compression ratio discussion

[John]

Again Flash, excellent presentation of important design parameters. Sticky this please as you will not find a more clear explanation. I am ask at the shop how much compression can we build it for? When the important question is how much compression are you wanting and willing to build for. I have seen stock rebuilds that pinged due to poor cam selection, improper ignition timing, tops of pistons left as furnished. Sharp edges left on head and block bores after machining. Air intakes swapped out for cool looking CAI's that only did a good job pulling heated air into the engine. It is within the grasp of the average builder to work with the I6 designed internal geometry, compression and quench to get a dependable DD stroker, but when we get away from the basic design we must think of the compensations necessary, I build OBDI because I can do so much more with it easily. OBDII is a blue haired witch to, outsmart, trick without it relearning around your changes, Are you willing to cc the heads and polish within a match, extra cc's here to remove some compression, are you willing to radius all sharp edges to reduce hot spots, polish the tops of the pistons to reflect heat and then have them sent out for coating, Are you willing to run alternate fuels, or use ethanol/water injection to reduce heat, insulate air intake systems or go to lengths to reduce under hood temps, run a stand alone fmu/ignition controller. or even have the head/block machined to o ring it and run some big numbers on the CR. I built one that was impossible to reliably start with a electric starter, but was scary how well that one ran. A builder has a lot of questions to answer before asking, how much compression can we run on that fuel. Back when the dinosaurs still roamed with carbs and without computers I would have said 10.25.1 on 100 octane. Today it is not as straight forward, look at the compression of some of the Honda's and Nissans, some of those will run on econo grade and will not ping, just loose optimum power. Some will make you cry they run so badly. The difference is in the build/fuel/ignition managment.
Not as eloquent as Flash
John

[Muad'Dib]

Split Flash's post out of here so it can be found easier... and moved to the FAQ section.

Closing this thread so it can resume in the other thread if need be.

If you feel it still needs to be open, please PM me.

[Muad'Dib]

Re-opened as the original poster made me aware that the original question was never answered! .. MY BAD

[Flash]

compression ratio discussion

It seems that most people where they build their stroker try to get the CR close to stock. Why?

Is it just for ease of getting it to run right and run on 87 octane? Or is there another reason that I'm missing?

If I wasn't worried about either, and was willing to have to run 93 octane how high do you think I can go on compression ratio?

Has anyone tried this?

What altitude do you live at, What cam are you going to us( more importantly what is the IVC spec) Are you willing to add a little racing fuel to the mix if you are off on you figures and 93 octane JUST ISN'T QUIGHT ENOUGH to keep the knock or ping away.

I, my self, will have to answer these question before i can decide what compression i will choose when i build my stroker
Answer these question and i will give you my educated guess.


Flash

[gradon]

I am in the process of getting the parts together for my stroker build this spring. I plan on having a high 9-very low 10 to 1 compression ratio that requires the use of the premium gas we have available. Within a week or two I'll be working on the head and will try to get rid of the sharp edges and hopefully increase the cc's a lil(how much have any of you added by doing this?). Hopefully the head is flat and won't need to be shaved so I won't lose any cc's. I do want to hear from those that have strokers with compression ratios 9.5:1 or greater to learn from their experience. My quench will be .053-.060 using the .043 HG and a lil deck height after it has been decked(If KB doesn't hurry up).

[Shark]

i have read all the posts and now i understand that having a good quench will help prevent ping at a given SCR. tell me if i understand this correctly. example: lets say i have a 9.5 SCR and a quench of over .08 and i experience some ping with a given octane fuel. then i deck the block and my quench is down to around .07 with the same SCR. i realize decking the block would also change the SCR but bear with me for simplicities sake. with the quench closer to stock, would i be able to run a lower octane fuel, or the same fuel w/ no ping, than i would with the higher quench?

also, DCR. im still confused about it. if i have a 10:1 SCR and a 8:1 DCR am i going to be able to run pump gas in a stroker? how do they relate? and has anyone added DCR to a stroker calc?

i know, lots of questions. just wanna make sure i get this straight. very interesting stuff.

[brewmenn]

What altitude do you live at, What cam are you going to us( more importantly what is the IVC spec) Are you willing to add a little racing fuel to the mix if you are off on you figures and 93 octane JUST ISN'T QUIGHT ENOUGH to keep the knock or ping away.

