Moody Blu'
01-05-2005, 10:55 PM
I found this on a site. they talk abotu the 750 cfm being badly engineered.
I have been running this carb and did notice a big drop in MPG compared to my friends edelbrock 600cfm.
I may swich to a 750 holley but the thing is i have all the jets for the edelbrock 600,650 and 750. should i just go with a 650 and tune it?
heres what i found.
and heres the link too.
http://www.centuryperformance.com/forum/topic.asp?TOPIC_ID=63&whichpage=1
We tried the 750 on a variety of engines and vehicles. Small and Big Block Chevys, Big Block Mopar and Big Block Ford. We tested on big inch streetable applications, and also high horsepower smaller engines. On each case the engines made more power (and lower b.s.f.c) with different carbs (Holleys, tweaked Quadrajets, and in some cases the smaller Edelbrock carbs made more power and torque). We even tried reworking/modifying the 750 Edelbrock carbs to try to find some improvements, nothing worth mentioning.
I do not have an exact count, but it was over 18 applications ... each one was frustrating.
Like I stated, the smaller Performer Series carbs are awesome pieces. Easy to tune, decent performance and efficiency.
The "correct" way to do your dyno testing is get the engine all set up, run it with the 750 Edelbrock (adjust it to optimum)... then, try not only a different brand carb, but also try the smaller Edelbrock carb. For a final test, go back to the 750. Remember, different carbs in some cases will relate differently to different jetting, timing, etc. Your dyno operator should know what to do (unless you are just going there to get a power and torque number with no actual tuning). Take the time for comparisons ... that is where the numbers show the variances.
One more concern ... a 750 cfm carb on a 331" engine is designed for REGULAR operation above 7800 RPM.
A 331" engine that sees 7,000 max RPM only needs 670cfm (at 100% volumetric engine efficiency). Most engines are typically only 80-90% VE. High output race-only applications can exceed 95-100% VE.
Engine size (CID) x maximum RPM / 3456 = CFM
CFM @ 100% volumetric efficiency
Offer me the specs on your engine (including head flow ratings, etc) and I could probably calculate your power and torque ouput to within 3% of what an engine dyno will actually show you. (and make a few suggestions to any changes ... if needed) I can tweak the settings to come pretty close on chassis dynos, but I need a ton of information.
Best Regards,
Sam - Pres/CEO
Century Performance Center
I agree on the Edelbrock 750 carbs. Here's an e-mail I sent to Edelbrock's warranty manager. Their only suggestion was to send the carb in to have it rebuilt and engine dyno tested with NO GUARANTEE any problem would even be corrected. Their suggestion to me was to enlarge the venturi passage in the primary & secondary feeds (as in drill larger!) I have concluded the 750's aren't worth anything but the $0.79 you can get in scrap aluminum. Love the 600's though.
(original e-mail to Edelbrock)
I've used three 600's so far on mild & mid HP 350 chevy engines and I haven't a single complaint. They were easily tuned, drivability was great, and fuel economy was fair. So, when it came time for selecting a carb for my street/strip engine project for my Fiero, naturally Edelbrock was my first choice. I looked at the specs of your RPM crate engines and found mine were quite similar: 355, 4-bolt, forged crank, H-beam rods, 10.5:1 forged pistons, sportsman II heads, weiand stealth intake (no room for RPM Gap or else I woulda used it), 1-5/8 shorty headers w/ dual 2.5" flowmasters 40 series, msd distributor and 6AL, and the cam: 234/244, .488/.510 but on a 114 lobe sep for the higher compression. At the time, you were using the 750 carbs on the crate engines, so the 750 manual was my choice. Upon receiving the carb, I found a couple points that struck me as odd. The throttle bore was the same dia on the primaries and secondaries (1-11/16) versus having 1-7/16 & 1-11/16 respectively on the 600's. Fair enough. One of them has to increase to increase CFM to 750. The difference in the secondary jetting raised an eyebrow when compared to the 600 as well. The 750 same throttle bore dia (as stated above)and venturi dia as the 600 on the secondaries, but the jets come in at .107 for the 750 and .095 for the 600. The required is basically a proportion of the air flow through the throttle bores. Being the throttle bores are the same size and ALL OTHER THINGS HELD THE SAME, the jets for the two should be roughly the same. However, the jets are an astounding .012 GREATER for the 750! This began me believing that the difference is making up for some inequity in another feature of the carb. Since the 750 is really a 600 with larger venturies and throttle bores on the primaries, there must lie some sort of problem. Is there a defiency in fuel delivery when the primaries are open past 65%? The whole idea of the larger primaries themselves worries me. From what I understand, a more radical engine has less than ideal air draw at RPM's under 2000. The larger throttle bores then are a hinderance to the fuel mixture because of the slowed flow of the wider bore cross section. As area for flow increases, velocity decreases, just like an intake. A smaller primary would have greater air velocity than the larger and thus produce a better mixture. Granted, the smaller primary will be restrictive to flow at higher RPM's as more air & fuel is demanded.
