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dynamic compression ideas

General engine tech -- Drag Racing to Circle Track

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Walline
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dynamic compression ideas

Post by Walline » Tue Jul 26, 2005 3:16 pm

Does anyone have a number range they use for deciding how much dynamic compression an engine can tolerate, on 87 octane. I realize there it is MUCH more involved than just a number. But I would like any ideas. What have others tried, with 87? I have never had a problem if the dynamic was under 8.2 with 92 octaneon SBC, but we are building a 454 that we want to run on 87,and we want to push it as high as we can. THANKS!

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Post by SStrokerAce » Tue Jul 26, 2005 3:20 pm

Part of it depends on how much VE and peak cylinder pressure you expect to see. The more of both of those the more you need more octane.

What's the rest of the combination look like? Chamber size, piston shape, quench, heads etc...

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Post by Walline » Tue Jul 26, 2005 3:32 pm

BBC TRW forged .150 domes, 119cc chambers. Zero decked for .040 piston to head clearance. Right at 10:1 static compression. have not done final numbers. just figures. Victor Jr. intake, 781 oval port heads, 3500 converter, and 4.10 gear in 3500lbs car. With the current cam it figures 7.22 dynamic compression. I am sure it will run fine on 91, but not sure about 87.

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Post by SStrokerAce » Tue Jul 26, 2005 6:25 pm

Well the burn isin't going to be the best with a dome in there and the larger the cam the more peak cylinder pressure if all is going right. Iron heads don't help either. 7.2 is pretty low reguardless, I wouldn't be really worried but you can always hook up a knock sensor to it, throw it on the dyno with 91 and then back off to 87 and see if you gain any knock.

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Post by b73 » Thu Jul 28, 2005 8:07 pm

Some info, and a great calculator to download here:

http://cochise.uia.net/pkelley2/DynamicCR.html

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Post by panic » Thu Jul 28, 2005 11:47 pm

This all looks depressingly familiar.
Q: if dynamic compression is an accurate knock barometer for the entire RPM and load range, does this mean that an engine with moderate CR and late intake closing (DCR = 8) will have lower power throughout its range than a motor with moderate CR and early intake closing (DCR = 9)?
A: if no, DCR does not predict octane requirement (despite what Mr. Kelley thinks).
If yes, we have all been wasting a lot of money, don'tch think?

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Post by SStrokerAce » Fri Jul 29, 2005 1:39 am

panic wrote:This all looks depressingly familiar.
Q: if dynamic compression is an accurate knock barometer for the entire RPM and load range, does this mean that an engine with moderate CR and late intake closing (DCR = 8) will have lower power throughout its range than a motor with moderate CR and early intake closing (DCR = 9)?
A: if no, DCR does not predict octane requirement (despite what Mr. Kelley thinks).
If yes, we have all been wasting a lot of money, don'tch think?
you do fit your name boss....

http://web.camaross.com/forums/showthread.php?t=382484

Start at post 15 after you get thru post 1 and that's the best I can do.

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Post by panic » Fri Jul 29, 2005 7:53 am

