It's all about the cubic feet

[bcjammerx]

So an old domestic tuner I once hung out with, in response to me quoting the adage, "it's all about the cubic inches (no replacement for displacement)"

said, "No, it's all about the cubic feet"

So I got to thinking tonight...how big is a 1 Cubic Foot engine in our understanding of liters.

from onlineconversion.com

(shortcut)
1 cubic foot = 28,316.846 cubic centimeters (cc's) = 28.316 846 592 liter

in other words...28 Liters...
3.5 Liters PER cylinder for a V8

Damn...that's stupid big. In fact...unless we are talking about big 'ol diesel engines or an all out dragster...that's almost impossible big for a street car. So what did he mean? Boost maybe?

I tried to do some looking around. From researching I gather you can not alter the engines displacement with boost, you increase the amount of air it displaces though. So how does that equate to engine cubic inches and psi? So now I'm again.

I guess I don't know what I'm asking and that doesn't help. I want to ask,
"Is there a formula or chart that tells you how much you increase the displacement of the engine at a given psi?"

..but I know that is incorrect thinking. I just don't fully know why I know how well the engine flows will effect that (volumetric efficiency) i.e. two engines that are identical and have identical turbo setups and tune setups...with the exception that one has smaller intake and exhaust ports...the one with smaller ports will not create as much power...and etc etc.

Anybody help a brother out? Cause this is all making feel quit ...stupid

[johnl]

"Cubic inches" is a reference to the cubic capacity of an engine, but capacity per se isn't necesarily an indicator of an engine's performance.

Even though raw capacity does have a huge influence, an engine of X cubuc inches may perform extremely well or extremely poorly relative to other engines of similar or disimilar capacity.

So, your 'old tuner' was right, it's not about cubic inches. So what does he mean by "it's about cubic feet"? My best guess is that he's referring to the amount of air that the engine can ingest in a given period of time, i.e. 'cubic feet per minute'.

Regardless of how it does it (and assuming close to correct fuel / air mixture), the amount of air an engine can ingest in a given time period is the major influence on how much horespower it can produce. Other things have an influence (e.g. compression ratio), but it's the ability to 'process' a huge volume of oxygen that ultimately makes big power numbers.

[bcjammerx]

is there any way to relate that to cubic inches and boost? .

HA HA...damn strings...always trying to figure out how long they are ain't I?

[johnl]

is there any way to relate that to cubic inches and boost? .

Not really.

HA HA...damn strings...always trying to figure out how long they are ain't I?

The trouble with stings is not only that their static length varies from string to string, but since they are all in varying degree somwhat elastic, their dynamic length also changes, and changes to differeing degree with different string materials...

[Potatoswatter]

is there any way to relate that to cubic inches and boost? .

Sure, the engine takes in so many cubic inches of air per revolution, times your boost ratio, and so many revolutions per minute. Multiply and convert to cfm through the compressor.

HA HA...damn strings...always trying to figure out how long they are ain't I?

WTF?

[johnl]

Sure, the engine takes in so many cubic inches of air per revolution, times your boost ratio, and so many revolutions per minute. Multiply and convert to cfm through the compressor.

The real world isn't that simple...

The biggest problem here is to do with your first sentence. Just because a cylinder has X capacity doesn't mean that it inducts X quantity of air (or that it will even induct the same quantity at different rpm, whatever that quantity may be). Usually it will induct less quantity than it's capacity suggests, but in some instances / at certain rpm it may well induct more (even without supercharging).

[Potatoswatter]

I didn't suggest "so many" equals the cylinder displacement.

Anyway, you can measure the CFM at 1 atm of any fan including a turbo. Hard to measure, sure, but not meaningless and it's an easy "bigger is more is better."

[bcjammerx]

string is a sort of inside joke

well how about compression ratio? Doesn't so much boost increase compression ratio? I've seen a chart somewhere...I think...

[bcjammerx]

another example of what I'm trying to ask.

Some dude was saying that with his 1.8 liter turbo he had to race in the same class as n/a 2.3 liters.

[deevergote]

Have you considered that maybe this guy didn't know what he was talking about?

[johnl]

There's usually an 'equivalance factor' applied to even out the playing field between NA and boosted cars, but of course it's quite arbitrary. A similar 'equivalance factor' is generally applied to cars fitted with Wankel engines.

[deevergote]

Yeah, rotary powered cars are usually put in the same class as engines twice their size (so the 1.3 is pitted against 2.6s)

Still, forced induction doesn't actually increase displacement.

[bcjammerx]

To the "cubic feet" thing yes...I'm pretty sure he's an idiot...that's not the only reason why though.

And yeah...I figured those "standards" were based on hp

...i've even seen guys on here say boost adds displacement. Guys who know their shit...but I'm not gonna go and quote 'em...this is something I wanna find out meself

It makes sense...your engine n/a can only take in so much air and will only add so much fuel to get to stoichiometric (well close to ideal). Any hoo if you looked at the volume of air it took in and compare it to what size an n/a engine would be that would also take in that much air.

Guess it doesn't REALLY make any difference, not something you use to tune...just cool to know.

[johnl]

It makes sense...your engine n/a can only take in so much air and will only add so much fuel to get to stoichiometric (well close to ideal). Any hoo if you looked at the volume of air it took in and compare it to what size an n/a engine would be that would also take in that much air.

Here's something to think about; with each induction stroke a well designed NA engine of X capacity in each cylinder can actually induct X+ air into the cylinder...

Volumetric efficiency significantly greater than 100% is possible with NA engines, but only with very highly tuned racing units over a very limited rpm, each side of which the VE probably falls away quite sharply. In effect such an engine is 'naturally supercharged' by the inertia of the mass of the air that is sonically resonating in the induction system.

[Potatoswatter]

Here's something to think about; with each induction stroke a well designed NA engine of X capacity in each cylinder can actually induct X+ air into the cylinder...

Yes, with carefully managed airflow



(It's turbo'd, I know…)