Cam / Compression Ratio

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Dan the man

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I know that I've been told this before and I tried finding it in my other post but had no luck and I'll write it down this time. Do I need to know what compression ratio I'll have before picking out a cam? I've been reading more about camshafts, I didn't even know what dynamic compression was until I joined this forum. Ivc affects the dynamic compression right? I'm wanting to get a cam that will work well with the compression ratio that my engine will have. I think that with all the help that I got here and reading more that I should have a decent running engine.
 
What I did was found the cam for my goals then planned the compression around the cam to get to the dcr I desired. There are calculators and formulas to get you close. I have them written down somewhere. But take my advice and plan everything out and shop parts and all that before buying anything. Because I ended up buying stuff twice because I didn't plan ahead very well.
 
I forgot to mention there are alot of ways to achieve your final compression if things don't go exactly as planned. And another thing don't get caught up in an exact number. Find a good a good cam that fits your needs and then get as close as possible to your desired compression. As a general rule you want a 8:1 dcr for pump gas. So get your calculator and see what compression you will need to achieve 8:1 with whatever cam you pick. And if you need it have the formula to get the IVC for the calculator. Wallace Racing: Dynamic Compression Ratio Calculator
 
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dartfreak75 said:
What I did was found the cam for my goals then planned the compression around the cam to get to the dcr I desired. There are calculators and formulas to get you close. I have them written down somewhere. But take my advice and plan everything out and shop parts and all that before buying anything. Because I ended up buying stuff twice because I didn't plan ahead very well.
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I'm looking at Howard's 255* / 261*. If I was to have a 9.42:1 compression ratio the dynamic is 6.43:1, is this a safe dynamic compression ratio for the cam I mentioned? No parts bought yet.
 
dartfreak75 said:
I forgot to mention there are alot of ways to achieve your final compression if things don't go exactly as planned. And another thing don't get caught up in an exact number. Find a good a good cam that fits your needs and then get as close as possible to your desired compression. As a general rule you want a 8:1 dcr for pump gas. So get your calculator and see what compression you will need to achieve 8:1 with whatever cam you pick. And if you need it have the formula to get the IVC for the calculator. Wallace Racing: Dynamic Compression Ratio Calculator
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I looked for the ivc specs for the Howard cam that I was looking at but had no luck. That cam has 208* / 214* duration at 0.050, 106* icl.Do I need a 8.0:1 dcr?
 
Dan the man said:
I'm looking at Howard's 255* / 261*. If I was to have a 9.42:1 compression ratio the dynamic is 6.43:1, is this a safe dynamic compression ratio for the cam I mentioned? No parts bought yet.
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Thats pretty low. Ideally your want 7:1- 8:1. Thats a pretty aggressive cam for a street car. What is the rest of the combo? Auto manual rear gear engine etc?
 
Dan the man said:
I looked for the ivc specs for the Howard cam that I was looking at but had no luck. That cam has 208* / 214* duration at 0.050, 106* icl.Do I need a 8.0:1 dcr?
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I mean no you don't have to but it won't have as much bottom end power. All the dcr does is show you the real world compression with the cam installed and with elevation. Its basically your engine cranking pressure if everything is perfect. So with a 6.5:1 your looking at around 116 psi cranking pressure. That's like a stock smog 318 or even less
 
Dan,
What number are you using for the intake closing point?
Is the cam you are looking at is the howards 711381-10? That's a relatively mild cam, your DCR seems very low for 9.4 static even at high altitude.
Intake closing should be around 54* installed at 106.
 
Depends what your doing but for Maiinly Street engine, I’d say cam then cr.

if you start out the other way around depends what cr you pick your cam choices narrow and could force to run lot larger cam than you want.

Eg… if you want a cam like xe250h your not gonna be able to run a lot of cr with pump gas.

But for the most part people that build engines know what level of performance their going after and know around what cr and cam they’ll need off the bat.
 
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Because you are on pump gas I would pick the compression ratio first. Why? Because you are limited by the fuel you can get at the pump. Maybe you only want to use 87 octane pump gas, or just run it on mid grade.

Whatever you decide that’s what you have to live with. If you pick the cam first, you then have to use a camshaft that meets that one criteria first, and then the cam needed may not run your power brakes or maybe it needs a looser converter or more gear than you want.

In fact, I almost always pick the cam last, after I have the compression ratio, head flow, tire diameter, gear ratio, shift rpm, trap rpm (if you are going to drag race it some) and highest cruise rpm you’ll see. Then you can make the best decision on cam timing, rather than picking cam timing first and making that dictate everything else.

