HaxbySpeed

The problem is they're not technical claims. I won't comment on Q-jets because you can't even buy them anymore. Edelbrock carbs are great for replacement on stock or very mildly modified engines. On a 475-500hp engine they'll cost you 10hp or more vs a similair sized Holley. Instead of spending twenty minutes refuting all his ridiculous claims, I'll just ask you this. Why does EVERY single aftermarket carburetor model the Holley, why does every major custom carb tuner use the holley platform and holley components, why is almost every high performance intake manifold designed for use with a Holley style carburetor, why does every performance crate engine come with a Holley carb. etc, etc..

TEXASRPM

Sorry, they are technical claims. If they are not, your quote is also not a valid technical claim. Not all tech claims are of a mathematical formula or spec they are from ones own observations over time.

I can understand not wanting to spend time on the Quadrajet but the Edelbrock Marine Carb which is much like the Weber is a popular piece.

I personally think that over carburation is done very often unless you use a A/F meter and have a o2 sensor bung installed on the riser in order to measure for the correct ratios. If not, it is then a crap shoot.

I often see posts regarding excessive soot complaints which often is caused by richness at idle. This can be a very important reliability issue due to possible oil dilution from excessive fuel.

If you try many of the carb calculators available you will see that large carbs are often not necessary for a given application.


Are the claims made valid or not? Lets go Pros.

Thx

SB

TexasRPM -

What is your definition of 'over carburetion.' ?

I ask because most people use this term inappropriately and it really has nothing to do with 'too rich.'

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More OE FI boats have dirty transoms than OE carb'd boats.

Do a search on 'dirty transom' and/or walk around at the local watering hole checking out the back of boats.

So, this means what ?

SB

You can use Holley's carb calculator if you are looking for regular performance.

If you want more performance, then don't use it or if you have to use it , then modify the math to what you see works best. Example as I mentioned before: some engines I'll plug in 120% or more VE for proper max performance airflow.

4bbl carbs cfm's are rated at 1.5" hg.
In my experience, and a quazzilions others, if you have 1.5"Hg at WOT then your still airflow choking your motor. Not only will top speed be down, but also acceleration.

Anyhow, way too much to write here. It's all been written a ton of times. There are books available that you (plus others including Dennis) can read that will shed more light.

Also remember, experience and gathering data will not always agree with what some people's 'formula's' lead you to believe.

articfriends

Scott, my old boat was a prime example, all the carb calculators said it only needed 600 cfm at 80% ve, I put a edelbrock 650 on it just to see what happened, boat would only go 57 mph with secondary's fully open , boat would go 62 with holley 830 cfm annular discharge on it, what the speedo and tach says on the water outweighs theory in a BOOK, Smitty

TEXASRPM

I use a formula as a starting point when choosing the correct CFM. Is it correct to say that larger CFM sizes are only good for much higher RPM environments such as up around 6000 to 7000 rpm. However, most boating and engines are either running from around the docks to around 4800 to 5500rpm.

When the boat is under a load, the engines vacuum drops and needs more fuel. Too large of cfm would only drop vacuum even more is that correct? It would then need extra RPM (increased vacuum) to draw in more fuel in order to justify a larger CFM carb.

Now here is just my opinion. If as a result of lower vacumm and less fuel being drawn in (bogging), could over jetting to make up for the lack of fuel contribute to a rich condition?

Would the larger bores in the larger carbs decrease airflow speed through the carb at lower rpms and decrease needed vacuum?

I generally follow this formula (350 x Max RPM x 0.9) / 3456
This is with 90% VE for a performance/mild build.

Using a 540cu X 6000rpm X .90 ve/ 3456 you come out with a 850 CFM which sound about right.

Using a 355 X 5300 X .90 /3456 you come out with 490 CFM which in this case would be a good candidate for a 600 CFM. Would you not conclude that this motor would be over carbureted with a 700 to 750 cfm?

I consider myself a novice and do not have any experience on a dyno. As a result I have to start from some point such as a formula as a guide. This along with an o2 sensor and a good A/F meter.

I agree that richness can be caused by several conditions such as fuel pressure, ignition timing, improper jetting, float levels or idle mixture screw settings if running rich at idle.

Hey, if anything said is incorrect please correct. Just learning!

SB

Air speed draws fuel thru the booster. Different booster designs not only provide differences in atomization, but also air speed thru the booster.

Remember, our atmosphers is under pressure. At sea level approx 14.5psi.

If you have lots of vacuum under the carb, you are losing available air psi. VAcuum is a measure of restriction. Therefore, a vacuum guage can be used as a live engine air flow restriction meter. I use them on intake tracts, flame arrestors / cleaners, and of course the carb itself.

Math is good. Testing live is better.


Remember, engines don't suck air. They make air voids that the atmosphere's psi fills.

Assuming we are using your engine as the example. 600 will be better on a single plane intake than the 750. The 750 better on a dual plane then the 600. Obviously, your engine build will be better with the dual plane.

Dual planes expose 1/2 the throttle bores to each cylinder and provide a smaller plenum each cylinder feeds off of. Single planes expose each cylinder to all throttle bores. Common plenum provides more storage and less 'signal' ie: airspeed thru the carbs boosters.

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Anyhow, if you want a 600cfm that's fine. Using the 600 I'd recommend a 1/2"-1" open spacer (taller of the two that you have room for) and a quick change secondary spring kit if you have to go vac secondary, but better yet a mechanical secondary carb. Make sure that you have metering blocks in the secondary. Easier/quicker to tune than if you get the metering plates.

The spacer will expose more throttle blade area to each cylinder. Remember above ?

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This is really further than I wanted to get into. Nothing personal. But you could write and write back and forth about a whole bunch of what if's and what not's on this subject.

But that's a good amt of meat and potatoes.

Hope it helps and somewhat satisfies your hunger.

sprink58

I think the best all around performance is for most guys a function of throttle response. My experience dictates that a smaller carb that maintains a high velocity with higher manifold vacuum is going to be more drivable and have better throttle response than a larger one.

However...if top end is the driving force then go big...pick a size that is going to provide the absolute highest output at high rpm/WOT.

Back in my drag racing days the hot set up was twin 600 double pumpers on a tunnel ram for the 8000 rpm small blocks and twin 750's for "balls out" big blocks. We were launching at 5000 rpm and didn't give a damn about power below 4K~5K RPM. In boats we are looking at 2500~5000 as an operating range and it's all about a "Fat and Flat" torque curve. In a word...keep the vacuum and velocity as high as possible. Just my "Old School" opinion.

SB

An I/O does not have a 1200rpm stall converter.

I wack my boat's throttle (or most other's boats for that matter) and the prop starts to grab at 2800 or so rpm. This is also why we cam for this range vs idle/low speed.

Throttle response will be slower with a slow opening secondary system.


I don't know about you, but I want power now. Not later. Throttle response is now.

This is a performance boat. Not a big car/truck with tight converter and not much rear end gear where you have to wait for rpms to get to where the engine works better.

Try it and then get back to me.

You'll like it and go wow !

TEXASRPM

Thanks for the input.