porting a BMC A-series 12G202 (Mini, Sprite etc) & dyno
Posted: November 10th, 2006, 12:20 pm
Hi.
Guy asked to post some pictures of what I was working on in another topic. Rather than veering further off course I started a new topic. Well here goes.
This is my first time porting what is known as an a-series closed chamber head (I normally work on big bore heads for 1275cc engines) instead of 850cc/998cc/1098cc heads. Much of what I have done so far is described by David Vizard in "tuning the a-series engine" but there are quite a few points that are only described in outline.
The GC-VW features a very similar head btw.
This type of head was used on the cooper 997 and 1098 engines and has slightly bigger valves (still tiny) and a 'squarish' intake runner for the siamesed intake ports. It will get a slightly oversize plasma treated stainless intake valve (same price as the stock ones).
Valve job and bowl:
20/45/70 with a 1.6mm seat which is a tad on the wide side but I prefer seat life and a bit more flow at low valve lift to outright flow.
Guided by many people in the know (Guy and many others) I try to keep the seat angles as discrete and sharp as possible. Which is easier said that done.
Valve back cut to contact face at 30 degrees.
Bowl is blended to the back cut at the long side. SSR goes up pretty much straight with only a fraction of the down cut remaining.
The port is biased (it leans in a bit) toward the cylinder center and I try to keep the sides pretty much flat
The chamber
The closed chamber head has a very deep and distinctive shape to the chamber.
This presents a few problems:
The shape causes quite a bit of shrouding of the valve one if temped to grind away generously to provide ample breathing space.
However, chamber volume is rather large and thus you need to be careful not to cut huge amounts away.
If you cut too much you will have trouble getting the CR up to a useful (in this case 1:10.1) as you will have run out of deck thickness for skimming the head.
As a result the objective is to deshroud the valves as much as possible by cutting away the least amount of metal.
The port
Port runner:
Just cleaning it up and grinding all the surfaces straight. I use a straightedge to check if there are still undulations.
Exhaust:
According to flow bench result published by Vizard the guide boss is a major obstacle to flow from about 0.200 inch lift. I have stared at the port shape (luckily Vizard published cut away images of the ports) for quite a bit and I will try something a bit different.
The casting is quite thick and this allow for some reshaping. My goals is to reshape the guide boss and SSR (which is rather chunky) and the rest of the port into a Laval nozzle shape. I know of one porter who works with this shape but funnily enough he does not want to tell me exactly what he is doing. But I suspect that the idea is to get the exhaust as efficient as possible and use more overlap in the cam but not too much duration..
First thing that needs to be done is to cut away metal in a way that will turn the large and horribly shaped sort side into something that will actually allow air to pass. This entails cutting quite a bit of metal trying to widen the SSR and provide a large as possible radius. Idea being to get a more active SSR with more volume to allow the air to slow down and negotiate the 90 turn at lower valve lift.
Secondly I aim trying to change the discharge pattern from the port and seat to something that is more aimed toward the centre of the cylinder instead of pretty much straight through into the combustion chamber wall.
As I have limited testing equipment at this point I have to rely on some very unscientific methods.
I use the dust from the grinding as tell tales where the flow is going. While very crude, it does tell you that a valve is discharging around the entire or only a portion of its perimeter at a certain valve lift.
By reshaping the SSR, throat and valve boss the pattern of dust when blown from the port has improved from nearly everything in a narrow jet along a small portion the long side to a much more even pattern indicating a more active port with less "dead air".
So far I¢ž¢m a bit disappointed of the air activity at the SSR at low lifts. Even after major reshaping. As this is with a roughly cut seat it might not tell me anything.
Joost
Guy asked to post some pictures of what I was working on in another topic. Rather than veering further off course I started a new topic. Well here goes.
This is my first time porting what is known as an a-series closed chamber head (I normally work on big bore heads for 1275cc engines) instead of 850cc/998cc/1098cc heads. Much of what I have done so far is described by David Vizard in "tuning the a-series engine" but there are quite a few points that are only described in outline.
The GC-VW features a very similar head btw.
This type of head was used on the cooper 997 and 1098 engines and has slightly bigger valves (still tiny) and a 'squarish' intake runner for the siamesed intake ports. It will get a slightly oversize plasma treated stainless intake valve (same price as the stock ones).
Valve job and bowl:
20/45/70 with a 1.6mm seat which is a tad on the wide side but I prefer seat life and a bit more flow at low valve lift to outright flow.
Guided by many people in the know (Guy and many others) I try to keep the seat angles as discrete and sharp as possible. Which is easier said that done.
Valve back cut to contact face at 30 degrees.
Bowl is blended to the back cut at the long side. SSR goes up pretty much straight with only a fraction of the down cut remaining.
The port is biased (it leans in a bit) toward the cylinder center and I try to keep the sides pretty much flat
The chamber
The closed chamber head has a very deep and distinctive shape to the chamber.
This presents a few problems:
The shape causes quite a bit of shrouding of the valve one if temped to grind away generously to provide ample breathing space.
However, chamber volume is rather large and thus you need to be careful not to cut huge amounts away.
If you cut too much you will have trouble getting the CR up to a useful (in this case 1:10.1) as you will have run out of deck thickness for skimming the head.
As a result the objective is to deshroud the valves as much as possible by cutting away the least amount of metal.
The port
Port runner:
Just cleaning it up and grinding all the surfaces straight. I use a straightedge to check if there are still undulations.
Exhaust:
According to flow bench result published by Vizard the guide boss is a major obstacle to flow from about 0.200 inch lift. I have stared at the port shape (luckily Vizard published cut away images of the ports) for quite a bit and I will try something a bit different.
The casting is quite thick and this allow for some reshaping. My goals is to reshape the guide boss and SSR (which is rather chunky) and the rest of the port into a Laval nozzle shape. I know of one porter who works with this shape but funnily enough he does not want to tell me exactly what he is doing. But I suspect that the idea is to get the exhaust as efficient as possible and use more overlap in the cam but not too much duration..
First thing that needs to be done is to cut away metal in a way that will turn the large and horribly shaped sort side into something that will actually allow air to pass. This entails cutting quite a bit of metal trying to widen the SSR and provide a large as possible radius. Idea being to get a more active SSR with more volume to allow the air to slow down and negotiate the 90 turn at lower valve lift.
Secondly I aim trying to change the discharge pattern from the port and seat to something that is more aimed toward the centre of the cylinder instead of pretty much straight through into the combustion chamber wall.
As I have limited testing equipment at this point I have to rely on some very unscientific methods.
I use the dust from the grinding as tell tales where the flow is going. While very crude, it does tell you that a valve is discharging around the entire or only a portion of its perimeter at a certain valve lift.
By reshaping the SSR, throat and valve boss the pattern of dust when blown from the port has improved from nearly everything in a narrow jet along a small portion the long side to a much more even pattern indicating a more active port with less "dead air".
So far I¢ž¢m a bit disappointed of the air activity at the SSR at low lifts. Even after major reshaping. As this is with a roughly cut seat it might not tell me anything.
Joost