Combustion Chamber Modifications

[howieschoon]

Any thoughts on modifications to this head? I was thinking about Somander/Singh (whatever they are) grooves in the quench area and aimed at the spark plug. The chamber is coated with a thermal barrier coating (the valves are also but I don’t know for sure that it is still on them, they didn’t clean up very easily). My main concern is gas mileage with additional power being a happy side effect. It is in a 2.0L 4-cyl that is mostly driven in town and occasionally on the highway, so my main concern is with lower RPM also, 3500RPM or so. Piston-to-head clearance is right around 0.045”. Bore diameter is 3.406”, intake valve diameter is somewhere around 1.6” and exhaust valve is around 1.4” (I’d have to look these up to know for sure) for an idea of the sizes involved. The valves are parallel to the bore. The intake port comes in from the right and the exhaust port exits to the left on the first picture. The depth from the head face to the bottom of the chamber is about 0.43” and max lift is only about 0.4”. I’m sure that shrouding is quite terrible but I would have to look it over some more to see where water is if I was going to try and decrease the shrouding. It is throttle body injected, so atomization is probably not the best, especially at part-throttle, although I have an idea that might help with that.

The pictures of the chamber that has not been cleaned up has about 25K miles on it but the last few may not have been the best. I had a blown head gasket (why the head is off right now also, I need to investigate why/how it blew, it didn’t break the fire ring, looks like it went over it but I haven’t cleaned that area off yet), although it wasn’t in either of these cylinders. I thought it might be a good example of what is going on combustion-wise in the chamber. In an article that DV wrote about maximizing compression he mentions quench area and says that too large of a quench area can increase unburned HC emissions, I was wondering if that may be one problem with this particular head. There is also a not-so good picture of the piston, the top is the intake side and the bottom is the exhaust side.

David, in the article I mentioned above you say that you shoot for a cranking pressure of 200psi when using 93 octane fuel and then reduce that by 5psi for every octane point below 93. Is this with iron heads and would it change with aluminum and/or thermal barrier coatings? If so, any rough approximations?

Mostly what I am hoping for would be anyone’s first impressions about this. Don’t go in to too much detail, there is a fair chance that I will only do a few minor changes, partly because I need to get it back together soon and partly because at the moment I don’t have any good way to check wall thickness. Wow, sorry this was so long, if anyone would like any more info on this feel free to ask and I will try to get it for you. Also, I have read the majority of the articles and posts that would relate to this and have my own ideas. I’m not being lazy and having everyone else decide what would be best, I am just trying to get other people’s opinions (who probably have more experience than me also).

[Pinhead]

Holler: Cylinder Head Porting, Part 2

That's what I would try to do.

[Brian P]

What engine is this (make model year)? I find it interesting that the shape of the bowl in the piston does not even remotely match the shape of the chamber in the head. Is the sparkplug offset to the same side as the offset in the bowl in the piston?

[MilesTugoh]

Doesn't appear the water passages match either ..

[howieschoon]

The car is a '91 Pontiac Sunbird. The engine is a 2.0L 4-cyl. I think it is made by Opel but it could be Vauxhall (sp?), I have ARP Vauxhall rod bolts but I think it is actually an Opel. Should also be very similar to the 1.8L engines in pontiacs, etc.

The dish in the piston is more-or-less centered about the spark-plug. The picture of the cleaned off chamber is for cylinder #1, the piston is from cylinder #4 but the water passages don't really line up anyway. The outer 2 areas do line up and the middle area just stops. I would guess that they did that to get the sand out of the casting easier or something but that is just a guess.

If you take the second picture and the last picture, the exhasut is on the bottom on both and the intake is on the top on both, that is probably the easiest way to compare the two. I thought the first picture was the easiest to see the combustion chamber and the third one is there to see how the chamber "colored". Also of note is that the vast majority of the last few thousand miles have been in-town and usually less than 5 miles.

Pinhead, thanks for the article. I don't think that I will be quite that extensive but I do hope to do some of that. I would really like to deshroud the exhaust since it has to make a 180* turn from where I assume most of it would go. I also plan on adding a single singh groove aimed at the spark plug. I need to find a decent, inexpensive way to measure the thickness of the walls to make sure they don't get too thin.

[howieschoon]

Does anyone know what the minimum wall thickness in the combustion chamber would be for a street driven NA car with sand-cast aluminum heads? In the article Pinhead mentions, there is another article about pistons and it says small areas can be as thin as 0.18" thick with larger spans being as thin as 0.22" thick but this is also for pistons. If I can get this info and measure my wall thicknesses it would give me a much better idea of how extensive I can rework these. Would a sonic tester be the best way to measure these? As you can probably tell my experience is lacking but I have read a lot of books etc and am trying to do more.

[Unknown]

Its a 2.0 8v Vauxhall/Opel engine (Depending whether your in the UK or Europe) engine code something like C20NE.

They have quite nice downdraft inlet ports if its one of the post 1988 heads.

You can certainly open the chamber up to the gasket line around the valves to reduce the shrouding without problems. As far as I can remember the chamber walls are fairly thick but dont quote me on that!

Its also worth getting the valve seats cut to match the diameter of the valves as the std seats are usually quite a bit smaller than the valve diameter (which means you might as well have a smaller valve).

Head gasket failures are not uncommon, usually blowing from the back of no.4 (flywheel end) across to the water jacket but I can't see any of the usual signs on your head.

