Boy this one hurts the brain.... a lot of years ago I used to follow and try to pick the brain of one Jerry Arnold. This guy is one amazing guy. He used to run an altered in comp classes with a 6cyl ford, 300 based engine. This thing was and I think he still runs it, is one incredible ride. It at one time had 6 carbs...not sure what they were, but it FLEW. Oooops I'm drifting again... Anyhow, it would be nice to know what rpm's he is running, what are the engine specs(comp ratio,cam, etc), what intake manifold will the three dueces be on etc. Post up some of these specs anf info and I'll give it a go. By the way, what carb/intake setup is he currently running? And what rpms does he launch at?
It is the formula law of physics...the formula being (CID x RPM)/3456 = 100%VE physically all engines are created equal therefore the standard of 3456 is dictating that (CID x RPM) will always be 100% efficiant volumetrically. THEREFORE IT IS LAW. QUOTED DIRECTLY FROM A PERSON WITH A BS(bull shitters) DEGREE I do not know nuttin bout nuttin except I hate these forumals.
I didn't know that it was any "law" in physics; but physics certainly enters the picture. We suggest the equation (formulae are for babies) be initially written: CFM = RPM x CID / (12 x 12 x 12 x 2) The three twelves are necessary to convert the (I)nches in CID to the (F)eet in CFM. The two is necessary because of the 4-stroke engine (and why this equation cannot be used for a 2-stroke engine). (12 x 12 x 12 x 2 = 3456 Jon.
Panic - I have seen you mention before that the equation is for multi-cylinder engines of 7 or more cylinders. I was taught (yeah, I am an older geezer) that the equation was useful for multi-cylinder 4 cycle engines of 4 or more cylinders. 4 cycle engines of 3 or fewer cylinders, or 2 cycle engines would need a conversion factor. Is there newer information that changes the base equation use from 4 or more to 7 or more? And you are certainly correct on manifold design being as important as the CID. 6inarow - I appreciate your watch comment (had not heard that one previously); but if more technical posts such as Panic's were done (and questioners actually read them), fewer individuals would have many of the issues which arise from the use of mismatched components. Of course, sometimes getting the questioner to read a long technical post is like the old cliche about the horse and the watering trough. Jon.
Getting them to read it is one thing, writing it so they can understand it is another thing. Seems that a table showing engine sizes and no. cylinders vs. rpm, with some correction factors for vac sec/mechanical and other various carb types would be pretty helpful. I usually just recommend a Qjet
Below are two engines with exactly the same dual carbs on them, but their intake-systems and motor volume are very different. On both motors, both carbs are 625cfm Carters, so a total of 1250cfm. The first one is a freshly built 496ci stroker with an original Chrysler longram intake system. It has OOTB aluminium heads, fairly stockish cam but 1.6 rockers, 1-3/4" shorty headers and a fairly restrictive dual 2.5" exhaustsystem. Motor is built for torque and does 17mpg by the way. Too much carb, not enough, just right? The second engine is a stock 354ci with a Weiand dual quad intake. Also with two 625cfm carbs on top. The Weiand intake is basicly a single plane intake with a divider wall in the center and 3 balancing holes in the divider. The rest of the motor is bone stock with a fair bit of wear in it. The timing chain has lost it's virginity a very long time ago as there's a noticable 'slop' to be felt when turning the crank both ways. Way too much carb, just right or indeed not enough engine? 496ci 354ci
OK, here is a prime example. I am building a 283 for a daily driver. I have roughly 292 CID. I figure the compression ratio should be around 9:1. I also have 2 cams to choose from. I have a 260H and a 268H Comp Cams. I will be runing long tube headers as well. It will be running an automatic tranny and a 3.25 ratio on the ring and pinion. I bought 2 carbs to experiment with. I bought a 500 cfm and a 600 cfm Edelbrock Performer carb. I am leaning towards the 500 cfm. What say ye?
Roothawg's real world sample, I like it. I'd say the 500 would "feel" better as a daily driver but the 600 would be fine. The 500 might have better manners if your commute is all city or a good amount of stop n go. If it's more highway I'd run the 600 and never look back. Passing power and total output would be notable with the 600 and if the majority of use was highway then mileage wouldn't suffer either. Add the fact that the low speed or primary portion of the carb can be dialed in for the engine's needs I think the 600 still gets the nod. I'd also use the 260 cam.
BigBlockMopar....That long ram motor is awesome...I just love the way those look. 17mpg is pretty sweet to! Roothawg, once again everyone will have a different answer/solution to your example. Either way you go it will run fine and more than likely be acceptable. If I was doing this engine I'd go with the 268h and I'd have a QJet on top. I am no fan of the edelbrock carbs. Once again the difference of peoples opinions. I spent 12+ years building and running QJets for racing, both drag and circle track. The Qjet is a highly hated carb, and I can accept that, but when they are understood and setup right they can and will be one of the best all around carbs. With that said, I would also think that the 600 cfm over the 500 would be the way to go. No matter what cam/carb combo you choose, it will be able to be tuned to run satisfactorily. Hove fun with the build and post up what you chose to build and what the results were with your comparison.
