I've been doing alot of research lately on calculation of Static CR vs. Dynamic CR. I will start off by saying that I do understand that a static CR figure has nowhere near the affect of determining pump gas safe CR levels as Dynamic CR.

Here is my dilemma.


It is generally regarded that a motor with a Static CR level around 12:1 and up is pushing it on pump gas. From the above statements we know this doesn't tell us much. Say we have a given motor that has a static CR of 12:1 and after proper tuning it can handle 91 octane fuel. Now say that we are at the limit of 91 octane fuel right now as far as tuning as what tuning can do to prevent detonation. What I am getting at is this motor obviously has a Dynamic CR below 12:1. Which means that motors are actually a LOT LESS tolerant of compression as we were to believe.

Main Inquiry: What is the highest DYNAMIC CR that a motor can take while still using a fuel at an octane rating at say 91.

Given the previous motor's setup it is going to be nowhere near 12:1. I would press to say that it wouldn't even be above 11.5:1

Any thoughts? Am I understanding this correctly? I believe this information would be enough to really change thinking about how people build motors. It would make for more reliable builds regardless of NA or FI applications. Everyone knows that dynamic CR is important but nobody ever talks about its limits.

NOTE: Base this assumption on a stock head motor, ie no welding of the combustion chambers etc.

 

Doesn't it somewhat depend on your cam as to what the dynamic CR is going to be?

 

It's almost entirely dependant on the cam. Compression can't begin until the intake valves are fully closed. Static compresson assumes the valves open and close right at TDC and BDC. Dynamic compression uses the point when the intake valves actually close. This takes up some degree of the crank rotation which takes up some of the stroke, making it shorter. You find out what the new stroke is and put that into your compression ratio formula in place of the stock stroke. For 91 octane pump gas you generally want to stay in the 7.5:1-8.5:1 dynamic range. Lower and you might get a dog of a motor like people putting huge cams into motors with 10:1 static compression and wondering where their low-mid power went. If it's higher you need more careful tuning or you can run a more aggressive cam profile.

The above was an over-view of the following article;
http://www.empirenet.com/pkelley2/DynamicCR.html

 

i think that it all matters on the ignition timing.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 91SiZ6 &raquo;</TD></TR><TR><TD CLASS="quote">It's almost entirely dependant on the cam. Compression can't begin until the intake valves are fully closed. Static compresson assumes the valves open and close right at TDC and BDC. Dynamic compression uses the point when the intake valves actually close. This takes up some degree of the crank rotation which takes up some of the stroke, making it shorter. You find out what the new stroke is and put that into your compression ratio formula in place of the stock stroke. For 91 octane pump gas you generally want to stay in the 7.5:1-8.5:1 dynamic range. Lower and you might get a dog of a motor like people putting huge cams into motors with 10:1 static compression and wondering where their low-mid power went. If it's higher you need more careful tuning or you can run a more aggressive cam profile.

The above was an over-view of the following article;
http://www.empirenet.com/pkelley2/DynamicCR.html</TD></TR></TABLE> T hat artical is absolutly correct if you are building a 5000 RPM 307 Chevy. If you are doing a higher reving 4 valve motor, find another sourse of info.

 

Yup 91SiZ6 is correct. Why not check out the available cams with their ABDCs? Then work from there. I recall that crower stage 3s only have 47 degrees ABDC which is actually quite mild and would not need an increase in static comp. But when you start looking at close to 50 thats a different ball game.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by DonF &raquo;</TD></TR><TR><TD CLASS="quote"> T hat artical is absolutly correct if you are building a 5000 RPM 307 Chevy. If you are doing a higher reving 4 valve motor, find another sourse of info.</TD></TR></TABLE>

how bout sharing some of that knowledge

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by camlob &raquo;</TD></TR><TR><TD CLASS="quote">Yup 91SiZ6 is correct. Why not check out the available cams with their ABDCs? Then work from there. I recall that crower stage 3s only have 47 degrees ABDC which is actually quite mild and would not need an increase in static comp. But when you start looking at close to 50 thats a different ball game. </TD></TR></TABLE> Just to ask you a stupid question, what is the ABDC closing of the non-vtec lobes.or are you running VTEC killers or locked VTEC lobes? Just so you know, DOHC VTEC engines are different than SOHC VTEC motors. You can actually change over-lap, and cam centerlines, how scary is that?
I promised myself i would not post here anymore along with Rocket, RLZ,JJ, Dave, J-Dogg, etc. But when i see wrong info, it does bug me. If you have adj. cam gears, or a feeler guage you can adjust the ABDC number. Now that is scary.
If alls you can do is buy a SOHC cam, with bad timing numbers, and hope it runs well, you can have opinions here.

