Compression Ratio

-
D

Dan the man

Well-Known Member
Joined
Aug 1, 2014
Messages
5,149
Reaction score
2,789
Location
missouri
I don't have a car at the moment. I'm just doing my homework sorta speak. Can decent performance be gotten from a engine with a 8.0:1 - 8.25:1 compression ratio and the rpm's staying under 5,500? I'm wanting to get a plan / idea of just exactly what I'm wanting to do with the car once I get it so I'm not bouncing all over the place. I've bought notebooks to write things down so I won't be re asking the same questions over and over. I'm wanting to take a totally different approach this new year. But the car will still be for cruising, but hopefully with a little extra zip.
 
The answer is yes, yes I believe so. lol
Can you do it? Don't know, you tend to jump back and forth.
 
fishmens67 said:
The answer is yes, yes I believe so. lol
Can you do it? Don't know, you tend to jump back and forth.
Click to expand...
I know, that's why I'm wanting to take a totally different approach and take notes and pay closer attention to what you guys say.
 
Check out a Hughes Whiplash cam
Designed specifically for low compression engines, sounds great
 
After reading many of your posts, I think a Dart with a 318, 4bbl, headers and a set of 3.73's would by a fun ride for you. also a small cam if you want.
 
I see how you all enjoy your cars and this hobby of ours. I told myself get a plan together and stick with it. Cost, that's all part of the hobby just like any other hobby. If a part seems high to me I might ask if anyone knows of a better deal, but no more griping about it, it is what it is. It's a new year coming up and it's over due for a better outlook and attitude on my end.
 
fishmens67 said:
After reading many of your posts, I think a Dart with a 318, 4bbl, headers and a set of 3.73's would by a fun ride for you. also a small cam if you want.
Click to expand...
I know that this has been said more times than I can remember, something like a 260* cam?
 
Dan the man said:
I don't have a car at the moment. I'm just doing my homework sorta speak. Can decent performance be gotten from a engine with a 8.0:1 - 8.25:1 compression ratio and the rpm's staying under 5,500? I'm wanting to get a plan / idea of just exactly what I'm wanting to do with the car once I get it so I'm not bouncing all over the place. I've bought notebooks to write things down so I won't be re asking the same questions over and over. I'm wanting to take a totally different approach this new year. But the car will still be for cruising, but hopefully with a little extra zip.
Click to expand...
Is compression ratio important yes, can you build power with low cr yes, with a mild build at best you want 8.8-9.2:1 with pump gas and mild cam, is gonna kill your performance to be a point lower no is about 4% power loss so a 250-300 hp engine will be 10-12 hp lower.
I think people over sweat cr.
 
Dan the man said:
Do you have something like this for a 300/318?
Click to expand...
Yeah, i had to think back many years. Same intake and carburetor as the 360, stock compression ratio 318 bottom end, mildly ported 360 heads, 273 adjustable rocker arms and a Comp Cams 270S solid flat tappet camshaft. Sounded like a big block when we opened the headers.
 
IQ52 said:
Yeah, i had to think back many years. Same intake and carburetor as the 360, stock compression ratio 318 bottom end, mildly ported 360 heads, 273 adjustable rocker arms and a Comp Cams 270S solid flat tappet camshaft. Sounded like a big block when we opened the headers.
Click to expand...
Thanks, I'm going to write this down so I don't forget.
 
I’m asking what YOU think the benefit of running less than 9.5:1 is.

I can’t see a reason to EVER run compression that low.

Your thoughts please.
 
Rat Bastid said:
I’m asking what YOU think the benefit of running less than 9.5:1 is.

I can’t see a reason to EVER run compression that low.

Your thoughts please.
Click to expand...
I was going by what a factory stock 318 would have for compression from the factory. I was thinking that if I was to get a car with a bone stock 318 and it ran fine and didn't smoke what performance could I expect from it with a few bolt ons. I'm taking about a LA 318. Of course if it would need a rebuild then I would raise the compression ratio.
 
Dan the man said:
I was going by what a factory stock 318 would have for compression from the factory. I was thinking that if I was to get a car with a bone stock 318 and it ran fine and didn't smoke what performance could I expect from it with a few bolt ons. I'm taking about a LA 318. Of course if it would need a rebuild then I would raise the compression ratio.
Click to expand...

this is actually a solid plan. get something that runs and cruise it around for the season while you build something to drop in later.

more than likely you're going to find something that's already had a go thru and if has an intake and carb that's great! heck, maybe a cam and headers, even better!

if that's the case just leave it alone and drive it. resist the temptation it "make it yours" or mess with it (unless it's radically over cammed or carb'd) and save all the work and money on building another motor exactly how you want it.
 
The only reason to go low compression is because you are reusing pistons you already have or you only want to run low grade gas. As mentioned, cam choice will be critical. I have bad memories of a low compression 440 I built man years ago. My buddy and I just looked at each other with disgust when I put the foot to the floor for the first time. :steering:
 
Dan the man said:
I was going by what a factory stock 318 would have for compression from the factory. I was thinking that if I was to get a car with a bone stock 318 and it ran fine and didn't smoke what performance could I expect from it with a few bolt ons. I'm taking about a LA 318. Of course if it would need a rebuild then I would raise the compression ratio.
Click to expand...


What you are missing is most factory stuff is a point or more under the advertised CR.

And you throw away way more giving up just a few percentage points of HP.

It‘s crazy to do that.
 
But, you must remember something else ....
Something I had heard for years, and passed off as something only needed for actual "race" cars, and used to never pay much attention to, but learned the hard way, and again verified on my most recent build, which was a /6 of all things. ..

Most stock engines as built from the factory, actually fall short (some "way" short) of commonly " advertised" compression ratio, largely due to something called "tolerance stack up".
Everything needs to be measured, CC'd, and all the math done to see what you really have ... even on a rebuild for a docile daily driver or else be prepared to be disappointed.
 
IQ52 just posted dyno figures on a mild low compression 360 less than hour ago. Looked great for a street cruiser, plenty of low end torque
 
str12-340 said:
IQ52 just posted dyno figures on a mild low compression 360 less than hour ago. Looked great for a street cruiser, plenty of low end torque
Click to expand...

As hard as this is for me to say, there is more to compression than horsepower.

Now I’m going to go throw up.













Now that that’s done, you can’t look at dyno numbers only and compression ratio. There is more to it than that and a water brake or eddy current dyno can not measure that.

Horsepower is KING, but you don’t get all the info from the dyno.
 
Rat Bastid said:
What you are missing is most factory stuff is a point or more under the advertised CR.

And you throw away way more giving up just a few percentage points of HP.

It‘s crazy to do that.
Click to expand...
what's wrong with rolling something for a bit while you build something bitchin in the mean time?

even if you threw a used/Hong Kong phooey intake at it, used carb and a new cheapy swizzle stick at it you'd be, what, 500 and a weekend to go party?

I'd splash that cash easy for a quick grin and scratch that itch while building up Ricky racer motor in the meantime to drop in during the off season.

there isn't a ton of lost investment there-- if planned appropriately and bought judiciously a new intake, carb and headers could all get swapped onto the motor you're building.

not everything has to be Johnny kick *** out of the gate.
 
You must log in or register to reply here.
-

Back
Top