Well said, and I

So, what is the magic number to run pump gas? Obviously, if so much credence is given to this one parameter, then someone has to have a number they are looking for. Otherwise, it is just a guessing game.

The fact is, there are engines that will rattle with less than 8:1, and others won't rattle at 10:1 DCR. How do you know what is correct? How can a tool be so valuable if the target is constantly moving, and the real world numbers don't correlate to the formula result? I'm not saying the DCR doesn't have some value if used properly, but when ALL parameters aren't considered, like is often the case, the DCR is almost meaningless.

I think Brad is showing his frustration that he built his engine to run on pump gas, using a DCR calculation, and it didn't work out that way. There are lots of guys out there blowing their stuff up from detonation, when they thought they were safe. And yet, guys are still recommending engines be built using the DCR. That being said, if that's what works for you, go for it.

The altitude your using the engine at would play a role in what you can get away with, correct? Boise is 2800 ft above sea level, and there are guys here with 10:1cr + engines running around on 91 octane (highest you can buy in this area) with no issues at all.

Several old school machine shops here that build every street engine at 9:1 cr and install a much larger than stock camshaft. The end result is something that often sounds nice, but is a pooch on the bottom end, and by the time the bigger cam comes on the heads (if they are stock) are signing off. This is how too many people end up with a rebuilt 440 with headers, etc. that runs 15 flat at 91 mph.

I don't have to "spin" anything and I never changed what I meant: DCR doesn't take into account how cylinder pressure changes across the RPM range due to VE and, therefore, is of limited or no value. It doesn't matter that DCR uses CR and IVC as inputs if the output calculated isn't of use.

Seeing that the DCRs of two different CR & cam combinations are the same still won't tell you the RPM at which the combination built with a higher CR and later IVC crosses over from less cylinder pressure to more cylinder pressure than the other combination.

If you want to know at what point in the RPM range an engine achieves peak VE and how well it carries the VE up to max RPM, you gotta put it on a dyno and collect the data.

DCR only applies at very low engine speeds such as idle and perhaps cruise. Once some inertia ram effect takes place then DCR goes out the window. So if a person is concerned with how an engine will run at low speeds then DCR is something to pay attention to.

I would imagine DCR is a hot topic over in the Tug Boat and Tow Truck section of the board, not sure why it is a topic of discussion in the Race section though. Most race engine live in the RPM zone between peak torque and peak power and DCR is irrelevant at that RPM.

Naaaah. I knew when the CR ended up at 11.5 compared to the old build's 10.8, while the cam duration & LSA specs didn't really change, that the likelihood of still being able to run straight 93 was pretty slim. I brought race gas to blend with pump gas at the dyno just to play it safe.

“DCR doesn't take into account how cylinder pressure changes across the RPM range due to VE and, therefore, is of limited or no value. It doesn't matter that DCR uses CR and IVC as inputs if the output calculated isn't of use.“


DCR= DYNAMIC compression ratio. The term dynamic TELLS you that it’s a changing value. If someone has chosen to use only one value of a constantly changing equation, the results will likely be less effective.
Throughout this thread, it appears that the mistake that people are making is they use “dynamic” as a fixed number/ point of reference.

So to me, It looks like (based on the first sentence in quotes) you assumed that dynamic is a fixed number.
Maybe that’s the whole basis of this thread???😳

How do we pick a combo? Voodoo? No, we use parameters. Cam, compression, head design, head material, operating altitude, engine temp,fuel type, and more. Miss any one of these factors and the desired goal may not be met. Static compression is just compiled number. Dynamic compression is also just a number. These are tools to point you towards the desired goal. Neither are set in stone. The more knowledge you have, the better chance of meeting the goal. Just like when you ask questions. Compile the answers, throw out what you don't believe in, and make your best stab at it.
Doug

Well said, Andy! I was thinking the Irritation Pump section, but tow truck and tug boat works, too.

My mistake, Brad. I was thinking you built it for pump gas.

Gee, why didn't I think of this?
Oh, wait... I wrote about it, HERE, 10 years ago.

This approach is understandable, since quantifiable data can be difficult to obtain for many of the parameters involved. That's where very expensive and time consuming research used by factory engineers, and high end race teams comes into play. The problem is the use of DCR as a singular factor in determining SCR and IVC. It just doesn't work by itself. Again, the engine doesn't care when the intake valve closes. It only sees volumetric efficiency. Get too much charge into the cylinder at too low an rpm, and it will rattle.

If Dynamic comp. doesn't mean zip then put a 290 @ 50 cam in a 440 that had a static ratio of 8 to 1. and see what et you get. Then put a 250 @ 50 and see what et you get. If dynamic comp. does't mean anything why even close the valve?

In terms of detonation. Why does my 1988 dodge truck ping, but power tour mountain motors don't? Bet it has something to do with cam selection (along with other things). Detonation is easier to control at higher rpm. there is a point, probably you can stop detonation with rpm. So if we can help control lower rpm detonation with camshaft ivc and the upper rpm detonation with rpm. Sounds like a win win, if you know what your doing.

I had to put into this pot

I work with effective CR because it's adjustable from 11:1 pump gas and as high as 20:1 if you have steel ones

DELETED.

Bingo

Show me a "DYNAMIC" CR calculator that accounts for the differences in VE that occur at 1000 RPM intervals from 1000 to 7000. If it's in an engine simulation program, then the program probably calculates BMEP, too, which IMO is a more useful data point.

What people typically use to calculate DCR isn't DYNAMIC at all because it's a single point-in-time calc that ignores VE completely. THAT'S really a STATIC CR calculator where the IVC event is changed from the default BDC to ### degrees ABDC; nothing DYNAMIC at all about it until it can factor in VE changes under different operating conditions.

I'm not the one who is getting confused between the meanings of DYNAMIC and STATIC, regardless of whether the subject calculation is named incorrectly.

/////////////////

This has been an enlightening discussion, if for no other reason than I am truly surprised by how difficult it seems for some people to grasp what I think is a pretty basic concept. However, it appears to be headed for a death spiral for that same reason.

I'm fine with agreeing to disagree, regardless.

If by "Race" you mean drag racing, sure. In the "Corners are Best" forum being able to drive and accelerate at part throttle and below peak torque is very relevant. Haven't found the "Tug Boat and Tow Truck" forum yet, is it a sub forum for the General board, where the penguins and hippos hang out?

Here's another I was just reading popcorn
you've probably read this one>>


I wrote it.
Observe the copyright notice: http://victorylibrary.com/mopar/cam-tech.htm