The point was his engine build was spot on to take that timing like it did. His engine builder knows the true secret of DCR, and not the DCR everyone goes around on this site blabbing about involving rod length and IVC events. The dynamic events that happen as an engine moves throughout its rpm range, Piston dwell time at the top, Exhaust Scavenging air ram effect, quench. Everything had to be working spot on to take pump gas under those conditions.

 

13-1 static is very doable depending on the IVC. Ya need to stay under 9.0-1DCR to get it done.
It's no big mystery.

 

That works great as long as you don't start the engine, as soon as you start the engine velocity takes effect and your DCR is well over 9:1 and the higher you rev the engine the more your DCR shoots up whilst stress shoots up as well. A late IVC event is not what makes 13:1 streetable. Its a dance between the parts and the IVC timing is only one step in the dance.

 

REALLY!

BS,velocity? VE goes up some so the DCR goes up a tad. I am/ have run 13-1 on 92 octane at 9.0-1, so your theory has a few glitches in it.
There is NO dance going on. Figure up your DCR and the static should be whatever makes it come out 9.0 or less.

 

then 20:1 compression would be streetable by just having a very late IVC (and very long stroke)

Velocity is what drives up VE. Velocity in the port, with velocity in the port it will continue ramming air into the cylinder even as the piston comes up, thats the whole point of the late IVC is to take advantage of the velocity ramming air in. NOT to drop your DCR down. DCR is just a number, a number with very little meaning. All it means is that if your engine is moving very slowly it effectively has x compression, it doesnt state anything for what the true compression of the running engine is, that is determined by runner length, port cross sectional area and piston speed.

 

Length of the runner and cross section do not have anything to do with DCR. Static is the number with no meaning.Read this and see where ya are wrong.

http://www.kennedysdynotune.com/Dyna...ion%20Tech.htm

 

DCR is only meaningfull in an engine that does not do more than one rotation, as soon as you've completed a second rotation the original DCR calculation is invalidated. DCR stands for dynamic compression ratio, and as its name implies it changes. That calculation only calculates where it starts, not where it is at 7000 RPM when you shift, your DCR is significantly higher than 9:1 at 6500 RPM where your power peak may be, in some engines DCR even exceeds SCR at certain rpms. RPM is merely a vague way of giving the piston speed, because when it comes down to it what matters to velocity is piston speed, not RPM.

 

Ya need a little more reading.
Tell me anywhere in this paper it says ANYTHING about head flow.

How do ya get piston speed without RPM's. DCR will NEVER be larger than static.

http://www.ls1tech.com/forums/archiv.../t-102153.html

 

I never said you get piston speed without rpms and yes DCR can be larger than static, ever heard of a supercharger or turbocharger? Its possible to happen even on a N/A engine. That article gives a poor definition of Dynamic Compression Ratio. If dynamic compression ratio does not change, Why is it called dynamic compression ratio? The effective compression ratio of an engine varies based on many factors, including IVC, SCR, Piston velocity, cross sectional area and runner length.

 

No, that's a good explanation of DCR. You need to read it more carefully. At the end, it mentions the differences which occur in an engine running at high speed. Here, anything that influences VE will alter the sylinder pressure. This includes heads, cam, intake, barometic pressure etc.

The real "bottom line" is cylinder pressure in a running engine. DCR, as defined, is much better correlated with cylinder pressure than static CR. But it is far from a perfect correlation. Unfortunately, we hobbyists don't have a way to readily measure cylinder pressure in a motor at peak torque, so we have to use surrogates like DCR.

Rich

 

I was reading this banter and hoping that you would step in Rich.

Dynamic compression is a combination of many different events, but Rich is right, the best way to make sure an engine will be "safe" is by figuring the cylinder pressure. If you have no way of doing so, DC is your second best bet. DC is what I personally use, and for 91 octane fuel, I like to keep my DC to no more than 9.2 max.

Good luck on the motor, and stick to what Lloyd recomends.

 

Then calling that DCR is a poor name, theres nothing dynamic about it at all. Its a good description of your static compression ratio after the cam but thats all it explains, calling it dynamic is like saying a wooden stool is a lazy boy.

 

I agree, the name is not adequately descriptive. But that is the way the term is most often used.

Cranking pressure, which is measured rather than calculated (as is DCR) correlated well with DCR. The reason cranking pressure is less useful as a surrogate for cylinder pressure is that it can only be measured AFTER all the parts are selected and the motor assembled. As such, it is not as useful as DCR for design purposes. But as a frame of reference, the maximum cranking pressure range for pump gas is 190-210psi or so. As with DCR, as you approach the maximum the rest of the combo becomes more and more critical (have good swirl, good cooling, plugs that are the right heat range, etc.).

Another elaboration/explanation. The higher the VE, the higher the cylinder pressure. This is important because when you compress a gas, the temperature rises. If the fuel-air mixture is heated too much due to compression (i.e. cylinder pressure too high), the result will be detonation. "Restricted" motors, using small carbs due to sanctioning body rules, use a very high CR if allowed by the rules, due to limited VE. I don't follow NASCAR closely, but I believe they now are limited to 12 or 12.5:1, with a small carb. Before the CR limit, I think theey were up in the 18:1 range. But if you look at Pro Stock, the pinnacle of OHV NA engine development, the picture is quite different. These motos have a very high VE (they are not "restricted") and use much lower CR, ~14:1. Even with exotic gasoline, they can't run higher because of the contribution of high VE to cylinder pressure.

With a blower, VE may be very high and hence cylinder pressure must be controlled by lowering the DCR. It is a well accepted fact that using lower mechanical compression with high boost to achieve the maximum cylinder pressure allowed by the fuel chosen produces more hp than higher compression with less boost. But in each case, detonation due to excessive cylinder pressure is what ultimately limits boost and mechanical compression.

Rich

 

If you look in my original post I specifically stated I was not using DCR in that sense...

 

For any discussion to be meaningful, the terms needs to have a mutally understood definition. This is the major problem in many "arguments". I think that Mr. LiENUS is using a somewhat non-standard definition of DCR, and that's where the confusion has set in.

Everything should be relatively clear now, so to get back to the original question of "11.5:1 + 91octane =?", the answer is, as it often is, it depends. You can guesstimate an answer using the concept of DCR as I and LR have tried to explain. If the original poster would give his engine specs and cam specs, we could figure that out for grins and giggles. But if Lloyd said the combo would work, I bet it will work. Facts always trump theory. Nonetheless, I'd like to know the DCR of the combo Lloyd said would work to see if the theory holds up. I am betting it does, provided the right question was asked of Lloyd.

One thing I would love to try is water injection on an NA combo. Theory says it ought to allow more cylinder pressure without detonation. I'd like to verify that and then see if it made more hp.

Rich