Compression Ratio

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Gotta say I just build what I want to build without input from anyone. I don't design engines for anyone and just report how I build mine. I don't tell anyone else what they need.

Have iron headed 360 based pump gas engines from 7.5:1 to 11.3:1 that make 260-620 horsepower normally aspirated. And normally aspirated pump gas 440 based iron headed engines from 7.5:1 to 11.9:1 that make 330-740horsepower.

They perform what function that I alone want and don't give a rat's behind what others think.

Go for your build Dan.
 
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IQ52 said:
Crap, I think I may have just contributed to the yawn.
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All the talk about compression was boring me.
What Dan want's is something to drive with a little pep.
I think you gave a great example.
 
fishmens67 said:
All the talk about compression was boring me.
What Dan want's is something to drive with a little pep.
I think you gave a great example.
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My two cents....nearly worthless... but the sooner this guy gets something going by the seat of his pants the better. I have a relatively low compression 400 with an awesome cam that will blow your sox off... I know it can be done. He needs to get on with it. :steering:
 
Not every engine has to be picture perfect. If it keeps you in the game and puts a smile on your face, that's all that matters. There are plenty of good running low compression engines out there with TONS of miles on them.
 
Stock 318 with bolt ons for cheap? Here's my result : 72 Duster . Stock 78,000 mile 318 . Summit 272 454 cam and lifters I got for $100 on Black Friday. $20 replacement timing set . Yanked the heads and did a quickie bowl blend, Comp 901 springs and new valve seals. LD4B intake and 650 DP. Dougs headers with 2.5 exhaust. 3.23 gears. Went 14.39 at 96 with stock converter on Cooper Cobras. Installed a PTC 3500 converter and M/T ET Streets and went 13.89 at 97. I only have nine passes on it with no real tuning so far. This was supposed to be a temporary engine until I build a 360 (or 408) but it may be in there for awhile.
 
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.
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Wow, I didn't know that there was that much difference. Thanks for pointing that out to me
 
Increasing hp with compression is NOT a linear scale. As the comp ratio gets higher, there is less gain. Going from 8:1 to 9:1 is worth about 2%.
 
It can be more in efficiency among a number of other things.
 
Post #41, correct, not much gain. Probably measurable on a dyno, but the butt-meter is unlikely to feel it. So if you have the parts already for a 8:1 engine, the decision becomes: is it worth spending more $$ for a one point increase in CR that might not be felt. The higher CR will help with idle quality/vacuum if you increase cam size, but then again if this is just a cruiser & the cubic inches are going to do the work, it would be fine with the lower CR & mild cam.
 
I'd think a mild Magnum build would do the trick, lots of pluses. Plenty of them around cheap too.
 
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Dan the man said:
That's not much of a gain
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Don’t buy that nonsense. As I already said, the dyno has limitations and that’s one of them.

Most of the guys spouting percentage gains from compression ratio increases get that from one (maybe more) of Vizards books. I’ve seen similar charts and graphs and they all say the same thing.

BUT…they never read the text under the chart/graph and if they did they would know that that percentage is the MINIMUM gain to be expected. Not the maximum.

If you are building something like the “Mission Impossible” engine, then I’d run bigger heads and live with the compression ratio I get.

If I’m building something that isn’t limited by rules or the constraints of some self imposed engine building flagellation then I put compression in it.

One last thing Dan. If you want to learn this stuff then you have to keep learning for the rest of your life. You should learn something every day and then apply it to validate what you’ve learned.

The learning curve is very steep, but today this information is out there from far more reliable sources than myself.

Verify everything, trust nothing until you PROVE it out.

To that end, spend some time learning about expansion ratio and how it affects engine performance. Once you get a grasp of that you will wonder why some many are willing to give up compression for old wives tales and fables.

That doesn’t mean you can build a 4200 pound car, use a 2200 converter and a 2.75 gear so you can run 95 MPH down the freeway and use 11:1 compression.

You have to apply some common sense and have a pretty decent grasp of ICE theory. But most guys can run quite a bit more compression than they do with very minor changes to the build.
 
