Thread: c.h.i.e.f 's 110 Isuzu county

The only reason I mentioned the bowl volumes was in the hope that it might help smooth out any imbalance in power pulses/reduce the vibes a little, otherwise I agree with John.

Having said that, if I had an engine of my own apart my OCD side couldn't help but 'keep going', I would actually find it really difficult not to go all the way

Interesting...thanks for this info guys.

Also been awhile since I've looked at anything remotely related to cars but don't the valves on the Isuzu's sit pretty much flush with the face of the head ? I'm assuming I'm missing the point of bowl volumes?

I'm talking bowl in the piston, just like a diesel, but either way it's still a combustion chamber.

If the chamber volumes are uneven you will get different combustion pressures between cylinders and therefore more vibration.

It may not make much difference on a 4BD/G1-T, I really don't know, but my thoughts were, if you're already there balancing rods and pistons to increase the smoothness, why not take the extra beef/difference of the pistons out of the bowl instead of the underside and balance the chamber volumes at the same time........

And FWIW the little Ford Kent 1600 crossflow engine, as used in Formula Ford worldwide for near over thirty years looks just like a diesel with the head off.
The head is dead flat, the valves protrude, no chamber, with the entire chamber sitting as a hemispherical bowl in the piston with a squish band all the way around the edge (and two valve cutouts) .

The only thing lacking (from a diesel perspective) is the spire in the centre of the bowl.

It's a limited design for a petrol race engine though.

The plug location coupled with the large, wide bowl means the need for large amounts of spark advance when looking for ultimate power at revs, and the flat head really limits flow/power potential.

A small kidney shape chamber in effect can use the chamber walls as extensions of the valve seat, maintaining a venturi and flow as the valve opens to maximum lift.
A dead flat head limits flow very quickly, even though the valves are unshrouded.

Of course this doesn't matter on our TD's as they are force fed air

Originally Posted by rick130

I'm talking bowl in the piston, just like a diesel, but either way it's still a combustion chamber.

If the chamber volumes are uneven you will get different combustion pressures between cylinders and therefore more vibration.

It may not make much difference on a 4BD/G1-T, I really don't know, but my thoughts were, if you're already there balancing rods and pistons to increase the smoothness, why not take the extra beef/difference of the pistons out of the bowl instead of the underside and balance the chamber volumes at the same time........

And FWIW the little Ford Kent 1600 crossflow engine, as used in Formula Ford worldwide for near over thirty years looks just like a diesel with the head off.
The head is dead flat, the valves protrude, no chamber, with the entire chamber sitting as a hemispherical bowl in the piston with a squish band all the way around the edge (and two valve cutouts) .

The only thing lacking (from a diesel perspective) is the spire in the centre of the bowl.

It's a limited design for a petrol race engine though.

The plug location coupled with the large, wide bowl means the need for large amounts of spark advance when looking for ultimate power at revs, and the flat head really limits flow/power potential.

A small kidney shape chamber in effect can use the chamber walls as extensions of the valve seat, maintaining a venturi and flow as the valve opens to maximum lift.
A dead flat head limits flow very quickly, even though the valves are unshrouded.

Of course this doesn't matter on our TD's as they are force fed air

Auh gotcha now...good idea about balancing pistons from the top as you said it will serve 2 functions in one but I'm curious as I'm not sure if grinding material from inside the bowl will cause hot spots ?

If you plan to remove any material from the combustion chamber bowls, be aware that there a subtle features that are extremely important for combustion and if you change any of these you may well stuff it up.

IMHO the safest place to remove metal is from the raised 'hill' in the base of the bowl.

Unless you know a lot about diesel combustion, don't change the slope of the sides of the bowl, or the re-entrant lip (changing the lip would be a disaster).

Don't let people who are 'expert' at performance mods on petrol engines, and don't fully understand diesel combustion, mess with your combustion chambers or inlet ports. As I said before, different beasts!

Because diesel engines do maybe 99.9% of the heavy lifting when it comes to transport around the world (shipping, trains, trucks, etc.), they have been subject to a lot of research and development. And what is correct for large cylinder bores is not correct for medium and small cylinder bores. Although the Isuzu 4Bxx engines are old, Isuzu engineers knew what they were doing.

Sage advice in the above post

Originally Posted by Bush65

If you plan to remove any material from the combustion chamber bowls, be aware that there a subtle features that are extremely important for combustion and if you change any of these you may well stuff it up.

IMHO the safest place to remove metal is from the raised 'hill' in the base of the bowl.

Unless you know a lot about diesel combustion, don't change the slope of the sides of the bowl, or the re-entrant lip (changing the lip would be a disaster).

Don't let people who are 'expert' at performance mods on petrol engines, and don't fully understand diesel combustion, mess with your combustion chambers or inlet ports. As I said before, different beasts!

Because diesel engines do maybe 99.9% of the heavy lifting when it comes to transport around the world (shipping, trains, trucks, etc.), they have been subject to a lot of research and development. And what is correct for large cylinder bores is not correct for medium and small cylinder bores. Although the Isuzu 4Bxx engines are old, Isuzu engineers knew what they were doing.

Given the importance of the bowl shape, what are the chances that you would have different volumes? I'd say minimal! However I have never measured the volume of a petrol head chamber little less that of a diesel piston bowl.

Originally Posted by slug_burner

Given the importance of the bowl shape, what are the chances that you would have different volumes? I'd say minimal! However I have never measured the volume of a petrol head chamber little less that of a diesel piston bowl.

Likewise I have not measured the volumes to know what the variations are like.

Rick was making a good point, but in a direct injection diesel, once ignition starts the combustion pressure rise is extremely rapid - this is what causes the noise and also results in high torque. Combustion is over in a very short time and the pressure drops as the piston descends.

Pressure rise in petrol engines starts low and reaches a maximum toward the end of combustion (opposite to diesel).

For higher torque the diesel has to burn more fuel at around TDC. The difficulty here (neglecting production of NOx and noise for this thread) is to get enough oxygen to where the fuel droplets are. This mixing is why the shape of the bowls and inlet ports is so important. Change the combustion between one cylinder and the other by upsetting the critical shape of the bowls could result in far worse vibrations than small variations in volumes.

If you could see the shape of the piston bowl and the spray from the nozzle holes in the injector, you can see how the bowl is shaped so the spay doesn't impinge on the walls of the combustion chamber. However this doesn't show what is done to promote mixing and the importance of the lip shape.