I went to a shop that specializes in porting motorcyle heads for drag aplications the other day (sorry, keeping the name/location to my damn self! ) ... I had been in touch with them over the last couple of months to discuss head preperation for a street/strip H22 motor. After looking a stock H series head over he asked me if anyone had ever played around with shrinking the exhaust ports to a more circular shape... I couldnt think of anyone who had done anyhting like that for the Honda scene other than Honda (K series). He was telling me by raising the port floor and adding material to the sides of the port you can create a straighter more efficient exit for the exhaust... I was just curious if anyone has had any experience with shrinking exhaust ports?

He then went in to how improving flow on the intake side has to be met with improved efficiancy on the exuast side... There are a lot of porters out there who spend conciderable amounts of time on the intake side of a head (square port/ taper porting, etc, etc) and dont do a lot to the exhaust side.

I can understand how exhaust port madification can be an improvement on turbo motors but Im not to clear on how it would affect a NA aplication...

COuld this be something the honda world hasnt quite stumbled upon yet? Or is this something that may just be a waste of time???

Opinions wanted...

 

yeah it would be important to have just as free flowing of an exhaust side as an intake side, if you think about it, if the engine is taking in more air faster and more efficiently, then it has to get rid of that air faster and more efficiently whether it is NA or FI. Thats cool that your trying new stuff tho, good luck man.

 

Yup, there is lots to gain on the EX. side, it should be high ported, rounded, and all 4 ports have a diff. shape and roof, but you already knew that right?

 

The other nite I was hanging out with some Suziki biker freaks at their shop. I noticed that the older GXRers had different EX ports than the newer ones. This was also the same deal with some other heads they had lying around, had different shapes and work done to them.

I wish I would have ask some questions since I wasn't really sober to begin with,lol

 

this is nothing new to the honda world. good head porters will do this on a race head, but its not really worth the time/effort for a street/strip head.
you really dont want to take much out of the port itself, cept to round it out a little more. right behind the valve seat, it should open up just a little more, than taper back down to cause reversion, which will increase exhaust flow velocity, and scavenging. but if you want to get trick, you put a bump on the intake cam right when the exhaust cam start to open the valves this way you get even better scavenging, increasing velocity even further due to zero vacuum in the cylinder.

 

What like this??



Modified by fkned at 2:04 AM 7/18/2004

 

What is the best method for adding material to the head (specificly the exhaust ports)?

Is there some type of liquid aluminum you can use to add to an exhaust port? If so, any specific brand/product that you recomend?

 

Welding is the only thing I'd trust

 

bigger is not always better....if u know a lil bout physics u would have a lil better understanding of it....but yea you would have to TIG weld that....

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by RABHonda &raquo;</TD></TR><TR><TD CLASS="quote">it should open up just a little more, than taper back down to cause reversion, which will increase exhaust flow velocity, and scavenging.</TD></TR></TABLE>

So reversion will increase velocity and help scavenging....

Ok buddy

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Tbone &raquo;</TD></TR><TR><TD CLASS="quote">

So reversion will increase velocity and help scavenging....

Ok buddy </TD></TR></TABLE>

If you take into consideration such designs as the Top Fuel/J's racing intake I can see how he may be somewhere close but I dont fully agree...

I think that with an exhaust port where fluid is pushed out as apposed to sucked in that the (small, taper out, taper in) may ubstruct flow... Im not basing this on any sufficient testing or data, just on the basic understandings of flow...

If you were to start with the stock port dimension into a slight tapred shrunken exit or even a constant diameter smaller port you will have a better chance of increasing exhaust flow over a somewhat "reverse hour glass" port...

Now if we were talking about in intake system I think he would have struck gold with his comment... talking exhaust ports though...


Modified by Rick Solis at 7:10 PM 7/19/2004

 

I dont think whats happening with the hourglass shape is classified as "reversion"

 

this sounds like what Hytech and Jim Fuelling call an anti-reversion chamber, like the ones in John's headers. see a pic (someone has one fs), but an hourglass shape, while a bit of a restriction, provides a Venturi through which to accelerate compressable gases, and the bottom of the hourglass helps to ensure that when gases pass through the bottleneck, once they expand, they don't back-up on the intake stroke at intake/exhaust cam overlap, provided that a sharp edge is properly placed. i'll try to draw it:

l l
\ /
/l l\
l l

****, my use of characters to draw won't turn out right, but maybe someone gets the idea and will better illustrate.

 

the thread for some reason reminded me of endyn's exhaust manifold gasket for those that never saw it. hte gasket larger then the port it helps with anti reversion.



o and here is his modified header flange.


o and like slofu is saying on the bottom of the hytech in this picture u can see the anti reversion chamber. but look at the top of the flange hmm.

 

The gasket doesn't help prevent reversion, it was just opened to stay out of the way of the larger exhaust ports.

The header ports were enlarged to prevent reversion. By being slightly larger than the exhaust ports, there is a lip for exhaust to hit in the event it started to reverse.

 

Back in the day I was always told not to let any machine shop touch your head unless they have worked on VTEC heads before.
Cause bascially vtec heads are different than all other heads and your better off having someone port the head who has worked on vtec heads for a while
just my .02 cents

-Brian

 

Well I still think what he wrote is wrong, reversion has nothing to do with the effect he was explaining..
Port entry and exit will be high pressure and low velocity , when you narrow it in between it accelerates the velocity and lowers the pressure. What does that have to do with reversion? Thats venturi.

 

Your absolutely correct about reversion and venturi. Venturi effect increases flow velocity (decreases pressure) not reversion. You want to prevent reversion to keep flow in the right direction.

Here is a nice visualiztion tool for venturi flow I found. Helps visualize the pressure relationship which is counter intuitive.

http://www.ce.utexas.edu/prof/....html

A properly ported head for max power should have exhaust flow that is 80% that of the intake flow. ie: Similar ratio to the stock head. 33mm vs 28mm valves gives a 72% based solely on valve area.