Actually, that was very rude of me to have phrased it as a question. I already have an opinion.
The responses in that thread ranged from "not a clue" to "I will now quote every non sequitur I can remember about engines, and flatly state that they prove my point". No one laughed when someone said (loosely paraphrased) "all race engines have the same torque per cubic inch regardless of RPM" as if compression ratio had no effect.
Quoting from my own comments:
"DCR Errors
“Dynamic Compression Ratio” (DCR) is offered (elsewhere) as a method of approximating how much power will be lost by extended intake closure (i.e., cylinder pressure is reduced). DCR is very useful in predicting what octane is needed at lower speeds to help setting idle (initial) spark vs. advance curve length, stall speed, &c. However, this is only partially accurate due to several errors and misconceptions.
1. The actual power loss is only present below the points where both full capture (where intake reversion ABDC stops) and positive flow-through at overlap occur (although these may not be close together). Above that point power is increased (over the milder cam), either by improved volumetric efficiency (and cylinder pressure, since a larger percentage is trapped in the cylinder), higher RPM reached @ the same efficiency level, or both.
2. The power loss is not solely due to reduced cylinder pressure (which is the output of the Kelley, RSR &c. DCR calculators), since the lower pressure is also acting through a shorter effective stroke (measured from intake valve closure ABDC) and therefore suffers from two reductions. My own method factors in the effective cylinder displacement as well, for a closer approximate at how much power is developed at low speed.
3. DCR is widely considered to be an accurate barometer of knock resistance (e.g., “up to 8-1 DCR can be used with XX octane”, &c.).
This is not true, and not safe, since (in a high compression motor with its DCR reduced by late intake closure) after the early low-pressure period expires (at the beginning of the torque curve) actual combustion pressure will be at least as high with the bigger cam than it was previously - even though the DCR is lower, and therefore suggests that lower octane is safe. However, in a motor with 14-1 static CR the gas pressure at its torque peak is not at all reduced by a very late intake closing point, although the (lower) DCR may indicate that 92 octane &c. is sufficient.
Since the knock will only occur at high speeds it may not be audible, and will reduce power (lower MPH) even if no damage can be detected. In my opinion, the power sometimes gained by retarding spark in high gear is actually an attempt to recover some of this loss - but would be better served by reducing the static ratio slightly.
DCR is a curve or slope of cylinder (not combustion) pressure, with Position 0 (the absolute low end) at cranking speed, then a small rise to idle speed, then another rise to the capture point &c. After this point (and especially near the torque peak) the static CR becomes more important, since almost the full stroke length is captured and compressed, regardless of the intake closing point.
Any cam will determine the “slope” (or rate of rise) of the DCR curve. A long-duration cam with its attendant late intake closing point will have a high degree of rise, a mild cam less, &c. A longer cam will also extend (stretch) the range of RPM that the slope covers, sometimes over several thousand more RPM.
The static ratio determines the height of the cylinder pressure line at Position 0 (cranking speed). With high static ratio the entire curve is higher, with the curve's upward intensity being governed by the intake closing point.
It's possible to design a DCR that looks promising, but will not provide any more power, by assuming that there is no limit to either static ratio or intake closure - and, of course, neither is true. Some motors cannot turn fast enough (due to stroke length, weak valve gear, high reciprocating weight, &c.) to reach their capture point if the intake closure is too late, and will produce more power with more conservative cam timing. A motor with limited static ratio (flathead) must conserve cylinder pressure by limiting intake closure for the same reasons.
Another error in use of DCR calculations for low-speed power prediction lies in the fact that a smaller volume of mixture being compressed to a higher ratio. Even though the pressure gauge reading taken during cranking or idling is higher, the total of cylinder pressure times the actual mixture volume captured may still be lower (compared to the original milder cam and moderate compression ratio).
To sum up: DCR a useful tool, but widely perceived to be of greater worth than can be supported by physics. "

Read the entire article:
http://victorylibrary.com/mopar/cam-tech.htm

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Post by Darin Morgan » Fri Jul 29, 2005 10:12 am

b73 wrote:Some info, and a great calculator to download here:

http://cochise.uia.net/pkelley2/DynamicCR.html

Two important points to remember:

The DCR is always lower than the SCR
The DCR does not change at any time during the operation of the engine


All this stuff is out of text books and is probably 20-30 possibly 45 years old and in my personal opinion its wrong! I get what they are trying to do, but I dont think it works.


Let me put it this way.

You have an engine with 15:1 static CR.

the engine operates at a peak VE of 124% with a minimum trapped volume of 118%. Now what is the dynamic CR again???????

That same engines VE ranges from 118% to 124%

The Dynamic CR doesnt change??? Come agian?


If I am all washed up on this please educate me becuse what they are saying does not ring true. Are they refering to somthing differant?
Last edited by Darin Morgan on Fri Jul 29, 2005 10:30 am, edited 1 time in total.
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Post by Walline » Fri Jul 29, 2005 10:23 am

Darrin, I fully understand what you are getting at. So? What method can I use to design engine combos, that someone wants to run on pump gas? Is there software that can help, or do we just have to test it. Can software numbers(VE) be used in a formula to recalculate dynamic comp. I realize there are a huge amount of variables. Thanks for the replay, that was the type of info I was looking for!

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Post by Darin Morgan » Fri Jul 29, 2005 10:47 am

Walline wrote:Darrin, I fully understand what you are getting at. So? What method can I use to design engine combos, that someone wants to run on pump gas? Is there software that can help, or do we just have to test it. Can software numbers(VE) be used in a formula to recalculate dynamic comp. I realize there are a huge amount of variables. Thanks for the replay, that was the type of info I was looking for!
Walline wrote:we are building a 454 that we want to run on 87,and we want to push it as high as we can.

