Edelbrock 650 or 750

[silver71mc]

motor being rebuilt, 402 being bored 30, cam 515/520 268/280 aftermarket aluminum intake, shorty headman headers, hei distributor. a/c and auto trans. street car, which would you use edelbrock 650 or 750. thanks

[Sam (Bones)]

750

[Mike 57]

Calculations say that the engine needs 708 cfm at 6000 rpm and 100% efficiency. Now your engine will never see 100% efficiency on the street. If you are going to stay under 6000 rpm I would say 650 would be the best carb for you. I am assuming that the posted cam specs are "advertized" not at .050 lift.

[Winston Wolf]

750

[Winston Wolf]

750. It's not like you have a 350...

[MCfan]

Hi, Paul,

 

For what it is worth, I will share a bit of my recent experience with both carbs on a very similar motor. I am not refuting the recommendation of any other responder to your question - just sharing my own experience and learning in case it might help you make a decision.

 

First, a couple things I learned that you may already know, the Edlebrock 650 AVS Thunderer is different from a 750 Performer in more than just CFM, although they use the same design for primary jets, metering rods, transition springs and secondary jets. The 650 AVS will work with any Edlebrock manifold but the 750 is NOT recommended for RPM or Torker II mainfolds (see the Edlebrock web site). My own experience may have proven this to be true.

 

Honestly, my recent experience has revealed pros and cons for both carbs so I believe your selection will depend on how you intend to drive your Monte and how much you want to spend to do it. Also, it might depend on which type of aftermarket intake manifold you select, as mentioned above.

 

I have a 402/.030 over with 11.8 CR, Sig Erson 286 duration/.540 lift cam, oversized valves, Edlebrock Torker II, long 1 7/8" headers and HEI. Also, I have a manual transmission but that may not be a significant difference. The motor was originally built with a 750 Holley DP but came to me with a brand new 650 Edlebrock AVS Thunderer (#1806). The previous owner replaced the 750 Holley in an attempt to "get better mileage".

 

Frankly, I have struggled with the tuning of this setup (mostly detonation under acceleration and run-on) for a couple of years. A very fortunate encounter at a local car show last December with the eight-time European Pro Stock Champion, Jimmy Alund, allowed me to put the car into the hands of a professional driver/tuner and has transformed its street performance and resolved its tuning issues.

 

After adjusting the valves to his "specs", ditching the vacuum advance and re-curving the HEI to be "all in" between 2600 - 2800, all of the moderate acceleration detonation and run-on issues disappeared. We still had some detonation at WOT so we replaced the R43TS plugs with R44T plugs and began the simple process of recalibrating the 650 to richer primary settings for both cruise and power and also larger secondary jets for richer WOT. Idle and low speed driving were excellent, instant cruise-to-power transition and no detonation, HOWEVER, we could now tell that acceleration began heeling over around 4500 rpm.

 

After test driving the car with the recalibrated 650, Jimmy recommended trying an Edlebrock 750 Performer to extend the high end power range. I bought a new 1411 from Summit, recalibrated it to comparable 650 richness settings and mounted it with the thermal insulation gasket he also recommended. Idle was excellent, throttle response was instant and WOT acceleration still strong when I chickened out around 5500. HOWEVER, the engine would now stumble when launching from low rpm in first gear during normal street driving - not good, but possibly not an issue with an automatic transmission. NOTE: This may be the reason Edlebrock does not recommend the 750 Performer on a Torker II manifold like mine. Also, although I didn't actually measure it, I could tell the richened 750 was slurping gas at a healthy rate - go figure.

 

I consulted with Edlebrock tech support regarding the 750's low rpm stumble and made their recommended changes to the accelerator pump linkage and nozzle diameters but that had little, if any, effect. So, until I can identify the cause of that problem, I have gone back to my 650 AVS Thunderer with a new calibration profile that richens the primary power mode and secondaries a lot and the primary cruise mode just a bit.

 

For the type of street driving I enjoy, I now love my recalibrated 650. Good idle (with no run-on), reasonable cruise mileage, excellent throttle response, and plenty of acceleration up to 5000. If I wanted to race on the street or track, I would remount the 750 in a heartbeat - but otherwise it's not going back on until I figure out the low speed stumble issue.

 

Which ever carb you choose (or get one of each) , be sure to get the appropriate Edlebrock Recalibration Kit containing a starter set of springs, jets and metering rods. Both carbs are very easy to recalibrate, although changing jets is more involved than changing springs and/or metering rods, as you may know.

 

Unless you plan to race on the street or track, I would think long and hard before I gave up on a 650 AVS, especially if you do a lot of normal driving and are sensitive to fuel consumption. Just my thoughts on the subject. Sorry for the long post. I have some detailed spreadsheets on recalibration combinations for both carbs if you're ever interested. Just PM me. Good luck!

[silver71mc]

Thanks for the info, and your opinions on which carb to use. The info shared on this site is always helpful. Thanks, Paul