might be a good topic???

Depends on what it will be used for. Port velocity and flow is directly related to where an engine makes power...or doesn't. A generalization would be to say that the best port is the smallest one that will flow what it needs to without going into sonic choke.

The Ford Boss 302 head is an example of the port design norm of the late '60s. They flowed well for the time, but are big - too big, which makes them sluggish. They are matched in flow and outperformed these days by much smaller inline-valve AFR 185s.
 

sCREamnClones

New member
Look at the present day NASCAR heads D-shaped ports Drag are turning to oval for the most-part
Agreed....however, the trend toward 'Ovals' in Drag Racing is more 'throtte-stop' related. We've been using oval ports for 'Super-category' for a long time but are using Big-ports in T/D & T/S.
 

mikey56

New member
dad used to say that porting a head is like a garden hose and a straw...get a mouth full of water and blow it thru the garden hose, might get a trickle...take that same mouth full of water and blow it thru a straw....your gonna get someone real wet....too big and you kill your power....keep it just big enough to get good flow and in relationship to the other things your doing to your motor.....

I'm learning!!!
 

subbietjw

Member
me I like a larger port... I always think of the 2 stroke dirt bike motors... most will have a large port... when you start to look at the way there made the higher you get in rpm the more the ports open up allowing the air to flow... 2stroke and 4stroke are a lot different!!! but I think that there is some good small engine info hiding that could carry over..

now lets change it up a bit... how do you like to do your exhaust ports???
 

Yakattack

Member
And food for thought... Just got done tinkering with our homemade flow bench...
Head1...stock Honda 14cc... Exhaust out flowed intake...head flowed the least amount at all lift .05-.350
Head2 stock jt109... Intake out flowed exhaust, head flowed ok on both in and ex about 10-15% better than head1
Head3 stock gx200 exhaust out flowed intake, same flow % as JT109 except ex and in, reversed
Head4 TG1 intake out flowed exhaust till .250 lift then exhaust out flowed intake flowed 10-15% better than head2,and 3
Head5 P&P unshrouded honda14cc head intake out flowed exhaust, intake out flowed head4, exhaust flowed same as head4

Next is flowing a 14cc head with a 27/25 valve combo, then a 28/25 combo
Also might P&P the TG1 and see what we find.
 
I agree with the above posts, the smallest that flows what is needed. A good flowing small port will out run a bigger port flowing the same amount, in my experience.
 

W5R

New member
when porting a head, the bigger you make the port, the more you sacrafice low end power. If you take too much off the ports, you kill the low end power of the engine, which is the power you need most with the clone engines unless you are running outlaw classes or something like that. There is a delicate balance to how much to port a head and improve the flow. Some heads will require more port work than others. That said, to answer the OP question i like a smaller port because i love the low end power of the clones, they are great engines for the torque they produce when you have them set up right with the right mods. Just my opinions of course.

I have one engine that i was just playing around with on a yard kart years ago, i went thru several heads learning to port and polish heads, porting can be a nice improvement when done right. It takes alot of practice to get to where you can do it smooth and perfectly every time without ruining a good head.
 
To make matters worse, flowbenches don't replicate the airflow conditions of a running engine. Computational fluid dynamics (CFD) isn't yet to the point of being able to simulate pulsed flow of a compressible fluid in a complex port. Until it can without taking weeks or months of computation on a mainframe, all we can do is keep trying to find a predictable correlation between what a flowbench says and how an engine performs.
 
Top