Dcr

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AJ/FormS

AJ/FormS

68 Formua-S fastback clone 367/A833/GVod/3.55s
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I thought I'd throw something out there;

Suppose you wanted to run a Dcr of 8.0, in your 367(40over) iron-headed streeter, so you could run just about any pump-gas. This will get you a cylinder pressure of about 160psi.
I'm gonna try and chart it so you can compare Scr to ICA, and approximate cam size(seat to seat,on the intake) to get it. All numbers are biased to achieve the 8.0Dcr.Keep an eye on the change in cam size as the Dcr rises.
You could run less Dcr, but if you are already buying pistons, what would be the point?
You could try a little more Dcr, depending on your chambers and tuning ability, but another 1/2 point would probably be pushing the limit.
This is just for informational purposes.


.... Scr........ICA req'd........cam*.......LDA ........ICL ...camchange ............notes
..... 8.4 ....... 31 ............ 202 ....... 114 ..... 110 .....zero .............Just try and find one
.... 9.4 ....... 55 ............. 254 ....... 112 ..... 108 .....52*
.. 10.4 ........ 69 ............ 286 ........ 110 ...... 106 ....32*
.. 11.4 ........ 78 ............ 308 ........ 108 ...... 104 ....22*
.. 12.4 ........ 85 ............ 310 ........ 104 ...... 100 ....02*

Now, I know some of you guys are not into Dcr, and that maybe don't see it as being that important. And I agree that if your long block is already assembled and in good shape, there could be a lot of money spent on very little gain.
This chart/exercise is more geared for those whose projects are in the planning stages, and who wish to attempt to optimize it. Sometimes minor changes, can lead to great benefits.
Check out the last two cams! They are basically the same seat-to-seat, yet the 4* tighter LDA, and the re-time allows/requires, a full 1.0point of compression to be come to the target 8.0Dcr!
Also check out the poor 8.4Scr engine. There is almost no way to even get to the target 8.0Dcr!
To be sure there are many other cams that could be thrown into the mix. I just chose popular and makes-sense LDAs and kept it installed to 4*advanced, as common denominators.The actual cams just fall out of the equation.
My last three combos have all been 10.7 to 11.2 SCr with aluminum heads, and the Dcrs have ranged from 8.6 to 9.2. One of them went 93mph in the Eighth at about 3650pounds,at 950elevation,a small cam, and a street set-up; so it made a believer out of me.
As a comparo; my stock 340 Swinger went 98 in the QUARTER back in 1971.Which is about where we all ran. Boy-oh-boy, I thought that car was fast!We all thought 98 was fast.
Have fun.....
 
BURN 'IM AT THE STAAAAKE! There, thought I would get that out of the way....LOL!

Very interesting way to look at this, A/J. As a data point, we are ending up pretty much on that chart's trendline with 10.0:1 SCR, 268 Crane cam, 112 LSA, 5 degrees cam advance, with added margin due to AL heads and good quench. DCR in the 8.2-8.3 range. I was originally thinking 262 but chickened out, and erred on the side of a bit more margin for my son to drive and enjoy it anywhere, any time, for many 10k's of miles and not have to worry about gas or detonation. Soon we'll see if the TC stall is too low....

BTW, DCR has been talked about and considered for 4-5 decades; 40 years back, it was referred to as 'effective CR'. But no online calculators existed , and I suspect only the advanced race teams and factory engineers had a way to work it out and relate it to performance. The internet and computers and online calculators have brought all of this within everyone's reach.
 
