Compression Ratios....drift..your sarcasm welcome hehe

[VTEC_boi]

What is the highest CR that u can safely run 93 octane on?

11.1:1?

Also...is dynamic CR changeable? If so..how?

This is for Naturally Aspirated things...


What's the dynamic CR of B16As? B18C1s? B18C5s?

Thanks for any help!

[fritz_269]

Originally posted by VTEC_boi
What is the highest CR that u can safely run 93 octane on?

11.1:1?

depends on the cam...

Also...is dynamic CR changeable? If so..how?

Of course, changing the static compression will change the dynamic compression, but aside from that, the cam pretty much determines the dynamic compression. The airflow through the head (e.g. porting) has an effect too.

Depending on when and how fast the cam opens the valves, there will be less/more air in the cylinder at various piston positions. Computing the exact number is a bitch, and generally takes a good computer program, accurate cam measurements, and accurate airflow numbers from the head. Generally, production cars usually run around a point under their static CR - but those numbers vary WIDELY.

You can do the computation with the help of a program like Mr. Gasket's Dyno2000, sold by Motion Software http://www.motionsoftware.com/dyno2000prodpage.htm

In reality, the important number is called 'Indicated Mean Effective Pressure' (IMEP) - it's related to the dynamic compression, but includes more stuff like friction and thermodynamic losses. It's measured in psi and is an indication of the effective peak compression pressure within the cylinder. Armed with the IMEP (at a particular RPM), the residual combustion chamber temp, intake charge temp, and a few good estimations of the head/piston design, you can predict what octane you'll need to keep away from detonation. Of course, that's all in theory, and reality usually has a few surprises; but it'll get you in the ballpark.

For an aluminum head with good quench area and adequate cooling, you can usually get away with around 240psi IMEP on 92 octane gas. A poorly designed iron head might kill itself as far down as 180psi.

[VTEC_boi]

So..the less aggressive the cam..the lower the static compression? or higher?

Thanx for the help Fritz!

[fritz_269]

Originally posted by VTEC_boi
So..the less aggressive the cam..the lower the static compression? or higher?

Read the edited post above. You replied too fast.

Nope. The less agressive the cam, the higher the dynamic compression gets. Conversely, the more agressive the cam, the lower the dynamic compression goes. That's why you can make a 15:1 static compression engine run great - it has a very long cam and input flow restrictors. Static compression is not changed at all by the cam - it only cares about the swept volume (bore x stroke) and the combustion chamber volume (left over space when the piston is at TDC).

The cam choice and static CR should be intimately matched. Many good builders start with the cam, then calculate the static CR necessary to hit their target IMEP. Just adding more and more duration to a cam at a fixed CR is a losing proposition - you'll lose IMEP, thus volumetric efficiency, thus power. Conversely, if you keep adding static CR without changing the cam, you'll quickly increase IMEP to the point where you start to detonate.

Like everything with automobiles, it's all a magical balancing act...

[fritz_269]

PS> FYI, cars in many races of the NASCAR Winston Cup series are limited to 14:1 static compression as well as an intake something like three square inches (called a 'restrictor-plate' under the carb). You can bet your bunny that they use some pretty sophisticated computer modeling to come up with the perfect cam shape to give them the maximum IMEP they can get under those harsh conditions.

[Someguy]

Originally posted by fritz_269
Nope. The less agressive the cam, the higher the dynamic compression gets. Conversely, the more agressive the cam, the lower the dynamic compression goes.

Wouldn't be the other way around? The less air flow though the head, ie lower volumetric effeciency, the less air would be compressed into the combustion chamber? Or are we talking at cross purposes?

Anyway, the design of the head/piston/combustion chamber has a huge effect on this with lots of potential "Got Yas!". With things happening pretty quick in the combustion chamber hot spots caused by irregularities in air flow, matrail differences, rough spots or jagged edges can make or break performance and pistons (at least the break part).

