MGB: Olds 215 MGB v8

Hi, Engine - Olds 215 MGB v8

Can someone tell me what changes in an engine when using .040 + pistons? Does the displacement change?

Also, does 10:1 compression require special octane gas? or can it run regular pump gas?

Thanks in advance

Can't wait to see what you get for replies. I know that I am running (or soon will be) .030 pistons, that raised my compression but with the proper headgasket it lowers back down to where I can run normal pump gas without any issues.

3.5" bore vs. 3.540" bore
Divide diameter by two to get radius.
Square the radius dimension and multiply by pi (a constant... 3.141).
Multiply by the engine's stroke, which is 2.8".
Multiply by the number of cylinders, which is eight.
Result: 215.5cid vs. 220.5cid

What else changes? You'll have to buy new pistons, so at that time you may have the opportunity to buy pistons with a slightly different dish volume or height and thus potentially your displacement and compression ratio may be effected. You'll have to put new head gaskets in. Most people use composite head gaskets, and if they're a little thicker than what you started with you'll lose a little compression ratio. While you have the heads off, you should get a valve job. When the machine shop has the heads, they'll verify that the heads are flat. If they're not flat and need to be skimmed, that'll boost your compression ratio a little. Aluminum heads are generally less susceptible to "pinging" than iron heads because they run cooler. There are other variables like how much ignition advance you dial in, but you'll certainly be able to run "pump gas". Worst case, you may have to run premium, but you certainly won't have to buy aviation or racing gasoline.

If you're interested, there are several online calculators for static compression ratio. Here's one that's fun to play with: http://www.rbracing-rsr.com/compstaticcalc.html

You're planning on installing a fresh camshaft, aren't you? The valve job and new camshaft will likely have a lot more effect on how your engine performs than the 5 cubic inch displacement increase.

Peter, both the displacement and compression ratio will change. With the increase in bore diameter the displacement of the engine increases. With the increased displacment in each cylinder being compressed into the same combustion chamber volume the compression ratio will also rise. For .040" over bore I'd expect to see a compression ratio increase of about .2 over the 10:1 you mentioned. You might be able to run regular gas at that ratio, but the timing and cam profile will have a lot to do with that. To be safe I'd think you'll probably be better off using premium gas with the higher octane rating.