Section : Car Preparation

Cylnder Head modification - theoretical stuff 

Posted By : Jason Chinn, 28-Feb-2004, 11:39am

Just thinking aloud here - everyone knows that if you want to improve the flow of air and fuel into an engine you can improve the route the intake charge has to flow through by "porting" the cylinder head - ie smoothing out or enlarging the ports and taking out anything that obstructs the flow, like reducing the effects of the valve guides sticking into the path of the intake charge. In professional engineering shops this development work is done on a "flow bench" - I've never seen one but I assume it's some mega bucks piece of kit with electrickery and sensors and a bloke in a white coat with a clipboard. It's measuring flow and giving readings in volume of air per second etc but if all I want to do is improve flow - ie speed up the flow of stuff in and stuff out can I just use a stop watch and a bucket of water - ie pour a measured volume of liquid into a hopper, start it flowing and time how long it takes to go through. Then I can do my improvements and re-test. That will give me an idea of any improvement I've made against say a standard reference piece.
Is this viable? Are the flow rates going to vary by so little and the timing so inaccurate that I will lose any improvement in the margin of error introduced by the lack of electrickery and boffin in white coat?
Or is such a DIY flow bench accepted practice and I've just come up with an idea that every one has known about since 5 mins after the wheel was invented?
Also, gven the art of grinding away bits of metal to create a "ported" head, how do the professionals make them all the same - ie what quality control do they have? Do they grind then flow bench it and ensure it reaches a standard or do they just grind it until it looks right, paint it up and flog it?
Just the ramblings of a bloke who should be out in the garage but is too soft to do so in the freezing cold

• Cylnder Head modification - theoretical stuff 

  Posted By : Jon Wolfe, 28-Feb-2004, 08:36pm

  I think you have brought up an interesting point here - about repeatability of cylinder head porting. Without a flow bench (which uses a manometer to measure air pressure at various points - the higher the flow rate the lower the pressure) it is impossible to gauge whether your carefully 'matched' ports are the same. Experience counts for a lot but that takes time and there are some bloody awful heads out there. Secondly flow rate is not the be-all and end-all and some turbulence needs to be introduced to get a good air/fuel mix. For example, shaving the valve guides off in line with the port wall usually has a nett bad effect - flow may be better but the air/fuel mix is compromised. To make matters more complicated very few engines have similar manifold tracts - a detail which too many engine builders neglect - you should never port a head without measuring the combined flow through the full induction tract.
  
  The trouble with using water (actually not a daft or new idea) is that it flows too fast. You can slow it down by submerging the whole head in water with the port face level with the surface. The back pressure slows the water giving you more time to measure the drop of water level from your header tube. Another trick is to use gear oil which is very thick, but very messy.

  • Cylnder Head modification - theoretical stuff 

    Posted By : Jason Chinn, 29-Feb-2004, 10:02am

    I've often thought about the inlet/outlet tract being matched to the head. I came to the conclusion that if you're going to do it right you would need to ensure the whole path of the fuel / air charge going in and the exhaust coming out had the most unrestriced path poossible. That means matching carb to inlet manifold and inlet manifold to head - dowling and smoothing, trimming any gaskets etc. I'm not so sure it's that critical with the exhaust manifold but that too should benefit from a smooth flow out. So, buying a head, some manifolds and carbs and bolting them all together is one thing but actually matching them to each other and "fitting" them together is another - just how much benefit is there in this labour intensive work?
    
    Some searching on the internet has revealed lots of plans for flow benches - some insanely complicated with aircraft instrumentation - some pretty basic. Something for further research methinks

    • Cylnder Head modification - theoretical stuff 

      Posted By : Nigel Gibbins, 29-Feb-2004, 07:39pm

      Jason,
      
      A simple flow bench can be made up from an old vacuum cleaner and a carburettor balancer gauge thingy. You just need to seal it to the port and switch it on, take a reading, grind a bit and repeat.
      
      Trial and error will produce the results... It's generally accepted that if you are going to start porting your own heads you should try a few scrap ones before attmepting the real thing.

      • Cylnder Head modification - theoretical stuff 

        Posted By : Jason Chinn, 29-Feb-2004, 08:32pm

        In my youth I strayed to the dark side and started messing with F*rds, a little fast road 1700 Fiesta - I had 3 heads in the boot of the car when I got it -I totally wrecked the first one practicing, made the most opened out and free flowing head I could with the second one - it was crap and couldn't hold a gasket down. Head number three was good - the other two kept the garage doors open on a windy day for years afterwards, long after the car had expired!
        Those were the days, electric drill with a flexi on the end of it could never find any cutters that lasted very long as I recall - B&Q specials were the order of the day - oh to be young and naive again!
        
        Now old and stupid