Robin-Subaru... better airflow

[graucho]

In my opinion/experience I found that the small intake on the stock RS35 can suck all of the air it needs the way it is. It's the "rear end" LOL, thats restricting of flow. Currently I have 2 RS35's and both are running a lot sweeter by "slightly" grinding the exhaust port. I didn't have time so I had it done for me by a local shop. I think you can mess with the intake all you want but it's not going to do much good if your restricting the burnt air/fuel out of the exhaust port. Its like running your furnace with 6" ductwork when you should have 8". Removing the furnace filter isn't going to make the furnace run less sluggish. Just my 2cents.

 

[give me vtec]

I re-read my last post and it sounded a little harsh. I didn't mean it to be. I guess my point plain and simple is any air flow on the intake side must be equally met at the exhaust side. Especially on a head this small, biggest benefit on the exhaust port.

As a kid growing up pops was a small engine wizard. If someone said their motor was having a breathing problem he instantly went to the exhaust side, slightly ported the exhaust and 9 times out of 10 gave it back to them with a smile. I guess i'm just passing on one of my family's flow solutions.

not harsh at all... what you said was absolutely correct.

 

[moped-dan]

graucho, I think you bring up a good point.

I ended up drilling 4 holes in the front of the air filter box, I really do believe it makes a difference. I went for a nice ride yesterday and it felt more willing to go up hills without slowing down and didnt slow down as much against a headwind.

I didnt really think about the exhaust side of things, I wonder if I could increase flow in that direction. I was thinking of going for a ride with the exhaust how it is now, then remove the muffler and try it again and compare the runs. Is it the muffler that would be restricting flow or the exhaust port itself?

 

[Old Bob]

You guys need to bone up on head design and porting. I've never seen so much misinformation in my life.

Look up David Vizard and read his book on head porting, before you mislead people into ruining their engines with half assed theory and conjecture.
Read this book or keep your mouth shut.

The Theory and Practice of Cylinder Head Modification
Author: David Vizard

 

[Happy Valley]

You guys need to bone up on head design and porting. I've never seen so much misinformation in my life.

Look up David Vizard and read his book on head porting, before you mislead people into ruining their engines with half assed theory and conjecture.
Read this book or keep your mouth shut.

The Theory and Practice of Cylinder Head Modification
Author: David Vizard

Well Bob, I've noticed a couple things about your contributions here, while you have displayed you have working knowledge and experience with small utility engines and have offered what I consider to be good information, you seem to awfully defensive and particularly disdainful of anyone who doesn't.

What is the purpose of a site like this?

Maybe you could offer a bit of synopsis of Vizard's work to those less informed?

 

[GearNut]

That's a lot of $$$ on a book. I hope it's good....
http://www.amazon.com/Theory-Practi...1_fkmr0_3?ie=UTF8&qid=1274315841&sr=8-3-fkmr0

 

[Old Bob]

Yes it is not cheap. Books on this subject are not cheap.

I can attempt to share the relevent information.It gets tedious at times and really needs some study in other areas of intake and exhaust tuning and camshafts to get the full picture.
There is only so much that can be done with a given displacement and rpm range.You won't make a monster out of one of these engines, but can improve it.
One of the first things to grasp is torque and horsepower relationship.
HP is a function of torque, without torque there is no hp.The primary objective of engine modifying is to improve torque in a given rpm range.

I'll dig up some info from my book.

Just for kicks, anyone know why intake valves are bigger than exhaust valves?

 

[GearNut]

Yeah, there is more mass to the intake charge, less mass to the exhaust charge since some of the mass got burned up during the combustion process.
I have been involved in building Harley engines for 17 years. The ones I built were not rip snorting asphalt burners mind you, just good reliable daily drivers. I am not a fan of short life HiPo engines, My mentor was all over that scene though.

 

[graucho]

Just for kicks, anyone know why intake valves are bigger than exhaust valves?

First off....
Who in the heck are you to tell anyone to shut up on a public forum? No one gave advise to do any porting themselves. No one explained what tools to use or how to do it. The bicycle movement will be slow growing by those who shut down a thread by making others afraid to post because you belittle them. This is a teaching place with all levels of experience. Anyone can have input here. I've seen thousands of threads with misinformation, with people finally chiming in with good bits of advise without telling others to keep their mouth shut.

Secondly to answer your exhaust valves question,

The flow over the valves (or any port):
m_dot = CD*A*rho_0*sqrt(R*T_0)*(P_r)^(1/y)*(2y/(y-1)*(1-P_r^(y/(y-1)))

where
CD = discharge coefficient
A = area of the port
rho_0 = gas density before the port
T_0 = temperature before the port
P_r = pressure ratio over the port
y = cp/cv

It's the pressure diference over the valves that drives the flow, and the pressure difference is greater on the exhaust side. Hence... allowing the use of a smaller valve.
Unless your burning nitrous. I know with nitrous you want a slightly bigger exh valve than just a regular NA motor. Different pressure ratios.

 

[Old Bob]

The reason the intake valve is bigger is simply because there is only 14.7 psi of pressure to push a/f mixture into cylinder while the cylinder pressure is typically 50-75 psi when the exhaust valve opens.