Compression ratio and boost

90sc35thann

Registered User
I know our compression ratio is about 8:1. I was doing some thinking and does anyone have an idea how this is affected by the increase in volumetric efficiency under boost? I am more curious than anything.
 
That calculation leaves a lot of things on the table. It's not the same as 17:1 N/A. The volume of any gas is relative, and so since the air is under pressure prior to entry into the cylinder, the end result is not the same.
 
ECR = [(Boost PSI/14.7) +1] x static CR.

And the stock static CR is 8.2:1, not 8:1, IIRC.

I was just taking a guess at the stock CR. I was more interested in the formula you had lsited above. I knew it was out there somewhere. That's pretty cool. Just so I better understand. I understand why you are dividing by 14.7 (pressure at sea level), but why add "1" ? Is that how you adjust for absolute pressure?
 
That calculation leaves a lot of things on the table.

It certainly does. It just sounded like that's what the OP was asking about (that equation).


I understand why you are dividing by 14.7 (pressure at sea level), but why add "1" ? Is that how you adjust for absolute pressure?

Adding "1" makes the multiplier greater than one. Otherwise, boost levels less than 14.7 psi would lower the ECR, which isn't correct.
 
That calculation leaves a lot of things on the table. It's not the same as 17:1 N/A. The volume of any gas is relative, and so since the air is under pressure prior to entry into the cylinder, the end result is not the same.

I totally agree with that. I know you can't bump up compression to 17:1 on an NA motor and get similar performance Vs. forced induction. My brain was just spinning some stuff around with things I read and I wasn't sure if there was some type of funky calc out there for something like this. Curiousity more than anything.
 
I totally agree with that. I know you can't bump up compression to 17:1 on an NA motor and get similar performance Vs. forced induction. My brain was just spinning some stuff around with things I read and I wasn't sure if there was some type of funky calc out there for something like this. Curiousity more than anything.

He's basically saying that every motor's cylinder pressure, NA or boosted, will result in some comparable static compression ratio at peak cylinder pressure. Compression ratio is measured as the volume of the cylinder/combustion chamber at the bottom of the piston's stroke divided by the volume of the cylinder/chamber at top dead center. It isn't a function of air.

If two motors with the same shortblocks and same compression ratio make 600hp then they have equal VEs. Regardless if one is NA and the other is FI.
 
LIke CMAC said though Boost and compression ratio are independent. I positioned my question improperly. I was more curious about how much the volumetric efficiency is typically increased via an M90 sitting on top of our engines. Obviously there are many factors but was curious about the average gain.
 
THATS UNDER FULL BOOST:confused: DON,T THINK THESE MOTORS WOULD LAST LONG AT 17 TO 1:eek:

You're missing the point of the equation.

LIke CMAC said though Boost and compression ratio are independent. I positioned my question improperly. I was more curious about how much the volumetric efficiency is typically increased via an M90 sitting on top of our engines. Obviously there are many factors but was curious about the average gain.

That will vary greatly based on blower drive ratio, amongst other variables.
 
LIke CMAC said though Boost and compression ratio are independent. I positioned my question improperly. I was more curious about how much the volumetric efficiency is typically increased via an M90 sitting on top of our engines. Obviously there are many factors but was curious about the average gain.

VE doesn't change just because you have boost. Air is not forced into the combustion chamber, it's still drawn in same as NA. The difference is simply that the air is now at higher density, but the system still functions the same way. Because of this, an SC engine at 14.7psi boost (2x atmospheric pressure), doesn't not make anywhere near 200% VE. So 15psi on your SC is nothing at all like 15psi on an LS3 engine.
 
Wow. :) All i want to say is again I positioned my question incorrectly. I never made any crazy claims and I was more curious but now wishing I had never asked.......... :) Again I positioned the original question incorrectly. Maybe I was tired, who knows.

I would never compare our performance gains to a modern engine with cylinder head designs that are way more efficient than ours. I know our SC's engine are essentially antiques. All I wanted to know if someone knew how much the average VE gain was on our engines with an M90. I never meant to start any of this.

DD check your private messages. :)
 
I don't think you started anything, I just said there is more to it than meets the eye, so stating a theoretic compression ratio based on a basic formula of atmsphere + boost is not going to give you any kind of accurate idea about what is actually going on. It's just a funny number and the comparison to the LS3 is just a random reference intended to get the thought process going in your head.

If an engine doesn't achieve 100% VE on it's own (ours doesn't even come close), then adding 15psi boost to that will only result in whatever the base is, plus 15 x whatever your existing VE is. So if your engine only makes 65% VE on it's own, then 14.7 x .65 = 9.5 in the motor. Based on this you start to realize that your 15psi boost really only means 9.5psi, and that is relative to the original 9.5psi generated NA by a 65% efficient motor, now you have 9.5 + 9.5 - 14.7 = 4.3psi actual boost vs. a 100% efficient NA motor. I've oversimplified it, but you start to get the idea.

Then there is the whole thing about increased density, speed of flame travel, cooling effect of the richer mixture + increased "volume" that you've squeezed into a smaller cylinder/combustion chamber, and you have something that plays by a whole different set of rules than something that is truly NA.

That's all I was saying.

As for what is common in an SC motor? At 15psi I normally record somewhere around 150% VE, but that varies a lot. I've recorded over 200% VE plenty of times, but the way the Ford EEC calculates VE is still just a shot in the dark because the EEC doesn't know how much is bypassing the combustion process due to valve overlap and over-scavenging.
 
Also, if you want to really use #'s that matter for calculating a compression ratio, you have to take into account dynamic compression ratio, because an engine never actually runs at it's static compression ratio. Back in the day the OE's used to run 12.5:1 compression ratio's on street cars and the gas wasn't all THAT much better than today. The difference is the cams in those motors had wide lobe separation angles to bleed off the compression until rpms go high enough to be able to take advantage of it without resulting in detonation. The wide lobe separation significantly reduces the dynamic compression ratio and reduces cylinder pressure in the lower rpms.

But I digress.......
 
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