I understand cam selection is sexy and exciting but that one choice will have more influence of how the engine behaves than any other single component.
 
Every engine I built for my car I planned thoroughly with the end result as my guide....for example, my current engine I wanted to be able to run at least 10.4 in the 1/4 mile in the heat of summer so I could comfortably run a 10.5 index class at the local track. This goal dictated what parts I needed to achieve the goal. I also wanted to be able to keep my car street legal so I could drive around town....this made a rather narrow path for me to walk with the build, but it gave me direction. I also over shot by just a little on the power, but hey, I'm not complaining, lol.

Like Rat Bastid said, if running pump gas is important to you, then make that a must in your build check list and plan around it. Most things you can make work....unless you have some crazy unrealistic goal like running 7's in the 1/4 whilst getting 30 MPG with the air-conditioner running.....now that I say that, I'm sure someone out there will be like "Challenge ACCEPTED!!" :lol:

Long story longer, start with a goal in mind, then build engine to fit.
 
I almost always pick the cam last
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Same here.

Just make sure the approach to the whole build is pointed towards the same goal.

Using the Wallace calculator........... for a 9.4:1 318+.030, with that 255 Howard’s cam, in at 106, at sea level, I got just over 8.0 dynamic, and 165 cranking pressure.
 
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Ted265 said:
Dan,
What number are you using for the intake closing point?
Is the cam you are looking at is the howards 711381-10? That's a relatively mild cam, your DCR seems very low for 9.4 static even at high altitude.
Intake closing should be around 54* installed at 106.
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That's the camshaft I'm looking at. Were did you find the ivc? I did another dcr and came up with 7.42:1 is that good for the cam I'm looking at? Why doesn't Howard's cam list this information? Is there a simple formula to figure inv?
 
PRH said:
Same here.

Just make sure the approach to the whole build is pointed towards the same goal.

Using the Wallace calculator........... for a 9.4:1 318+.030, with that 255 Howard’s cam, in at 106, at sea level, I got just over 8.0 dynamic, and 165 cranking pressure.
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I came up with a 7.42:1 dcr and a 9.69:1 scr. I used a online scr / dcr calculator.
 
Ted265 said:
Dan,
What number are you using for the intake closing point?
Is the cam you are looking at is the howards 711381-10? That's a relatively mild cam, your DCR seems very low for 9.4 static even at high altitude.
Intake closing should be around 54* installed at 106.
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Can I use mid grade gas or do I need premium?
 
MOPAROFFICIAL said:
You can run 87 on that.
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Awesome. Hopefully gas prices will drop again soon. I'd like to get 300 flywheel horsepower from the 318. Looking at speedmaster cylinder heads. A 650 carb, I'm not sure what intake to run since I changed plans on the cylinder heads and cam that I'll use
 
What am I missing here?

8A0655CC-7844-4C47-AE94-AAB6EFDAF987.png
 
PRH said:
What am I missing here?

View attachment 1715886499
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Cylinder head cc's, head gasket thickness, piston to deck clearance, valve relief size. When I put in all of the required information I got a 9.69:1 scr and a 7.42:1 dcr. I had to enter the connecting rod length and ivc for that I enter 54*. I believe that information is top secret. Comp cams is the only one that lists all required information.
 
PRH said:
What am I missing here?

View attachment 1715886499
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It's becoming to be more hassle than it's worth to me. I used a scr / dcr online calculator and I put in all of the required information that it asked for and I checked it twice before I hit the calculate button. I can't understand why 2 or 3 people are getting different results.
 
You do see the screenshot of the info I put into the Wallace calculator, right?

If you change the CR to 9.69, in that calculator, the dynamic CR would just go up.

Why don’t you post up a screen shot of the info you used?
And which calculator is it?
 
PRH said:
You do see the screenshot of the info I put into the Wallace calculator, right?

If you change the CR to 9.69, in that calculator, the dynamic CR would just go up.

Why don’t you post up a screen shot of the info you used?
And which calculator is it?
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How can it go up? Are online calculator's a joke? The calculate simply shows what I posted I have no idea why yours is different. It's beginning to much of a pain in the butt
 
PRH said:
You do see the screenshot of the info I put into the Wallace calculator, right?

If you change the CR to 9.69, in that calculator, the dynamic CR would just go up.

Why don’t you post up a screen shot of the info you used?
And which calculator is it?
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Thanks everybody for your help and advice, but I'm thinking about leaving it bone stock.
 
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