How will a xr258hr run in a 355 with 10:1. I just bought one for my 305tpi almost new and got it very cheap. Id like to reuse it in my budget 355 build. thanks

 

I'm not 100% familiar with that particular cam, but based on the family of cams its probably too small for your application. I consider the 268 a good fit with most street 355's. Hell, mine is a custom with a 274 intake and 288 exhaust. Runs surprisingly good.

 

Should work fine in that combo.

Reason being, it's most likely not 10:1; or even, real close.

If it has the usual .045" of deck clearance that rebuilt motors almost always have, .039" gaskets (1004 or similar), 6cc valve reliefs in the pistons, and even giving the heads the benefit of the doubt and calling them 62cc as they often are instead of their 64cc "spec", it's still only 9.5:1. If the heads are REALLY 64cc, it's 9.3:1.

Probably just fine. It won't lope or anything, but it should have lots of torque.

 

High compression and small cam means high cylinder pressure which means high octane fuel.
Vortec heads are about as good as it gets with iron though. Keep the timing low-ish and you should be fine. But as mentioned, unless you measured it accurately as 10:1, it probably isn't

 

I have a set of new forged pistons my friend came across for free for .030 over and I was told(find out exactly tonight) is flat top with valve reliefs,and 9:1 with 76cc heads with stock deck clearance .