Just a note about a carb ordeal on the 64 Impala "family 4 door rod" I've been building. I must apologize for my absence around here for work etc. I've learned that Demon Carbs are way under rated! The 725 road demon carb that some retailers recommended turned out to be way too much carb for my warmed over 327, no matter how hot the cam. I've had this engine built about as much as one can without requiring extra high-octane fuel. After jetting down the 725 road demon to 58 on the primary and still having too much flow I sold the 725 to an individual with a 451 stroked mopar, perfect for him. I called Barry Grant and gave the specs on the 327 that's bored .40 over with a balanced 350 crank, 218 intake and exhaust as far as the stock head would allow. A 262 extreme energy cam that has 218 on intake and 224 exhaust at .050 duration. The folks at Barry Grant, (which I should have called with these specs in the first place) recommended the Speed Demon 575 carb, jetted 58 on primary and 66 on secondary, since I live at 6000 feet. Runs perfect right out the box, so perfect it's got more snap than any fuel-injected engine I've ever heard. Moral question of this story by this novice... Why do retailers of performance carbs always recommend too much carb? I can't believe how pervasive this problem is. Talked to locals that have junked perfectly good demon carbs going back to regular Holley or edelbrock. Yet now that I simply give the specs to Barry Grant, install the carb.... I cannot believe how perfect it runs right out the box! No Smoke, no stumble, just snap! Running it in at 2500 for 15 minutes tomorrow on 20w 30 castrol and on to 20w 50 afte that unless anyone has nother suggetion??
Ive read that demons are actually around 100cfm higher than advertised. (as compared to a holley ect...) Think i read that in chevy high performance. but im not sure.
There is a formula to help get a rough idea how much carb is needed. Displacement x RPM x Volumetric Efficiency / 3456 V.E. depends a lot on how well built the motor is. A stocker might be around 75% a hot street maybe 85%. A full on race motor can reach 100% with tuned intake and exhaust lengths. It's suprising how little carb most engines really need. Bigger is not always better. The most common engine building mistake is too much carb and/or cam.
I thought most magazines, books, tv shows etc. always recommend too small of carb. I have a 355 chevy and it ran ok with a 600 cfm holley, if I jetted it way up. Out of the box it was dangerously lean. Later, I changed it to a 750 Holley DP, and it ran way better. I've never tried a Demon though, from your post sounds like they're worth a try. I've also heard that if you have to go up or down more than a few jet sizes you have the wrong size carb.
The only reason I've jetted down so much is al***ude. I drive mostly between 6000 to 9000 feet above sea level in Northern New Mexico and the air is very dry which lowers the firing ability even more. Humid weather gives a good boost around here. I do believe however after several car shows etc. that most of us run too much carb fouling out plugs and boging down what could be much better performance unless of couse you're in the humidity and oxygen rich enviroment close to sea level, where engines go nuts. I'm surprized just renting cars on business when I travel back east.
i'm curious what "too much flow" means to you? most times people go trying to put in smaller jets because a carb is "too big" they should really be changing the idle air bleeds, adjust the accelerator pump, etc. not the main jets.
The reason your carb worked so well as compared to the local guys recommendation is because you talked directly to the guy that designed it. He should know what to recommend. It does make a difference. Also, in the early days of Demon carbs, they were junk, and now everybody is afraid to use them. Today, they are a good carb.
Looks to me like the only way to change air flow is to change the venturi, ****erflies, etc. Kinda hard to do that on most carbs. Changing jets, etc. just changes the mixture-not theflow. Larry T
Simply too much fuel and air. Changing the air bleeds was an opiton on the 725. I have not messed with the accelerator pump or air bleeds. After calling Barry Grant's tech line and selling the first carb for $250.00 I decided to let them do what they recomend. The speed demons are built when you order, with boosters / air bleeds designed to give your engine what it needs. I'm just learning, but am amazed at how well this Speed Demon works for less than $400 built for me. I still may have a friend that does mess with air bleeds, timing, fuel, etc fine tune this engine, as he does a good job locally on cars, airplane engines etc. I suppose I could have made the 725 work, but after bolting on the 575 Speed Demon, I could not be more pleased with the results. The smaller jets were simply recomended by Barry Grant for this al***ude.