Some the the best engine builders in the world have tackled that problem and the ones I talked to all did it pretty much the same way. They tried to keep the duration small , little overlap and as much valve velocity they could get away with on an endurance/street cam. They then kept bumping up the CR until it melted and backed off a notch. Its still trial and error is what I am trying to say. You can dig around for every calculation in the book and it will help but it wont tell you how. Talk to people who build these types of engines because high performance pump gas engines are tricky. When it comes to making SERIOUS power with pump gas, Air fuel management is your BEST FRIEND don't forget that!!!! EFI will let you do things that Carburetors cant get close to. Chamber heat, cam duration, overlap duration, air fuel management and don't forget chamber/ piston design! OH boy is that important! After talking to Kasse about everything he went through to make power on low octane fuel, I wont touch it with a twenty foot pole. Its very difficult when your trying to make Serious power with 92 octane, making power with 87 octane,,,,, I wouldn't even try it unless you have a lot of time and money.
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Post by Wolfplace » Fri Jul 29, 2005 1:11 pm

This again :(
I have my own opinions which I have been slammed for more than once on the DCR crowd board but I will add my thoughts to this discussion that I have posted before to the question of "DCR" which I feel in this context should more aptly be named "ECR" or effective compression ratio in regards to where the intake valve closes as dynamic implies something in trasnsitions or change.

Here is one older post I saved for what it's worth :lol:

Ok,,, just to add my thoughts regarding DCR here as my name was mentioned earlier as agreeing with Ryan which I pretty much do but,,,,

Just to be clear, the difference is I NEVER said it was meaningless.

I do not presume to know all there is to know in the universe regarding engines.
Fact is after about 40 years I find out daily how much I don't know :(

I have posted at different times my thoughts on DCR as it is used here & here is a couple of them:

DCR or more correctly "effective compression ratio as Dynamic implies something in motion or transition as used in Pat's DCR calculator is a very important part of the equation when it come to cylinder filing,, or lack of cylinder filling but,,,.
There is a bit more to cylinder pressure than just when the intake valve closes.

At low RPM, DCR as it is being used here will suffice when trying to run crap gas with high compression.

But, when you start getting up near peak torque & the VE is starting to climb everything changes & you better damn well have fuel with enough octane to support the cylinder pressures you are making.

This is not as simple as where the intake valve closes, it now becomes, what I feel is truly what Dynamic compression ratio is all about & includes where the intake valve closes ALONG WITH how much cylinder filling you have done taking into account all the other factors like how efficient the intake & exhaust system is at a particular RPM, which is essentially what volumetric efficiency or VE is about.

VE is a ratio of how much you filled the cylinder verses what you could have filled the cylinder (100%) at atmospheric pressure & in a well tuned system (or forced induction) can reach well over 100%.
IE: "overfilling"


Here is the rest of another post:

"First let me say I think that Pat's DCR calculator is an excellent tool when used with a bit of common sense.

Here's some of my thoughts about DCR & high compression that I have posted before.

With a big cam you will see lower cranking compression but you also have to use a little common sense with the slam a big cam & lower the DCR syndrome
It may have low cylinder pressure at cranking & low rpm's but somewhere it is going to start getting efficient & the engine is going to see whatever compression you have.
If the fuel isn't good enough when this happens ugly things can & eventually will happen.

My own opinions are that the calculator as a tool needs to be used properly & as I said before, with a bit of common sense.

Unfortunately I feel there just is no set compression for either aluminum or cast heads.
As was stated there are way too many varibles.
It would be nice if engines could read all the theory & behave accordingly but it doesn't always seem to work out that way :D

I don't really care what RPM you are at, detonation can & will occur if the fuel doesn't have the octane or is to lean for the circumstances at the time.
This is true of gas, alcohol or nitro.
The heavier the load & the lower the rpm the more likely it is to detonate but it can happen at 8000 or 3000 if things ain't right.
As for hearing it on a dyno, you usually will not unless it is really bad.
You will probably see it if you happen to be measuring crankcase pressure & you will see it in the torque curve when the power goes to hell again if it is bad.
The exhaust temps may or may not go up & sometimes they will go down.
The best deal is the in cylinder pressure measurement stuff that is available but it starts around $25-30,000 for one cylinder :eek:

Short of that, it pays to keep an eye on plugs & start on the rich side with conservative timing just like at the track.

