Octane rating vs CR

[BLSTIC]

Don't know how that line got through the later part of that post??

putting in [ s ] without the spaces opens a strikethrough tag in the code. You've done this where you wrote country[ s ]

[hoodeng]

DAMN!! learn something every day! Fixed now, Thanks BLSTC.

Cheers.

[My427stang]

Frank, this forum is not a fan of DCR, and partially for good reason. First is the name, as it really isn't looking at dynamic changes, just cam-event adjusted SCR, and second, because the argument always is "you can't kill big compression with a cam at all points on the curve" which is very true, but in the end SCR never happens during operation either

However, with many street FE builds under my belt, built with expected cam combos, and tuning, and SCR to match, I can give a loose correlation that at least gives us a "probably shouldn't exceed number" but it's not a hard and fast rule.

With the assumptions up front that you have cam and compression to match the use, a sub .050 quench, a proper timing curve, engine temp control and good a/f mixture AND using the same calculator I have for about 15 years (PKelley's) Under 8.0 is generally safe for 91 octane if you actually measure everything. It's not that the PKelley calculator is better, I have just used it over and over and with a common street FE, 400+ cid, flat top or dish piston, you can see trends using the same tool over and over. Different calculators have slightly different numbers, which are no less right or wrong, r so all you can do is build experience with the tool you like

With a set of Trick Flows and a .041-.046 quench, at about 10:1 the engine wants less timing and can handle quick curve, on a hot streeter, you could creep up. With a set of old iron (any FE iron, even the ones that look like they might be good, aren't) 7.5-7.8, corresponding to a drop in SCR not adding cam to kill it. FWIW, most of my pump gas builds range from 10-10.3:1 and with hyd rollers, end up at 8:1-ish DCR, my own 489 inch, is 10.7:1 and 8.33 but it also is EFI with a complex timing curve, .041 quench, and I ran the numbers with advertised durations, but run it slightly tight-lashed, (.014 cold) which makes the cam bigger than the numbers would indicate.

FWIW, my iron headed 462 EFI FE in my 4x4 truck is .045 quench, 9.75:1 SCR, 280/286, 112 on 107, iron head, which works out to a 5000 rpm peak and 490 HP, 540 ft lbs, with 7.62:1 DCR. It's pretty mellow, but it's for a slightly lifted short box 4x4. Not sure I would trust much more compression with a crappy D2 chamber

If you are trying to run 87, I'd build it with less compression, probably 7.5-ish DCR, BUT, honestly, if you are doing that, you likely are running less cam and would drive compression down anyway. So it really becomes chicken vs egg discussion, but it shouldn't be, cam for use, match compression to cam and the rest of the engine design, and if you want, check the DCR for reference, it IS a good indicator, just not a driving characteristic

I will say that the rest of the build is more important, keep the quench tight, cam correctly, etc, then when you check DCR you probably won't find any magic, although I always plug the numbers in. An FE with a cam for a mild streeter, typically wants 9.5:-ish and it ends up at a 7.8-ish as a check figure. Where I see the odd behavior is when someone tries to cam for compression, as opposed to camming for use.

I would be happy to give my 2 cents on the build you are considering, it would likely be more beneficial than a rule of thumb

[David Redszus]

SCR and DCR are often misunderstood and therefore misused.
Both are simply ratios that do not predict actual combustion events. To predict actual combustion events we
need real numbers like compression temperature and compression pressure. Neither SCR nor DCR provide this
information. SCR is incomplete since it only represents a ratio based on full stroke. DCR is based on stroke at IVC.

Frank, this forum is not a fan of DCR, and partially for good reason. First is the name, as it really isn't looking at dynamic changes, just cam-event adjusted SCR,

The term DCR is accurate and appropriate for what it is. It incorporates the effect of the inlet valve closure into the ratio calculation, and nothing more. This ratio does not change with a fixed IVC, or rpm, but it does change with each piston position. We are primarily concerned with DCR at TDC. But we still do not have useful information and must rely on anecdotal observations. Or not.

and second, because the argument always is "you can't kill big compression with a cam at all points on the curve"which is very true, but in the end SCR never happens during operation either

We we can't kill big compression with a cam but we certainly can reduce the DCR ratio, but not the SCR.