Bewy said:
Post #41, correct, not much gain. Probably measurable on a dyno, but the butt-meter is unlikely to feel it. So if you have the parts already for a 8:1 engine, the decision becomes: is it worth spending more $$ for a one point increase in CR that might not be felt. The higher CR will help with idle quality/vacuum if you increase cam size, but then again if this is just a cruiser & the cubic inches are going to do the work, it would be fine with the lower CR & mild cam.
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Rat Bastid said:
Don’t buy that nonsense. As I already said, the dyno has limitations and that’s one of them.

Most of the guys spouting percentage gains from compression ratio increases get that from one (maybe more) of Vizards books. I’ve seen similar charts and graphs and they all say the same thing.

BUT…they never read the text under the chart/graph and if they did they would know that that percentage is the MINIMUM gain to be expected. Not the maximum.

If you are building something like the “Mission Impossible” engine, then I’d run bigger heads and live with the compression ratio I get.

If I’m building something that isn’t limited by rules or the constraints of some self imposed engine building flagellation then I put compression in it.

One last thing Dan. If you want to learn this stuff then you have to keep learning for the rest of your life. You should learn something every day and then apply it to validate what you’ve learned.

The learning curve is very steep, but today this information is out there from far more reliable sources than myself.

Verify everything, trust nothing until you PROVE it out.

To that end, spend some time learning about expansion ratio and how it affects engine performance. Once you get a grasp of that you will wonder why some many are willing to give up compression for old wives tales and fables.

That doesn’t mean you can build a 4200 pound car, use a 2200 converter and a 2.75 gear so you can run 95 MPH down the freeway and use 11:1 compression.

You have to apply some common sense and have a pretty decent grasp of ICE theory. But most guys can run quite a bit more compression than they do with very minor changes to the build.
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Very well said
 
I used one of those compression ratio calculators and I put the information in that I thought I might be using such as deck height and chamber size and I got just a little over a 9.0:1 compression ratio. Please keep in mind that I was only doing this to see were I would be as far as the compression ratio goes. After the new year holiday I have a couple of cars to check out.
 
Rat Bastid said:
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.
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couldn’t agree more. As an example, take a 10 to 1 small block stroker, and a 12 to 1 stroker.
the difference might be “ supposedly “ 25 horsepower or so, but when you get that car to the track, the acceleration and shift recovery are going to be night and day different.
compression really effects a time slip, way more than the 3 or 4% gain typically bantered around on non stratosphere type compressions…..ie 16.5 pro stock motors.
any chance/ way I can to raise squeeze, I do. Partially because I will run whatever fuel is required, but also because the torque and crackle are great byproducts. And the slip is definately gonna look much nicer
 
Rat Bastid said:
Don’t buy that nonsense. As I already said, the dyno has limitations and that’s one of them.

Most of the guys spouting percentage gains from compression ratio increases get that from one (maybe more) of Vizards books. I’ve seen similar charts and graphs and they all say the same thing.

BUT…they never read the text under the chart/graph and if they did they would know that that percentage is the MINIMUM gain to be expected. Not the maximum.

If you are building something like the “Mission Impossible” engine, then I’d run bigger heads and live with the compression ratio I get.

If I’m building something that isn’t limited by rules or the constraints of some self imposed engine building flagellation then I put compression in it.

One last thing Dan. If you want to learn this stuff then you have to keep learning for the rest of your life. You should learn something every day and then apply it to validate what you’ve learned.

The learning curve is very steep, but today this information is out there from far more reliable sources than myself.

Verify everything, trust nothing until you PROVE it out.

To that end, spend some time learning about expansion ratio and how it affects engine performance. Once you get a grasp of that you will wonder why some many are willing to give up compression for old wives tales and fables.

That doesn’t mean you can build a 4200 pound car, use a 2200 converter and a 2.75 gear so you can run 95 MPH down the freeway and use 11:1 compression.

You have to apply some common sense and have a pretty decent grasp of ICE theory. But most guys can run quite a bit more compression than they do with very minor changes to the build.
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The first guy I heard who talked about those percentage increases was John Lingenfelter.
But realistically, once those cams get big enough, you're more concerned about "how" you're going to get "enough" comp in there... LOL
 
Bakerlite said:
The first guy I heard who talked about those percentage increases was John Lingenfelter.
But realistically, once those cams get big enough, you're more concerned about "how" you're going to get "enough" comp in there... LOL
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For my goal, do I really need more than a 9.5:1 cr? I'm not going for big power, but I do want a improvement over stock.
 
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