car is slower with new carb

[mustangs4me]

I shouldnt.

[mustangs4me]

Maybe I could.

[mustangs4me]

Naaaa I wont.

[Welfare Stang]

....

[jayhawk261]

Originally Posted by Welfare Stang

i'm building a 306 that's 11 1/2 to 1 with ported twisted wedges that flow 220 exh 270 intake, a performer rpm air gap intake, trick flow stage 2 cam 1 5/8" longtubes what carb should i run?

I know you think you are setting me up for some kind gotcha type fail here, but I'll play along. What cam specs? With 11.5:1 I'm guessing pretty good lift. Probably over .600. Any idea what the base rpm range for the cam is? Roller?

[Red84GT]

its okay guys, never had anyone ever get into a debate in any thread i have ever started on the internets lol. Anyways yes i need some suspension work on my car, thats in the pipeline i have SFC's i havent had them welded in yet and Troque box reinforcement plates so i can get some aftermarket upper and lower LCA's

another question: what all is required to go from hyd flat tappet to hyd roller cam? the spider in the lifter valley and the dist gear? anything else besides those things and the cam and lifters?

[Welfare Stang]

trick flow stage 2. that is a roller cam 224I 232ex @.050 and 542 intake lift 563 ex. lift yes it is a roller, a hydraulic roller.

[jayhawk261]

Ok, that cam is listed as a 6000 rpm cam by Trick Flow. Figure a couple hundred rpm variance for whatever might be affecting things. At 306 CI & 6300 rpm, the volume of air the engine can move at 100% efficiency is 558 cfm. If it were me, I'd start on the dyno with an out of the box 600 DP and go from there. It's about the A/F ratio as to how efficiently the thing is going to run.

With 11.5:1 compression, I would probably have gone with a bit larger cam. I'm not a camshaft expert, but my experience has told me that that high of compression in a small motor will really work well at higher rpms.

Way back when I used to run a motorsport M-6250-A333 solid lifter flat tappet cam in a 351W in a 79 Mustang. The specs aren't too far off of the cam you are using. The timing events are different, and obviously not being a roller, the opening and closing of the valves was not as efficient. That cam with a set of crazy worked over stock heads with chevy valves and 10:1 compression worked very well after I got the carb issues settled. I started with a 750 and never could get it to work. Tried re-jetting to lean it out, but it just ran too rich. Following the advice of a friend who had lots more experience than me, I put a 600 on it. It practically tuned itself. It ran way better than it ever had. No more richness, just good running.

I guess what this boils down to is not that the CFM is the issue for me, but that with a carburetor the fuel is mixed with the air in the carb, so for a larger CFM carb, it is set to deliver more fuel. The engine itself will regulate CFM. That's why a 1000+ cfm throttle body will work on a fuel injected engine. The engine will take only as much as it needs, and fuel is regulated through the injectors. A 750 carb will flow more fuel than a 600 due not only to bigger jets, but larger passages.

I know, you run a 750 and it's perfect. That's great. I'm glad it works for you.

[Welfare Stang]

what i'm saying is not everything goes "bye the book" i once built a 351-w similar to your 351 build for a friend of mine, in a 83 mustang gt. it was a 69 windsor bored .030" over with flat tops , a motorsport hydraulic cam, edelbrock performer intake, and ported 69 351-w 4-v heads. he stuck a 625 carter on it. i drove the car a week and put a 750 carter i had laying around and it completely woke it up. night and day difference. complete opposite of your situation with your 351 why? that's just how things work sometimes, no book or volumetric effeciency chart can have an awnser for that. as for the motor i spec'd out according to your calculations 588 cfms, that motor ran best with a 700 dp. it ran 11.60's-11.70's another motor i built that was similar was 10.5to1 with box stock edelbrock performer heads, rpm air gap intake ran best with a 650 dp car goes 12 teens to 12.20's . a 600 cfm wasn't enough and it doesn't matter what chart you look at or calculate, the numbers don't lie. i'm not trying to be an ass just trying to get you to see the "whole picture" just because a ratio chart says this or that doesn't mean it's gonna work, it may in some cases though, but i go with what's tried and true.

[jayhawk261]

I understand the big picture, and I also understand the numbers don't lie. That's why I'm big fan of dyno tuning. Then you have real world numbers that you know mean something. As this point in time, dyno testing is one of the best tools to get these issues put to bed.

