Carb thoughts

[tfeverfred]

I responded to Butch's post in my journal, but decided the responses would reach people better in it's own thread.

Quote:
Originally Posted by butch27
IMHO most of us are OVERCARBED. I don't believe we really need over a 500 cfm from what I've read and calculated on my stock 302".

You just may be correct, Butch. I have heard that before and I was curious as to why Edelbrock recommended the 500 as well as the 600 for a stock SBC. I have also read that the Quadrajet comes out to 600CFM.

The 500 seems like it would be the way to go and yet, the 600 is seen everywhere. Can anyone chime in on this? I'm not changing the 600 because it seems to work very well, but it would be interesting to see what other opinions are.

 

[Francis Blake]

I am getting a 4.3 v6 ready for use and had a 600 holley so I put it on and it runs ok but its too much so I called speedway and talked to their tech he said a 500 Edelbrock was what I needed for my v6 and a 600is right hor a 305. I hope to have the 4.3 runnlng tomarow and will let you know how it runs. Francis

 

[Photoman]

A 600cfm carb on a ~stock~ 350 is fine. Even more so if the carb has vacuum sensing secondaries like the Holley or AFB.

With anything more than a stock cam, you'll generally get better results with a 750cfm and vacuum secondaries or a 750cfm AFB as the 600cfm will limit breathing at the upper rpm ranges.

 

[sstock]

I responded to Butch's post in my journal, but decided the responses would reach people better in it's own thread.

Quote:
Originally Posted by butch27
IMHO most of us are OVERCARBED. I don't believe we really need over a 500 cfm from what I've read and calculated on my stock 302".

You just may be correct, Butch. I have heard that before and I was curious as to why Edelbrock recommended the 500 as well as the 600 for a stock SBC. I have also read that the Quadrajet comes out to 600CFM.

The 500 seems like it would be the way to go and yet, the 600 is seen everywhere. Can anyone chime in on this? I'm not changing the 600 because it seems to work very well, but it would be interesting to see what other opinions are.

Either way would be a good choice, thing is with vacuum secondaries it doesnt really matter much. I've seen 750 vac. holleys on 302 work well although would not be my first choice. Cant go wrong with a 600 cfm carb on any small block of any manufacturer.

 

[RexRod]

Hi Fred,

I think the idea that most engines are over carbed makes sense. Back in 1980, I built a 502 big block chevy for a 75 Pontiac Trans Am I owned. This was before the 502 was really very popular for street cars. My engine builder recommended that I NOT go bigger than a 750 Holley. I thought the carb he chose was way too small at the time, but he knew what he was doing. He said I would be much happier with the smaller carb. He was right.

I never put the car on a chassis dyno, but that 502 pushed that 3,500+ pound car around like it was a flee! That car was super fun to drive. So much torque, if you found yourself in the wrong gear, it didn't matter. All you had to do was stand on it.

Based on my experience, I would be willing to bet that a 500 Holley on a 307 would be more than enough. Even with a little cam and a header.

My two pennies.

David

 

[der Spieler]

I had a Holley 600 on my 350. It had a 292 cam and headers and I changed to a weaker spring (I think it was the white one) in the vacuum secondaries. When I had the 3.70 gear in the rearend it would boil the 18" M/Ts. I changed to a 3.00 gear for mileage but at freeway speeds it still drank gas like a fish. The weaker spring had the secondaries partially open most of the time.

 

[sstock]

Not good, out of the box the universal small holley works fairly well and is close on the tune on medium sized motors. I just had the wideband out on Saturday tuning my neighbors just completed cobra kit car. It has a 351 windsor and a 750 holley vac secondary on it. The entire fuel trim was way rich out of the box. It is not uncommon to have them too lean on the choke, too rich on the warmed up idle, too lean on the accelator pump squirt, too rich on the main metering and then too rich on the secondaries. Get my point, there is about 5 ways to tune a holley and you just cant do the job 100% right without using a wideband. It is possible to get them so they are not gagging you with unburned hydrocarbons, but you need a jet kit and converted secondary metering plate. I wouldnt be too afraid of a small double pumper with mechanical secondaries either on a light car like a T, if you are not on the pedal too much.

Steve

 

[TheAntiBoyd]

My dad has a pretty warm 327 with a 4spd; and my camaro had a 383 with a 700r4; and both of us used the same mathematical equation to size our CFM requirements... double the CFM and when youre in between common carb sizes, pick the smaller one for a street car He chose a 600cfm Edelbrock and I at first went with a modded Q-Jet (LOVED IT) followed by a 750 Holley Great at the track but I had trouble in parades, cruises and stop and go traffic with it. you can do all sorts of mechanical combinations, permutations, voodoo etc. Yes cams matter, Yes intake type matters But if you have a mild 350, and street drive it a 650cfm carb is great. you blower guys really throw it for a curve unless its a 177ci or 142ci blower.

Anyway those are my thoughts. For a street car, I like vac secondaries and a slightly smaller carb. And dont be afraid of the lowly quadrajet

 

[Coupefreak]

I have ran Holleys on everything from my daily drivers, to my stock cars, they require a little "tweaking" to get them just right. I am a firm believer that there is no such thing as a "plug and play" carb. I have never found one that runs great "straight outta the box".
I have a Barry Grant 625 CFM Road Demon on my Coupe. I fiddled with it for about a month before I finally got it running good. nearly drove me to insanity (its not a far trip anyway) out of the box, it had 78 jets in the PRIMARIES, and 70's in the SECONDARIES......bassackwards

I'm gonna invest in one of them wide band air/fuel thingys, they make it alot easier to tune a carb.