Thanks nm9, for the encouragement.
Yeah we knew about effective, but I never really got into it, what with raising a family and all, through the 80s, 90s, and the last child, my baby-boy, left in 2004. So in 1997 when I decided it was time, I had a lot of research to do, and you might remember the internet back then? Naww it was magazine articles for the most part, and nobody was talking. They were just delivering recipes and expecting us to blindly follow them.Well, I'm no follower.And, I got a thick head, but I did learn something in high-school, namely trig and algebra. But again, no one was publishing Dcr targets. So I had to back-engineer a lot of recipes to generate some data. In the end the numbers were all over the place and almost totally non-helpful. So I bought the Desk-top Dyno program and started generating my own data. I was not looking for absolute numbers, but rather,trends. And of course my library of hard copy books grew and grew. Some of that data actually made it into my head! I finally figured out that Dcr was a special key. I still didn't know how to use it though, cuz nobody was publishing targets. All this was going on during the time the body was in the bodyshop, and I was collecting new parts as money became available.
Eventually I resigned myself to the possibility that I was gonna have to entertain the idea that water injection might be mandatory, cuz by this time I had made up my mind to push the CR envelope.There was just no way I was gonna spend money that was so hard to come by on a lame 9.5 engine.No, I wanted 11 or better. And I was willing to create the target. So I picked what I had come to believe was a reasonable Dcr target,namely 8.5. And I had made up my mind that I wanted the 292/509. With a 70* ICA,that required an11.2Scr. Right on target.Unfortunately that was not possible with the parts I had; 65cc head,6.5 gasket 5cc eyebrows and 0 deck, for a total 76.5..This yielded a 10.83Scr/8.24Dcr @104ICA. I advanced it an extra 4, and this put the Dcr up to8.52. Since I was using the .029 gasket on a zero deck, I was done.
Well I had a hard time getting that past anybody. But I didn't care; what did they know? Had they spent months and months and months, doing the research? No they were stuck in old-school thinking, that 11.2 was beyond do-able. Perhaps I was more foolhardy than brave.
Well, much to my surprise, that stupid thing ran on 91non-oxyginated right out of the gate! A little tuning and 87E10 was easy. That was the beginning, in 1999.
Since then, I've learned a few other things about Dcr. It's not a fixed thing.It varies with rpm and other factors too.In my engine, it might start out at 8.5, but as cylinder filling improves through-out the rpm range, there will/may come a point where it becomes the Scr(10.7 in my case). With exceptional filling, it may even become greater. So even though the chart shows Scrs over 11.4, I have no idea if it's runable.
10.83/8.52 worked for me.(EDIT; 11.2/8.5 formerly posted,was in error.)
Your results may vary.
Turns out that 292cam is a bit of a pig on gas, but worse, it's not very streetable. The bottom end where I was spending most of my time was pretty soft. So the first guy that showed interest got that cam cheap.
My next cam was much smaller. The Hughes HE2330.This is a 270*cam, and has a 61*ICA and produced a Dcr of 8.62. This too ran on 87E10, with aluminum heads.And it was a tyre-fryer!
The last cam a Hughes HE3037. This is a 276*cam, in at 64*ICA came in at 8.75Dcr. But by this time the well-used .029 headgaskets gave up, and so I swapped to .039 Fellpros. This dropped the Dcr to 8.53, and I'm still on 87E10.
Long story short, I guess the target was conservative,as it has no problems at this 8.5 level.

For my next iteration, I think I will target a Dcr of 9.5 with the Eddies and and the .039 gaskets. Yeah I'm being brave. Problem is I've run out of adjustability. I guess pop-ups will be in my future.There will be other changes as well.....Then again maybe I'll just stick with what I know works.
EDIT I redid some of the math. Apparently, the highest I've run is 10.82/8.52 with the 292cam.And 10.54/8.62 with the HE2330. And the current is 10.66/8.53 with the HE3037. My bad.It was a long time ago.My apologies.So no pop-ups afterall :(
 
To that last part... DCR is important IMO, but as you note - it changes. You can put in parts to give you a certain DCR and end up creating a lot of detonation in the midrange. So using that knowledge properly is as important as having it. Over the last year or so I've pulled away from high DCR builds. There's really no reason to push limits - you can make safe power and efficiency in other ways.
 
AJ/FormS said:
Now, I know some of you guys are not into Dcr, and that maybe don't see it as being that important.
Click to expand...



If someone doesn't see DCR as important, they need to take up stamp collecting. Whether you build an engine to push the boundaries of pump gas or not, DCR is an important consideration.
 
moper said:
To that last part... DCR is important IMO, but as you note - it changes. You can put in parts to give you a certain DCR and end up creating a lot of detonation in the midrange. So using that knowledge properly is as important as having it. Over the last year or so I've pulled away from high DCR builds. There's really no reason to push limits - you can make safe power and efficiency in other ways.
Click to expand...
Hey Moper, just curious, what do you consider 'high DCR'?
 
I am currently building engine on the high side of DCR. This kind of stuff has always been interesting to me, so I wanted to build an engine like this.

