Don Terrill’s Speed-Talk

I'm wondering how both manufacturers like Holley do it, and how the carb modification shops do it.

I'm not an expert, but I can tell you this, it comes from tremendous amounts of testing and data. There are numerous things that they have to take into account and it's more than just a simple explanation. Fuel demand requirements, emulsion packages, circuit timing, carb sizing, engine applications, and about a million other things. Well, maybe not a million, but you get the idea. It takes alot to come up with a combo that works, and even with the best tuners out there, someone else will tweak it and tune it just a little better......

The same way you get to Carnegie Hall!

Donny

Years ago I had the opportunity to spend several hours in a discussion with the head engineer at Walbro. I asked about the calibration of various size carburetors and he replied that they have a carburettor flow bench that is extremely accurate. I asked where I could buy a similar one. "No problem," he said, "we will build one for you. The price will be $250,000. It comes with full instrumentation."

But no instruction manual.

I read this somewhere a long time ago. "A carburetor is a uniquely designed device calibrated to deliver the wrong amount of fuel at any RPM!"

robert1 wrote:I read this somewhere a long time ago. "A carburetor is a uniquely designed device calibrated to deliver the wrong amount of fuel at any RPM!"

David Vizard, "How to build horsepower, Vol 1 , page 79, quoting a "well known physicist."

"A carburetor is a wonderfully ingenious device for providing an incorrect air/furl mixture at all engine speeds."

Damn close to being correct.

"A carburetor is a wonderfully ingenious device for providing an incorrect air/furl mixture at all engine speeds."

Damn close to being correct.[/quote]

This statement is very close to being true when you try and run a production, mass produced performance carburetor on a high performance engine.There might be one application where the carburetors fuel curve meters correctly, spot on for that motor. Put it on a different motor and it might not be so good. When carburetors were used for OEM road applications the carburetor was calibrated to the specific production engine used in that vehicle. This calibration was verified on a wet flow bench using mineral spirits as a test flow fluid. This calibration could be controlled close enough to match a fuel curve standard that was considered acceptable for the application.

When you get into real race applications things change. Every different motor configuration requires a specific calibration to properly match the fuel curve requiremnts. A lot of this can be achieved through tuning the calibrated restrictions in the carburetor. The ones having threads being the easiest to change.

When you purchase a carburetor from someone who specialises in custom calibrated, high performance carburetors you should be getting a devise that is correct for your application. The price you pay should be directly proportionate to the work involved. This is not unlike race engines themselves. You can compete in limited classes with a "crate motor" and do quite well, but don't try and compete in a Dirt Late Model show. In Drag Race applications a Supergas motor can be built for a fraction of the cost of a Prostock motor. Do they perform the same? I think we all know the answer.

The higher the quality and the better the piece the more you are going to pay. It all depends on what level you want to play.

Not too long ago it was basically impossible for an average guy to custom calibrate a carb but these days a sharp guy can get it done. The newer carbs have billet metering blocks which are fully adjustable so you don't have to mill, drill and fill anymore. Add in a wide band 02 setup and your average Joe can now sort out a carb if they pay attention to what they are doing. It is also easier than ever to get lost so a person needs to take good notes and think hard about what changes they are making.

The new Holley Ultra carbs are a good place to start if a person wants to try building their own combination. Those carbs allow you to adjust all of the circuits and they pretty good right out of the box. They usually only require some minor tweaks to get a nice fuel curve on a street/strip type of car.