Vance

 

[Youngster]

SStock, Talk to me about this wideband. I'm not familiar with it. Would it work to set up multi carbs, like 3X2's?

Ron

 

[TheAntiBoyd]

Wide Band O2 sensors are great for tuning. It helps you find that *magic* 14.7:1 Air Fuel ratio. If you can solder, there are kits out there to make your own Wide Band O2 sensor and read it with a multimeter. you really need to specify wide band though. There is a version on the market that has 3 lights. red for lean, yellow for rich and green for dead on or something akin to that. I dont like color coded gauges and things...gimme hard numbers. That being said; it COULD help you with 3x2, 4x2, etc but as far as I can see only still to the aspect of rich or lean. There is no tool better on earth (to me) than a 25 dollar Uni-Syn from edelbrock for setting up (balancing) multiple carburetors.

Just my digital grain of salt

 

[RexRod]

This is the wideband controller kit I have had good luck with:
JAW

The wideband I would recommend is the Bosch LSU-4.2. You can get them on ebay for between $60 and $80 bucks. Don't pay more, they are out there. Here is the one I use:
eBay Motors: Bosch LSU4.2 Wide Band O2 Sensor AEM 30-2001 LSU 4.2 (item 230272241132 end time Jul-26-08 16:18:15 PDT)

The Jaw and a Bosch LSU-4.2 is the cheapest way to do it I have found. You can really spend a ton on this stuff if your not careful. If you tig another O2 bung in the exhaust, you can move the sensor between cars. They will last forever that way.

Hope that helps.

David

 

[Youngster]

I donno...That looks pretty scarey. I think I'll stick to my Uni-Syn. I had nightmares after seeing The Matrics. I'm real thankfull there's a cover over my 'puters inards!

Ron

 

[donsrods]

I agree that most of us are overcarbed, especially with stuff like a tunnel ram or lowriser manifold with a couple of four barrels sitting on top, but there is nothing that says hot rod more than lots of carbs sitting on top of the engine.

The general rule, if you want to be in the ballpark for correctness a 302/305-ish motor needs something around 500 CFM, and a 350 sized engine needs a 600. A 454/460 motor would take an 800 CFM well.

I planned on running the 600 CFM on my T (350 Chevy) only to get it running, then swap to a tripower setup. But the thing runs so good and gets mid to high 20 mpg milage that I don't want to mess around with it. However, that single carb sure looks lonely to me perched up there. :lol:

Don

 

[Brucer]

the wideband O2 sensor is basicly a sniffer... you put one on each side of your exhaust and it reads your air/fuel ratio... its the o2 sensors that are on your newer vehicles today, i believe its 1995 and up vehicles all have dual o2 sensors, 1994 and down have a single o2 sensor..

they basicly sniff your air fuel ratio and send signals to your computer so the computer can adjust for differences in the weather and such and adjust accordingly..


you can get a standalone wideband o2 sensor and guage, but for tuning a carbed car for balancing the carbs it wouldnt work.. you would need a sensor on every exhaust port to balance each cylinder..

on fuel injection cars you can adjust each individual cylinder if you have the correct software..

 

[TheAntiBoyd]

Personally, I dont subscribe to the "tuning every cylinder" theory on a carbed street car - for the "proper" A/F ratio. And heres why. My friend Brian had an I-6 Mustang when we were in college. I had a turbo dodge daytona (13.20@103.64mph) and we were tossing around the idea of turbocharging his inliner. So, we decided we needed to learn more about inliners; and tore it apart. I had a HELL of a time trying to get it dialed in because the 2 end cylinders were always more lean than the middle ones and thats without the turbo. We ended up going efi (MegaSquirt) and he sold the car before it was dialed in for crazy money.

If you have a single plane manifold, good luck dialing each cylinder in for the street...its nigh impossible for most guys and not even reasonable. If one cylinder is off enough it'll show at the end of the collector on an O2 sensor. Its very hard to tune a carb for one cylinder. Im not saying dont bother, but if both cylinder banks are even, youre fine.

The 87 firebird that my girlie is lovin right now - 305 TPI. Threw an adjustable fuel pressure regulator on it, and bought an old laptop for custom programming. I soon discovered she makes a slight bit more power running very close to lean; BUT gets the best fuel economy running slightly rich. This to me is counter intuitive, but a lot of the EFI guys report the same thing

Bottom line, if you like techie things, an O2 bung on both sides of your engine is a nice tool to play with to get it really dialed in, BUT Id still read my plugs (on the off chance one cyl is 25% lean and another is 25% rich) and use my nose.

Sorry for the long winded post. I had the JAW unit...it works good.
Once again these are my real world observations and not law, take it for what its worth

 

[ACJC]

I've always wished Holley or someone else made 390 cfm carb with a choke tower installed. I believe a 390 would be a gracious plenty for the majority. Here in SC we could probably get away without a choke except on the coldest of days.

Just a thought.

JC

 

[TheAntiBoyd]

I think carter afb boat carbs came in around 400cfm