Cylinder wall thickness, thrust side and boost.

[TheSilverBuick]

As I'm now considering the next step in my OHC Pontiac straight six I need pistons and want to look at what possible overbore's can be had, so I have a few cylinder wall thickness questions.

Do straight six engines have a sidewall thrust side like a V8? I'm guessing the driver's side of the block is the thrust side if so

Also what is the general minimally accepted values for cylinder wall thicknesses at various levels of boost? 10psi? 15psi? 30psi? etc.

According to the machinist the thinnest portion of any of the walls in the block I'm going with is .238", and the block has been over bored .030" already. The thinnest portion is supposedly between the Number 1 and 2 cylinders (on both blocks).

[Caveman Tony]

I'm gonna take a WAG, and say there is definitely side thrust in a 6.

Whether it has more than a V8, well, I'd think that is more of a function of stroke length & rod length than whether its a V8 or I-6...

[TC]

I concur on the thrust side being on the drivers side.......

I'm think that you'll have more problems blowing head gaskets, than blowing out cylinder walls........

As for boost how big of a hair dryer you want to put on this thing?? Big Turbos are great for big power, but usually suck when it comes to street manners, this is the one thing I've been dealing with in picking the turbos for my twin turbo setup......And just wait until you get into A/R's and their effects on how the turbo operates. And the sad fact is after all these months I'm still not 100% sure which turbo I want, I've narrowed it down some, but there is still about 7 different turbo's I can use, but now I want to find the one that fits what I want do with the car. Then start Plotting your setup on the turbo maps, that can really start you thinking........Though I suggest you get a Turbo with a "ported" turbo housing, they also call it an "anti-surge" housing, it will move the surge line on the map farther to the left and give you a bigger range of boost to work with.........First I thought I wanted T61's, but now I'm looking more towards T67's or T70's, thought T72's would probably work, I think I would be giving up to much street manners for turbo's that want to see high rpm.........And then there are the GT35's and GT45's, they would work to, but all of them have different turbo maps and my setup plots differently on all of them.....I found this site, so far it seems to be the easiest to use and is really helping me pick the turbos I want, check it out.......It's even named after one of our favorite members.......

http://www.squirrelpf.com/turbocalc/

[TheSilverBuick]

It's kinda off topic since I'm looking at cylinder wall thickness (i.e. 4.00" bore would give me a .143" wall thickness, which I'm sure is too thin), but the head gasket is a thin steel shim type. Don't know if they are better or worse than a standard type but seem pretty stout.

It's sitting onto of the head there.


I'm not really looking at turbo's yet. One step at a time, and starting with pistons to get me in the 9.0:1 compression ratio zone, and once again looking to see if there is an off the shelf one that will work, but the compression height remains the common problem.

[Darren N.]

Whether it has more than a V8, well, I'd think that is more of a function of stroke length & rod length than whether its a V8 or I-6...

I think you are correct. This is simple physics, no? At every instance other than TDC and BDC, the piston and rod assembly is acting on the throw of the crank. Because the throws are offset, there is always force acting on the walls.

Now determining exactly "how strong" the block is for your intended use I'm guessing depends on geometry and weight. The stroke, rod length, compression height, piston skirt length along with the weight of each component will determine how much dynamic load is placed on the cylinder walls in the intended RPM range.

Have you found anyone who's pushed a block like this to its limits?

[TheSilverBuick]

Have you found anyone who's pushed a block like this to its limits?

No, at least not of the OHC Pontiac, or at least published on the internet. There are a couple Chevy inline sixes, and none I've seen mentioned anything about the cylinder wall thickness, just a .040 or .060 overbore pistons and running.

The one that stands out is this one: http://www.inliners.org/ubbthreads/u...=43253&fpart=1 and he only runs it up to 10psi, and I'd like to go higher than that, at least 15psi.

[Loren]

Thinking about the thrust side thing, I wonder if I'm missing something here. Seems to me, looking at the front of the car, the motor's turning clockwise, therefore through the power stroke the crank pin is to the right of TDC, so the rod angles to the left going up to the piston, so the combustion force is gonna push it to the left (again as seen from the front), therefore the passenger side of the cylinder would be getting pushed on by the piston more.

[TheSilverBuick]

I would think the most force is right after the cylinder ignites (fighting the load) and the rod would be on the right of the centerline (looking from the front), where as the force of the piston going upwards, left of centerline, is relatively light (fighting compression).

[Loren]

The rod would be to the right of c/l...but figure the angle it's at and imagine that were you, like, leaning against a tree. Your feet (or the rod journal) are to the right, the tree (or cylinder wall) is to the left, the tree is taking a certain amount of force caused by you leaning on it. Anything pushing on your head, causing additional force onto the ground, would also cause additional force against the tree.

(I should write kid's books, huh...this is kinda fun)

[TC]

I just look at like this, on the pistons upward motion of travel the crank is pushing the rod at an angle that would push the piston into the drivers side of the cylinder......On the downward travel of the piston the crank would be pulling the piston away from the passenger side of the cylinder.......

[SpiderGearsMan]

Procharger

[Barry Donovan]

Thinking about the thrust side thing, I wonder if I'm missing something here. Seems to me, looking at the front of the car, the motor's turning clockwise, therefore through the power stroke the crank pin is to the right of TDC, so the rod angles to the left going up to the piston, so the combustion force is gonna push it to the left (again as seen from the front), therefore the passenger side of the cylinder would be getting pushed on by the piston more.

I was thinking that too..
inline, and overhead.. it may be symmetrical?

love the idea of the ohc..

maybe get strongest valve seals. is there a means to a forged cam?

any boost pressure at all is going to be smashing the heck out of it with "neutrinos".

[CDMBill]

I think the answer is simpler than all the foregoing. If it'll clean up at .030 over buy pistons for that and focus on ring package and pin placement which is likely fixed if you already know what rod you are running. Tiny incremntal increase in bore sizes will get you tiny tiny incremental power, whereas leaving yourself room for the next overbore will save finding another block as I'm guessing these things aren't laying all over the wrecking yards. .238 is good for 2HP/cu.in. easy, how far are you trying to get? BTW its the thrust faces you are concerned about most, but in a boosted applicationI wouldn't sweat the last posible bit of displacement, there are other things to to get sweaty about. I'd worry more about the the combustion chamber and the piston face, quench pad etc.

[Stich496]

how much are you going to need to shave off the engines deck to get some quench.. ??
and will angle milling the head help any (yes I know it's an inline) but angle milling can get the valve a little fathar away from the wall.. for a little extra, and he needs some compression.. whats the cc of those chambers

[TheSilverBuick]

Other than the combustion chambers being machined (good 'ol Pontiac) I don't know much about what to do with the chambers. I did pick up a new die grinder yesterday and did some "practice" work on the head I decided to call a spare on the short side radius, they were quite sharp. But while looking at the heads I did notice that the spark plug is rather recessed under a lip on the head. I need to try and cc the chambers again. I tried once and the two chambers I tried on one head the valves leaked (carboned up). Visually the chambers on the two heads look the same, but according to the OHC forum one of my heads should be 65cc's ('66 1bbl 230) and the other 80cc's ('69 1bbl 250), so maybe one has a bit more depth to it Maybe I should be re-evaluating which head is my practice head I don't want a 11.5:1 compression engine using the wrong head.

Angle milling is viable since I have an adjustable timing gear, how ever with the valves only sticking maybe .10" below the head deck at full lift I'm not sure I see a benefit. The intake valve is 1.92" and the exhaust 1.6" (Turbo Buick's are 1.71", 1.5"), so only about .36" of space around the bore.