car is slower with new carb

[Red84GT]

I'm not going to try and type a novel here but i'm going to start off by saying i finally got a new carb for my mustang, basically new holley 670 street avenger. Yesterday i raced in mods vs rods at TRP, well first of all i havent raced in a year. Friday night i got the carb put on and adjusted as a was instructed to, 1 1/2 turns out on both screws, took it for a test run, felt faster on the street, got to the track and made my first run, launched at WAY too high an rpm and spun all the way through first and most of 2nd gear, was rewarded with a lowly 15.8, next pass i launched at a lower rpm and netted a 14.7@92mph with a 2.4 60ft time, i still spun somewhat through 1st. We only got 2 runs for qualifying, so i went into eliminations with only 2 passes under my belt, well i was pitted against an aero coupe with a 4.6 4v cobra motor and drag radials, he ran a 13.8 against my 14.97.

So this is my problem, before i blew up the factory 7.5 3.08 geared rearend i was trapping 103mph in the quarter @ 14.3-14.4, then i got the 8.8 with 3.73 gears and i dropped down to 14.1 but at 98mph, so i lost mph, then i took off my stock carb and found this:



if i was to guess i'd say the secondaries never opened on the factory carb and i was running 14.1 on a 2 barrel lol, okay so i put on a new holley 670 and i lost 6 tenths of a second and lost 6mph!!! What is the deal?? Did i overcarb the motor or what is going on??

[cb84capri]

670 is a big carb for a 302. you're probably going to have to lean it out from the out of the box settings.

did you play with the carb to see if the secondaries on the factory one work? or are you guessing? i doubt you were running a 14.1 on 2 barrels.

cale

[Red84GT]

670 is a big carb for a 302. you're probably going to have to lean it out from the out of the box settings.

did you play with the carb to see if the secondaries on the factory one work? or are you guessing? i doubt you were running a 14.1 on 2 barrels.

cale

notice in the pic the primary butterflies are clean and the secondaries are grimy and dirty? i'm guessing if the fuel ran past them they'd be clean

both the adjustment screws on the 670 are 1 1/2 turns out

[TheSick'86]

Im sorry I do not know what the possible issue could be with your carb. I actually want to know how to post a thread on my own. If anyone could please explain to me how to I would very much appreciate it! im new to the site so bare with me brothers!

[rancheronut]

I'm not going to try and type a novel here but i'm going to start off by saying i finally got a new carb for my mustang, basically new holley 670 street avenger. Yesterday i raced in mods vs rods at TRP, well first of all i havent raced in a year. Friday night i got the carb put on and adjusted as a was instructed to, 1 1/2 turns out on both screws, took it for a test run, felt faster on the street, got to the track and made my first run, launched at WAY too high an rpm and spun all the way through first and most of 2nd gear, was rewarded with a lowly 15.8, next pass i launched at a lower rpm and netted a 14.7@92mph with a 2.4 60ft time, i still spun somewhat through 1st. We only got 2 runs for qualifying, so i went into eliminations with only 2 passes under my belt, well i was pitted against an aero coupe with a 4.6 4v cobra motor and drag radials, he ran a 13.8 against my 14.97.

So this is my problem, before i blew up the factory 7.5 3.08 geared rearend i was trapping 103mph in the quarter @ 14.3-14.4, then i got the 8.8 with 3.73 gears and i dropped down to 14.1 but at 98mph, so i lost mph, then i took off my stock carb and found this:
[/IMG]

if i was to guess i'd say the secondaries never opened on the factory carb and i was running 14.1 on a 2 barrel lol, okay so i put on a new holley 670 and i lost 6 tenths of a second and lost 6mph!!! What is the deal?? Did i overcarb the motor or what is going on??

from what i reading in the first post, is you now having traction problems. that where your losing the extra time and MPH. if the tires are spinning! then you will run slower ET &mph.


been awhile for me playing with carbs but last time i checked. the factory fords had vacuumed secondary's.
so unless you apply vacuum to the secondary , they should not open all the way.
allso if your vcuum secondary spring is too stiff, that to will slow opening the secondary's.


only double pumper have manual linkage to open the secondary

[Red84GT]

i was recommended to buy a double pumper with mechanical secondaries, the 670 is vac secondaries.

oh and for those who dont know my motor mods
306
TFS TW heads
1.6 RR's
comp xe-262 cam
weiand stealth intake
holley 670 carb
Ceramic coated headers
offroad H pipe
Mac pro dump exhaust
8.8 rearend with 3.73 gears
msd coil

last year i launched at a lower rpm and bogged off the line but was rewarded with 6 tenths faster times. My suspension is bone stock and my tires are just BFG T/A's.