Here's the facts: all my settings were ideal. electric fuel pump regulated to 5.5 psi, in dash pressure gauge - the needle never moves off 5.5. Floats properly calibrated (and checked several dozen times afterward while tuning). 10 deg advance base timing with the lightest springs for earlier advance, and full vacuum advance at idle. my engine idles at 900 RPM @ 14" vacuum. No erratic idle or vacuum fluctuation. Now, here's the frustrating part: the mixture is ALWAYS rich. There was no sign of fuel leaks or fuel dumping, just the thick exhaust and excessive carbon build up inside the pipes. The plugs indicated the same rich condition. The only way I could get a good mixture was to decrease the primary rods/jets down to .073x.052/.107 and secondaries to .101. By the time I restrict the fuel that much, it is far too lean on warm up and intermittently lean on in all stages. On a positive note, the step-up springs as usual, worked best with the pinks. But unfortunately, that was the only solid tuning I was able to rely on. Frustrated, I exchanged carbs thru Summit and figured something was amiss internally in the carb. I got the new carb and bolted it on. After more extensive testing, the same problems ensued. I decided I must be missing something. I replaced and reset the floats, put in all new gaskets, and new needles and seats in the carb. I bought a new electric fuel pump and fuel pump relay for the hell of it, replaced the lines from tank to carb completely, and routed them even more carefully to ensure there we no kinks or restrictions. I even bought a A/F mixture gauge and welded the bung 8" from the shorty collector. I tried various tuning set ups and it was no different than the first 750 carb. The A/F gauge verified everything I believed to be happening under the initial and latter tuning attempts. Too rich, so I'd go leaner. Too lean, so I'd go richer. There was no happy medium, just too rich or too lean. At that point, I thought my initial hunch about the 750 might be right. I unbolted the 600 from the mild motor in my truck and onto the 355 in my fiero. I retuned it with .068x.047/0.101 primaries and .098 secondaries. The difference was like night and day. So, I found my problem, the 750 either wouldnt work with my set up or the carb design is fundamentally flawed. So, I set out in search of my next carb to use. The 600 killed my high end and that wasnt a sacrifice I wanted to make. So, I started researching. Holleys, Demons... and Edelbrocks. I found nothing but praise of the 600's online in forums and reviews but very mixed feelings on the 750. Curious. Then, I noticed Edelbrock has a new EPS 800 out, which replaced the 750 on their crate motors! I couldnt find details on the throttle bore and venturi sizes, just that the venturi clusters were redesigned. The stock calibration caught my eye the most. Even though the 800 has a higher cfm rating than the 750, the base calibration is .071x.047/.113 primary and .101 secondary. Hmmm... no excessively rich secondary jet on a even bigger carb? It's a whole .006 smaller, two stages. The 800 hasn't been out too long so I couldnt find much feedback on it, so I'm hesitant to give it a try. I need a carb bigger than a 600 for sure. I don't want to "switch teams" to a Holley or similar and give up a low price, ease of tuning and most of all, tuning familiarity. On the other hand I don't want to take another risk. I do not claim to be an expert on air/fuel distribution or flow through carburetors, but something does not seem right with me with the 750 in my experience. I would like to believe that the frustration endured in tuning the 750 was beyond my control, but maybe I still was missing something. But, if the 800 was released to remedy the 750's poor design I would like to return the 750 and pay the difference for the 800. I'm not looking for a handout, just answers and a carb that works as good as the 600. Any information you can possibly provide me on the subject would be greatly appreciated.
so would one gain performance by switching to a holley 750?
