Pump Gas and Compression ratio

[Guest Zpeed]

I always hear about people saying that if a engine has like 12:1 compression or something it cant be done because of 92 octane,

 

if its 92 octane does that mean that the ratio must be 9.2:1 or something i'm not sure....

 

i do not really know, and i'm not goin to start building an engine that has 10:1 or something like that if i can't run it...

[74_5.0L_Z]

On pump gas with iron heads you are limited to about 9.5:1, if you go to an aluminum head you can run about 10.5:1. The higher your compression ratio, the higher the combustion chamber pressure will be. If the combustion chamber temperature reaches a critical temperature before the spark is introduced, you will get early combustion. A flame front will be started by compression then another initiated by spark. The colliding flame fronts cause knock, and huge pressure spikes. The result is broken engine parts. This is somewhat of an over-simplification. Your actual maximum compression is affected by the cam profile. If you run a cam with a lot of overlap, then you can use more static compression. For a motor that will live on the street I'd sick with the limits given earlier.

 

Thanks,

Dan

[Kevin Shasteen]

ZSpeed,

 

Comp.Ratio (9.2:1) has nothing to do w/the Pump Gas Octane rating...other than a coincidental link that street performance engines wont run very well on a pump gas w/an Octane Rating less than 92.

 

If you wish to understand Fuels/Oil's & how they get their ratings I'ld suggest you purchase a book entitled "High Performance Automotive Fuels & Fluids" by Jeff Hartman & Published by Motorbooks International w/a Copywrite date of 1996...very insightful book.

 

As far as you question on "What is Comp.Ratio"...that is as involved as the octane ratings of fuels. But here's the skinny on Comp.Ratio's as I see it.

 

The simple way of stating what Compression is...is as follows: Cyl.Displacement Vol + Cyl.Head Combustion Volume / Cyl.Head Comb.Volume.

 

Keeping in mind there are tow actual Comp.Ratio's that effect eng.performance-& they are: 1) Static Comp.Ratio 2) Dynamic Comp.Ratio.

 

Static Comp.Ratio is the comp.ratio while the engine is not running while the Dynamic Comp.Ratio is the actual comp.ratio while the engine is running: The latter is different as Valve Events (determined by the camshaft design) will effect the cyl's ability to "Seal Itself" prior to combustion-& therefor will differ slightly at times to Static Comp.Ratio. When people speak of Comp.Ratio's they are usually referring to Static Comp.Ratio.

 

Picture if you can, one of your engine's pistons...as it rises & decends w/in the cylinder. As its displacement allows (Displacement = Bore^2 x Stroke x .7854 x #of Cylinders) your engine is allowing a certain amount of air into the cylinders for combustion. As this air is entering the cyl's for combustion you have to consider the volume in the cylinder when the piston is at Bottom Dead Center (BDC) & you have to consider the volume remaining while the piston is in Top Dead Center (TDC).

 

As a brain teaser-you also have to include all the factors w/in the engine that effect the cyl's ability to "Seal The Cyl" during the combustion process....AKA: The Camshaft & its Valve Events; such as a) Intake Valve Opening, b)Intake Valve Closing, c)Exhaust Valve Opening, d) Exhaust Valve Closing. To tease the brain even further-you now have to consider the qualities of "What Makes A Camshaft Work": such as Lift, Duration, Lobe Seperation Angle, Lobe Centerline ect ect, but that is another discussion.

 

Back to the subject at hand; compression ratio below the deck of the block while the piston is at BDC is called Displacement Volume while the volume of air in the cylinder head's combustion chamber while the piston is at TDC is called Combustion Chamber Volume. Because the Comb.Chamber Volume is also apart of the Displacement Volume when the piston is at BDC it too must be included when working the formula for Static Comp.Ratio: this is where the "1" comes from when citing a comp.ratio such as "9.2:1". The "1" represents the Comb.Chamber w/in the quoted ratio: whereas the "9" in the quote is the ratio w/in the cylinder if you were to divide the "1" into the Displacement Vol.

 

In other words; a 9.2:1 Comp.Ratio means that the "1" (represents Comb.Chamber Vol) will fit 9.2 times into the the Displacment Vol. Or, 1 x 9.2 = Displacement Vol while 1 x 1 = Comb.Chamber Vol; thus the 9.2:1 cited Comp.Ratio.