 

http://kb-silvolite.com/calc.php?action=comp2

Like Don F said you need to know the hard numbers.You will need some engine knowledge to use this calculator.I have never tried to use it on imports but I have compared my personal experiences with small block chevy engines vs the results of this program.It seems to be pretty accurate for a domestic v8.The are some good articles on piston design and engine dynamic on this site.

 

I know the dymanic cr largely depends on the cam. DUH. LOOK AT THE QUESTION!

WHAT IS THE HIGHEST DYNAMIC CR YOU CAN RUN ON 91???

Everyone seems to have rules of thumb on static cr, but why not dynamic cr which is the more important factor.

 

If you get a chance read the WD-40 shoot out with some respectible engine builders. That should answer most of your questions. By the way Dynamic Compression depends on air flow. If you cannot fill the cylinder, (restrictor)you increase compression, If you can fill the cylinder, or above, VE, forced induction, you lower it. Then you talk to your fuel supplier, he know's more than you.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by DonF &raquo;</TD></TR><TR><TD CLASS="quote">If you get a chance read the WD-40 shoot out with some respectible engine builders. That should answer most of your questions. By the way Dynamic Compression depends on air flow. If you cannot fill the cylinder, (restrictor)you increase compression, If you can fill the cylinder, or above, VE, forced induction, you lower it. Then you talk to your fuel supplier, he know's more than you. </TD></TR></TABLE>

DonF, is right. Basically, you need to run a engine simulator to get a analytical value for dynamic compression. Simply looking at ABDC and stroke my get you in the ballpark if the underlying assumptions pertain to engine your trying to evaluate.

One thing about engine sims, they are only as accurate as the information supplied to them.

 

I'm aware of that, and I've been able to come up with a number for my dynamic compression, but my question would be, what is the highest dynamic compression I can run. I know by increasing static compression, the dynamic compression should go up.

Am I wrong in thinking this?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by DonF &raquo;</TD></TR><TR><TD CLASS="quote"> T hat artical is absolutly correct if you are building a 5000 RPM 307 Chevy. If you are doing a higher reving 4 valve motor, find another sourse of info.</TD></TR></TABLE>
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by DonF &raquo;</TD></TR><TR><TD CLASS="quote"> If alls you can do is buy a SOHC cam, with bad timing numbers, and hope it runs well, you can have opinions here. </TD></TR></TABLE>

I actually like how much I threaten your pride. It makes me belly laugh.

Motor building basics are motor building basics no matter how much you want to deny it. As I said to you before, it makes more sense to start with an easier to understand system. People learn far more efficiently that way.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by DonF &raquo;</TD></TR><TR><TD CLASS="quote"> Just to ask you a stupid question, what is the ABDC closing of the non-vtec lobes.or are you running VTEC killers or locked VTEC lobes? Just so you know, DOHC VTEC engines are different than SOHC VTEC motors. You can actually change over-lap, and cam centerlines, how scary is that?
I promised myself i would not post here anymore along with Rocket, RLZ,JJ, Dave, J-Dogg, etc. But when i see wrong info, it does bug me. If you have adj. cam gears, or a feeler guage you can adjust the ABDC number. Now that is scary.
If alls you can do is buy a SOHC cam, with bad timing numbers, and hope it runs well, you can have opinions here. </TD></TR></TABLE>

Pls enlighten me why I should be familiar with ABDC closing of the non-vtec lobes? It was my impression, from various research, that what mattered was the ABDC closing for the vtec lobe. Is'nt the long duration and lift on the big lobe(not to mention overlap) that causes dynamic compression to drop? So what is so scary about changing overlap and centerlines? Thats why I go to a builder and tuner. Pls let me know if I got it wrong because I dont think so. I can read a little. BTW my sig says I have a Jun 3 and not a VTEC killer cam.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by camlob &raquo;</TD></TR><TR><TD CLASS="quote">

Pls enlighten me why I should be familiar with ABDC closing of the non-vtec lobes? </TD></TR></TABLE>

