Well I never did tell about the difference in the motor when we went from the Edelbrock 500 cfm to the Street Demon 625 cfm polymer, but it was a big kick in the pants to say the least, well a friend had a Edelbrock Performer 750 cfm that he kept saying lets put it on the 302, he says I'll guarantee it will make it scream! Well I tried to explain that based on cubic inchs, yada, yada that 750 was too big, that it wouldn't run as good as it does now........well, we put the 750 on it and here is what I learned.....he was right, WAY right to a point. On the low end the car didn't want to run, sputtered, backfired, etc. BUT once it got past 5k rpm WHOA, it made that motor absolutely scream, it would roast through 1st, 2nd and just barely started to get traction when I hit 3rd.......now I'm sure alot of you understand the why of how come it will scream with a carb that is suppose to be too big to be effective but I was impressed, stunned, quite shocked. Now let me be clear it only run like a raped ape in the 4500 and up rpm, trying to take off easy was very difficult, seems like they could put together a carb that would act like say 600 cfm then go to 750 or whatever once the engine could accept it, anyone wanna explain why it can't be done because I know for a fact that this 302 will handle 750 cfm very well in high rpm's
What is the power range for your cam - seems like you might benefit a bit by having a cam that begins to make power at around 2,000 rpm unless you are looking for the big top end power
The formulae you find for determining carb cfm are very conservative. For a four barrel they are based on a depression of 1.5 in. mercury. I don't know of any performance engine that meets it's potential with 1.5 in. vacuum at wide open throttle. Typically you want your vacuum level to be about half that. I just recently replaced my 650 dp with a 625 Street Demon and love it so far. A 3-barrel carb where the secondaries flow more than twice what the primaries flow.
Just got back from another drive btw and spent a bit more time driving after tuning it a bit and actually the low end below 2 k was only difficult time, it wanted to stall when we came to a stop, had to feather the pedal to keep it running and just driving at 35-40 mph could feel it surging or stumbling, however you look at it but when you get on the gas and it passes 4000-4500 rpm somewhere right in there it literally sounds like it's screaming, some neighbors several miles up the road came by and said it sounded almost painful it seemed to be winding so tight, actually never got past 7000 rpm, got right to it but not past......................I don't now, it's probably just a matter of I haven't had the motor tuned right completely and it was more of the combination of a well tuned 750 cfm, then well set timing (by friend) and when we p I don't have a clue what you said in regard to mercury depression and whatnot but we did put the demon back on the car and while embarrassing to admit, after said friend tuned the motor with the 625 (he used a vacuum gauge, did something inside the distributor, what I don't know) and NOW it runs as good or better than the 750 did on top end plus it idles smooth, starts with one crank, basically what you would expect out of a well tuned car..........I guess I just didn't have everything dialed in right, btw next Monday is to be the first day of letting the son take it to school, it will be his first outing in it alone, he has had good seat time but I'm nervous, I only wish I had the extra $$ to get the rest of the MSD 6AL -2 package where I could set those RPM's at a max, not that I don't trust him, it's just well I know how hard it is to resist some punk in his 4 banger with oversize muffler revving next to me at a stop light.....I want to see him pulling back up beside me at the next light coughing from inhaling my tire smoke
Bigger carbs will work on small engines. It's usually not the best answer but they will work. As for the idle issue, that's carb tuning needed. The higher the RPM range, the bigger carb you need. How do you think drag racers use dual 1250 Dominators? It's a narrow, high rpm band they are tuning for. How often will your car actually see RPM's on the high side of 6000? Honestly. Very seldom unless it's a race only car. Your exhaust will be the restriction that stops the gains, or compression, cam, intake manifold, ignition, heads....anything hooked to the engine will limit what you are actually getting out of a given carb. What may work for me may not work for you because of minor changes to the whole combination. I've run 980 carbs on 351W circle track motors successfully. I did it to kill the sharp bottom end and still have a good top end for a slicker track. Not recommended but it does work. My 13.5:1, 364" Ernie Elliot headed, full roller engine that we turned to 9000 RPM, really liked an 850 Quick Fuels carb the best. The Holley 750HP outdid it on a dyno but not on the track. The 980 worked well on a hammer down style track. We even had to run it on a 390 4bbl Holley HP for some sanctions. You can do a lot with a carb if you are willing to spend the money. You can skirt a lot of problems with a proper tuning and understanding what is really going on. That is why carb estimators are so conservative. They are trying to average idle to about 5-6000 RPM max. If all you do is turn high RPM's it isn't a useful tool. If you re driving in stop and go traffic with an occasional blast, they are fairly decent, depending on your engine combination. SPark
OK, now this makes sense, I have been thinking when I use a CFM calculator and it says that optimal cfm for a 302 is 482 cfm street carb that is where it will make the most power, it's not, far from it I see now and I consistently read most are running at least a 600 cfm, many 650 cfm with a few above, well above on the 302 cubic inch motor.......Thanks for clearing that up.