When this debate started, I was commenting on the OPs original set up. With his mild cam and intake set up, and dyno tuning, I don't believe that there will be a need for more than 600cfm. In my experience, I would bet money that is how it works out.

Because I was focused on the one set up, I really didn't expand on my thoughts well. I know I covered my belief in using the dyno, and I think I even said that the carb size derived at was a starting point. If I didn't, call me out. I'm not up for reading it again. Once on the dyno and running, it's possible for whatever reason that the best running carb may be larger or smaller than what the math says. I can only guess that the ports in the heads and the intake manifold can act as air storage volume at higher rpm essentially expanding the volume of the engine. If the air in the heads and manifold move fast enough, maybe it can have a slight compressive effect meaning better volumetric efficiency and more air flow. Edelbrock claims as much as 120% VE on their Victor Jr manifold if I remember correctly. These are just some thoughts I've come up with on it.

To top it off, MM&FF published an article this month on their 400m buildup that hurt my argument a bit. They ran the hopped up 400M with an Air Gap and a 750cfm (mathematically correct) to 519 hp @ 6500 rpm and then switched to a 1000 cfm to 528hp at similar rpm. The next big step was a CHI big single plane with a 1050. That made 568 hp, but they didn't say RPM. Why this can happen may be rooted in my thoughts above. Maybe the efficiency of the manifold is so good that it increases the VE by a substantial amount. This is, as you guys say, where the experience makes a difference. There is no substitution for dyno tuning. The article never stated what the A/F ratios were.

It's all good with me, and I'm not looking to make enemies. I think mustangs4me and I may have called a truce at this point. Although he did talk about my boat a bunch, so I added it as my avatar. Maybe he'll have a better understanding of why I have a boat after seeing it.

I STILL say that a 600dp model 0-4776 is the correct carb for this app. With the mild cam and intake manifold, even based on experience for me, should be it.

[Welfare Stang]

i'm not looking to make any enemies either. at some points we can be both wrong. why some combos take bigger or smaller carbs is anybodies guess. i built a 400 m at one time (hot rod did a budget build 400m years ago 380h.p. for under $2,000 threw exh manifolds) i did a similar build .040" over milled the heads .030 aluminum intake cam that was 224 234 @.050 and .510 .534 lift headers aand i started with a 600 holley vacuum secondaries which the motor ran stronger on but being in a full size 3/4 ton truck i couldn't get the secondaries to open soon enuogh for when i was towing, so i put a 650 dp on it for the mechanical linkage for the secondaries, but it ran better and had moeere power with the 600 vacuum. why? who knows that's just the way it worked best on that engine. talk at you later, pete

[mustangs4me]

I bet that the reason that engine ran better with the vacuum secondary carb was because a heavy vehicle can not burn the fuel as fast as a lighter one with a DP carb so you give up some performance there. This is why I will never run a vacuum secondary carb on a mustang just because they are pretty light and the rpms come up faster.

[85 GT Kid]

All I'm going to say about this is that from what I have seen at the track and on the dyno (brake) has been that a bigger carb has worked better every time.
Anybody can read what they want in a magazine or on the net but going out there and doing it has always seemed to work for me.
It is a fact that an engine is nothing more then an air pump but there are alot of other things that come into play when building a combination.
I had a 10 second Mustang with a 306 in it over 20 years ago and have helped a friend build alot of different combos from a 546HP 306 to a 700 plus hp 327.Keep in mind this was over 15 years ago when that was big numbers.

The guy that first told me that the bigger carb works better (700/750 ) on a 302 was John Healy. Search his name, He held an NHRA comp record for years and built engines for Tasca Ford, Ronny sox, Warren Johnson ECT.

Keep in mind Im talking about a hot street type motor. A real mild combo wont as much carb.

[jayhawk261]

I talked with a friend of mine about this subject who has WAY more experience than me. He pointed me to this article:

http://www.carcraft.com/techarticles...arb/index.html

In essence, it says that you can't just rely on the VE because of all of the additional factors involved. Reading the vacuum to determine whether or not the carb is a restriction is a big point. It is a good read.

Unfortunately this does hurt my argument, but I would still be money that the OP's engine will run right with a 600.