That's the one that you'll have trouble with. If you build the motor to have the same dynamic compression as stock cams and stock static compression with the bigger cams and higher static compression then you'll have much more dynamic compression (perhaps dangerous amounts) when you're on the smaller cam profile. If you were always running on the big lobe, that's not a problem and the motor is more like an sbc

 

on a street car you prolly do 95% of your driving on your dinky little primary lobe. thats why you need to pay attention to the primary lobes intake valve closing, oh and good luck BTW finding that listed somewere.

i think the scary part regarding cam adjustments might be the fact that making cam gear changes will affect your dynamic compression. you could drive in the dyno fine, and drive out pinging on your primary lobes and have no idea why.

if your worried about detonation and the only thing your looking at is dynamic compression you basically are wearing blinders. theres alot of factors to consider and i suggest playing it safe on compression. you can always increase it later with milling, headgaskets ect.

 

Different cylinder head chambers and piston domes will change the octane requirement for different compression ratios. What will work on a chamber that has been modified, and proper quench is different from stock. Different head designs have different requirements, so in answer to your question, max DCR for 91 octane there is no 1 answer.

 

Personally, the only thing I use either compression approximation for is cam selection. I check what the DRC is on all three lobes and make sure it all looks good. Quite obviously this is only one piece of the puzzle and not the end-all-be-all of building. How well the head flows will obviously have an effect on how well the motor runs over all. But I don't think you'd get really great all-motor results with Crower 404s at 8.5:1 static compression no matter how well the head flowed.

To reiterate something. I am not a professional nor do I claim to be.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by DonF &raquo;</TD></TR><TR><TD CLASS="quote">Different cylinder head chambers and piston domes will change the octane requirement for different compression ratios. What will work on a chamber that has been modified, and proper quench is different from stock. Different head designs have different requirements, so in answer to your question, max DCR for 91 octane there is no 1 answer. </TD></TR></TABLE>

I understand this Don. But I'm asking if there is a rule of thumb figure for a stock b16/type-r head on pump gas. I know there is a way to collect this data and get a roundabout figure, but it seems that either everyone likes to keep these figures to themselves as their own R&D or just plain most people don't calculate it all out.

 

11.7:1

Are you happy?

 

very much so.

Thank you kindly.

 

Rule of thumb, read the WD-40 shootout. The better builders in CA all chose 11.5-1 CP pistons. They all had good heads and proper cams and had 100 % VE or above theirfore dynamic compressions or BMEP was above 11.5-1, probably 12.2-1 Dynamic at peak TQ. We have 93 here, and have done 11.8-1 static, this keeps the timing in a good range BTDC to keep the max cyl pressure in the 12-14 ATDC area and get peak TQ and BHP.
Unless you can measure BMEP you will not know the DCR. Next best is VE where you measure the actual CFM an engine uses. This generally only avail. on engine dynos, not sure about Roto-Test, I will have to ask Alan.

 

I had my head built wherein my builder re-shaped my combustion chambers ala endyne to accomodate my high dome pistons. I actually think that my mid-range is so much better. I dont think I am going back to 11:1 at this point. So the exercise I did of using the ABDC of the Jun cam to get the ideal DCR worked out fine. If you check out endynes article section, they have a civic running at 13:1 SCR and is a daily driver. But since I have not driven a 11:1 SCR with a relatively decent cam that has good CFM I cannot make a proper judgment. I will still think that if someone overcams an engine, then the money he used to buy the cam will be put to waste.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by camlob &raquo;</TD></TR><TR><TD CLASS="quote">I had my head built wherein my builder re-shaped my combustion chambers ala endyne to accomodate my high dome pistons. I actually think that my mid-range is so much better. I dont think I am going back to 11:1 at this point. So the exercise I did of using the ABDC of the Jun cam to get the ideal DCR worked out fine. If you check out endynes article section, they have a civic running at 13:1 SCR and is a daily driver. But since I have not driven a 11:1 SCR with a relatively decent cam that has good CFM I cannot make a proper judgment. I will still think that if someone overcams an engine, then the money he used to buy the cam will be put to waste. </TD></TR></TABLE> What was the ABDC number? And what was the DCR @ 7800 rpm ? What is your total timing @ peak TQ, what fuel? Sorry Rocket, now I am gone. LOL