My build is a 71 318 bored .030 over, with Icon 844 pistons, 302 heads, and a Comp XE284 cam. The machine work is not finished yet and so I don't have the exact specs, but it could be 9.75:1 SCR and 8.0 DCR. I have been considering 10:1 SCR and 8.2DCR... but I fear that is pushing past the limit on 91 octane on iron heads.

Maybe I should go for .035 quench and the 8.2? Does good chambers mean you can run 87 on 8.0 DCR and iron heads(Successfully)? Some people seem to think 8.5 DCR is max for 91 on iron(but nearing impossible) and 8.0 is more reasonable.

Another question, 87E10?(Not available in my area) 87 with 10% ethanol boost? Is it like 91 or better?
 
In terms of anti-knock, 87 is 87.

"High is kinda relative. It depends on the fuel, the load,the build,and how you got it."
-Start with fuel cuz that's easy. We streeters have just 3 grades to choose from, and they cover a very narrow range.
-Then there's load;
A V8 A-body can be as trim as 3100pounds or as fat as 3600. That's a 16% spread.
Then gears. You can run 2.76s to 4.30s and beyond, with first gears from 3.09 to 2.45, and clutches to all kinds of stalls.And the there's altitude.And carbs vary from around 500cfm to 750 and beyond. And fresh air versus hot underhood air. This is really complicated.
-Next is build;
things like chamber shape, quench,material, ability to shed heat,induction/exhaust,and all kinds of other parameters.
- And finally,How you got it.
If your combo has a large spread between Dcr and Scr, you might be in trouble. The fuel still has to support the Scr sooner or later. If your combo has a small spread, it might be fuel friendly.
-------------------------
As to your combo; a 1.75spread, topping out at 9.75/10.0, With your listed parts in a lightweight Swinger, it should be fine. And if it turns out not to be fine, there are still several things to try, before digging into it. That's the beauty of a Zero or near-zero deck engine. Options.
Now I have to admit, there's lots of smarter guys than me on this topic. Lots and lots.
I don't know if it's possible to design a perfect Dcr to Scr spread, cuz it's kindof ethereal. Every time you change one parameter it effects others. This is where trusting your builder comes in. The Mopar builders who have been doing it for years, know what works with what.So if you just want to drop it in and drive, find one you can trust.But if you enjoy the getting there,like I did,and have the resources,aim high.

And please reread post #4.It would be hard to top moper's comments using as few words as he did.
 
What is the preferred online dcr calculator?
 
Completely agree. But for the few that would never be happy with easy horsepower... pushing the limits can be awesome, or disappointing...

My car is on the fairly light side(3200ish) so that may help me out some. And I am not afraid of lots of tuning to get it right.

I don't have an engine builder, just a machinist and me. But I do enjoy the getting
there, and hope to help any one else out trying to build something similar.
 
AJ/FormS said:
I thought I'd throw something out there;

Suppose you wanted to run a Dcr of 8.0, in your 367(40over) iron-headed streeter, so you could run just about any pump-gas. This will get you a cylinder pressure of about 160psi.
I'm gonna try and chart it so you can compare Scr to ICA, and approximate cam size(seat to seat,on the intake) to get it. All numbers are biased to achieve the 8.0Dcr.Keep an eye on the change in cam size as the Dcr rises.
You could run less Dcr, but if you are already buying pistons, what would be the point?
You could try a little more Dcr, depending on your chambers and tuning ability, but another 1/2 point would probably be pushing the limit.
This is just for informational purposes.


.... Scr........ICA req'd........cam*.......LDA ........ICL ...camchange ............notes
..... 8.4 ....... 31 ............ 202 ....... 114 ..... 110 .....zero .............Just try and find one
.... 9.4 ....... 55 ............. 254 ....... 112 ..... 108 .....52*
.. 10.4 ........ 69 ............ 286 ........ 110 ...... 106 ....32*
.. 11.4 ........ 78 ............ 308 ........ 108 ...... 104 ....22*
.. 12.4 ........ 85 ............ 310 ........ 104 ...... 100 ....02*