[mustangs4me]

It will run right I will give you that, but it wont run to its fullest potential. Plus I dont make it a normal habit to sorce my information from magazines. Just my .02 cents dont shoot me. LOL!

[jleews6]

Originally Posted by mustangs4me

It will run right I will give you that, but it wont run to its fullest potential. Plus I dont make it a normal habit to sorce my information from magazines. Just my .02 cents dont shoot me. LOL!

Its all good with me. Im just trying to help and tell what I have seen thats all. These boards are here to help each other and thats why we have threads like this.
By the way I didnt realise that my computer was loged in under my sons name *85GTkid*
LOL

[mustangs4me]

That is kinda funny. That is what I have been doing also too but this time I had someone choose my carb for me and its a 750.

[Welfare Stang]

Originally Posted by jayhawk261

I talked with a friend of mine about this subject who has WAY more experience than me. He pointed me to this article:

http://www.carcraft.com/techarticles...arb/index.html

In essence, it says that you can't just rely on the VE because of all of the additional factors involved. Reading the vacuum to determine whether or not the carb is a restriction is a big point. It is a good read.

Unfortunately this does hurt my argument, but I would still be money that the OP's engine will run right with a 600.

hmmmm, didn't i mention that back in the beginning of this whole debate?.... the other factors, compression, cam, cfm flow of the heads etc.etc that's what i meant bye the whole picture and ve, vs. more cfms than a stock motor and such. all the parts have to work effeciently together, not a mismatched intake with killer heads, or vise versa (or yates heads and a stock cam LOL me being a smart ass earlier in this debate) it will run good with a 600 but will it run better with a properly tuned engine with a 650 dp or 650 vacuum? is the question. i'd say if he has 10 to 1compression or better his motor will take all a 650 dp has to give it if the motor is properly tuned.i'd like to get my hands on the car myself and dial it in. jayhawk i'm glad everyone came to their senses and this didn't turn out any uglier, kudos to you for admitting that you could be wrong in your argument, and we all can have a friendly debate, with our varied opinoins and supporting info. pete

[Welfare Stang]

Originally Posted by mustangs4me

I bet that the reason that engine ran better with the vacuum secondary carb was because a heavy vehicle can not burn the fuel as fast as a lighter one with a DP carb so you give up some performance there. This is why I will never run a vacuum secondary carb on a mustang just because they are pretty light and the rpms come up faster.

yeah your theory is exactly right, on the heavy vehicle and dp carb. i always wondered what that engine would run like in a fox body.lol probably would have screamed with that double pumper in a vehicle that weighed 2,000 lbs less.

[85GTTops]

Are your vacuum lines and all vacuum amplifiers correctly set up for your distributor?

Lack of spark advance will kill your performance.

[KANARMAN]

I have been using a junk 650 holly double pumper mech secondaries for years and it runs pretty good.

WITH 650 Double pumper performer rpm duelplane stock ignition complete stock heads cam pistons rings bearings. Mods 4.10 in a 7.5 et streets flowtech equel headers x pipe
flowmasters underdrive pullies no front bumper aircomp or smog pump
t-5 trans ran a 13.52 in my 1985 t top gt

[mustangs4me]

Not to shabby!

[KANARMAN]

Thanks that post wasnt ment to be about me more to state that a biger carb works fine on a small cube junk mild mod motor. But then again when I ran that 13.52 I spun a little so? From what I hear 750 dp is the max for a 302 after all if you spray it you can cram more fuel LOL

[Welfare Stang]

Originally Posted by KANARMAN

I have been using a junk 650 holly double pumper mech secondaries for years and it runs pretty good.

WITH 650 Double pumper performer rpm duelplane stock ignition complete stock heads cam pistons rings bearings. Mods 4.10 in a 7.5 et streets flowtech equel headers x pipe
flowmasters underdrive pullies no front bumper aircomp or smog pump
t-5 trans ran a 13.52 in my 1985 t top gt

don't expect that7.5 to last long, it's gonna go like the wind.

[Bracketbomber]

Originally Posted by 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

I really don't want to contradict anyone here but our 83GT will run 12.50's all day long with a 2bbl carb. It runs 11.90's with a 575cfm Demon. You may need to put a lighter spring in the secondaries but the 670 will work.