[rancheronut]

last year i launched at a lower rpm and bogged off the line but was rewarded with 6 tenths faster times. My suspension is bone stock and my tires are just BFG T/A's.

see told you. sound like you need better tires(slicks)
stay with the 670 but invest in some better traction helpers(tires/control arms)

[Red84GT]

thats the next step, plus torque box reinforcement

next question is: my vacuum advance hose goes from the diaphragm on the dist to the front of the carb, and then the pcv goes from the pass side valve cover and pcv valve to the other vac hookup on the front of the carb, this is correct??

[dy85merc]

A set of tires is needed here. I only run a 650 on mine, switched from a 670 street avenger and went faster...never liked the 670 much, kinda problematic.

[Cappn Tripps]

Post both time slips, 60ft etc... and was the weather the same?

[86_Notch]

i always run mech secondary carbs. I've had great success with either holley or quick fuel 650 double pumper carbs.

but like the other guys have said, get your traction problems rectified first before you start messing with the carb.

[black1980fiveoh]

I haven't have much luck with my Holly 670. I brought a Holly 570 and it feels beter on the street. I too am having issues of my car going too slow but different issue.

[mustangs4me]

Dude 103 in the 1/4 is 13.23 you really need some traction. When you find that you will prolly be in the 12s. A 2.40 60 ft s#@* I had a stock GTA that would do better than that back in the day and you have a whole lot more under the hood if you know what I mean. Traction is the problem. Plus your combo is a little wack with the Dp intake, tw heads and a vacuum secondary carb. You need a victor jr intake and a good ol holley 650 Dp. Get the traction first and you will be on your way dont worry about what I said about the combo think TRACTION.

[cb84capri]

notice in the pic the primary butterflies are clean and the secondaries are grimy and dirty? i'm guessing if the fuel ran past them they'd be clean

both the adjustment screws on the 670 are 1 1/2 turns out

secondaries don't open unless you are into the throttle quite a bit. judging by how dirty they are, they aren't opening all the way. fuel hits the opposite side to what you're looking at if they don't open fully.

cale

[cb84capri]

thats the next step, plus torque box reinforcement

next question is: my vacuum advance hose goes from the diaphragm on the dist to the front of the carb, and then the pcv goes from the pass side valve cover and pcv valve to the other vac hookup on the front of the carb, this is correct??

hook the vacuum advance up to port vacuum, it should be higher up on the front of the carb than manifold vacuum. the lower one is manifold vacuum and should be hooked to the pcv.

cale

[jayhawk261]

A 670 is too big of a carb for a 302 - 306 based engine. If you run Holley's math. I'm guessing that a 600 would be just about right. I would go with a 600 dp. A Holley 0-4776 would be my choice.

Also, everyone is correct. Traction is your biggest issue. A set of Nitto drag radials, MT drag radials, or even some full blown slicks will net you WAY better 60ft/1/4/mph numbers. The MT 26x8.5 slick fits on a set of 10 holes quite well. Bolt them on and go!

[mustangs4me]

Hell no, a 670cfm is not to big for a 302. With his combo TW heads and the cam He could go with more carb in my opinion, but I wasnt going to tell him that because he just needs to focus on traction. Lets keep the myths to a minimum here.

[jayhawk261]

Myths? I'm pretty sure that volumetric efficiency is not a myth. A 4 stroke engine (carnot cycle) has only so much efficiency. If I remember from thermodynamics it is in the 60-70% range (long time ago, so I could be off a little bit)? The volume of air the engine can move combined with rpm (how quickly it can be moved) defines how much air can be introduced. 600 cfm will run just about any naturally aspirated 302-306 engine with no problem. If he's running it up to 7500 - 8000 rpm, it might be a different story. If he had a blower, that would be a different story as well. Putting too big of a carb on will necessitate smaller jets. If you have to go too small on the jets, then you create a lean condition. It won't balance out well and you'll constantly be chasing the tune.

The equation goes:

(Cubic Inches)x(Max RPM)x(Volumetric Efficiency)/3456 = CFM

In this case I'm putting some guess numbers to it. Say 6500 rpm and 85% VE.

(306)x(6500)x(.85)/3456 = 489 cfm

Even if you leave VE out of the equation, the number is 575 cfm. That is for a 100% efficient engine that can not exist.