Following are a few calcs to illustrate the point:
IVC @ 110deg BTDC
SCR.....DCR
6.5.....5.0
7.2.....5.6
8.1.....6.3
9.3.....7.1
10.0...7.7
11.0....8.3
13.4...10.2
15.2...11.5

SCR 10.0
IVC....DCR
90....6.2
95....6.6
100...7.0
105...7.3
110..7.7
115...8.0
120...8.3
125...8.6

The bottom line
DCR is only a ratio and does not provide actual numerical values that can be used to determine combustion effects.
With the addition of inlet air pressure, inlet air temperature, fuel ratio, we can calculate compression pressure and temperature at any crank angle. Compression pressure can be validated by use of a simple compression test; the results will be much closer to DCR calcs than to SCR calcs.

While DCR ratios might not change, the actual compression pressures and temperatures will be impacted by a number of variables. It is important to make the distinction between compression ratio and actual compression pressure values.

[My427stang]

David, we chose some different words, but agree completely.

I will say that my opinion of the word dynamic would indicate change during the event. Specifically ... (of a process or system) characterized by constant change, activity, or progress: "a dynamic economy"

You have shown how at certain positions the value is changing, so yes you are correct using that definition and example, but as used by the majority of DCR advocates, they reference the top and bottom of the effective stroke with the valve closed, or a single ratio, not changing, i.e.7.5 in Frank's original question.

As far as your indicator of the ability to change compression,again, literally of course we CAN change DCR, but, if you dove into the message of my post, JUST countering too much SCR with later cam timing, creates two things, one is a mismatched cam, and two, potentially too much pressure at high VEs.

I likely should have said "we should not just kill SCR that is too high with cam timing" Because despite being one of the first guys to counter the DCR naysayers, I still try to teach to pick a cam based on all the other components and desired use, compression ratio second. Sometimes you have to back in, but if starting from scratch build for use

Regardless, good info and in the end, the message was clear from both of our posts, there is no specific ratio that is a get out of jail free, and certainly not common across engine designs

[Steve.k]

Then Ofcourse there’s the variations within pump gas 87 octane fuel that only compounds the situation.

[David Redszus]

The selection of a cam shaft should not be related to compression ratio. Select the cam that allows the best air
flow at the intended engine speed.

The piston should be selected based on required compression and chamber turbulence.

The two are separate issues.

Raising compression can easily change the squish area ratio (and chamber turbulence) inadvertly. Surprise!!!

That said, I can't tell you how many times an engine builder has installed high compression pistons, along
with a longer duration camshaft, only to find a lower compression pressure than stock.

[Truckedup]

All I know is the vintage hemi head air cooled bikes I mess with have a high octane requirement despite long duration cams compared to car engines.

[My427stang]

The selection of a cam shaft should not be related to compression ratio. Select the cam that allows the best air
flow at the intended engine speed.

The piston should be selected based on required compression and chamber turbulence.

The two are separate issues.

Raising compression can easily change the squish area ratio (and chamber turbulence) inadvertly. Surprise!!!

That said, I can't tell you how many times an engine builder has installed high compression pistons, along
with a longer duration camshaft, only to find a lower compression pressure than stock.

I am not sure if you are disagreeing, not reading, or agreeing LOL

Camshaft and compression is absolutely related, no component in a build is unrelated, but my point is do not select a camshaft to match compression, instead, along with other inputs, select compression ratio to support the cam timing events and other characteristics to meet desired performance (not just peak, but all the measure associated with an engine being appropriately matched to end use).

I will say I disagree that one should cam for a single engine speed, unless we are building engines with CV transmissions or running on a governor. I do agree that you pick a piston based on required compression, but you pick a compression ratio based on use, fuel, chamber, and camshaft selection, not to mention all the other variables that affect controlled and uncontrolled burn.

Regardless, again, Frank if you list what you are trying to do, happy to help

Trucked up, I am no hemi guy, but, the design seems to inverse the shape I want in a a piston and chamber, but man can they breathe well, which sure can offset in the power department.

[F-BIRD'88]

Shortly, Comp Cams will come out with their new line of

Comp "Octane Cams" ... Leap over tall buildings on any gas.

