Pro's and Con's of higher compresson ratio

[Barracuda67]

As some of you may know I've been toying with the idea that I would like to put some higher compresson pistons in my 273.
I am now approx. 8-8.5:1 and want to increase to 10.5:1 same as stock Commando engine.
Will there be a noticeable difference? in anything?
(Besides the fact, I may have to add a boost to my feul, or at least burn premium.)
Hopefully a little more power? :cheers:
So what would be some pros and cons of doing this?
Thanks.

 

[1968FormulaS340]

Long and short of it….

The good…. Power goes up.

The bad…. Fuel requirement goes up.

 

[Max340]

Increased compression with no other changes will raise cylinder pressure, which translates to more power. The CR you mention shouldn't be a problem as far as the mechanical parts, including starter and cooling system.

One downside is fuel requirements. I'd check and be sure what your final CR is with the pistons you install, just to be sure you are hitting the mark you want. It would be more than frustrating if you found more compression and didn't know it.

Another downside might be valve to piston clearance. You'll want to check that before fully installing the heads.

 

[flyfish]

I see no downside at 10.5:1...it's still pump gas, just a few cents more for good pump gas....and more power always sounds good to me.

 

[cudavert]

I've always been a big fan of higher compression.....sure, you've got to deal with the possibility of detonation, but there are little tricks here and there to help alleviate it. Running a cooler thermostat, a little richening of the mixture, a cold-air intake setup, playing with timing (or sometimes getting rid of the vacuum adv.), and of course, a little bit hotter cam can help "bleed off" some of that higher cyl pressure @ lower RPM's. As far as I know, higher compression is more efficient and increases mileage too.

 

[godfatherofchry]

i have to agree with the past two posts and you will be glad you did wen you here it

 

[rumblefish360]

I see no downside at 10.5:1...it's still pump gas, just a few cents more for good pump gas....and more power always sounds good to me.

Cam depedent not to mention the ocatane that is avialable to him, could make that a challenge or a no go for him. I'm also thinking about cam size. A stock cam of the everyday mill isn't going to cut it.
A reproduction or like similar cam would do great though!

10.5-1 is/was an advertised ratio. Actual ratio will be different. Do you know how many cc the heads are?

Egge will make you pistons for this engine. If you need valve reliefs, you'll have to spec them so they can be added.

 

[RedFish]

If the cam grind will allow the inhale stroke to bleed back wouldn't it load up and drown after idling for a while ? What works on the race track might not work in street traffic.

 

[caferacerx]

I would go with around 9.5:1 compression ratio and a really good set of heads

 

[rumblefish360]

If the cam grind will allow the inhale stroke to bleed back wouldn't it load up and drown after idling for a while ? What works on the race track might not work in street traffic.

What you may be thinking off, at least I think... maybe.. is a cam on a low numeric centerline and a bit on the large side for the size of the engine.
Something like;
244 duration @ .050/whatever lift on a 106 centerline for example can be a bit large for this size engine. No mention of gear or stall, but just assume a Hwy. like set up. EEEEK!

I agree, race track grinds will act as such and there not the friendliest cam to drive the street with. Stop and go traffic can be a bummer.

 

[C130 Chief]

Raising compression ratio will, all else being equal, increase torque throughout the entire RPM range.

10.5 was a factory combo for the 273, and I wouldn't consider building one any other way. You will probably need to run pump premium. Make sure you polish the combustion chambers, or at least knock down any sharp edges.

 

[1967DartGT]

A con for the 273 will be the cost of the 10.5:1 pistons.

 

[RedFish]

We can cuss and discuss the pros and cons of higher compression all day.
In the end, ma mopars engineers got it right when they put a lighter rotating assembly in the 340.

 

[rumblefish360]

Raising compression ratio will, all else being equal, increase torque throughout the entire RPM range.

10.5 was a factory combo for the 273, and I wouldn't consider building one any other way. You will probably need to run pump premium. Make sure you polish the combustion chambers, or at least knock down any sharp edges.

Goooood idea!

I think I read somewhere that 1 pt. in the ratio is good for 3% increase. Anybody want to comment on that? Going from his.... I'm sure, under 8.0-1 ratio, he would be looking at a 10% increase in power alone with new pistons and nothing else.

A con for the 273 will be the cost of the 10.5:1 pistons.

As allways the problem with engines like this.

 

[cudavert]

Whatever you do, don't put a hot cam, giant combustion chambered 360 heads, flat-top 8:1 pistons (maybe .050 "in the hole") and that crazy open plenum Commando intake manifold.......then we'd have to talk you down from 10th story ledge.

I've made a couple of these mistakes already. 20-something years ago, me & a buddy decided to put a 308 adv. duration cam in a stock low compression 273.....no low end, and probably not much more on the top end either. It sucked.....the only saving grace was that it sounded wicked like a mo' fo'.

I guess there's millions who are guilty of this @ one point or another.....mismatched engine combos......(good thread?)

 

[Barracuda67]

Ok... Thank you all for your feedback. All good.

A little detail on engine spec.
I rebuilt this engine approx 1yr ago.
40 overbore but only piston available to me at that time were 4-valve relief slugs with a concaved (dished) top.
Mech. Cam and lifters and matched springs from comp cams. lift is I think .468 not sure on duration (I will get specs later, at home, I'm at work).
20 thou off heads, cleaned up and port matched.
Weiland Stealth intake.
Edelbrock Thunder series 500cfm carb. (adj. secondaries)
1970 340 hp exhaust manifolds and 2" dual stainless
Mopar Orange box electronic. ignition.

904 trans with shift kit and 800rpm over factory stall torque converter

71/4 rear factory suregrip and still running 276 gears. (but I have a set of 323's in a box somewhere in the garage)

Thanks,
Marque

 

[moper]

I think you will find even teh "10.5:1" pistons are not even close. You should have a custom set of pistons made. It's more money, but you'll get exactly what you want in the end. I believe your 273 has closed chambered heads, so you need to get a zero deck piston, preferably a flat top with valve reliefs, and then you'll be stylin'.