Engine compression after a re-build?

[hat_man]

I posted this question on Ranger forum and wondered what the forum braintrust here thought. I am no mechanic so it may be a silly question.

If you were to re-build a 2.3l Lima and they bored the block (say .030) to get everything back to smooth and round, are the connecting rods in the re-build kit "de-stroked" or slightly shortened to compensate for the increased volume of air being compressed?

And....if they are....could you use the stock connecting rods along with all the other new parts to get any measurable increase in compression? Would it be to any advantage or, if there was any increase, would it be so slight that it really wouldn't matter?

Thanks

[user removed]

If the block was bored .30 over there should be no need to change the connecting rod length.

regards
Mech

[hat_man]

That's why I ask here.

Just out of curiosity....at what point would you have to change the length and would my theoretical idea come into play?

[user removed]

My 97 has some spark knock at light loads and I think the compression is close to 9.3 to 1 stock (cast iron head). If I had to rebuild my engine I think I would look at the combustion chamber areas on the head surface and make sure there were no rough spots that could create a hot spot and encourage preignition. Not really removing a significant amount of material, just making sure the combustion chambers were clean, fairly smooth and maybe even check the volume of the individual chambers to make sure they are very close to the same volume. If not then work a little more on the lower volume chamber.

I would use one of the wizz wheels you use to remove gasket material. Might clean up around the plug threads with the plugs installed, looking for partial threads where they were cut for the plugs and getting rid of them.

Matching up the ports to the manifolds to make sure there were no issues that would affect flow.

Use the old head gasket to make sure you do not remove any material that would effect proper sealing of the head gasket.

regards
Mech

[mort]

Originally Posted by hat_man

That's why I ask here.

Just out of curiosity....at what point would you have to change the length and would my theoretical idea come into play?

Hi hat_man,
When I was rebuilding engines nobody would care until the compression ratio increased by more than 5%, and even then it was a big nothing. The biggest oversize I ever saw was .060. In the cases where the CR was imagined to get too high a thicker head gasket was used instead of short connecting rods - which would be custom made for too much money.
Assuming 3.78 inch diameter and 3.126 inch stoke, and that the combustion chamber is all in the head, so boring doesn't change the combustion space; .030 over size gives you 1.6% more displacement so 1.6% higher CR. 9:1 goes to 9.144:1 (or 9.4:1 goes to 9.55:1)

.060 would be 3.2% so 9.4:1 becomes 9.7:1
In general if the engine didn't ping before a .030 re-bore it wouldn't after either.
-mort

[2000neon]

Stock connecting rods, I had the engine machined for my El Camino it had to go 0.030" over to clean up the cylinders, the rotating assembly stayed the same with the exception of +0.030" pistons. Which is why it's actually a 355" motor now, not a 350.

[Rusty94cx]

You would need rods only if you were to get the crank stroked. Ofset ground to allow more travel. Don't bother. To raise compression if you wanted to, can create problems. Would be to bore it say .030 over and have the head shaved down a bit also say .050. Your still not looking at huge gain.

[hat_man]

Thank you all for the information. I was trying more to understand how all the "pieces parts" work. I used my engine only because I am a little more familiar with it.

Thought I might have been onto something, but not knowing how all the parts worked together I needed to ask.

What I was seeing in my head was something like this:

Stock cylinder/piston/connecting rod creates X amount of compression.
Larger diameter cylinder+stock length rod=more volume=greater amount of compression.
Larger cylinder+shorter connecting rod=same X amount of compression.

I was seeing more volume squeezed into a larger space(created by shorter rods) as being equal in compression to stock volume squeezed into stock space. For lack of knowing the correct term...having the same psi. My line of thought was then if you had more volume (in the bored cylinders) squeezed into the stock space (in the head) using stock (imagined slightly longer) connecting rods it would increase the compression to some extent.

It seems I was a bit off in left field. Oh well. I have to learn somehow.

Thanks to everyone for the info.