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

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KevinB

KevinB

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Can someone please check my calculations for compression on my 318?

I plan on swapping in 920 heads from a 273 which *should* be around 63 cc. Keith Black 167 pistons *should* put me .02 in the hole. I plan on using .030 gaskets, and a .030 bore over job.

When I get a chance I'll cc the 920 heads, but until then I'm just going off other numbers I've seen posted.

When I plug all this in I get just about 9.5:1 which I understand is the best compression you can use and still avoid premium gas.

I also understand from research that the 920 heads flow as well as my stock '68 318 heads.

Oh and don't worry, the 920 heads are off a '66, so the intake bolt angles should be fine.


Thanks!
 
I added up the deck height numbers for the KB167 and got 9.588". That is around .012" in the hole instead of .020".
(3.31"/2 + 6.123" + 1.810" = 9.588" vs a nominal 9.600" deck height

With that, I get a 9.6 SCR. I am a bit skeptical you are going to avoid Premium at 9.5 or 9.6.....I'd just be ready to use it. Not a terribly big deal IMO....
 
nm9stheham said:
I added up the deck height numbers for the KB167 and got 9.588". That is around .012" in the hole instead of .020".
(3.31"/2 + 6.123" + 1.810" = 9.588" vs a nominal 9.600" deck height

With that, I get a 9.6 SCR. I am a bit skeptical you are going to avoid Premium at 9.5 or 9.6.....I'd just be ready to use it. Not a terribly big deal IMO....
Click to expand...

Ok So maybe I'm basically in the neighborhood...perhaps I should shoot for 9.4 then. If I do get pinging and I'm right at that limit I'm sure swapping to a slightly thicker head gasket would help, or even doing a rough polish of the chamber. I figure if I'm right up against it, fixing it should't be too difficult.

Thanks for double checking my math. My engine builder is gonna have to reassemble to get my true piston depth, but for now I think it's a solid enough plan. Once I get my true numbers I'm hoping to make any tweaks using the gasket thickness.
 
Peyote said:
If you are doing a lot of the work yourself, just do that now. I have been porting my heads, and I am polishing the chamber to an extent.

That, in of itself, will affect the CR, although not a bunch. But getting rid of any sharp spots will aid in fending off detonation.
Click to expand...
 
Whats the rest of the build, if it's mild I don't see why the need to push the limits of CR. 8.8-9:1 will allow cheap gas and only sacrifice a few hp.
 
273 said:
Whats the rest of the build, if it's mild I don't see why the need to push the limits of CR. 8.8-9:1 will allow cheap gas and only sacrifice a few hp.
Click to expand...



Well its a budget build to an extent, and I suppose "mild" is a relative term, but I'm aiming for 340hp. Street/strip cam, TTI headers, edelbrock dual plane intake, holley 650, and a 8.8 rear with 3.73 gears.
 
Depends how big the cam you might get away with 9.5:1

Stock 273 heads will have a hard time making the power numbers you want.
 
273 said:
Depends how big the cam you might get away with 9.5:1

Stock 273 heads will have a hard time making the power numbers you want.
Click to expand...

I'm hoping its doable with bigger valves and porting work
 
Peyote said:
I'm hoping its doable with bigger valves and porting work
Click to expand...

You check out the sticky on porting 318 heads (same thing) ? Sounds all good then, which cam where you leaning to?
 
273 said:
You check out the sticky on porting 318 heads (same thing) ? Sounds all good then, which cam where you leaning to?
Click to expand...

Well I know jack crap about cams, but here's the one I was leaning to:


Comp Cams XS268S

Duration:
Intake 268
Exhaust 274

Valve Lift:
Intake .488
Exhaust .501
 
You'll need more cam than that, IMO to run on pump gas successfully.
 
It seems like you thought up a pretty decent combo, someone with more cam experience might be able to tell you if there's a better cam choice but looks all good to me.
 
Yeah, now I can get behind that one. That would work.
 
273 said:
Instead of more cam, run less CR
Figure what cam you what and pick the DCR that will allow you to run the gas you want and build the CR that gives you that.
Click to expand...

Why? He'll make more power with more cam with the compression left alone.


That will be a rowdy little 318 with that 236 @ .050
 
I am doing a similar build right now. It will have the ICON pistons instead of the KBs and I am using 302 heads. I have 1.88 1.60 SS valves in and have not checked CC on them yet. I am aiming for 9.75 SCR and will be running the comp XE 284, but I plan/hope to run on premium.

I want to run ~.040 piston to head clearance, but I don't know exactly where the pistons will sit yet.

I know the 920 heads are quite a bit different than the 302s but some of the same rules should apply, and I bet our 318's will end up pretty similar.

Good luck on your build, sounds cool.
 
Dartboy52 said:
I am doing a similar build right now. It will have the ICON pistons instead of the KBs and I am using 302 heads. I have 1.88 1.60 SS valves in and have not checked CC on them yet. I am aiming for 9.75 SCR and will be running the comp XE 284, but I plan/hope to run on premium.

I want to run ~.040 piston to head clearance, but I don't know exactly where the pistons will sit yet.

I know the 920 heads are quite a bit different than the 302s but some of the same rules should apply, and I bet our 318's will end up pretty similar.

Good luck on your build, sounds cool.
Click to expand...


Thanks, and good luck with yours...If I end up having to run on premium that won't be the worst thing in the world, I'll only drive it one a week anyway, but I'd like to pass it to my son one day, so I'd prefer the regular octane option.

What's your reasoning behind the choice in piston clearance?
 
Because it supposed to help prevent detonation.

http://www.crankshaftcoalition.com/wiki/Quench

" A quench distance of ~0.040" will allow a high performance engine to run without detonation using less octane than would otherwise be needed. Of course this is providing that all the other important areas are also covered, like the static and dynamic compression ratios, correct air/fuel ratio, correct plug heat range, good ring and valve guide seal, etc. "
 
I have heard different numbers for perfect quench, but it seems to be .035-.045" from most sources and if your in that range it may help your engine run on regular with 9.5 SCR. Though one note is the KB pistons state minimum .040" clearance.
 
RustyRatRod said:
Why? He'll make more power with more cam with the compression left alone.


That will be a rowdy little 318 with that 236 @ .050
Click to expand...

If he wants to run more cam that's fine. I'm just saying pick the compression that works with you cam choice not the other way around.
 
Dartboy52 said:
I have heard different numbers for perfect quench, but it seems to be .035-.045" from most sources and if your in that range it may help your engine run on regular with 9.5 SCR. Though one note is the KB pistons state minimum .040" clearance.
Click to expand...

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!
 
273 said:
If he wants to run more cam that's fine. I'm just saying pick the compression that works with you cam choice not the other way around.
Click to expand...

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.
 
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