Interesting thing about those Edelbrock carbs...they have a little air valve thingy in the secondaries, so they don't act as big as their CFM rating most of the time
Roothawg To echo 1/2 evil, another vote for the Q-jet (or the Carter TQ), both of which are spread-bore design; but if you do the spread-bore, use a spread-bore manifold. The choice has to do with the size of the PRIMARY. Your 283 will probably never use the 750 (or more ) CFM from the Q-Jet or the 800 from the TQ, but the 150/200 on the primary side will give you excellent drivability on the street. In the for what its worth category: for square-bore carburetors, Carter released a 400 CFM (part number 9400 superceded by part number 9410) specifically calibrated for the 283. Both are 200 CFM on the primary side. I won't offer a suggestion on the cam, not my field. Jon.
Re: Squirrel's comment--Not only interesting, but useful. If the engine can't haul the air valves open, it can't overcarburate itself so easily, like it would with a too large mechanical secondary Holley.
or is that when theory becomes irrelevant? Most guys don't care about numbers, they want their car to drive nice
Thanks. I have a set of 1.6 rockers to use with it as well. I didn't opt for the full roller but they are the roller tip. This is just a knock around car. It should weigh in at around 2500#. I have a square bore manifold, an old C4B.
I will weigh in on one thing Carter vs. Edelbrock. I can get all the tuning supplies I need for the Edelbrock at my local parts house. I can only get rebuild kits for specific Carters there. Tuning supplies for the Carter I have to order. Just a thought.
RootHawg, I have been running a +.040, 283 CID, stock '63 power pack heads, 260H cam, shorty headers, Edelbrock Performer, Edelbrock 600CFM, T-5 and 3.73 gears for sometime. I could smell gas strongly at the tailpipe so I stepped down 3 sizes on jets and rods at the Edelbrock techs advice. That helped some and I get 20-21 mpg highway. I still had a little richness and got back on the phone with the tech. He started talking about fuel pressure and said that it should be regulated at 4.5 - 5.5 and finally admitted that I really should be running a 500cfm carb for my setup per his calculations. He said that overpressuring the carb could cause some of the problem. I couldn't see how but went ahead and installed a regulator, the only thing it did was stop a loading up problem I had at idle. I am thinking about ordering the 500 and setting the 600 on the shelf to see i fit runs better that way. Larry
It's easy, just like on 94's and 97's. More pressure than it likes and it's like having the floats to high and gas just goes into the engine. You can get a decent one pretty cheap. I think the last one I ordered was from Summit. Their plain white box one had a Holley inside.
Roothawg, PM me and I will give you my telephone number if you like so we can jaw at one another. I am just a little SE of Tinker.
Zman, Makes that makes sense to me. I picked up my regulator from O'Reilly's along with the gauge. It wasn't too expensive.
Certainly a consideration for most. It really isn't for us, as we make the parts as needed. (Opinion) Carter outdumbed Holley in the later years as far as performance. We were a Carter WD for years, and received all the Carter sales, tech, etc. bulletins. A friend in Carter Engineering once sent me the listing of the complete Carter inventory, knowing that we specialize in the older stuff. In going through the inventory, I found the item "parts cabinet". Searched and searched, and found no reference to the part number for the cabinet. Called my friend who said he would find out. He called back the next day to say noone at Carter knew anything about it!!! He ordered one out of the master warehouse for Carter Engineering. When it came in, he called to describe. Basically, a master strip kit. A metal cabinet with eight drawers each about 3 inches tall by 6 inches wide by 1 foot deep. Contents were hundreds of jets, step-up rods, higher flow fuel valves, step-up springs, etc. for the performance carbs. Racers were crying for these parts; and noone at Carter knew that these cabinets existing. Price in the early 1980's was only 99.00. We ordered four of them just to get the parts! What might have been! Jon.
I think that we need not assign the VE formula more or less credit than it deserves. It is simply a starting point to get the into the ballpark. Drivability is king. For the numbers guys, I have a stamp set.
zman - remind me next spring when it warms up, and I will get you a picture of what to look for. My digital camera works best outside. Jon.
I'm really enjoying these Q's and A's. The 454 in the greaseslapper is a stock head 8 1/2:1 comp. truck engine with a 289 degree hyd cam. I went to the carter web site and put in the peramiters at 4500-5000 rpm, and they suggested a 625- 650 cfm carb! I'm like "the've lost their minds!" Then I was talking to a friend that has spent his life involved with real world carburated engines and he said they were right because I wasn't planning to spin the engine any faster than 4500. His comment was "yes you could put on an 850, but if you are only making 4500 rpm the carb is only going to draw around 600 or so cfm, why fight all the tuning problems of too much carb?" I began to realize the concept of yes a 454 will support an 850, but if you balloon foot the engine around town all the time and never run up top, you are asking that big carb to do something it wasn't designed to do. So what's on there now is a 625 cfm Carter comp on a Turantula manifold. I'm guessing it will still boil the tires, and my throttle response will be crisp with no bog. Just a guess on my part. I know dick about carbs. But I like to hear from those that DO know. Thanks
This is why I love the HAMB. After all the years of fooling with cars, I still learn lots of new, good stuff.
The Jalopy Journal