I have been running this carb and did notice a big drop in MPG compared to my friends edelbrock 600cfm.
I may swich to a 750 holley but the thing is i have all the jets for the edelbrock 600,650 and 750. should i just go with a 650 and tune it?
heres what i found.
and heres the link too.
http://www.centuryperformance.com/forum/topic.asp?TOPIC_ID=63&whichpage=1
We tried the 750 on a variety of engines and vehicles. Small and Big Block Chevys, Big Block Mopar and Big Block Ford. We tested on big inch streetable applications, and also high horsepower smaller engines. On each case the engines made more power (and lower b.s.f.c) with different carbs (Holleys, tweaked Quadrajets, and in some cases the smaller Edelbrock carbs made more power and torque). We even tried reworking/modifying the 750 Edelbrock carbs to try to find some improvements, nothing worth mentioning.
I do not have an exact count, but it was over 18 applications ... each one was frustrating.
Like I stated, the smaller Performer Series carbs are awesome pieces. Easy to tune, decent performance and efficiency.
The "correct" way to do your dyno testing is get the engine all set up, run it with the 750 Edelbrock (adjust it to optimum)... then, try not only a different brand carb, but also try the smaller Edelbrock carb. For a final test, go back to the 750. Remember, different carbs in some cases will relate differently to different jetting, timing, etc. Your dyno operator should know what to do (unless you are just going there to get a power and torque number with no actual tuning). Take the time for comparisons ... that is where the numbers show the variances.
One more concern ... a 750 cfm carb on a 331" engine is designed for REGULAR operation above 7800 RPM.
A 331" engine that sees 7,000 max RPM only needs 670cfm (at 100% volumetric engine efficiency). Most engines are typically only 80-90% VE. High output race-only applications can exceed 95-100% VE.
Engine size (CID) x maximum RPM / 3456 = CFM
CFM @ 100% volumetric efficiency
Offer me the specs on your engine (including head flow ratings, etc) and I could probably calculate your power and torque ouput to within 3% of what an engine dyno will actually show you. (and make a few suggestions to any changes ... if needed) I can tweak the settings to come pretty close on chassis dynos, but I need a ton of information.
Best Regards,
Sam - Pres/CEO
Century Performance Center
I agree on the Edelbrock 750 carbs. Here's an e-mail I sent to Edelbrock's warranty manager. Their only suggestion was to send the carb in to have it rebuilt and engine dyno tested with NO GUARANTEE any problem would even be corrected. Their suggestion to me was to enlarge the venturi passage in the primary & secondary feeds (as in drill larger!) I have concluded the 750's aren't worth anything but the $0.79 you can get in scrap aluminum. Love the 600's though.
(original e-mail to Edelbrock)
I've used three 600's so far on mild & mid HP 350 chevy engines and I haven't a single complaint. They were easily tuned, drivability was great, and fuel economy was fair. So, when it came time for selecting a carb for my street/strip engine project for my Fiero, naturally Edelbrock was my first choice. I looked at the specs of your RPM crate engines and found mine were quite similar: 355, 4-bolt, forged crank, H-beam rods, 10.5:1 forged pistons, sportsman II heads, weiand stealth intake (no room for RPM Gap or else I woulda used it), 1-5/8 shorty headers w/ dual 2.5" flowmasters 40 series, msd distributor and 6AL, and the cam: 234/244, .488/.510 but on a 114 lobe sep for the higher compression. At the time, you were using the 750 carbs on the crate engines, so the 750 manual was my choice. Upon receiving the carb, I found a couple points that struck me as odd. The throttle bore was the same dia on the primaries and secondaries (1-11/16) versus having 1-7/16 & 1-11/16 respectively on the 600's. Fair enough. One of them has to increase to increase CFM to 750. The difference in the secondary jetting raised an eyebrow when compared to the 600 as well. The 750 same throttle bore dia (as stated above)and venturi dia as the 600 on the secondaries, but the jets come in at .107 for the 750 and .095 for the 600. The required is basically a proportion of the air flow through the throttle bores. Being the throttle bores are the same size and ALL OTHER THINGS HELD THE SAME, the jets for the two should be roughly the same. However, the jets are an astounding .012 GREATER for the 750! This began me believing that the difference is making up for some inequity in another feature of the carb. Since the 750 is really a 600 with larger venturies and throttle bores on the primaries, there must lie some sort of problem. Is there a defiency in fuel delivery when the primaries are open past 65%? The whole idea of the larger primaries themselves worries me. From what I understand, a more radical engine has less than ideal air draw at RPM's under 2000. The larger throttle bores then are a hinderance to the fuel mixture because of the slowed flow of the wider bore cross section. As area for flow increases, velocity decreases, just like an intake. A smaller primary would have greater air velocity than the larger and thus produce a better mixture. Granted, the smaller primary will be restrictive to flow at higher RPM's as more air & fuel is demanded.