Now, I know some of you guys are not into Dcr, and that maybe don't see it as being that important. And I agree that if your long block is already assembled and in good shape, there could be a lot of money spent on very little gain.
This chart/exercise is more geared for those whose projects are in the planning stages, and who wish to attempt to optimize it. Sometimes minor changes, can lead to great benefits.
Check out the last two cams! They are basically the same seat-to-seat, yet the 4* tighter LDA, and the re-time allows/requires, a full 1.0point of compression to be come to the target 8.0Dcr!
Also check out the poor 8.4Scr engine. There is almost no way to even get to the target 8.0Dcr!
To be sure there are many other cams that could be thrown into the mix. I just chose popular and makes-sense LDAs and kept it installed to 4*advanced, as common denominators.The actual cams just fall out of the equation.
My last three combos have all been 10.7 to 11.2 SCr with aluminum heads, and the Dcrs have ranged from 8.6 to 9.2. One of them went 93mph in the Eighth at about 3650pounds,at 950elevation,a small cam, and a street set-up; so it made a believer out of me.
As a comparo; my stock 340 Swinger went 98 in the QUARTER back in 1971.Which is about where we all ran. Boy-oh-boy, I thought that car was fast!We all thought 98 was fast.
Have fun.....
Click to expand...

Some good information there,A/J. I always preferred quick action ramp cams,with wider lobe centers( depending on the setup). FWIW ,always up for learning.. You have a current link,to a DCR calculator?
Damn it.. Read your post,missed the obvious stake. My apologies.. Let me post my stuff in ,I will respond honestly..
Current Magnum j.y combo (ootb EQ Mag replacements,1128 MR Gasket .028" thick stuff, Howards 229/233 hydraulic roller ,Crane 1.5 conversion ( I like my rockers adjustable, and reliable) Static compression ratio of 9.4:1.
Effective stroke is 3.58 inches.
Your dynamic compression ratio is 9.40:1 .
Your dynamic cranking pressure is 195.63 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of PSI is 9.40 :1.
V/P (Volume to Pressure Index) is 211
My first combo: hydraulic rollers expensive, I worked it:Static compression ratio of 8.8:1.
Effective stroke is 3.24 inches.
Your dynamic compression ratio is 8.06:1 .
Your dynamic cranking pressure is 160.52 PSI.
Your effective boostconon ratio, reflecting static c.r., cam timing,
I did punch in "0" for sea level. I usually race at Lompoc Ca's " S-20" , about 20-50 feet below street level. Speaking of "cold air, ", does D.C.R., have a response ,or is it considered ????
Edit: Of course I screwed up,on first calculations..Static compression ratio of 9.4:1.
Effective stroke is 3.27 inches.
Your dynamic compression ratio is 8.67:1 .
Your dynamic cranking pressure is 176.38 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 8.67 :1.
V/P (Volume to Pressure Index) is 174
 
O.k. posted my used stuff.. Was curious if you after absolutely crisp driveability, ( I'm sure of that...), or any fun in life with E.T.'s?
 
nm9stheham said:
Hey Moper, just curious, what do you consider 'high DCR'?
Click to expand...

That's the problem.. Every build will have it's own "problems" threshold based on everything from mechanical parts, machining accuracy, tuning, fuel type, and weather. Nevermind the differences in calculators and their assumptions... I use the KB site. I prefer having the numbers used at .050" rather than seat to seat. The figures below are based on using that site. Seat to seat or .006" numbers do not tell the whole story so I'd rather use later (and IMO more accurate) numbers, and fudge a little based on experience than let the calculator fudge for me.
In terms of pushing limits - with an engine built to my standards (sorry for the way that sounds but I'm beyond picky in terms of machining quality and how I do things) and assuming the typical 91 octane w/10% ethanol CT uses...
An open chambered head (no quench) - max dynamic ratio of 8:1
A closed chamber iron head (piston to head clearance no larger than .035") - max dynamic ratio of 8.25:1
A closed chamber aluminum head (piston to head clearance no larger than .035") - max dynamic ratio of 8.5:1

These do have some fudge factor. If you've read my old posts on Moparts arguing with Eddie Elvis about "pump gas" you'd understand I tend to rely on the fuel quality being crap at your local Quickie Mart. Pump fuel comes from the ground and is not consistently blended. Not out of a Rocket or VP drum that is perfectly consistent in every way. I've run DCRs as high as 8.5:1 with iron open chambers & no quench. But those ran on pump 93 and I tuned them and admittedly upon one teardown there were signs of detonation even though it was never heard. I used to be about 1/4 point higher for all of those but decided I wanted some cushion and realized that the power that might get me could be made in other ways.