I, my self, will have to answer these question before i can decide what compression i will choose when i build my stroker
Answer these question and i will give you my educated guess.


Flash

I live in Michigan at around 600 ft.
I haven't decided on a cam. I need to do more research on this subject as well
I don't have a problem with adding some racing fuel or octane booster to the mix, but I'd rather not have to.

I'm also considering getting some type of chip or tuning computer so I can mess with the ignition timing if need be.

[brewmenn]

i have read all the posts and now i understand that having a good quench will help prevent ping at a given SCR. tell me if i understand this correctly. example: lets say i have a 9.5 SCR and a quench of over .08 and i experience some ping with a given octane fuel. then i deck the block and my quench is down to around .07 with the same SCR. i realize decking the block would also change the SCR but bear with me for simplicities sake. with the quench closer to stock, would i be able to run a lower octane fuel, or the same fuel w/ no ping, than i would with the higher quench?

also, DCR. im still confused about it. if i have a 10:1 SCR and a 8:1 DCR am i going to be able to run pump gas in a stroker? how do they relate? and has anyone added DCR to a stroker calc?

i know, lots of questions. just wanna make sure i get this straight. very interesting stuff.

From what I understand, having a good quench height gives you better air movement through the combustion chamber and that gives better cooling, with fewer hot spots. Hot spots is one thing that can cause pinging. So if the pinging is caused by hot spots which are caused by to high of quench then reducing quench should help the situation. Going from .08 to .07 may not yield great gains, ideally you'd want to get it down around .045. In a perfect world you'd want quench to be 0 but you have to allow for rod stretch, bearing wear and other build factors. Other factors can cause pinging also so if a given quench isn't the problem changing it may not help.

I believe that DCR is ultimately what you need to calculate to decide if it will run on pump gas. Flash's post, moved here: viewtopic.php?f=27&t=73 tells the relationship between SRC and DRC. I'm sure it could be added to a stroker calc but I've never used one so i don't know if it's been done yet. I just put together my own spreadsheet for stroker calculations.

[Flash]

What altitude do you live at, What cam are you going to us( more importantly what is the IVC spec) Are you willing to add a little racing fuel to the mix if you are off on you figures and 93 octane JUST ISN'T QUIGHT ENOUGH to keep the knock or ping away.

I, my self, will have to answer these question before i can decide what compression i will choose when i build my stroker
Answer these question and i will give you my educated guess.


Flash

I live in Michigan at around 600 ft.
I haven't decided on a cam. I need to do more research on this subject as well
I don't have a problem with adding some racing fuel or octane booster to the mix, but I'd rather not have to.

I'm also considering getting some type of chip or tuning computer so I can mess with the ignition timing if need be.

when you have you exact compression ratio and have you cam selection down to a couple of cam, let me know and i will figure out you DCR as i have a program on my computer to do it with.

flash.

[Flash]

i have read all the posts and now i understand that having a good quench will help prevent ping at a given SCR. tell me if i understand this correctly. example: lets say i have a 9.5 SCR and a quench of over .08 and i experience some ping with a given octane fuel. then i deck the block and my quench is down to around .07 with the same SCR. i realize decking the block would also change the SCR but bear with me for simplicities sake. with the quench closer to stock, would i be able to run a lower octane fuel, or the same fuel w/ no ping, than i would with the higher quench?

also, DCR. im still confused about it. if i have a 10:1 SCR and a 8:1 DCR am i going to be able to run pump gas in a stroker? how do they relate? and has anyone added DCR to a stroker calc?

i know, lots of questions. just wanna make sure i get this straight. very interesting stuff.

From what I understand, having a good quench height gives you better air movement through the combustion chamber and that gives better cooling, with fewer hot spots. Hot spots is one thing that can cause pinging. So if the pinging is caused by hot spots which are caused by to high of quench then reducing quench should help the situation. Going from .08 to .07 may not yield great gains, ideally you'd want to get it down around .045. In a perfect world you'd want quench to be 0 but you have to allow for rod stretch, bearing wear and other build factors. Other factors can cause pinging also so if a given quench isn't the problem changing it may not help.

I believe that DCR is ultimately what you need to calculate to decide if it will run on pump gas. Flash's post, moved here: viewtopic.php?f=27&t=73 tells the relationship between SRC and DRC. I'm sure it could be added to a stroker calc but I've never used one so i don't know if it's been done yet. I just put together my own spreadsheet for stroker calculations.