ok. pretty much what i thought. another question, you said in your original post, that you were going to run the motor in for a while tomorrow, is this a fresh motor just being broken in? no doubt the smaller carb will be better for what the cars intentions are...a family hauler. i'm sure at low rpm's the smaller carb feels much more responsive, it's getting a better vacuum reading than the big one, but at WOT i don't think there's such thing as too big a carb(within reason), which kinda answers your question, why do retailers suggest big carbs? cause most guys want to go fast, bottom end torque matters on the street, but many guys overlook that for the big numbers at high RPM's, thus big carbs. i'm running an obviously too small carb on a 250 CI engine, the carb is intended for an engine more than 100 cubes smaller(just the primary carb in a soon to be 3 carb setup) at the low end it's a torque monster thanks to the tiny carb, but runs out of steam by 4000 rpm. but it feels quick. like i said, the smaller carb is right for what you're doing, just trying in a roundabout way to answer your question why they recommend big carbs. really, square bore carbs are only good for high performance and not much else, the best carb for balancing daily driving with some power, is a spreadbore carb like a quadrajet, the tiny primarys give good mileage and torque at low RPM's, but can feed any real street motor when you get into it too.
Yes, a totally fresh motor. Block is original but has been bored, cross hatched, decked etc. The pistons are hypereutectic with crome moly rings. I have been very plesaed with the new carb and have only reved it a couple times and shut it down, no smoke, no hesitation. I now have the gauges installed and am ready to run it in for the first time presumably to avoid flat spots on the cam. After which I change oil again to 20 W 50 castrol and run the next 500 miles, no? I am a bit concerned about the high rpm ability of this carb, but have been told this will flow better than most 650 carbs. I suppose these demon carbs are simply under rated. We will see, but when I started it and reved it there was no hesitation. Thanks for your words!
I run a 725 on my 330 olds. big heads, huge cam, big *** headers and 3 inch exhaust. 10.5 compression and lots of mondello parts. this little thing SHRIEKS!!!. the only carb I have evevr had that was better was a Holley we built when I was at the dragster shop, and I traded it for a RUNNING 454 chevy. the demons secret is in the main body. that is all I will say about it. The reason Demon is so conservative with its flow numbers is ;when you tune a carb for a specific motor (and we are talking about miles away from re-jetting) you have to put it on a flow bench. the problem is about 75-80% of the benches in the nation are either out of calibration, or are just plain stingy. (kinda like dynos. joe schmoe's may say your motor makes 500 hp, but sam spades says it makes 525. who is right?) so to combat this, Barry just rates them lower. Makes about as much sense as a soup sandwich, I know. but it is also how you make a good carb GREAT. if a 700 actually flows 730, chances are it will be better all around than a 700 or a 750. kinda one of those " the porridge is just right" moments.
My brother bought a demon carb for his car. He loves it too. I rebuilt my old 850 Holley with a BG kit. My buddy swore he would only rebuild with BG after doing my carb.
Lux, do you run a Demon? Maybe a Demon is the way to go on my 383, although I was trying to keep the price tag below $300.
Just like you stated in your post, BG set you straight and got you set up with the 575cfm carb, and you're very happy with it. The retailer (and I have a bit of experience in this dept) may have an unknowledgeable person answering tech questions. It isn't right, and you should be able to return it if they are decent people, if you calmly and rationally explain the problem and what the product's manufacturer said. I think what the moral really is, is this: If you want a straight answer, talk to the guys who manufacture the product. Ben
Ive had a demon in my day its was a older 850 on a built buick 455. That motor loved that carb. i loved the carb it was back in the day when i was still learning carbs (hell i still am) and that carb was the easiest to set up and worked flawless. Im a big fan of demons but just cant afford em (my old one came on the engine when i bought it) But not to stray off the topic much but for good low buck carb if you cant afford a demon you cant beat a edelbrock for out of the box perfection. and for a real mild motor there great. but i think demons are happier on a "hotrodded" motor just my 02.
I agree... as long as he didn't have the carb on the motor for a year and decide that it was wrong. Its like ordering a meal, eating 3/4 of it and complaining to the managment that it was horrible and you want your money back... If its newish, just tell them that it leaks. That is a usual defect return that most retailers will buy.