[fritz_269]

At low RPM, where head airflow is not so much of a factor - a long duration cam will give you less dynamic compression. Primarily, you want to look at the intake valve close (IVC) event. In a stock cam, it's a 10-20 degrees ABDC. In a really 'agressive' cam, it might be 40-50 degrees ABDC. That means that the piston is pushing a lot of air right back out the intake valve - so when it finally closes, there is less air in there to compress, so you get lower dynamic compression.

The reason a late IVC is useful is found at high RPMs. There, because the air is being rushed through the restrictive intake port, the piston has formed a little vacuum in the cylinder as it pulled down - so it's useful to wait a moment ABDC to let the air continue flowing in, even if it means the piston comes back up a bit.

Due to that same airflow restriction, dynamic compression is always highest at low RPM. Thus, the less agressive the cam, the higher the dynamic compression gets - all the way across the RPM range.

Make sense?

[Someguy]

Miller Cycle Engines. Hee hee.

But it seems knocking is much of a prevent problem at high rpms... Fuel delivery?

[fritz_269]

Originally posted by Someguy
Miller Cycle Engines. Hee hee.

But it seems knocking is much of a prevent problem at high rpms... Fuel delivery?

Umm... I have completely the opposite impression. Low RPM, high load, WOT is where detonation is most likely to occur.

low RPM = higher dynamic compression
WOT = higher heat from full air/fuel charge
high load = you stay at the same low RPM & WOT for some time
end result = detonation

Do you disagree?

[VTEC_boi]

Can you explain what you guys are talking about?


Hey...I'm about to go to the library - Fritz or someguy - any titles that would be good?

[fritz_269]

Internal Combustion Engines : A Detailed Introduction to the Thermodynamics of Spark and Compression Ignition Engines, Their Design and Development
by Rowland S. Benson
--Probably a little complicated for what you want, and somehow I doubt you'll find it at your local library.

Engineering Fundamentals of the Internal Combustion Engine
by Willard W. Pulkrabek (1997)
- this one's a fairly good introduction; no frills & easy to understand.

Some others:
Internal Combustion Engine Fundamentals
by John B. Heywood (1998)

Introduction to Internal Combustion Engines
by Richard Stone (1991)

How to Choose Camshafts & Time Them for Maximum Power (Speedpro Series)
by Des Hammill (1998)

Four-Stroke Performance Tuning
by A. Graham Bell (1999)


Here are some more web resources on cams & dynamic compression:
http://www.rehermorrison.com/techTalk/08.htm
http://www.mashoc.com/tech/Camshaft.htm

[fritz_269]

Originally posted by VTEC_boi
Can you explain what you guys are talking about?

What specifically are you confused about?

[VTEC_boi]

Originally posted by fritz_269
PS> FYI, cars in many races of the NASCAR Winston Cup series are limited to 14:1 static compression as well as an intake something like three square inches (called a 'restrictor-plate' under the carb). You can bet your bunny that they use some pretty sophisticated computer modeling to come up with the perfect cam shape to give them the maximum IMEP they can get under those harsh conditions.

Everything past that

[Someguy]

Originally posted by fritz_269

Umm... I have completely the opposite impression. Low RPM, high load, WOT is where detonation is most likely to occur.

low RPM = higher dynamic compression
WOT = higher heat from full air/fuel charge
high load = you stay at the same low RPM & WOT for some time
end result = detonation

Do you disagree?

Hmm... I guess it depends on the limiting factor. In my and similiar applications detonation is usually caused by the fuel rails not being able to flow enough fuel to keep the pressure up for the injector to deliever enough fuel fast enough. That occurs at high RPMs in 500+ rwhp applications.

But for what we are talking about now, that makes sense.

[SPOONFED_VTEC]

There are pistons for my car that can give me either 8.5 CR or 11.5 CR depending on the set i buy. If I went with Skunk2 stage 2 racing cam, and supposing i go all motor would the 8.5 or the 11.5 be better?

93 Honda Civic EX
1.6 VTEC
Bored .20" over
AEM Intake
Flowmaster exhaust
B&M short shifter
3" drop on Eibach springs and Koni Shocks