I used to drive a blown alcohol dragster & you would not hear detonation but you would hear & feel the engine "lay down" & it was a good idea to consider getting out of the throttle at this point if it wasn't a race ;)
This was usually on the top end so the rpm was definitely on the high side as this thing went through the lights well over 10,000 rpm.

If the plan is to use & lean on your engine with pump gas most of the time I would prefer to limit the static compression with aluminum heads at 10.-10.5 for 91 octane & 10.5-11 for 93 & change the cam accordingly.
There are combos that can & do run higher & some that will not tolerate this much but I feel the risks far outweigh the advantages of pushing the limits with compression.

You are right in that cranking compression which is part of the DCR is determined from when the intake valve closes.
The higher the static compression, the higher the cranking compression will be the sooner you close the intake valve.
This I think we all agree on.

Now there is a lot of other things that will effect cylinder pressure to a certain extent, like RPM, Barometric pressure, cam intensity, engine temp, air temp, humidity & probably a lot of other little things I missed as well.

Here's another thought to screw up the works,,
You can build an engine with say 13.0 & overcam the crap out of it & on paper the DCR will tell you it will run on 91 octane.
The problem comes when that engine gets up in the rpm & becomes efficient & starts actually seeing that 13.0. Now that "DCR number" becomes less of an issue as you are starting to keep pressure in the cylinder & with enough load without proper fuel, it will detonate.

The load part is why lighter low gear cars will usually tolerate more compression than a heavier or higher geared car.

It also works the other way.
We do restricted engines you just could not put a load on at low rpm in most cases.
Small cam hi compression deals like 13.5+ with a cam of say 240 or so @ .050 & at low speeds they will detonate themselves to death if you ain't careful but from say 4000-4500 up they will never see that 13.5 compression as the intake is too small to let enough air in to fill the cylinders :(
Is this enough confusion yet??

Anyway, you are only compressing what is in the cylinder from the time the intake valve closes & this can be completely different depending on a number of things but most of them are listed above I think :confused:

Ok,,, these are just a few of my uneducated thoughts on the merits of of just using when the intake valve closes to calculate DCR.

Someone else can pick up the VE, quench, swirl, chamber efficiency & timing issues :D "
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Re: dynamic compression ideas

Post by bobqzzi » Fri Jul 29, 2005 9:25 pm

Walline wrote:Does anyone have a number range they use for deciding how much dynamic compression an engine can tolerate, on 87 octane. I realize there it is MUCH more involved than just a number. But I would like any ideas. What have others tried, with 87? I have never had a problem if the dynamic was under 8.2 with 92 octaneon SBC, but we are building a 454 that we want to run on 87,and we want to push it as high as we can. THANKS!
I'm curious- why the 87 octane requirement? Sounds like a dangerous game to play with a no doubt expensive engine- especially one that has poor detonation resitance to start.

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Post by Walline » Sat Jul 30, 2005 9:04 am

I guess I should have made myself more clear. The owner wants the engine to run on 87. BUT, the engine is being built for nitrous, so when it is at the track it will be on race fuel if needed. When enjoying it on the street, he wants to run 87 and I am a little concerned. 91 is one thing. They always say they will limp it around, but everyone knows, you will get on it at some point.
especially one that has poor detonation resitance to start.
I do not have a lot of BBC experience. What makes you say that? The big domes to acheive compression, I have my ideas, just interested in your experiences. THANKS ALL!!

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Post by bobqzzi » Sat Jul 30, 2005 9:57 am

Walline wrote:I guess I should have made myself more clear. The owner wants the engine to run on 87. BUT, the engine is being built for nitrous, so when it is at the track it will be on race fuel if needed. When enjoying it on the street, he wants to run 87 and I am a little concerned. 91 is one thing. They always say they will limp it around, but everyone knows, you will get on it at some point.
especially one that has poor detonation resitance to start.
I do not have a lot of BBC experience. What makes you say that? The big domes to acheive compression, I have my ideas, just interested in your experiences. THANKS ALL!!
I was just commenting on the very large bore and generally poor combustion chamber (as compared to more modern designs).

I have to laugh at your customer promising to just limp around...sure he will! Any idea why he wants to run 87 octane? Seems to me, if you are concerned with fuel costs a big block may not be the most economical choice!!!

Good luck with this project

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