Need more compression in my 318, does this add up?

[273]

What difference would it make?

I'm not trying to be difficult, it's a serious question. Say I have 9.5 compression. What's wrong with picking the cam last?

I actually think that's the best way to do it.

A fair question, when you got a pre built engine your pretty much stuck with the CR it has so it got to be part of the basis for you decision. But right now nothing is set in stone and if the xs268s cam fits all his requirements, idle, powerband, stall, gear, performance, mileage etc why step up a cam or to fit some random picked CR amount. But if the bigger cam seems to fit with what he wants the go for it.

If it was me I'd pick what gear Im willing to run then pick a cam that would work with that. And go with the rest of the package the would compliment those choices.

 

[nm9stheham]

Correct on the KB pistons, and I'm assuming I have .07 like most do in 318's.

Considering the multitude of combustion chamber shapes, etc, I think the figure you're going by may be a bit too generalized. I'd love to talk to someone that's built a 318 with 273 heads and KB pistons!

If you are referring to quench gap, I know one local loooong time engine builder (40+ years) who pushed the quench gap down and down and found that at .025" he started having the piston just kissing the heads. So the .035 minimum looks like a solid general number, based on that, on bores of 3.5 to 4 inches or so. But, the .040" will allow for variations in things like journal heights and rod lengths that sometimes show up. There is nothing new in KB pistons that should be any different than other AL pistons relative to quench gap; expansion rate is pretty much the same for hyper vs non-hyper AL alloy, and piston rock depends on piston to bore clearance.

BTW, Peyote, what is the intended use for this car/engine? That may influence your choices.

 

[RustyRatRod]

Lol, you're not alone.






Reasoning? I'm still trying to follow youz guys on this cam stuff.

Sorry Bruce. I missed this.

Because at 9.5 or 9.6.......whichever, he will likely have too much cylinder pressure with a mild cam like the 268. The larger cam will have a later intake valve closing event and will start building cylinder pressure later in the compression stroke and create lower cylinder pressure allowing the engine to run easier on pump gas.

It "might" run on pump gas anyway with the 268, but it will be close.......especially in Summer idling in hot traffic during heat soak. He would have trouble IMO with detonation in that situation.

I don't like giving people advice that skirts the fence. Why take chances? Yeah, he can probably run 10.5 compression with perfect quench and yadda yadda yadda, but that's very costly to get that precise.

Doing it how I recommended is a lot cheaper and there's less room for error running on pump gas.

I like it when I build something I can actually drive and have fun with, instead of worrying about where I can find race gas.