 

To understand Dynamic Comp.Ratio...think back on the 60/70's Muscle Car era when engines were being built w/350-450hp & alleged Static Comp.Ratio's in the 12's. Yes, they were running on Ethyl 106 & even higher leaded pump gas octanes at the times...but their camshafts had so much had so much Overlap that the cylinders would not seal until a good amount of displacment volume would leak out (due to the cam's excessive overlap) that their Dynamic Comp.Ratio would actually be in the 8:1 to 9:1 Comp.Ratio's...thus their ability to be ran on the streets.

 

Hope this helps you and didnt confuse you. It takes time to understand some issues but dont give up. It took me about 5-10 years of reading & tripping my way thru trial/error before understanding...& I didnt have the Internet nor HybridZ buds to help me understand; nor were there as many Auto Perf.Books back in the late 70/early 80's as there are now. Your on a much better learning curve than I & many others had.

 

Dont get discouraged & always/always remember to have fun. If you're not having fun then you're doing it wrong!

 

Kevin,

(Yea,Still an Inliner)

 

[ July 25, 2001: Message edited by: Kevin Shasteen ]

[Guest Anonymous]

Zpeed,

 

FWIW, the Australian fuels, at the moment, have the following octane ratings:

 

Unleaded: 92

Lead replacement (LRP): 96

Premium unleaded: 98

Avgas!: 100

 

Not that it's legal to drive a car on the road using avgas

....

 

[ July 25, 2001: Message edited by: zedskid ]

[Guest Zpeed]

thanks alot... even thou i don't fully understand what you are saying... i'll have to read it a few more times... to understand more

 

And if it involves my Z, i'll be having fun!!

[Guest Anonymous]

Ok, here's how this works out. Running an engine of 12:1 compression on pump gas is not only idiotic, but it is very damaging to your head, valves, pistons, plugs, rings, and gasket. 92 octane doesn't mean a thing after 11.0:1 compression. That is the highest compression ratio that 92 octane will run without spark knock. Spark knock is a big problem. Don't let it happen. You'll burn holes through your pistons and melt the hell out of your plugs if you let it happen too long (I did it, I should know). Now, since your average engine compression ratio is 9.0:1 (in the US at least), it is not neccasary, nor does it help, to run 92/93 octane. All your doing is pissing your money into your tank then. If your engine runs fine on 87 octane, run it on it. The octane rating is its resistance to burn. 87 octane burns at about 2600 to 2800 degrees farenheit, if my memory serves me correctly. 92/93 octane burns at about 3400 to 3600 degrees farenheit. When you increase your compression ratio, you increase the pressure inside the compression chamber. An increase in pressure will cause an increase in heat and what temperature the fuel will burn. If you run an engine built to burn fuel at 3400 degrees farenheit, and run 87 octane (2600 to 2800 degrees farenheit), the fuel will burn too quickly and will spark at the opposing end of the spark plug in the cylinder. When the spark from the plug and the unintentional spark from the low octane fuel meet, it is called spark knock, and you will hear a pinging noise coming from the engine. Here's the kicker though. Running a 10.5:1 compression engine, which will run fine most of the time on 89 octane, but should run 92/93, will obviously get you more power, but will not affect fuel mileage. Want to know why? If you run fuel injection, and run an average of 2500 rpm, cruising speed, you will get more power from the same amount of fuel because your compression ratio is higher, and it takes the same fluid amount of fuel as an engine running 8.5:1 compression running at 2500 rpm. The theory behind this is that since the compression ratio is higher, you must run higher octane fuel, but the amount of space in the cylinder for fuel to occupy is smaller than in 8.5:1 or 9:1. You probably think I'm full of ****. Well, there's a $14k education backing all that up. the real asskicker in running the higher compression ratio is the obvious price jump in fuel. That'd be your main problem, as long as you can avoid spark knock. Hope this answers your questions.

[Guest super280z]

tailights,

 

if you read up on the 383lt1's by morgan motorsports, they build a motor to run 11:7.1 comp ratio because of those reverse cooled heads. just thought you'd like to know

 

p.s. where'd your 14k education come from?

 

[ August 01, 2001: Message edited by: super280z ]