Very interesting. The 725 was obvioulsy a great carb and this novice (me) did go for it. After sellling it and talking to BG I found that the cam had everything to do with the problem not the SIZE fo the carb. In other words, BG redirected me towards the CARB SERIES, not the carb size. Their first words to me were this, the cam at duration .050 with intake at 218 and exhaust at 224 simply disqualified the engine for ANY "road demon". That anything close to 220 or over 220 warrants speed demon, that the road demons are built for 215 - 220 and below. They pretty much said go with the 575, it will perform as well as the 650 speed demon and will give much better drivability, yet still give plenty of flow at 5500 should I so choose now and then. They also siad the 650 SPEED DEMON would work well but is more for high reving most of the time, which is not what I'll be doing. It's a heavy car that needs lots of low end power to get going and not drian the gas tank. Just a new guy finding out the hard way.... I'll try to get picutres out soon!
The retailer seemed to believe that once gas hits it, it's mine. I did however find a buyer right away for only 80 less than I paid, so lesson learned. Maybe one of you are about to purchase and will get your specs together, giving ther manufacturer a call first....
1. Limited application expertise of retailer personnel: This is just one of how many thousand products they sell. 2. Customer problem also: Most customers will overstate their engine capabilities or project what they hope for them to be "someday". 3. Customer problem, part 2: Asking a retail sales clerk in the first place for meaningful information. It's like my grandmother asking the 19 year old kid working at Best Buy, "Which one of these 19" TV sets will perform best?" 4. Retailing 101: When in doubt, always offer the more expensive item. Just one guy's opinion.
I agree. Unless you know the guy behind the counter and trust his input... talk to the pros. Usually the guys behind the counter have a good general knowledge. If they were really smart in one area, they would probably be a tech guy for some company. I get it all day long... people wanting me to put together an engine combination for them... I work phones, I'm not an engine builder!
Some of the CFM difference you seeing from the barry grant stuff as compared to say a holley or edelbrock comes from the fact that the BG's are wet flowed where everyone elses shelf stuff is all dry flowed. This does lead into some variation in the commonly looked at CFM ratings. If the demon was flowed on that same bench as the holley a 725 demmon will flow more then a 750 holley by quite a little bit. At least this was how it was the last i knew. So just something to look at and think about when looking at these carbs
Yes I agree about the oil and wonder myself why so many around here go for the 20 W 50. I'm a novice. It does get hot here and dry as well. But I would lean more towards 10 W 30. Do you think the 20 W 30 Castrol is a good choice for the run in tomorrow morning? I've been told the straight 30 WT I wanted to use does no give the slippery motion for new cam / valves that the run in needs and that the 20 W 30 is perfect for that "run in", 2500 rpm for 15 minuites, then oil change to 20 W 50 for the rest of the time. I lean towarsd the lighter weight for the first 2000 miles, or maybe just the fact that it's multi weight is thin enough? All ears....
I build quite a few engines for customers and I always break them in with 30 wt. I also sleep better at night if I use GM EOS (engine oil supplement) when I first crank them. It's a camshaft break in lube. And for what it's worth, I run 20-50wt in every vehicle I own ('cept the Panhead, it uses straight 60wt. Larry T
My understanding is that by wet flowing the carb Barry actually gets an accurate measurement for his carbs and that the others are overrated for their flow.... I really like the Demons... they are what a Holley should be...
Damn!you WERE drunk at the drags! Yep, the olds has a demon on it. and as I mentioned while we were sitting on the poncho, you can usually pick one up on e-bay for 50-60 bucks cheaper than retail. But then again, I would also say to **** it up and go to your local speed shop and buy it there. (wait, that was a previous rant...or was it?) your 383 yearns for a demon. watcha runnin for intake btw?
Just tell them the truth, you asked their tech support what size carb you needed for your application, and were told something that absolutely didn't work. They are supposed to be knowledgeable about the items they sell, or at the very least, should contact the manufacturer, and get back to you with a good solid, reasoned answer. On the other hand, if you're dealing with a low-down-crooked retailer, then yeah... do what Thirdyfivepickup said. It's sad, but true. Ben
Cool thread, and couldn't have come at a better time for me...I've been looking at new carbs for my 360...
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