 PM

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

Dark Knight · 08-03-2001, 11:58 PM

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...

------------------
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 ;-)

untamed351 · 08-04-2001, 10:28 AM

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

dragracer · 08-04-2001, 10:31 AM

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

Dark Knight · 08-04-2001, 11:32 AM

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...

88PONY · 08-06-2001, 10:09 AM

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

red82gt · 08-07-2001, 03:04 AM

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.

------------------
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).]

PKRWUD · 08-07-2001, 10:06 AM

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

------------------
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 !!!

Dark Knight · 08-07-2001, 02:42 PM

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 ;-)

------------------
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 · 08-07-2001, 05:03 PM

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

------------------
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 !!!

Phantom5oh · 08-07-2001, 05:26 PM

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

------------------
CHECK OUT MY MODS

http://www.mustangworks.com/cgi-bin/...splay.cgi?1350

juiceman · 08-07-2001, 06:02 PM

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.

MiracleMax · 08-07-2001, 11:55 PM

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

88PONY · 08-08-2001, 07:08 AM

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!!!

Thanks Again

PKRWUD · 08-08-2001, 10:05 AM

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.

------------------
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 !!!

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

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08-03-2001, 11:50 AM	#1
88PONY Registered Member Join Date: Nov 2000 Location: INDIO CA USA Posts: 141	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

08-03-2001, 12:13 PM	#2
84_GT350 Registered Member Join Date: Apr 1999 Location: Overland Park, KS, USA Posts: 1,631	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. ------------------ 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

08-03-2001, 12:38 PM	#3
BlackFly Registered Member Join Date: Feb 2001 Location: winnipeg , manitoba , canada Posts: 83	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.

08-03-2001, 03:24 PM	#4
juiceman Registered Member Join Date: Jan 2001 Location: ny Posts: 197	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.

08-03-2001, 03:30 PM	#5
79Stang The Stang Gang Join Date: Jan 1999 Location: St. Petersburg Posts: 63	I would go with a 650DP - You can always rejet if you need to, but thats a stong carb for your setup.

08-03-2001, 09:53 PM	#6
PKRWUD Junior Member Join Date: Mar 1999 Location: Ventura, California Posts: 8,981	Get a Holley 4779. You won't regret 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 !!!

08-03-2001, 11:58 PM	#7
Dark Knight Registered Member Join Date: Oct 1998 Location: Az Posts: 854	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... ------------------ 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 ;-)

08-04-2001, 10:28 AM	#8
untamed351 Registered Member Join Date: Jun 2001 Location: Belmar, NJ Posts: 52	The magizine car craft sept2001 has a great article on the right size carb for the applications, maybe that could help you.

08-04-2001, 10:31 AM	#9
dragracer Registered Member Join Date: Aug 2001 Posts: 3	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

08-04-2001, 11:32 AM	#10
Dark Knight Registered Member Join Date: Oct 1998 Location: Az Posts: 854	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...

08-06-2001, 10:09 AM	#11
88PONY Registered Member Join Date: Nov 2000 Location: INDIO CA USA Posts: 141	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

08-07-2001, 03:04 AM	#12
red82gt Sober voice of Reason Join Date: Nov 1998 Location: Kelowna, B.C., Canada Posts: 1,514	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. ------------------ 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).]

08-07-2001, 10:06 AM	#13
PKRWUD Junior Member Join Date: Mar 1999 Location: Ventura, California Posts: 8,981	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 ------------------ 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 !!!

08-07-2001, 02:42 PM	#14
Dark Knight Registered Member Join Date: Oct 1998 Location: Az Posts: 854	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 ;-) ------------------ 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 ;-)

08-07-2001, 05:03 PM	#15
PKRWUD Junior Member Join Date: Mar 1999 Location: Ventura, California Posts: 8,981	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 ------------------ 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 !!!

08-07-2001, 05:26 PM	#16
Phantom5oh Registered Member Join Date: Mar 2000 Location: Boston, MA Posts: 334	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 ------------------ CHECK OUT MY MODS http://www.mustangworks.com/cgi-bin/...splay.cgi?1350

08-07-2001, 06:02 PM	#17
juiceman Registered Member Join Date: Jan 2001 Location: ny Posts: 197	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.

08-07-2001, 11:55 PM	#18
MiracleMax Registered Member Join Date: Apr 1999 Location: Hayes, Va, USA Posts: 798	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

08-08-2001, 07:08 AM	#19
88PONY Registered Member Join Date: Nov 2000 Location: INDIO CA USA Posts: 141	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!!! Thanks Again

08-08-2001, 10:05 AM	#20
PKRWUD Junior Member Join Date: Mar 1999 Location: Ventura, California Posts: 8,981	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. ------------------ 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 !!! [This message has been edited by PKRWUD (edited 08-08-2001).]