Cam / Compression Ratio

[PRH]

All else unchanged, if you raise the static CR...... you raise the dynamic CR.

I looked at the UEM calculator, and it estimates the CR too high because of their “estimated” ivc point, instead of the actual ivc point.
Their estimated ivc point(.050 close + 15deg) is 9 degrees off.

54abdc is not the .050 close + 15deg point.
So, if you don’t use the actual closing point of 54abdc....... it’s a case of GIGO.

 

[PRH]

Addressing your original question, where you wanted to know how much compression for a certain cam......
This simple calculator gets you there.

You want to juggle the static CR and the ivc point to end up at no more than 8.0:1 for pump gas.
You’ll also want to use the average uncorrected barometric pressure for where you live.

9.25:1, 255deg advertised duration hyd cam in at 106, near sea level:

 

[Ted265]

T
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?

So your advertised duration of the intake lobe is 255*, intake centrline is 106*.
So split the duration and it will give you 127.5*. So the advertised intake opening and closing will be 127.5* either side of the installed centreline.
Intake closing will be 106 + 127.5 = 233.5* after TDC or 53.5* ABDC. (Round up to 54*)

 

[Dan the man]

All else unchanged, if you raise the static CR...... you raise the dynamic CR.

I looked at the UEM calculator, and it estimates the CR too high because of their “estimated” ivc point, instead of the actual ivc point.
Their estimated ivc point(.050 close + 15deg) is 9 degrees off.

54abdc is not the .050 close + 15deg point.
So, if you use the actual closing point of 54abdc....... it’s a case of GIGO.

What is the actual ivc of that cam or is it something that Howard's doesn't want people to know? I don't want to use guessing games or close numbers

 

[PRH]

See post 28

 

[PRH]

The UEM calculator, but using the actual ivc point(53.5) instead of their erroneous .050 ivc + 15deg(which is 45abdc).

So, if it were me....... I’d be shooting for about 9.25:1 to use that cam.

 

[RustyRatRod]

Figure out what your maximum RPM will be on a regular basis and cam according to that. In other words, where the engine will spend most of its time with tire size and gear ratio taken into consideration. Too small or too large from that point and you may not be happy with it.

 

[273]

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.

Over thinking this. It's a mild cam I'd go with 9:1 Cr call it a day.

 

[Dan the man]

The UEM calculator, but using the actual ivc point(53.5) instead of their erroneous .050 ivc + 15deg(which is 45abdc).

View attachment 1715886565

So, if it were me....... I’d be shooting for about 9.25:1 to use that cam.

That's what I'll do. I was getting pretty aggravated there for a while

 

[273]

A point of Cr is worth around 5% hp, increase are little better at the lower end of Cr eg. 8:1 to 9:1 vs 10:1 to 11:1. So a half or quarter point is gonna be what 5_10 hp at best. I'd rather sacrifice a small bit of power then push thing and have pinging mess.

 

[Dan the man]

A point of Cr is worth around 5% hp, increase are little better at the lower end of Cr eg. 8:1 to 9:1 vs 10:1 to 11:1. So a half or quarter point is gonna be what 5_10 hp at best. I'd rather sacrifice a small bit of power then push thing and have pinging mess.

Me too

 

[RustyRatRod]

Over thinking this. It's a mild cam I'd go with 9:1 Cr call it a day.

Exactly.

 

[RustyRatRod]

A point of Cr is worth around 5% hp, increase are little better at the lower end of Cr eg. 8:1 to 9:1 vs 10:1 to 11:1. So a half or quarter point is gonna be what 5_10 hp at best. I'd rather sacrifice a small bit of power then push thing and have pinging mess.

It's actually only around 3%, which makes raising compression even less desirable in some cases.

 

[Dan the man]

It's actually only around 3%, which makes raising compression even less desirable in some cases.

Sounds like it

 

[PRH]

I'd rather sacrifice a small bit of power then push thing and have pinging mess.

My thoughts as well for something that “has to” run on pump gas.

 

[Dan the man]

My thoughts as well for something that “has to” run on pump gas.

I can't afford racing gas anyway, besides it wouldn't be practical for a daily driver

 

[K.O. SWINGER]

What kind of heads are we talking about stock open chamber / aluminum closed chamber, I would be curious with gear ratio automatic 4 speed? It all plays into it.

 

[Dan the man]

What kind of heads are we talking about stock open chamber / aluminum closed chamber, I would be curious with gear ratio automatic 4 speed? It all plays into it.

Speedmaster, closed chamber aluminum heads. Stock 904 trans. 3.55 posi. Probably a small stall speed converter.

 

[K.O. SWINGER]

Here's my take on information given. And I will say these cats on here probably know a lot more than I do I'm just going from experience. That camshaft will give you a lot of bottom and and great throttle response. I would go a full stage bigg something in the 220 to 226 on intake duration at 50. In my opinion probably be worth another 30 40 horsepower and your gear ratio and other supporting cast should work together nicely.

 

[Dan the man]

Here's my take on information given. And I will say these cats on here probably know a lot more than I do I'm just going from experience. That camshaft will give you a lot of bottom and and great throttle response. I would go a full stage bigg something in the 220 to 226 on intake duration at 50. In my opinion probably be worth another 30 40 horsepower and your gear ratio and other supporting cast should work together nicely.

Awesome

 

[Phreakish]

If you want valve events, they can be backed out of advertised (not @.050) numbers in most cases.

Wallace Racing - Cam Degree Calculator

 

[Dartfreak75]

So your advertised duration of the intake lobe is 255*, intake centrline is 106*.
So split the duration and it will give you 127.5*. So the advertised intake opening and closing will be 127.5* either side of the installed centreline.
Intake closing will be 106 + 127.5 = 233.5* after TDC or 53.5* ABDC. (Round up to 54*)

I have never did it that way but it works out the same way as I've done it. That is alot easier that how I've done it.

 

[MOPAROFFICIAL]

I can't afford racing gas anyway, besides it wouldn't be practical for a daily driver

180 cranking takes 91 octane....185 and it's really sensitive and the tune will need changing with the slightest of weather changes.
K.i.s.s
160-170 cranking is what id shoot for. Wiggle room.
Use the actual closing point on wallace and the numbers are spot on. Ime

 

[Dartfreak75]

180 cranking takes 91 octane....185 and it's really sensitive and the tune will need changing with the slightest of weather changes.
K.i.s.s
160-170 cranking is what id shoot for. Wiggle room.
Use the actual closing point on wallace and the numbers are spot on. Ime

My 10:1 360 worked out pretty close to the calculator. My goal was 8:5:1 dcr and a 160-170 cranking pressure.

 

[PRH]

For sure, the altitude matters.