Velocity vs Volume in the intake

[Charlie Cheap]

I have a 1973 Ford 200" six to replace the same size engine in my 1965 Mustang. The .060" overbore in the original engine is too much making the walls too thin. It works but hard starting when hot, so I got a newer 200. The intake is cast with the head so the intake manifold can be a problem.

Some machine the manifold for a 2-barrel which works fine but costs about $500.00 to $800.00 not counting the valve job. I am not looking for maximum HP but Torque. My cam is designed for Torque in the 1500 to 3000 rpm range which is perfect for my purpose. The intake opening on the 73 is 1.75" while the 65 is 1.5" so I have a 1/4" bigger to start and the valves are the same as the 65.

Thinking "velocity" and not volume, it appears the Autolite 2100 carb at 287cfm should work well with the later head by only being adapted and not machined to fit direct on the intake. The carb worked fine adapted on my 65 with the 1.5 opening so the 1.75 should allow better flow into the manifold just using a bigger adaptor which is available.

The small valves may offer a venture effect into the combustion chamber, the Autolite has annular-atomization for better fuel vaporization, I built a cool-air intake for better combustion, along with a hotter ignition and better timing. As an old 1960's - 80's engine builder, is my thinking correct?

The slightly larger intake opening is ample for improvement of air/fuel flow, and the same size valves (smaller than later head) could help velocity into the chamber?

The older head is 1100 cfm and the 73 is 1240 cfm while the OEM 1-barrel was 1100 cfm and the 2-barrel I use is 287 cfm. This carb on my old engine got 24.5 mpg without the cool-air intake on a 2,275 mile trip running the air conditioner all the way, hauling 2 corn-fed adults. The carb adaptor mounts the 2-bbl to the 1-bbl manifold with a funnel style reducer.

Because the 73 manifold has a 1/4" larger opening and the volume is 1240 cfm as opposed to the 1100 cfm original, is my thinking in line with air-flow?

It has been decades since I needed this type of info and the guys I deal with hat to do machine work on classics.

 

[jack action]

The larger intake volume will raise slightly your low-torque rpm range. If the valve ports and camshaft stay relatively the same, you shouldn't see any change in top-end power.

Note also that the 73 combustion chamber is larger (62 cc vs 52 cc), which means your compression ratio will go down unless you do some modifications, thus losing power and increasing fuel consumption.

Just to make everything clear, the volumes are 1100 cc and 1240 cc (cubic centimeters) and your carburetor is rated by volumetric flow rate, 287 cfm (cubic feet per minute). They are not related at all.

What really counts are the area change and pipe length, independently (i.e. not the volume). To get a better understanding of the effects from the intake design, you can read this post. It is about exhaust design, but it is the same thing. Intake is even easier because the air temperature doesn't vary as much as the exhaust temperature.

 

[Charlie Cheap]

Thanks Jack, and I understand the difference between the carb flow-rate and the intake volume...I think...it has been a while. Also, the head will be milled to get the bigger head chambers down to about 55cc and the deck cut to lower the height from .019" to .014. Both engines have .019 decks. The 65 has 13cc dish pistons while the 73 has 7cc (some say 6.5) dish. Between that and the milling work I should be around the same compression as the 65. Also, the cam I will use closes the Intake valve at 57 degrees keeping the dynamic compression in a safe 7.5 - 7.8 range...if my math is correct. Static is about 8.7-1 which should be safe for regular driving on pump gas 89 octane. After my bad trip on bad gas I jumped up a notch to 89 octane and sometimes higher, with octane booster in the trunk just-in-case.I will definitely triple check that. The real reason for my .060" overbore was too much compression and cheap Mom & Pop gas station gas...maybe 85 octane. Number 6 ring broke and scored the wall.

 

[Charlie Cheap]

I took your advice and read the post you posted...that sounds grammatically wrong but I think I am correct. Kinda like this volume vs velocity question I asked. I run the OEM cast exhaust manifold to keep heat under the hood down, plus I ain't going racing. We built scavenger headers back in my racing days and my brain is carved with 18436572 for SB Chevy firing order. Getting those pipes to fire in a clockwise pattern in the collector was a lot of fun for us junk-yard racers. When an engineer type told us that gurgle...his term not ours...we heard in the carb was the exhaust sound wave barking back through the intake valves. Our reaction was, SURE IT IS, as if we had any idea what it was. When he walked off smoking his pipe (I believe an engineering grad requirement in the 70's) we looked at each other and made guesses at what he was smoking. The consensus was Maui-Whowi or something close that was not purchased at the tobacco store. After reading your post, I think we owe that fella an apology. I did read about sound waves working FOR or AGAINST an engines performance when I got serious into motorcycles engines in the 1980's. I kinda understood what the discussion was about, but fell asleep with a headache. THANKS for your tech information.

 

[Ranger Mike]

Ahh the 60s! As you can see the flat top pistons of 1965 were replaced with deep dish pistons that dropped the CR biggly!
hope this helps

i had to go back and get the cam lift specs..odd that 1973 had slight increase in the bump but lower duration to meet EPA specs. plus unleaded crap Gasoline back then

rm

 

[Charlie Cheap]

The 65 six piston had 6.5cc dish while the 73 had 13cc with a slightly different cam, though I have seen those cc numbers reversed... hello INTERWEB INFO! The head chambers are 51cc for the 65 and 62cc for the 73. With those numbers the 1973 compression is about what old tractors had when they could be hand-cranked. The pistons in the picture are V8 pistons, while mine are dished in a circle with no valve relief cuts. Info on-line SUCKS and is wrong about half the time. You guys seem to be accurate-freaks...in a nice way...and truly seem to want to help. Because I answer questions about the early Mustang 6-cylinder on-line, to guys who are not long-time Hot Rodders, I need facts they can trust. There comes a time when some ask where I got my info, and can they trust it? This page gives tech answers with references I can quote and/or pass on for them to check personally. I have tech references from the 1960's thru 1990's and it is good, but newer is better with the changes to the internal combustion engine seen in the last several years. A double OHC cross-flow head with 2100 carb on a nice (best length runners for MPG), hot non-computer ignition, tuned exhaust and tied to a 5-speed or non-computer controlled overdrive automatic, could get well into the 30's MPG. Match the right rear gears and tires with a lock-up converter...it may sound wild, but high 30's could be possible with no computer. It would run clean also. The Mustang 6-cylinder is rare for the 65-66 year compared to the V8...roughly 0ne-in-four. So, if a guy messes up the machine work, a new motor is hard to find, while 260, 289, and 302's can be purchased at the local 7-11. Okay, I exaggerate, but you get the idea. I hope I am not taking up too much of your time on this subject, but I learned years ago, if I do it on paper first, things seem to work better in reality.

 

[Charlie Cheap]

Oopps, insert INTAKE after the MPG carb comment above.