I guess the GM engineers who used a 750 CFM Quadrajet on your engine should have watched the video. Also never heard of lower octane related to more gasoline used.
Good luck.

 

I have no idea where the octane comes into the discussion....wow.

Having a carb "maxed out" at WOT isn't the best plan either.

All that said...you don't sound like a hardcore hot rod type.....so I doubt you'll be WOT very often. The 650 will run fine for what you're doing......but it will be leaving some performance on the table.


JIM

 

I believe the octane comes into play as it supplies the explosive energy per unit volume of fuel delivered to the cylinders by means of an approximate 15.7 to 1 ratio of air to fuel. Many, many years ago a used 103 octane from Sunoco ("Power to be Used Not Abused" ). But now with the lower octane fuels, more fuel has to be delivered to get the same amount of energy as would have been provided by a higher octane fuel in the past. The only way to deliver more fuel energy is to increase the CFM (based on the preset air to fuel ratio). My vette came with a 600 CFM carb in 1969 when octanes were much higher. That's why I feel the 650 would be a better choice rather then jetting down a 750. Hey, I could very well be wrong . . . again.

 

Your car came with a 750 Quadrajet from the factory and your gasoline theory is not correct. You have people in this thread who professionally rebuild carburetors and then test them on an engine. They told you to go with the 750. Your choice.

 

I thought this was a discussion not an argument. You are right on the 750 Quad that used a very different way of handling fuel delivery with metering rods vs jets. But fine. You are wrong regarding the fuel octane info I provided. Granted I am not a mechanic, just a hobbyist that study internal combustion engines at the US Coast Guard Academy and later sent by the Coast Guard to the U of Michigan for two of my three Masters degrees in Mechanical Engineering specialties. I do have the utmost respect for hands on mechanics, my dad was an excellent ( rebuilt my first 327 with him). And I truly enjoy using "book knowledge" and hands on mechanics with my experience running gasoline engines, enormous diesel engines and as was installed on my last ship, 2 × Fairbanks-Morse 38TD8-1/8-12 12-cylinder diesel engines generating 7,000 hp (5,200 kW) and 2 × Pratt & Whittney FT4A-6 gas turbines producing 36,000 hp (27,000 kW). But, you are wrong on the octane info. Hope we can remain friendly.

 

Octane is a measurement of a fuel's resistance to detonation. By design, higher octane fuel burns slower.

I've back to back dyno and track tested my engine (871 HP N/A and 1240 RWHP on boost) numerous times with 93 octane and much higher octane race fuels as well as oxygenated fuels. I can tell you when it was N/A and 11.56 compression ratio it made absolutely no difference between 93 octane and 115+ octane fuels. Literally swapped on the dyno and at the track. Jetting and timing were tested to see if it could handle more timing or fuel. NADA. With boost....it actually lost 20+ RWHP moving to MS109 oxygenated fuel until I added timing. Then it finally could use the benefit of the fuel's ability to fend off detonation. When N/A...it just didn't need any more octane to make power. It was doing all it could. When adding high octane oxygenated fuel you often have to rejet to provide more fuel because of the added oxygen released when it burns, but that has nothing to do with airflow. Sort of like adding nitrous. Nitrous doesn't make power..it provides oxygen to allow added fuel to burn. But non-oxygenated high octane fuel just burns slower and typically only makes power if you can add timing. Nitromethane releases tons of oxygen as it burns, but it burns VERY slowly and might need 60-70* of timing to get it burned. Incredible detonation resistance and slow burning allows them to overfuel the engine nearly to the point of hydraulic'ing it.

Adding additional fuel without adding airflow/oxygen just makes for an over rich pig of an engine. Increased CFM only comes when you're at WOT....or less throttle opening providing the same airflow (large vs small carb) which "might" change velocity depending on boosters, venturis, intake, cam etc etc. But regardless...whatever airflow is going in there is hopefully going to remain at the proper A/F ratio. Adding airflow without maintaining A/F properly will have it going lean.

You can adjust jetting mixture for any A/F ratio you want on any carb...big or small. BTW...that 15.7 is an awful lean cruising mixture for sure. Way off stoich.

But all that said...your 650 will run fine. It "might" have perceived sharper throttle response on a mild engine than a 750....but you'd be hard pressed to measure it. Top end and mid range power would tell a different story. The difference in a 750 Dominator and a 1050 Dominator on my street 427 was night and day. But again...that only works if the engine can use it. On my 540 I tested a 1400 cfm carb against my 1050. No difference in power...the 1050 could provide all it could use. Now if I'd have changed intake. heads, cam...maybe would have seen an improvement. On a 632 we tested my 1050, an 1150 and a 1250....the 1250 was up about 10 HP....the 1150 was a wash.

JIM

 

How many of the engines you have worked on had a carburetor?

