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Notes on air flow and engine efficiency.

This information is based on my sole observations and of course my particular engine. Your combination may differ in results. Information is based on observations made with an Autometer air/fuel gauge and “feel” of performance, and gas mileage records.

Engine is a basic crate model.
8.5 to 1 compression.
Stock cam and heads.
Performer manifold.
Edelbrock 1406-600 cfm carb.
Flowtech long tube headers Jet Hot coated.
2 ½ inch exhaust pipes with Dynamax mufflers.
MSD 6A and Pertronics Igniter.
MSD 8.5 Super Conductor Wires.

Drop base air cleaner with a 2 ½ x14 air cleaner. This air cleaner gave me ¼ inch clearance to a closed hood, and the air cleaner lid had ¾ inch clearance over the carb air horn.

The air/fuel gauge has 20 LED’s reading from lean to rich. With the above combination the gauge usually read rich at low speeds and at highway cruising would swing from very lean to slightly rich, using about 15 of the LED’s, making a real air/fuel reading difficult. At WOT the gauge read deep into the rich zone.

With the installation of my L88 high rise hood, vented for cowl induction, I gained another 2 inches of clearance.

I added a 1 inch Swirl Torque Spacer under the carb.
A 3 inch filter.
A ½ inch less drop base air/cleaner, and the top of the GM 14 inch lid which has a bowl up shape.
A K & N Sub Stack for smoother air entry
I now have 2 inches over the air horn of the carb.
I was able to eliminate two 90 degree bends in the fuel line to the carb.

Now I can feel an improvement in low end and top end performance. The air/fuel gauge is much more stable, allowing the possibility of actually tuning the carb metering with the gauge. At low speeds the gauge reads two LED’s varying at the stoich to rich separation. At highway cruising it reads 4 to 10 LED’s from lean to full stoich. (Which is desirable) When I punch it, the gauge reads stable at two LED’s in the center of the rich zone. I gained one mpg with no change to jetting or timing,.

My conclusions are that the added clearance over the carburetor, and the K & N Sub Stack, are the primary factors resulting in smoother (less turbulent) airflow into the carb which makes the fuel metering more consistent and accurate. I believe I should now meter towards a slightly leaner cruise mixture which is what the gauge and performance “feel” indicates. I might gain another fuel mileage improvement and don’t expect any reduction in performance.

Comparison mpg notes.
Last year on the Stars & Stripes Tour I averaged 15 ½ mpg over about 3000 mostly highway miles.
This year on Hot Rod Power Tour (3556 miles) I averaged 16 ½ mpg with the mentioned changes.

Other note of interest. Almost an entire tank was driven in the rain on Power Tour. That tank calculated at 18 mpg and was not factored in the 16 ½ average.
 

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Discussion Starter #3
The 3556 included a side trip to San Antonio. That loop was worth about 600 miles.
 
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