tarafied1

I don't have a picture of the plugs but I can take one. They are black and sooty. The cam specs. are
Dur. @.050” lift = 218/228 advertised duration 276/286, gross valve lift .509”[/align][/align]The fuel I have been running is BP 93 octane pump gas, I don't rev over 5500 RPM as it has tons of torque and a 2.9:1 rear-axle ratio so she is flying along pretty quick.[/align]I'm running about 14 degrees initial timing with about 36 overall. Other than the plugs being black and sooty it seams to be pretty good runner.[/align]thanks[/align]Craig[/align]

67mustang302

Is that a roller cam? Anyway though, you could try jetting it down a size or 2, but you have to balance lean best power vs detonation.

tarafied1

No it's not a roller, thanks for the advise. I will play around with it when it gets warmer out. Didn't mean to hyjack the thread.
Craig

Aussie66Fastback

Some great information from 67mustang302. Thanks!!!

my engine is a mild cam 302 with ported 351 heads, 600cfmvac sec Holley,tri-y's into 2x2.25" with an H. It pulls hard to 5500, then drops off pretty rapidly.

So I have (302*5500/2820)*VE = 600cfm.
600/(589) = VE
VE = 102%...Not likely! So therefore my carb is "too big".

How do i know my VE in advance to choose the right carb?

I'd like to suggest that you start another post which could be a sticky or in the FAQ, with a guide to carb selection by engine capacity and level of development (stock -> wild), pros and cons of various brands and in what situation would you want one of my77's expensive (but more adjustable) carbs.

thanks again 67

1967mustang

i started this form lol.i am have learned alot from it 67mustang302 is the man

musnicki

It's bored .30 over with TRW Flat top pistons, Radical Cam, gt 40's, old school torker manifold. When I press on the gas, itresponds nicely. You can check out the engine bay in my photos section. I think I get about 7-8 miles to the gallon. Those short trips too and from FT Bragg kill my gas tank.

my77stang

you guys need to take note of two things that haven't really been mentioned.....

a) using a dual plane intake will trick your motor into letting you run a larger carb. this is because when any given cylinder is sucking air thru the carb into the motor it can only go thru two barrels instead of four.

b) using vacuum secondaries lets you run a little bigger, because when dialed in properly the back two barrels will not open up more than they need to.

so, this is why its commonly accepted to run a slighty larger carb than the mathmatics call for.

kalli

b) using vacuum secondaries lets you run a little bigger, because when dialed in properly the back two barrels will not open up more than they need to.

thanks for confirming. I ujnderstood it that way as well. the secondaries are pulled open by engine, so it's harder to overcarb with those.

Kalli

silverblue66

autolite 4100 480 cfm. perfect for a mild 289 spinning up to 6000. plenty of + carb 21 mpg + snappy throttle = happy mustang + driver!

67mustang302

Well, VE can only be accurately calculated if you know horsepower, engine rpm/size and fuel consumption. Fuel consumption can use a BSFC of around .45 for a well tuned performance setup and get real close. Otherwise it's more of an educated guess.Good heads and a decent flat tappet cam plus all the supporting stuff and some compression would be around 80-85% in a nice street setup. Roller cams are good for another 5-10%. The better breathing the components and the better matched they are to one another, the higher the VE is. Morerpm also creates more VE at higher rpm, less at lower rpm, which is why high rpm cams get crappy mileage. Extreme cases of high n/a VE are Formula 1 engines, which at 19,000rpm have a VE of around 135%. Insane. They're producing over 5hp per cubic inch all engine, but also produce no usable power below an rpm that most street sport bikes redline at(12,000-14,000). Purpose built.

As far as the 2 things that my77 mentioned, those are both true. Since on a dual plane each cylinder's induction pulse is only drawing off half the carburetor, the volume of air inducted in the pulse will be at higher velocity in the barrels than with a single plane, which means better metering signal. That lets you get away with running enough carb to make good top end, but still having good low rpm power and throttle response. That's why dual planes are widely reccomended for street use, you can be off a bit on carb size and accelerator pump tuning and it won't hurt performance enough to notice.

Also vacuum secondaries are great for keeping metering signal in the back barrels high, so throttle response and overall power at WOT stays good. There is a problem with going TOO large on a vacuum secondary though. If the throttle blades are only ever partly open then when the fuel/air charge hits them, if they're at an extreme angle, it slows the charge down at the blades and increases the pressure, which can cause the fuel to plate out on the throttle blades, turning back into a liquid, and if severe enough, dribble down into the manifold. That can hurt power since atomisation drops off. Also if the front barrels are doing more work than the back barrels, then the front cylinders on the engine are going to make more power than the back cylinders, since the front cylinders breathe more off the front of the carb, and the rear cylinders off the rear of the carb. That's why all out drag cars are mechanical secondary with sqaure bores, and even jetting on all 4 barrels. As long as the secondaries are open all the way by the time you get up near peak rpm you're fine, it's when they never open all the way that power can suffer if the thottle blade angle is poor enough. Even a slightly larger than needed vacuum secondary carb if tuned right can still get the back barrels open well before peak rpm.

Vacuum secondaries + dual planes = great for the street, cuz you can run enough carb to have minimal restriction in breathing at peak rpm, but still keep the signal high enough to meter and atomise well while at lower rpm WOT. It may sacrafice some top end hp vs a single plane setup, but will generally make more average power across the entire rpm range. Single planes and mechanical secondares can be very unforgiving of the tune and setup, If it's off a bit, it lets you know it.

With dual planes and vacuum secondary carbs, you can overestimate VE a bit to make sure you have enough carb to breathe at high rpm, but still end up with more than enough signal to meter at low rpm. Also vehicle weight and gearing will play a factor. The lighter the car, the larger the carb you can get away with, and the more gear you have, the larger the carb you can get away with. The faster the rpm can spin up, the faster you generate a strong signal. It also comes down to what you want. I went with 570 cuz it's right at what I need in terms of peak rpm cfm, but my car is also geared tall, so I can't get away with going to a larger carburetor, and I also wanted a bit better mileage at lower rpm, since I do a lot of highway driving with no overdrive.

Another thing to consider as well is that the (cid*rpm/2820)*VE uses PEAK rpm and PEAK VE, and here's the kicker....peak VE occurs at peak torque, and peak hp(peak rpm)where torque is lower, VE is lower also. So an engine with a peak VE of around 90% at peak torque, has a VE at peak horsepower that's closer to 80%, which means that the calculation will overestimate cfm needed by a little bit. If you subtract 10% from your peak VE value and plug that into the equation, the carb cfm needed is even smaller than you'd think. Mine for example, assuming a peak VE of 90% and around 80% and peak hp at around 6,000rpm, I only need 520cfm, but run 570. With a dual plane I could easily get away with 650, but again, with my gearing I didn't want to push it. Regardless though, my carb is already 50cfm larger than it needs to be. And that's with AFR 165's, a Crane roller cam, long tubes, intake yadda yadda.

As far as adjustable carbs like Proform etc, the only drawbacks are that they may cost more, and if you change air bleeds with the carb still on the car, you may drop them in the intake. [&:]If you know how to tune a carb, one with more adjustability is always better. Especially in my77's case, where the non adjustable air bleed carbs that come off the shelf, aren't right for his setup. Once I get a 5/6 spd and better gearing, I'm going to get a better carb. The Street Avenger is a nice little carb, but I didn't see the point in getting a good carb now for more money, that I'll just end up replacing later anyway.