if you want to run a 750 CFM carb you need to have an intake, heads, cam and exhaust system to flow it. then there's the converter and gear to factor in.
Anything other than a 750 and my car runs too lean. Unless I tune the crap out of it. I ran out of tune-up on my 600 Edelbrock, and didn't want to start drilling holes in my 600 Holley DP, but when I put the 750 Holley it ran great, low end and top end. Recently, I had to tear it down and rebuild it and THEN it started to run poorly at low speeds, but I actually had to RICH it up to make it run good again. I have the largest pump cams installed, and riched up the jets a bit, and now it runs fine once it is warmed up (no choke on my model). When cold, it stalls and is hard to keep alive, but give it a couple minutes and it runs flawlessly. Pretty much any carb you take off of someone else's car and put on yours will need some tuning, since it is tuned for the original car, and not your setup.
Ok I got two guys in this thread with the same 625 street demon. I picked mine up and put it on my itty bitty 306 and it seemed like it was really a big carb for the 302. I took it apart to find 80 jets in the secondary, couldn't tell what the heck the primary were since they are not normal jets ( unless I seriously overlooked something ). Did either of you have to do any tuning or make any changes to the jets, squirters, or that funky lock nut and flat tip secondary goggle tensioner deal???? I put it on my car and was getting a real bad bog and fuel smoke when I would romp it.......... took it back off in frustration. Thanks
The Street Demon uses standard Holley jets in the rear barrel and it's own proprietary jets w/ metering rods in the primaries. The secondary (one large barrel) flows something like 425 cfm, thus the large jets. All I adjusted on mine was the idle speed, idle mixture, choke, and secondary air valve. If you're getting a bog at WOT you need to tighten up the secondary air door until the bog goes away. Hint: turn the screw counter-clockwise to tighten up the door tension. Do this in 1/4 turn increments. I know, "real men don't read instruction manuals", but... LOL
bmcdaniel I did all of those as well. My air fuel mixture was pretty much spot on with 18hg of vac at idle. Choke seemed to be set up fine when i got it so I didn't mess with it. I turned that air goggle thing as much as 2 full turns past the 1 full turn its set at and it didn't help my bog at all lol. But I've had that bog across 3 different carbs and haven't been able to succesfully tune it out. I'm getting ready to do a compression check and go from there. I suspect my heads have some issues.
Well as I stated earlier mine is running great since friend tuned the car, it may have been perfect right out of the box but I didn't give it a chance before I stated tuning (big mistake) all I can say now is this is the smoothest the engine has run yet, it starts on one crank (if we turn on the electric fuel pump 5 seconds before cranking) transitions from part throttle to WOT very well, no bog....really have to go easy to keep from spinning.....take off easy in 1st, run up to 15-20 mph with a light throttle, slap second and stomp it and it will catch the tire on fire....really having fun and very impressed with how well this motor has turned out...thanks to all
Good to hear. I'll be holding on to my street demon for a while. In the works of picking up a roller motor and auto so hopefully that goes as planned.
