Is an engine with a high static compression ratio (SCR) and a cam with a relatively late intake valve closing (IVC) event a good approach to achieve power and torque optimization?

From time to time, folks talk about designing a cam to lower the dynamic compression ratio (DCR) so the engine will run on pump gas. My thought is that emphasis on a relatively low DCR via a late IVC is a more back door approach. It seems to me that a properly selected SCR for the desired octane paired with a cam to give valve timing events that shape the desired power curve is really the front door approach and would result in a stronger torque and power band. What are your thoughts?

Steve

 

You said it well.

 

Yup SCR and IVC are the important factors

I've said it many times DCR is basically pointless

 

DCR = "Back Door Approach" that is if you are into that sort of thing, yuck

Jes

 

How can you say DCR is pointless when its a direct relation of how the IVC effects the SCR? BTW if you look at just about every well engineered motor that is supposed to run on 93 octane their IVC and SCR dictate a DCR of about 8.3-8.7.

 

It is a calculation based on SCR and IVC.

Also, please define 'every well engineered motor'

It has alot more to do with your target RPM range and SCR than just being a value to target. Having a target DCR may be great for a certain target RPM, displacement, etc.. but change those and your DCR for a well engineered engine is out the window

IVC is chosen for rpm range, SCR is chosen based on many factors

a 15:1 SCR and 8.3 DCR engine is going to be significantly different than a 9.0:1 engine with an 8.3 DCR. DCR does not tell you enough information, the ratio between SCR and DCR tells you ALOT more!!

 

LS1/2/3/4/6/7 all run DCR's in the 8.3-8.7:1 range STOCK. So does every Honda B-series and K-series. My old FZR600, 02' R6 and my friends 06' R6 all were acutally closer to 8.9, but that's because of the smaller surface area of the piston/combustion chamber. The Lexus 4.0L V8 I'm messing with right now also has a DCR in this range. These are all motors I would consider well designed for their applications. They all run well on premium gas and have been optimized for their application.

I agree that DCR isn't the only number to look at, but to say its irrelevant and "snake oil" could not be more incorrect. It is IMO the number that dictates a SCR adequate for a given IVC. I think all motors when being built should choose an IVC that fits the powerband wanted for the motor and then select a SCR that achieves the DCR goal of 8.5-8.7:1 for 93 octane.

 

I like the discussion that is being generated about this topic. My main point is that often, people go overboard with SCR by shaving heads, etc. Then they find that they have detonation and try to back away from the problem that they've created by making the IVC relatively late to lower the DCR. If you have heads with a good combustion chamber design and are running a pretty optimal quench, then I feel you should stay away from the high SCR's that some people favor and rely on a well-selected cam that puts the power band where you want it and get a good tune. You're probably better off with an 11.0:1 SCR than 13.0:1 SCR with a the right cam, head, quench and tune.

You can calculate DCR for the desired RPM range, but I think that DCR is mostly a trendy thing to talk about or a means of adjusting too high a SCR.

Steve

 

Ok, DCR will help to see "if your in the ballpark", i agree. But i would look at alot of other factors long before DCR.

My point is that DCR is WAAYY overplayed on this website. Does it have some merit? of course. Should it be THE driving decision on your engine design? No

What happens with direct injection? what about forced induction? Water/meth injection? EGR? etc..

 

David Vizard had an article dicussing compression and detonation. I agree that DCR is just one factor. Couple of others I remember from the article are a good cold air intake and a cooler running motor such as 180 degrees versus a 205 degree motor. As mentioned above combustion chamber design and a good quench play a factor.

 

This is how I might say it: On Forced induction cars are those numbers under boost, or idling?

 

DCR under boost can be calculated too

Here's a good calculator. Yeah the website's Harley based but the dcr doesn't care who's name is on the valve cover.

http://www.rbracing-rsr.com/comprAdvHD.htm

My thing is. I've always thought you should build and engine starting with a good set of heads, then you pick a cam that suits the heads and driving style and to get good Power/Torque Optimization you need to select your SCR for the given engine combination by getting your DCR close to the magic number for that engine combo.

DCR's arn't super dynamic like most people think, atlest for natually aspirated engines. Helmholtz ignored your dynamic compression is the same every compression cycle of the engine.

It is somewhat dynamic under different boost pressures. You will have a higher DCR at 12 psi than at 8 psi on the same motor. However once it reaches full boost pressure DCR becomes static again.

I guess this arguement boils down to who thinks about an engine more theoretically.