If you have the head skimmed then check the top for straightness and have that skimmed if required. If you dont then the cam will snap along side the center cam bearing due to being forced to bend every time it rotates.

[howieschoon]

Hello Unknown,
Thanks for the info on the Vauxhall/Opel engine number. I looked it up on google images and it does look like it is the same engine, although mine has a substantially different induction system. Mine is a TBI with a not fantastic looking manifold. The head is off of a '92 Pontiac Sunbird (as is the engine). I have done a valve job on it, about 4 years ago, but I really didn't know much then (as far as head modifications go, still don't know all that much either) so I pretty much just cleaned up some falshing in the ports and did very little to the area around the seats. I was very worried about hitting water. I will take the valves out this weekend and see what it have. The longer I wait the more I want to do.

It was indeed the #4 cylinder that had the failure, it went back to a water passage. When I rebuilt it both the head and block were decked, so they should have been flat. I will check both before I put it back together. I am not absolutely positive that the fire ring in the head gasket doesn't overlap the bore a little (which would be quite terrible I would think, especially since the pistons come out of the block 0.010"), I need to check. I don't think that the fire ring is round either, is there any reason for that? It gets slightly larger in one area.

I would still like to know what the minimum wall thickness should be and what the best way to measure this is. I have never used a sonic tester, which is what I assume would be best, but I really don't know. I think the very little that I have seen was in regards to measuring bore wall thickness, I think. Do they work in aluminum (I would assume that they do)? Wouldn't it be nice to have every tool you could possibly need.

Thanks,
Howard

[howieschoon]

Well, I fianlly looked at my old head (from a '91, rather than the '92 as shown in the pictures above) and I think I will be using it instead. The valves are already less shrouded so I wouldn't have to do so much work there and there is a quench pad on top of the spark plug area (any thoughts on this?). The minimum thickness in the chamber that I found with a sonic tester (woohoo, I found one I could borrow for the weekend) was 0.34". I still need to clean up around the exhaust valve a little, clean up the ports and coat everything with a thermal barrier but I think the '91 head will be better in the long run. I will try to post some picures of it when I get it cleaned up. Hoepfully I can do a better job on this one than I did on the '92.

Oh yes, the head gasket does not overlap the cylinder bore (the best that I can tell) but it does come very close to it.
Howard

[howieschoon]

Alright,
This is rather long and I do apologize for that. I have decided to keep the ’92 head since it has already had some work done to it and the chamber modifications shouldn’t be too hard. Here is what I have so far. It is a bit rough and it is apparently a bit harder to cut a decent groove with a die grinder than I thought. These modifications added about 2cc to the volume of the combustion chamber but I should be able to get that back by taking about 0.017” off the head. Once I have it milled off I will blend it all together a bit better also so that the edges are more rounded.

Does anyone have any recommendations for compression ratio/cranking pressure for this engine? It is just a somewhat boring street driven car that will rarely see WOT and I am mostly concerned with mileage. I was thinking about having the compression be rather high so that part throttle would be better and then maybe have some sort of a throttle stop so that it doesn’t go into WOT. I would like to be able to run 87 octane crap gas but if 89 octane e10 would be better (mostly mileage again) then I would certainly run that. If I did end up needing to run 91 octane that would be OK as well, but I would rather not unless the mileage gain would offset the extra cost. Where I am now I can get e85 also, so a mix of that wouldn’t be out of the question although I am a little worried about it eating up components. I have run about 30% e85 and 70% 87 several times with no problems, not even a check engine light, although I have had the check engine light come on with slightly higher amounts of e85. In other words, I would like to stick with using crap gas unless the benefit from using the higher octane outweighs the extra cost. I was also thinking that the option of using the higher octane would be a good buffer incase it didn’t work with the lower octane fuel or if I did have a throttle stop of some sort then I could remove it if I wanted to run the higher octane for more power. With 9.5:1 compression the cranking pressure was 180psi on a cold cylinder.

Here is some info for the engine:
*Stock replacement camshaft: *Duration at 0.050” lift 214deg intake and exhaust
*120deg lobe separation angle (yes, 120deg and no it’s not a turbo)
*Max lift both intake and exhaust is about 0.4”
Unfortunately I am not sure how it is installed in stock form, I should obviously measure it but I have not.
*Intake valve diam=1.646” (I also have a 1.69” valve, stock ’91, that I am thinking about using)
*Exhaust valve diam.=1.44”
*Bore=3.406”
*Stroke=3.386”
*Rod length=5.6”
*Piston to head clearance=0.038”
*Pretty much everything coated with a thermal barrier coating (chamber, pistons, valves ports)
*Stock TBI

The fire ring in the head gasket didn’t break (unlike the first one I blew on a completely stock engine several years ago), it looked like it just was letting gas go over the top of it. Any thoughts as to why this might happen (improper torque, thin casting, weak bolts, detonation, too high cylinder pressure, over advanced ignition timing……). I don’t think I have much of a problem with engine knock, be it from detonation or pre-ignition, but I do have an automatic (ouch, stop hitting me, I’d rather have a manual) and at low speeds the torque converter locks up and the RPMs drop too low (close to 1500RPM or so) and it will knock under load. If I drive it I make sure that it doesn’t do this by down-shifting but I cannot guarantee that it is always kept out of this condition. This is the only thing out of the ordinary that I can think of that would have caused the failure.

If anyone would like to know anything else feel free to ask.

Thanks again for all the info,
Howard