I am planning a new motor build, so naturally this question came up. I know that 8.8 DCR is usually considered the "max safe" DCR for a motor that will run on 93.

I did some research and some reading on here and I can't even find a good rule of thumb guideline for determining what octane gas is needed for a specific DCR range. All I have been able to find is 9.0:1-9.2:1 is considered "race gas" only but what about 9.3 or 9.5? Is it bad to get that high of DCR at all? I don't even know what the article meant by "race gas" 93 with some torco in it? low-lead AV gas? Sunoco 110? There are many different octanes of race gas and I know that when running excessive octane for the application, power is left on the table. It would be nice to have some sort of guideline, i.e. for every point increase, add 5 octane points or something like that.

I know there probably is not a precise way of figuring this, but is there any type of testing that has been done or just a basic "rule of thumb" chart to get an idea?

 

You ask for information I would also like to know.

 

I hope somone chimes in with some good information, all i can find is articles like this: http://www.empirenet.com/pkelley2/DynamicCR.html

great article, good information, but not enough.

 

mainly because DCR is pointless to the extent you are taking it..

What really means something is the ratio between SCR and DCR.. thats what will dictate your engine performance...

I personally 'dislike' all of this emphasis on DCR...

for example..

15:1 SCR and 8.8 DCR
or
9:1 SCR and 8.8 DCR...

you tell me what one you want and why..

 

AGREED.

What if your motor operates at 85 or 105 VE? What does that do to DCR?

 

9.0-1 is the highest you can go on 93 octane according to Patrick G. Of course you would need an excellent tune. 8.5-1 is the max for 91 octane. Anything higher than 9:1 DCR needs race gas. Having a thinner head gasket like a .040 will have a tighter quench and thus reduce the risk of detonation.

 

Exactly. I never understood the point of worrying about DYNAMIC CR calcs based on STATIC formulas. This always seemed to me an error in logical thinking. Any singular formula would seem much too overly broad to be useful.

 

its just a name given to it, it technically is not "dynamic" it is set in stone once you build the motor and slap a cam in. period.

I'm just looking for guidelines as to what DCR requires what octane basically. I understand that anything over 9.0:1 is going to need "race gas" but what if it could run fine with 94 octane gas? The whole reason I'm wanting to know this is because I want to run the minimal octane required to get the job done in a particular engine and maintain a small margin of safety, not only is this cheaper but it will make more horsepower. Putting 110 race gas in a motor that is barely too radical for 93 is gonna leave some power on the table, not to mention break your wallet if all you need to do is add some toluene in a light mix to achieve a minimum of X octane to make it work. Get my point?

 

Bingo.

It is a way overhyped term. I would not even worry about it. I don't even like to say it.

 

Really? Care to elaborate? I would dissagree with that therory.

 

the definition of DCR is basically a 'static' calculation based on SCR and intake valve closing position.. so technically, there is nothing dynamic about it...

The true dyanmics of cylinder airflow and filling events have to do with alot of varibables.. but for some odd reason, isnt discussed here much

 

I was refering to the Set in Stone part.

 

I think the reason for this is beacuse the number o variable to take into account would probably involve sometheoretical physics equations that would take up a whole thread.

If we truly know ALL the variables that go into account in a dynamic motor, then wouldn't every motor run at 100+ VE and get incredible MPG?

 

Well we do know most of the variables.. however, just because we know them, doesnt mean we can change them.

Every variable affects the other.. thus comprimise

 

SCR 11.3
DCR 8.8

What min octane pump gas will be safe to run? Anyone?

 

93 should work fine for you.

 

I do not understand what is so bad about using DCR as a legit calculation to help determine fuel requirements of a given engine. If we utter the words "DCR" can we not sit at the cool kids table?

How come you can have a 2-stroke dirt bike that is 13.5:1 with a 32:1 of oil in the gas and it still manages to run on pump gas just fine? Probably because of the always open exhaust and intake ports that BLEED OFF the pressure, just like having the intake valve open when the piston starts up for TDC on the compression stroke. What is wrong with this, they have a set DCR that is rather low...it apparently works for them? Apply the same principles to a car engine and why would it not work (the part about having the intake valve open, not the 2-stroke part)?

Air has mass, thus it has inertia...It does not like being pushed around, going through small spaces, or forced to change directions abruptly. People who argue that DCR is going to effectively change because the higher the rpm, the less air can escape back out the intake port before the valve closes means the DCR goes up. To them I say there is no free lunch here, the air comming in has to follow the same laws of physics so if that is the case there would be less able to come in, therefore it would cause less pressure to have to be bled off. Does this make sense? All I know is that using DCR apparently works, so why dismiss it? It must have some validity, it certainly is not "useless". I know Bane knows hs stuff, and in the post above he assured bluecamaro that his motor will be fine running on 93. Not knowing any other specs other than DCR and SCR. What about VE? Are we stuck on VE now? If enough air comes in to create a situation where there is 100+ VE naturally aspirated, if that much "extra" air can get in it can also get out... right?

I'm not trying to argue with people or start arguments, I'm just trying to learn. If not DCR then what do we use to figure out octane requirements on a GENERAL basis? not including temperature and environmental variables, but just a good rule of thumb way of seeing if you are at least somwhere in the damn ballpark.

Also, about the set in stone part I meant that when the cam is installed and degreed in, the calculated DCR will remain as it is for the life of the motor until you change something like the degree of the cam, the cam itself, change the overall SCR etc. etc.

 

So many problems, but to start off. . . What about, say a pro stock engine with a 15:1 Static compression, but a dymamic of nearly 18:1? How can the method you claim work? Do you see where VE comes in?

 

Thanks for posting a reply to my question.

I would like to know if there is a way to calculate that or is it based on common sense or people's experience.

 

noway, I can see a VE of maybe 105% but that still aint gonna make the DCR higher than the SCR. nope, sorry.