MC
16-04-04, 02:34 PM
I?ve lost track of the number of times that the question pops up about cylinder head choice and compatibility. I?ll give a brief summary of the different cylinder heads, their compatibility and the best choices.
To start off with, I?ll group the cylinder heads into type, and then look at the individuals. The heads can be broken down into three categories, carbed, multi point injection and single point injection. Sounds simple, but we?ll group the 1.4SR in with the single point injection group. To clear one thing up, all 8v Nova engines have the same exhaust ports, so any exhaust manifold will fit to it, you?ll just need to make sure they join well to the rest of the system.
The Carbed group, including 1.2 and 1.3SR have a large cathedral shaped port. The flow is direct and there is minimal restriction. The identical port shape means that the inlet manifold, and therefore carb from the 1300 can fit on the 1200 cylinder head.
The spi group including the 1.4SR have the worst design of cylinder head. The ports are small, square in shape, and twist down to the valve. There is also minimal room for improvement due to the close proximity of the water jacket. Once again the identical port shape means that the manifold and carb/body are interchangeable between the 1.2i, 1.4i and 1.4SR. It is beneficial to fit the body from the 1.4i to the 1.2i and possibly the 1.4SR to the 1.4i.
The best of the cylinder head designs is the mpi head. It has large round ports that flow well and directly to the valve, and the valve itself is larger. Because of the larger valve, you?ll get the best flow from one of these heads. There is no point switching any of the gear between the 1600 and 1400 as they utilise the same stuff. The best of the intakes is the later design without an AFM.
The point of interchanging any of these parts is to gain bhp, and this is achieved in two ways, improving breathing, and increasing compression. The compression on these engines is easy to calculate. You are working out the ratio of the maximum volume of the cylinder to the amount the combustion mix is squeezed down to. As the cylinder returns to the surface of the block at TDC, the majority of squished mix is in the cylinder head. This enables you to work out what compression ratio you will get from changing heads. For example, simplistically, a 1600 at 10:1 ratio will squash the reaction mixture down to 160cc (in the head), and a 1400 at 10:1 will squash down to 140cc (in the head). Now if you put the 1.4 head on the 1.6 block, you?ll be squashing the 1600 capacity down to 140cc, which is a ratio of about 11.4:1, and more power, and probably some detonation, and the inability to stop the engine running??It works with the other engines too.
The other side to gaining more power, is the increased flow, and this will come from head changes and/or inlet changes. This is self explanatory, the less restriction the better (to an extent).
The final thing to consider is fuel atomisation. A complete mix of fuel and air will give the most power, so the last thing you want is fuel dropping out of suspension and not burning properly. This is where the mpi heads are superior to the cathedral ported carb head. The port on the carb head may be bigger, and you may think that with large valves, it would be better than the equivalent mpi head, but this isn?t true. The majority of and fastest airflow through the port happens at the top and back surfaces, entering at the far side of the valve. Airflow at the bottom of the port is much slower, and gives the fuel time to drop out of suspension. Having a slightly narrower port ensures that all airflow is fast and prevents this, several respected tuners have established that adding metal into the bottom of a large port actually increase bhp, and this is why the mpi head is better. It has the same shape at the top of the port and can handle similar volumes of air, but the bottom is filled in and reduces areas of low airspeed.
So armed with all of this information, you can make the choice of part combinations:
You only have one real choice with the 1.2 carb engine, and that is to fit the inlet manifold and carb from the 1.3SR. It will bolt straight on, and reduce resistance to flow considerably at the carb. You could then go further and fit a Weber carb. Of course if you want maximum fun, then the twin 40 set-up for the 1300 will also fit straight on. Fitting an mpi head will give better flow due to the larger valves, but it will need to be skimmed greatly to increase the compression ratio. You would also have to either fit the mpi system, or modify the SR inlet manifold to match the head, which is possible, or fit twin 40?s for which a manifold is available.
The 1.2spi engine can be improved by fitting the manifold and body with injector from the 1400spi, or the manifold and carb from the 1.4SR (not sure how easy an emissions test will be, and you?ll need to change the electrics and a few other gubbins). The most powerful combination would be to use a 1200 carb head with a manifold and carb from a 1.3SR, but it is complicated with regard to fuel and electrics.
The 1300 is also limited to choice, as with the 1.2, an mpi head will need a lot of skimming, and apart from that, there is nothing better. Fitting a 1200 head would increase the compression somewhat, so all is not lost.
The engine that can be improved the most is the 1400SR. The simplest route to gaining decent power is to fit the 1300SR head, inlet manifold and carb. You could use the 1200, although the compression will be quite high. Use of the mpi head is the same as the others, although it wont need skimming this time if you use a 1400mpi head.
The 1400spi will benefit from changing to a better head and inlet such as an SR manifold and carb, but you will face the same issues as with the 1.2spi with changes to electrics and fuelling. Possibly the 1600spi body from a late cavalier might be useful, but I'm unsure.
So that?s about it, the 1400 and 1600mpi engines are best modified as they are. Hopefully this should cut down on the questions of what?s compatible. Don?t forget that you can use the reverse of these principles. If you have a 1300, and want more power, the bigger blocks will bolt on and save you changing any of the top end.