Here's the facts: all my settings were ideal. electric fuel pump regulated to 5.5 psi, in dash pressure gauge - the needle never moves off 5.5. Floats properly calibrated (and checked several dozen times afterward while tuning). 10 deg advance base timing with the lightest springs for earlier advance, and full vacuum advance at idle. my engine idles at 900 RPM @ 14" vacuum. No erratic idle or vacuum fluctuation. Now, here's the frustrating part: the mixture is ALWAYS rich. There was no sign of fuel leaks or fuel dumping, just the thick exhaust and excessive carbon build up inside the pipes. The plugs indicated the same rich condition. The only way I could get a good mixture was to decrease the primary rods/jets down to .073x.052/.107 and secondaries to .101. By the time I restrict the fuel that much, it is far too lean on warm up and intermittently lean on in all stages. On a positive note, the step-up springs as usual, worked best with the pinks. But unfortunately, that was the only solid tuning I was able to rely on. Frustrated, I exchanged carbs thru Summit and figured something was amiss internally in the carb. I got the new carb and bolted it on. After more extensive testing, the same problems ensued. I decided I must be missing something. I replaced and reset the floats, put in all new gaskets, and new needles and seats in the carb. I bought a new electric fuel pump and fuel pump relay for the hell of it, replaced the lines from tank to carb completely, and routed them even more carefully to ensure there we no kinks or restrictions. I even bought a A/F mixture gauge and welded the bung 8" from the shorty collector. I tried various tuning set ups and it was no different than the first 750 carb. The A/F gauge verified everything I believed to be happening under the initial and latter tuning attempts. Too rich, so I'd go leaner. Too lean, so I'd go richer. There was no happy medium, just too rich or too lean. At that point, I thought my initial hunch about the 750 might be right. I unbolted the 600 from the mild motor in my truck and onto the 355 in my fiero. I retuned it with .068x.047/0.101 primaries and .098 secondaries. The difference was like night and day. So, I found my problem, the 750 either wouldnt work with my set up or the carb design is fundamentally flawed. So, I set out in search of my next carb to use. The 600 killed my high end and that wasnt a sacrifice I wanted to make. So, I started researching. Holleys, Demons... and Edelbrocks. I found nothing but praise of the 600's online in forums and reviews but very mixed feelings on the 750. Curious. Then, I noticed Edelbrock has a new EPS 800 out, which replaced the 750 on their crate motors! I couldnt find details on the throttle bore and venturi sizes, just that the venturi clusters were redesigned. The stock calibration caught my eye the most. Even though the 800 has a higher cfm rating than the 750, the base calibration is .071x.047/.113 primary and .101 secondary. Hmmm... no excessively rich secondary jet on a even bigger carb? It's a whole .006 smaller, two stages. The 800 hasn't been out too long so I couldnt find much feedback on it, so I'm hesitant to give it a try. I need a carb bigger than a 600 for sure. I don't want to "switch teams" to a Holley or similar and give up a low price, ease of tuning and most of all, tuning familiarity. On the other hand I don't want to take another risk. I do not claim to be an expert on air/fuel distribution or flow through carburetors, but something does not seem right with me with the 750 in my experience. I would like to believe that the frustration endured in tuning the 750 was beyond my control, but maybe I still was missing something. But, if the 800 was released to remedy the 750's poor design I would like to return the 750 and pay the difference for the 800. I'm not looking for a handout, just answers and a carb that works as good as the 600. Any information you can possibly provide me on the subject would be greatly appreciated.
so would one gain performance by switching to a holley 750?