[mustangs4me]

Volumetric Efficiency works great for your everyday go to work car but if you look at his combo you would understand that his engine is more efficienct than the run of the mill 302. Tw heads increase his flow and therefor the demand upon the A/F mix going into the engine and the exhaust leaving the engine. The "Volumetric Efficiency" has been changed with these different heads and cam and can handle more CFM. I have a stock bottomend 302 with a custom cam and GT40p heads and I am running a 650 DP Holley with a proform body that flows 750 cfm, because of the changes it will work just fine. Ive seen it many times. We arent racing formulas or dynos but in reality we are racing cars. What might work on paper doesnt always work in the real world and vice versa. I just hate to hear people say " thats too big of a carb" when it really isnt and by the MPH he put down its not. Why would he all of a sudden have traction problems besides he thinks that the secondarys arent opening, I believe that the carb can now deliver what it needed more fuel and air.

[jayhawk261]

? Huh? Volumetric efficiency works for an average every day engine but not a modified one? Apparently the laws of physics and thermodynamics don't apply to an engine that can flow more air through the heads than stock?

While, yes, making improvements to airflow via a better cam and better flowing heads will increase the efficiency, it still won't make it 100%. No 4 stroke engine is 100% efficient. It isn't possible.

Better top end speed and more wheel spin does not indicate that the new 670 cfm carb is correct vs a new 600 cfm. It only indicates that it is better than the carb that came off. Looking at the pictures, it's obvious that carb has some problems. Put that engine on a dyno and optimum air/fuel ratio can be attained using a 600 cfm carb with small adjustments. No more is necessary. A bigger carb will not make more horsepower or torque at optimum A/F. Optimum A/F is what it is. When tuning an engine, this along with EGT, is one of the main factors in determining a correct tune.

*EDIT* Oh, and if you read his first post, he actually lost MPH after changing the carb. From 14.1 @ 98 mph to 14.7 @ 92 mph

[Mr Joshua]

psst
http://www.summitracing.com/parts/CCA-12-262-4/
rpm range 1,500-5,500
if thats his cam he doesn't want a VR JR
http://www.summitracing.com/parts/EDL-2921/
rpm range 3,500-8,000

even that weiand stealth is a little big for that 262 cam
http://www.summitracing.com/parts/WND-8012/
Idle-6,800
sounds like you need some tuning and cam that'd match your rpm range of your heads and intake..
then work on your traction issue... via, springs, shocks, tires, etc..
and here is your carb sizing chart for the street avenger series..

[rancheronut]

[QUOTE=jayhawk261;877791

*EDIT* Oh, and if you read his first post, he actually lost MPH after changing the carb. From 14.1 @ 98 mph to 14.7 @ 92 mph[/QUOTE]

well you forgot part of it he said "So this is my problem, before i blew up the factory 7.5 3.08 geared rearend i was trapping 103mph in the quarter @ 14.3-14.4, then i got the 8.8 with 3.73 gears and i dropped down to 14.1 but at 98mph, so i lost mph"

i don't see carb any where in there!

he also said "launched at WAY too high an rpm and spun all the way through first and most of 2nd gear, was rewarded with a lowly 15.8, next pass i launched at a lower rpm and netted a 14.7@92mph with a 2.4 60ft time, i still spun somewhat through 1st. We only got 2 runs for qualifying, so i went into eliminations with only 2 passes under my belt, well i was pitted against an aero coupe with a 4.6 4v cobra motor and drag radials, he ran a 13.8 against my 14.97

still nothing about carb in there also!

then he also said;last year i launched at a lower rpm and bogged off the line but was rewarded with 6 tenths faster times. My suspension is bone stock and my tires are just BFG T/A's.

so if he spun all the way in 1st gear and part of 2nd gear! that means he was past the 60ft mark on the track before he got traction! which would show up as slow mph plus in his ET!
UNTIL he gets his traction problems fix, his et are going to bounce around! so playing with 600 or 670 or double pumper is NOT going to help! that like getting the cart in front of the horse!
any body who has time racing at drag race track, knows spinning tires mean not going any where fast.
the only sport ,i know where you want spinning tires is in drifting!

[jayhawk261]

What, do you work for the New York Times with those selected quotes? :-)

So this is my problem, before i blew up the factory 7.5 3.08 geared rearend i was trapping 103mph in the quarter @ 14.3-14.4, then i got the 8.8 with 3.73 gears and i dropped down to 14.1 but at 98mph, so i lost mph, then i took off my stock carb and found this:

This says that his time after changing gears and before changing carb was 14.1 @ 98 mph.

okay so i put on a new holley 670 and i lost 6 tenths of a second and lost 6mph!!!