One last thing to add, is that if you are planning on building a fast engine with twin 40's, use an mpi head if possible, although manifolds are available for all three head designs.
Good luck,
MC
To start off with, I?ll group the cylinder heads into type, and then look at the individuals. The heads can be broken down into three categories, carbed, multi point injection and single point injection. Sounds simple, but we?ll group the 1.4SR in with the single point injection group. To clear one thing up, all 8v Nova engines have the same exhaust ports, so any exhaust manifold will fit to it, you?ll just need to make sure they join well to the rest of the system.
The Carbed group, including 1.2 and 1.3SR have a large cathedral shaped port. The flow is direct and there is minimal restriction. The identical port shape means that the inlet manifold, and therefore carb from the 1300 can fit on the 1200 cylinder head.
The spi group including the 1.4SR have the worst design of cylinder head. The ports are small, square in shape, and twist down to the valve. There is also minimal room for improvement due to the close proximity of the water jacket. Once again the identical port shape means that the manifold and carb/body are interchangeable between the 1.2i, 1.4i and 1.4SR. It is beneficial to fit the body from the 1.4i to the 1.2i and possibly the 1.4SR to the 1.4i.
The best of the cylinder head designs is the mpi head. It has large round ports that flow well and directly to the valve, and the valve itself is larger. Because of the larger valve, you?ll get the best flow from one of these heads. There is no point switching any of the gear between the 1600 and 1400 as they utilise the same stuff. The best of the intakes is the later design without an AFM.
The point of interchanging any of these parts is to gain bhp, and this is achieved in two ways, improving breathing, and increasing compression. The compression on these engines is easy to calculate. You are working out the ratio of the maximum volume of the cylinder to the amount the combustion mix is squeezed down to. As the cylinder returns to the surface of the block at TDC, the majority of squished mix is in the cylinder head. This enables you to work out what compression ratio you will get from changing heads. For example, simplistically, a 1600 at 10:1 ratio will squash the reaction mixture down to 160cc (in the head), and a 1400 at 10:1 will squash down to 140cc (in the head). Now if you put the 1.4 head on the 1.6 block, you?ll be squashing the 1600 capacity down to 140cc, which is a ratio of about 11.4:1, and more power, and probably some detonation, and the inability to stop the engine running??It works with the other engines too.
The other side to gaining more power, is the increased flow, and this will come from head changes and/or inlet changes. This is self explanatory, the less restriction the better (to an extent).
The final thing to consider is fuel atomisation. A complete mix of fuel and air will give the most power, so the last thing you want is fuel dropping out of suspension and not burning properly. This is where the mpi heads are superior to the cathedral ported carb head. The port on the carb head may be bigger, and you may think that with large valves, it would be better than the equivalent mpi head, but this isn?t true. The majority of and fastest airflow through the port happens at the top and back surfaces, entering at the far side of the valve. Airflow at the bottom of the port is much slower, and gives the fuel time to drop out of suspension. Having a slightly narrower port ensures that all airflow is fast and prevents this, several respected tuners have established that adding metal into the bottom of a large port actually increase bhp, and this is why the mpi head is better. It has the same shape at the top of the port and can handle similar volumes of air, but the bottom is filled in and reduces areas of low airspeed.
So armed with all of this information, you can make the choice of part combinations:
You only have one real choice with the 1.2 carb engine, and that is to fit the inlet manifold and carb from the 1.3SR. It will bolt straight on, and reduce resistance to flow considerably at the carb. You could then go further and fit a Weber carb. Of course if you want maximum fun, then the twin 40 set-up for the 1300 will also fit straight on. Fitting an mpi head will give better flow due to the larger valves, but it will need to be skimmed greatly to increase the compression ratio. You would also have to either fit the mpi system, or modify the SR inlet manifold to match the head, which is possible, or fit twin 40?s for which a manifold is available.
The 1.2spi engine can be improved by fitting the manifold and body with injector from the 1400spi, or the manifold and carb from the 1.4SR (not sure how easy an emissions test will be, and you?ll need to change the electrics and a few other gubbins). The most powerful combination would be to use a 1200 carb head with a manifold and carb from a 1.3SR, but it is complicated with regard to fuel and electrics.
The 1300 is also limited to choice, as with the 1.2, an mpi head will need a lot of skimming, and apart from that, there is nothing better. Fitting a 1200 head would increase the compression somewhat, so all is not lost.
The engine that can be improved the most is the 1400SR. The simplest route to gaining decent power is to fit the 1300SR head, inlet manifold and carb. You could use the 1200, although the compression will be quite high. Use of the mpi head is the same as the others, although it wont need skimming this time if you use a 1400mpi head.
The 1400spi will benefit from changing to a better head and inlet such as an SR manifold and carb, but you will face the same issues as with the 1.2spi with changes to electrics and fuelling. Possibly the 1600spi body from a late cavalier might be useful, but I'm unsure.
So that?s about it, the 1400 and 1600mpi engines are best modified as they are. Hopefully this should cut down on the questions of what?s compatible. Don?t forget that you can use the reverse of these principles. If you have a 1300, and want more power, the bigger blocks will bolt on and save you changing any of the top end.
One last thing to add, is that if you are planning on building a fast engine with twin 40's, use an mpi head if possible, although manifolds are available for all three head designs.
Good luck,
MC