DCR is a functional result of a lot of other things - not a specification. Keep it in mind - but don't build specifically for a certain level. There are some engines with DCRs down in the low 7s that make a ton of power safely on pump fuel.
 
Thank you, Moper. I appreciate the considered and thorough reply. I tend to want engines with a low end torque that is greater than is needed for best ET's in the 1/4 miles, and so like to push CR and limit duration for that end. But that runs straight towards detonation territory, so I am very interested in all things related to that. We are pretty much at your last combo with AL heads in our 340 build BTW.... so good to see that..... .040" quench but a little less DCR.

But I also don't push the limits; on the street and driving 10's of kmiles, I don't need to be worrying about detonation late at night with bad gas. And when rallying, it sucks to have to walk 12 miles out of some remote forest after midnight if your motor expires! BTDT.....
 
Abodybomber said:
Speaking of "cold air, ", does D.C.R., have a response ,or is it considered ????
Click to expand...
FWIW, I tend to think of cold air more related to AFR tuning and not engines internal tuning. You can't change the engine internals to adapt for cold, dry air, but you should be looking at AFR tuning, 'specially if turbo'ing. Kinda like for altitude.....
 
"Dy-na -mic".
The output of the calculators assumes atmospheric pressure, and assume the geometry of the lobe and the affects of racker ratio and angles on motion, and assume there are no dead spaces above the piston ring, and assume a low piston speed. They do not take into account the physics of a running engine. So you are computing pressure at 60rpm. Here is a link to a page that has a great graph. Sorry I can't attach a picture from this PC:

http://performancetrends.com/Definitions/Cylinder-Pressure.htm

Go to the page and look at the graph posted there. This is a pressure graph for a cylinder during all four strokes. Note the curve up when the piston is moving from BDC up towards TDC on the compression stroke. The shape of that particular curve, before the ignition event, is crutial. Too fast a rise and you can get detonation or ping from other deficiencies. Too slow and the engine will react sluggish because the cylinder filling is not what it could be. Now as you quicken the time for these cycles (raise the rpm) you can see where that particular curve could be really affected by inertia of the exhaust blowdown event and the intake charge within the intake tract and by the larger area exposed to atmospheric pressure due to open throttle blades (more "push"). The changes in the curve due to rpm are what one should worry about. Not the actual dynamic ratio. Cranking pressure at cranking rpm means next to nothing IMO in regard to output but it can indicate problems and is fun to talk about.

PS. Disclaimer - I own Engine Analizer Pro.
 
Using the two calculators posted above my SCR is 11.46, .48-1 and the DCR is 9.76-.82-1. I wonder how much timing I'll have to pull out to make it run on 93?

**EDIT**
Ok guys what's going on here? The numbers from the Jeep forum when entering the 55* ABDC from the cam card in the "Intake Valve Closing Angle" input field I get 9.82 DCR.

When entering the advertised cam intake duration from the Comp lobe chart, LSA and installed 4* advanced the calculator comes up with a DCR of 8.65-1 and a intake valve closing angle of 72.

What am I missing here? Is the closing angle asking for something other than the 55* from the cam card?

Ahaaa, the 55* closing point is at .050".
 
^^^ Roger.... the actual time of building pressure is supposedly around the advertised closing, not the .050" closing. I think KB says that they take the .050" and assume a .015* later closing.

I prefer this tool by Pat Kelley, which you can find on the 2nd page of this link. It has better and more inputs on the variables, IMO. A bit more work to use, but it gives me more confidence than the KB tool for example. The link offers a good explanation of what this tool is doing.

http://cochise.uia.net/pkelley2/DynamicCR.html
 
....Assumed to be 100% for the calculators, I do believe....I view DCR like head flow; bench flow numbers are not what the heads flows in real action, but are useful numbers for prediction and comparison and making mods.
 
As your VE decreases, so does your DCR, and as it increases, so does the DCR. That's something to factor into the equation. That's why threads like this can mislead people into just putting some numbers into a calculator and think it's all good.
Also the RPM will get to a point that the combustion cycle is happening so quickly that it does not have time to detonate..
 
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