[hoodeng]

We actually all build to an application, the limiting parameters are where we start from, clean sheet engine, very good basis for performance engine, existing engine that could fall in the historic or limited development engine, lastly a shed full of leftovers project engine.
In Truck's application he has an older open hemi chamber design that can be difficult to get high compression [over 11:1] into, he also has an unsupported crank center, not to mention dimensional limitations.

Others have mentioned older engines that were pretty stout in their day, but today all we are doing is beating up on an old horse, fine if you have a good stock of core parts [blocks, limited release heads, cranks etc].
We initially start out with an project idea, is it going to race in a class? Is it going to be a historic project? Is it going to be a sleeper? Is it a hauler? a wild dream of a completely inappropriate selection of parts left over in the back of the workshop? will it fit into the chassis?

It will need to work on paper first if you don't want to be spending on the same parts multiple times.

When we know what we are looking for we will look at other limiting factors, local laws for some if on the street, maybe high octane fuel is sketchy in our area?, [ i know a guy in country SA that keeps 200L drums of fuel for his kit], are we actually going to get to practically use it?

Once we have the brand and series of engine identified we now make the build decisions based of four fixed parameters.
First Capacity, will the intended capacity be accommodated soundly in the engine choice? or will it have paper thin walls with no post service options.
Second,Compression, will the comp choice be practical for the build? as stated earlier in Truck's application, some engines are hard to get comp into without nearly completely masking the combustion area with a brick on top of the piston.
Thirdly, Cam, will what you intend fit? great looking at a bunch of figures only to find it won't fit/won't turn in the stock location, or no tdc clearance without a stack of swarfe [compression]on the floor.
Fourth is head flow Capability, can the head even keep up with all your other desires? is there even a readily available alternative head for your project.

That last statement was precipitated by actual experience of customers wanting a high end build for an older engine showing up with heads they have bought off of some retired racer that were presumably bolted onto a winning engine in its day, only to be told they are basically junk and were junk when the person who sold them had to have them prised from their bleeding, dying hands in a moment of delirium weakness to let them go.

Another was in a limited production class that had open rules on heads as long as it used the std induction and exhaust manifold. Story goes, guy goes to race meeting, one racer is clearly passing and out running the other competitors regardless of grid position,, guy goes to fast racer and says name his price on the cylinder head, fast racer says,"Too much trouble to remove that one, but i have an identical spare head in the trailer that we keep for in case" ,,,but,,,, as it's the end of the meeting i will sell it to you for what it would take me to replicate the spare so we still have one', they are that good. Guy pays ransom then goes home and fits head..Next meeting, they have not even changed their lap time by a decimal point, guy goes to fast racer and says he has been ripped of, fast racer says "i'm not driving your car"

Cheers.

The fuel we are going to use or stuck with is going to underwrite all the above.

Cheers.

[Steve.k]

Very true Hoodeng. One thing I will say. Out of the engines I’ve built the ones that surprised me the most in their output were the 9.5 pump gas 91 octane engines. Far exceeded what I thought they’d do. Sometimes we get caught up in compression numbers. I’m working on a cast iron 9.5-10 compression motor now in my head i wanna do. All cast iron. I’ll fill you in when i get details worked out.

[David Redszus]

I do agree that you pick a piston based on required compression, but you pick a compression ratio based on use, fuel, chamber, and camshaft selection, not to mention all the other variables that affect controlled and uncontrolled burn.

Now that we have picked a piston based on some required compression (on whatever basis), how do we know what the compression actually is?

We can measure chamber volume and displacement to determine SCR. That tells us almost nothing with regard to combustion behavior; anecdotal folklore aside.

We can include IVC to obtain DCR. That still does not reveal the impact on combustion performance, since we do not know compression pressure or temperature.

But certainly, using SCR and octane index values are practically worthless; dozens of exceptions to our personal experiences will be easily found.

[BLSTIC]

Can we determine true compression ratio by multiplying static compression by not-MAP-corrected VE?

But you can only determine that with sims or a completed engine on a dyno.

[David Redszus]

Can we determine true compression ratio by multiplying static compression by not-MAP-corrected VE?

But you can only determine that with sims or a completed engine on a dyno.

Static compression ratio, multiplied by any number, will never produce a measured pressure.

VE is the ratio of air masses which is even more difficult to obtain.