The closest E85 station to you might be in Homestead, but if you really care about operating your car at the bleeding edge of potential, you'll be miles ahead if you EFI swap, or better yet LS or LT swap, your car. Running E85 will give you the high octane you want. Then use Holley's software, or HP Tuners, or Megasquirt, or whatever, to really dial it in.

 

I think you made a good choice. I have a 600 Holley. Works great on my 350 that dyno @ 343 gross horsepower. I also have a 750 Holley on my 408 Cuda that Dyno @ 496 gross horsepower.

 

I went with an AED 750 HO double pumper on my 70 BB. It puts out 575 HP verified on the dyno. My engine builder, Mark Jones was surprised the carb worked so well for its size. the driveability is great. I may be giving up a bit on the top end but its a street car that never really sees more than 6000 rpm. its mounted on an edlebrock rpm air gap intake. dont go too big for street use.

 

I went with an AED 750 HO double pumper on my 70 BB. It puts out 575 HP verified on the dyno. My engine builder, Mark Jones was surprised the carb worked so well for its size. the driveability is great. I may be giving up a bit on the top end but its a street car that never really sees more than 6000 rpm. its mounted on an edlebrock rpm air gap intake. dont go too big for street use.

 

Holley tech rep and tech rep at summit said 650 CFM with mechanical secondaries (I have a 4 speed).

 

Read post 37, trust Lars more than some unknown person.

 

Ok, good for you. Two "unknown" persons. One a tech rep at Holley the other sells Holley and other carbs at Summit. By the way, the octane greatly affects the explosion in the cylinders. A better controlled explosion, the more energy released, which means more available chemical energy released from the same amount of gasoline. Its really not that hard.

 

Holleys sizing has always been goofy. Why did Chevrolet size a 780 cfm carb on the 302 Z/28 why did Chevy spec a 780 Holley for the LT-1? If you trust the counter guy from summit let him pick a cam for you.

 

By unknown I meant experience and skill. The energy in a gallon of regular versus a gallon of premium is the same. The octane is only related to the compression of the engine. More compression requires more octane. Your original statement though was in regard to miles per gallon not energy.
Again you were given the original specs Chevy used for the 350/350, 750 CFM, and the advice of a leading rebuilder of carburetors in the Corvette world, also 750.

 

THAT..I can agree to wholeheartedly......but that's not how you phrased it earlier. You were discussing adding fuel/air volumes it seemed. You said lower octane fuel required the addition of more fuel to get same effect....more fuel will just be richer. No need to use any higher octane than the engine requires...in fact it will often make less power with a slower burning fuel if you can't increase the burn rate somehow.

JIM

 

 

Higher octane fuel having more chemical energy is a myth. Octane ratings are only a measure of a fuels resistance to detonation versus octane. At the pump the difference in actual “energy” between the different grades is negligible.

For example, E85 is generally about 100-105 octane, but actually has less chemical energy per unit than regular gasoline. To compensate for this engine builders will use larger injectors or fatter jetting in a carburetor. This is the reason your miles per gallon will go down when using E85.

 

Umm... lots of misinformation here.

Gasoline has a nominal energy content of 124,300 BTUs.
There is some variation from 115,000 to 124,000 for different blends, summer & winter blends, blends from different refineries & manufacturers, and whether or not you include the heat of vaporization of the water that gets produced.

There is very little to no real BTU (energy) difference between 87 and 93 Octane.

Octane rating refers to the difference in their combustion characteristics, as in how fast they burn when lit by a spark, and how resistant they are to detonation without a spark. It has nothing to do with the energy released.

But if you Google BTU content of gasoline you will get numbers from all over the map.
One myth says 93 Octane has more energy, one myth says 87 Octane has more energy, but both are wrong.
Typical Google nonsense, you need to consider the source, and the test method.

BTW I am a chemist.

Different Octane fuels, even different blends from summer, winter, manufacturer, may burn at a different flame front speed. That means if you change your fuel you may need to change your ignition timing to have the (MBT) Maximum Brake Torque still occur at roughly ~15* after TDC to extract the maximum power (energy) from the fuel. Yes that may mean new distributor timing settings. If you leave the timing alone, that could easily result in either detonation or reduced power output. Just like better combustion chambers significantly increase flame front speed, and require lower timing settings.

That is one of the main advantages from racing fuel, consistency. Something you can not get at a random gas station pump.

 

Just for reference...it's going to be tough to match Lars' background/experience/education on the subject of carburetors (and rocket science among other things). Great guy willing to help anyone...I've been to his garage to see him in action.

I know a few of the engineers at Holley and have been to the factory multiple times. Holley knows how to dial in a carb for sure and make good products. As usual, with sales reps...it's a luck of the draw to get one who truly understands what they are selling. Sales reps are coached to sell the '"safest" one from a warranty perspective which is typically smaller and vacuum secondaries. Some can pick up on exactly what the customer is saying and are comfortable suggesting a double pumper...but even then, there are a myriad of choices once you look at the details of the various offerings...so they need to truly understand their products and you have to ask enough questions to see if they do.

JIM