Gas-flowing a cylinder head
Discussion
How do you do this then?
I understand the basics; smoother, more rounded surfaces lead to free-flowing air/fuel mixture.
Engine breathes better, more power.
Wonderful. The question is, what tools/expertise do you need to actually do it?
Is it something a keen amateur can do on a trial-and-error basis?
I've been googling and (as usual) all I get is adverts for people that'll do it for me...
I understand the basics; smoother, more rounded surfaces lead to free-flowing air/fuel mixture.
Engine breathes better, more power.
Wonderful. The question is, what tools/expertise do you need to actually do it?
Is it something a keen amateur can do on a trial-and-error basis?
I've been googling and (as usual) all I get is adverts for people that'll do it for me...
You need a set of high-speed grinding stones and an air tool to spin them, a flow bench to measure your results... and an excellent appreciation of the sometimes non-intuitive physics of gas flow. If you go beyond the obvious stuff like cleaning off casting flash and such, it's just as easy to make things worse... so you also need a large supply of cylinder heads to practise on.
Furthermore, since modern engines are designed with the benefit of CFD software and cast using more precise techniques, there's not a lot you can usually do to make things better...
Furthermore, since modern engines are designed with the benefit of CFD software and cast using more precise techniques, there's not a lot you can usually do to make things better...
I supply of heads to practice on won't be a problem, the specialist stuff will.
I was primarily thinking about removing flashes/seams/manufacturing defects. I don't want to port it/reshape the valves or anything. Would there be any significant improvement?
I'm thinking of an engine designed in the late 80s, just for the interest really.
I did two CFD modules as part of my Uni degree, focussed larely on flow optimisation within the models. Will that cover the 'knowledge of gas flow' issue?
I was primarily thinking about removing flashes/seams/manufacturing defects. I don't want to port it/reshape the valves or anything. Would there be any significant improvement?
I'm thinking of an engine designed in the late 80s, just for the interest really.
I did two CFD modules as part of my Uni degree, focussed larely on flow optimisation within the models. Will that cover the 'knowledge of gas flow' issue?
Make sure to leave the inside surface of the heads on the rough side if doing them for anything that isn't going to be port or direct injection. If the surface is left too smooth for a carbureted head for example, the air/fuel mixture can fall out of suspension and pool in the head causing poor running and lean mixtures. Taking off casting marks, etc, will certainly help but not near as much as porting and shaping will. Port shape and size is critical for maximum air flow.
A common mistake is to not consider what is actually going on and opening out everything massively.
If you imagine hacking a port in half lengthways, you'll see that the key restriction is around the valve guide boss, that is the area which needs to be modded generally, not the bit of the port which flows well. A lot of it is intuitive like putting radii on big corners. Ideally you want the gas flow to be as circular in cross section as possible, any corners being "D" with the straight edge on the maximum flow side. That kind of logic. Another point to remember is 'zorst logic, putting huge tubes to your engine with little carbs will cause a reduction in gas speed and make the engine less tractable at low speeds, if the rest of the gubbins can't deal with massive flows/revs, big openings out are a bad thing.
Personally, I'd stay away from die grinders as they are too chubby and get a modern big dremel style thing with a flexible shaft. A good £100 should see you with a bombproof one. You need lots of patience and a steady hand, plus knowing what you are going to achieve, when you de-shroud the valves, how are you going to get your compression ratio back up?
Going back to key restrictions, before the port "elbow" the biggest restriction is the valve, how do you modify this? Does it have 3 angle seats? Are the valves big enough? How would bigger valves influence gas speed and how would that come into play with your cam and ports? I reckon bigger valves are a good thing. Going from a 29mm exhaust valve to a 32mm gives you 10% greater area for flow at any lift. That is a considerable increment, if you go from 1.3 to 1.5 rockers, that is worth about 15% extra area for flow. The thing with engines is that all the variables are dependent on each other. You have to have the lot in your mind and see how a small tweak would distort the big picture.
First step is to get "Theory and Practice of cylinder head modification" by David Vizard and read it at least 10 times before you pick up a grinder!
Another misconception I think is the current thinking on port finish, polish is indicative of good work. In my opinion, a matt surface does not keep fuel suspended more than a shiny surface. Heads have massive snots of metal and sometimes quite a rough casting finish, especially old ones, you can sort this out with relative ease.
Anyway, enough from me.....Go for it.
If you imagine hacking a port in half lengthways, you'll see that the key restriction is around the valve guide boss, that is the area which needs to be modded generally, not the bit of the port which flows well. A lot of it is intuitive like putting radii on big corners. Ideally you want the gas flow to be as circular in cross section as possible, any corners being "D" with the straight edge on the maximum flow side. That kind of logic. Another point to remember is 'zorst logic, putting huge tubes to your engine with little carbs will cause a reduction in gas speed and make the engine less tractable at low speeds, if the rest of the gubbins can't deal with massive flows/revs, big openings out are a bad thing.
Personally, I'd stay away from die grinders as they are too chubby and get a modern big dremel style thing with a flexible shaft. A good £100 should see you with a bombproof one. You need lots of patience and a steady hand, plus knowing what you are going to achieve, when you de-shroud the valves, how are you going to get your compression ratio back up?
Going back to key restrictions, before the port "elbow" the biggest restriction is the valve, how do you modify this? Does it have 3 angle seats? Are the valves big enough? How would bigger valves influence gas speed and how would that come into play with your cam and ports? I reckon bigger valves are a good thing. Going from a 29mm exhaust valve to a 32mm gives you 10% greater area for flow at any lift. That is a considerable increment, if you go from 1.3 to 1.5 rockers, that is worth about 15% extra area for flow. The thing with engines is that all the variables are dependent on each other. You have to have the lot in your mind and see how a small tweak would distort the big picture.
First step is to get "Theory and Practice of cylinder head modification" by David Vizard and read it at least 10 times before you pick up a grinder!
Another misconception I think is the current thinking on port finish, polish is indicative of good work. In my opinion, a matt surface does not keep fuel suspended more than a shiny surface. Heads have massive snots of metal and sometimes quite a rough casting finish, especially old ones, you can sort this out with relative ease.
Anyway, enough from me.....Go for it.
andys2 said:
Try readig this months 'Practical Performance Car' magazine, there is a good article by David Vizard about the basics of gas flowing a cylinder head.
Seconded. Vizard is a tuning God and PPC are currently running a series of articles by him on the subject. Probably tell you everything you need to know.
grahambell said:
Try readig this months 'Practical Performance Car' magazine, there is a good article by David Vizard about the basics of gas flowing a cylinder head.
Seconded. Vizard is a tuning God and PPC are currently running a series of articles by him on the subject. Probably tell you everything you need to know.
Thirded, try to get last months article also.
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