Don Terrill’s Speed-Talk

Port energy graph. I know which one I prefer. Thanks, Charlie

Charlie, I will dig out the flow numbers and the dimensions tonight and post them up tomorrow moring.
What I can tell you now is that the head uses a 1.77" valve with a 82 or 83% Throat , that much I do recall!
Getting to the end of these 2 bbl C Heads thank God!

Here's the flow numbers Charlie.
The stock valve size was 1.66 and in ported form I used a 1.75" with a 30 degree back cut.
This was on the convoluted center ports on a Pontiac head which also means that there long runner form makes CC numbers useless for camparison !
I did not time to pull these mule heads off the sheilf to get the dimension for you , but I think the Throat was taken up to 1.48".
The stock flow numbers where with the factory 45 seat, the 1.75" got 3 angles.

No under cut stems on either valve.
Stock 1.66"
100----200----300-----400-----450----500----550----600----650----700
44.6---90.3---123.2--142.2--145.2---147---148----149----150

Fully ported 1.75"
66.5---130.7---168.4--184.2--195----198.9--201.6--204--206.8---209.4

A flow test with a 1 3/4" tube @ .600" lift produced 214 cfm.

Solid numbers Mag. The little ford exhaust port hit 208 cfm with a 1 7/8" pipe. I feel this pipe is too large for a road race application. I think the longer ports give the air a chance to organize and actually flow better on the bench than a short port would. Does that mean it will make more power?

The nice thing with the width of a 302 is having headers that can have a gradual bend which makes a bunch of difference, and yes that tube size will kill off too much of the average power for certain!

Does David's program have a way to integrate the exhaust header into the calculations? It's very easy to get tunnel vision and lose sight of the big picture. Our goal is to make as much power as we can. The headers make a huge impact on engine output. Engines don't operate without headers (or manifolds). The header designer can have more of an impact on the final power numbers than the guy grinding on the exhaust ports of the heads sometimes. I believe header design and exhaust port design should be integrated together, not independent of each other.

My standing at the flow bench pre test is this.
If my flow numbers at .400" lift and up do show atleast a 3% gain when I slap on a 45 degree bent tube , then my tube size is too big.

The dyno, then ultimately the race track performance, dictate the ultimate header design. Using this experience helps to design the exhaust port and header to compliment each other. Often times the power curve that makes a car fast on the race track is more broad and doesn't have the impressive peaks that most people seem to think they want.It's really easy to end up with too large of a pipe. You basically need to have a crystal ball and know how the engine is going to perform in advance to be able to get the ports spot on. Flow bench info seems to be less helpfull on the exhaust side. NOT useless though!

With a duel pattern Cam and compressions above 11.5 even a 68% Exh to Intake ratio as shown in flow bench testing will work well many times!

BILL-C wrote:The dyno, then ultimately the race track performance, dictate the ultimate header design. Using this experience helps to design the exhaust port and header to compliment each other. Often times the power curve that makes a car fast on the race track is more broad and doesn't have the impressive peaks that most people seem to think they want.It's really easy to end up with too large of a pipe. You basically need to have a crystal ball and know how the engine is going to perform in advance to be able to get the ports spot on. Flow bench info seems to be less helpfull on the exhaust side. NOT useless though!

"Exhausted" has reported extensively here on grappling with ports that have been oversized chasing airflow when the engine really needs a much smaller area one to match the pipe diameter for optimum tuning.

MadBill wrote:

BILL-C wrote:The dyno, then ultimately the race track performance, dictate the ultimate header design. Using this experience helps to design the exhaust port and header to compliment each other. Often times the power curve that makes a car fast on the race track is more broad and doesn't have the impressive peaks that most people seem to think they want.It's really easy to end up with too large of a pipe. You basically need to have a crystal ball and know how the engine is going to perform in advance to be able to get the ports spot on. Flow bench info seems to be less helpfull on the exhaust side. NOT useless though!

"Exhausted" has reported extensively here on grappling with ports that have been oversized chasing airflow when the engine really needs a much smaller area one to match the pipe diameter for optimum tuning.

So is a flow chasing 53cc 1.6" valved exhaust that flows 170 cfm oversized for a relatively high rpm 289? Flows about 190 cfm with a 1 5/8" pipe. Thanks, Charlie

Ideally I think one would determine the primary pipe size via experience/dyno and/or PipeMax or other good software program, translate its ID into area and use that as a target maximum for the port while you develop its flow. I recall Calvin saying he sometimes had to stretch the flange end of the correct tube as much as 20% to cover an oversize port opening.

Update:
I found a graph by David Vizard in his How to Build Horsepower book; it also can be seen about 1/4 of the way through his Max Small Block book as seen here: http://www.chevydiy.com/chevy-small-blo ... t-systems/

The chart suggests pipe size vs. exhaust flow. For a street/strip engine with 170 CFM flow, a pipe ID ~ 1.60" is shown.

MadBill wrote:Update:
I found a graph by David Vizard in his How to Build Horsepower book; it also can be seen about 1/4 of the way through his Max Small Block book as seen here: http://www.chevydiy.com/chevy-small-blo ... t-systems/

The chart suggests pipe size vs. exhaust flow. For a street/strip engine with 170 CFM flow, a pipe ID ~ 1.60" is shown.

that methodology for choosing pipe size would perpetuate the issues of an oversized port. i think pipe size has hardly anything to do with port flow on a flow bench.

I'm not advocating that approach, merely providing a numerical answer to your flow-based pipe size question: "So is a flow chasing 53cc 1.6" valved exhaust that flows 170 cfm oversized for a relatively high rpm 289? Flows about 190 cfm with a 1 5/8" pipe. Thanks, Charlie"

As per my previous response, I believe pipe size comes first. Theories on the subject have certainly evolved over the years. The factory race system (developed in the late forties/early fifties) for the Vincent 61 c.i. V-twins I raced in the sixties was a pair of 2" x 54" pipes and a number of 30.5" singles in the pre-war era ran dual pipes of ~ 1.5".