This says that after changing the carb to the 670, all other things being equal, he was 6 mph and .6 seconds worse off.

As far as traction goes, yes he needs to address it. I said that several posts ago. Talking about the efficiency of the engine, it just isn't going to be that much greater than a stock engine. His cam is a 1500-5600 rpm cam. He has a good set of heads, but I would assume they are out of the box stock. His manifold is actually fairly well matched with the cam being a straight forward dual plane. This combo on a set of slicks tuned properly should run mid 13s or better. I had a set up years ago that was close in cam and intake. heads were factory heads with a nice port job as no aftermarket was available then. With 3.27 gears and good tires I could run 13.4 all day long. If i remember correctly I was around 100 - 103 mph. Been a long time. Oh, this was with a 600 DP.

Yes, traction needs to be addressed so that he can then run more consistently. Once he is running consistently, then the tuning can be done. What I and others are saying here is that, based on experience and the math, he would be better off with a smaller carb.

well you forgot part of it he said "So this is my problem, before i blew up the factory 7.5 3.08 geared rearend i was trapping 103mph in the quarter @ 14.3-14.4, then i got the 8.8 with 3.73 gears and i dropped down to 14.1 but at 98mph, so i lost mph"


i don't see carb any where in there!

he also said "launched at WAY too high an rpm and spun all the way through first and most of 2nd gear, was rewarded with a lowly 15.8, next pass i launched at a lower rpm and netted a 14.7@92mph with a 2.4 60ft time, i still spun somewhat through 1st. We only got 2 runs for qualifying, so i went into eliminations with only 2 passes under my belt, well i was pitted against an aero coupe with a 4.6 4v cobra motor and drag radials, he ran a 13.8 against my 14.97

still nothing about carb in there also!

then he also said;last year i launched at a lower rpm and bogged off the line but was rewarded with 6 tenths faster times. My suspension is bone stock and my tires are just BFG T/A's.

so if he spun all the way in 1st gear and part of 2nd gear! that means he was past the 60ft mark on the track before he got traction! which would show up as slow mph plus in his ET!
UNTIL he gets his traction problems fix, his et are going to bounce around! so playing with 600 or 670 or double pumper is NOT going to help! that like getting the cart in front of the horse!
any body who has time racing at drag race track, knows spinning tires mean not going any where fast.
the only sport ,i know where you want spinning tires is in drifting!

[rancheronut]

you can change the words all you want! until he fix's his traction problems, we are just wasting time! like spinning our wheels,
going NO where fast but at least the speedometer IN THE CAR say we are!

What, do you work for the New York Times with those selected quotes? :-)



This says that his time after changing gears and before changing carb was 14.1 @ 98 mph.



This says that after changing the carb to the 670, all other things being equal, he was 6 mph and .6 seconds worse off.

As far as traction goes, yes he needs to address it. I said that several posts ago. Talking about the efficiency of the engine, it just isn't going to be that much greater than a stock engine. His cam is a 1500-5600 rpm cam. He has a good set of heads, but I would assume they are out of the box stock. His manifold is actually fairly well matched with the cam being a straight forward dual plane. This combo on a set of slicks tuned properly should run mid 13s or better. I had a set up years ago that was close in cam and intake. heads were factory heads with a nice port job as no aftermarket was available then. With 3.27 gears and good tires I could run 13.4 all day long. If i remember correctly I was around 100 - 103 mph. Been a long time. Oh, this was with a 600 DP.

Yes, traction needs to be addressed so that he can then run more consistently. Once he is running consistently, then the tuning can be done. What I and others are saying here is that, based on experience and the math, he would be better off with a smaller carb.

[jayhawk261]

What words did I change?

AND, I said I agree with you. He needs traction! What more can I say? Are we only allowed to interject requested opinions in a step by step format? Once he fixes traction, then we can have an opinion on the carb?

OP, sorry to fill up your thread with the bickering! You should probably forget everything I've said as I am being told over and over that I am wrong. I'm not, but if everyone wants to say I am, that's ok too. :-) You should tub it and put in a narrowed 9", and then add a 1050 dominator! ;-)

you can change the words all you want! until he fix's his traction problems, we are just wasting time! like spinning our wheels,
going NO where fast but at least the speedometer IN THE CAR say we are!