Yeah, i thing a copy of that need to be brought back to this discussion.

Flash.

[Flash]

i have read all the posts and now i understand that having a good quench will help prevent ping at a given SCR. tell me if i understand this correctly. example: lets say i have a 9.5 SCR and a quench of over .08 and i experience some ping with a given octane fuel. then i deck the block and my quench is down to around .07 with the same SCR. i realize decking the block would also change the SCR but bear with me for simplicities sake. with the quench closer to stock, would i be able to run a lower octane fuel, or the same fuel w/ no ping, than i would with the higher quench?

also, DCR. I'm still confused about it. if i have a 10:1 SCR and a 8:1 DCR am i going to be able to run pump gas in a stroker? how do they relate? and has anyone added DCR to a stroker calc?

i know, lots of questions. just wanna make sure i get this straight. very interesting stuff.

If your compression ratio stayed at 9.5 and you quench improved by .010......and the ping was small......it would be eliminated.
If you only shaved the head with out bring the compression back down, i thing you would only compound you problem.

When the air is squished thru the tight spot of the piston into the bowl it cause a cooling affect....................Every one has us these, air in a can thing, that you us to blow dust out of a computer or what not. When you start spraying it, the air is room temp, but as you continue to spray it the air coming out of the can get colder and colder.
The air in the can is at hi pressure and as it is pushed in to a lower pressure it start to cool (same basic concept that a air conditioner work in a car)
The rushing air help to keep the fuel suspended in the air, for a better even burn as well as help to drop the temp a little.

OK let see if i can do a little bit better on DCR..............Think of DCR as the cam controlling affect of SCR.

Here is a couple stories that might help......Don't take the number to hart, JUST THE THEORY OF IT.

Lets say were build a stroker with a SCR of 10:1 and with the stock cam has a DCR of 8.3:1. Both SCR and DCR are consider boarder line street compression ratios. this motor is used at sea level ( "0"altitude)
The thing ping if you look at it cross eye. You decide to pull out a compression gauge and see what kind of compression you new motor has . All cylinder have a compression of 175 P.S.I
so we go out and looking for the biggest cam that can be found, and install it. DCR is not at 7.3:1 You loose lots of bottom end power but runs real good on top end with out any ping. So you pull out the compression gauge again, and the compression has dropped for 175 to 140 P.S.I

Now, let's look a how altitude affect our discussion on compression ratio.
We still have the same 10:1 SCR (DCR 8.3:1 and 175 P.S.I compression) motor with the stock cam that needs a 50/50 ratio of racing fuel to keep it happy.

Tired of all the fast pace of the city, we decide to move to the mountains for a change. (7,000 feet) We seam to be out of breath all the time now but, the stroker is running fine with out any racing fuel........once again decide to do a compression check and find that our compression has fallen to 140 P.S.I ......SCR is still 10:1 and DCR is still at 8.3:1(Boarder line street)
But no ping.


Flash

[oletshot]

Keith Black has a compression ratio calculator on their web site that will give both static and dynamic ratios.

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

Hope this helps.

[Shark]

i guess im trying to get an idea of what CR needs to be to run certain a certain octane. is anyone runnning a stroker on 87 octane? if so what is your CR? what did you do to get that CR? hows it run? the whole goal of building a stroker is high performance. that said, is it just a matter of fact that a performance motor needs to run performance fuel? or can better performance be attained even with a lower octane fuel?

[Flash]

Keith Black has a compression ratio calculator on their web site that will give both static and dynamic ratios.

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

Hope this helps.

Yes that will help out a lot.

You should go to some of you favorite cam sights and right down the cam spec of you favourite cams then build you compression and then add the cam. its actually a hole lot of fun.......and you will get lost for hours with............Hmmm wht if i changed this spec or what if i did this.........

There is on thing that you must understand before you start!!!!!!!!!!!! valve Duration is figured two different ways, a seat to seat duration and a "@ .050"..............You should only us cam spec's that give @.050" duration spec with this calculator.......or it won't be accurate.
The one i us is similar but DCR is figures with cams that us the "seat to seat" duration spec.
Any one who is building a stroker or planning on doing one should try it out.
Just keep moving the ### around on the SCR part, until you get to the compression ratio you want then add you cam spec to see how much difference there is between cams

The only thing i wish this one had was quench spec figured in to the mix..........Thats when i really began to understand quench and how to get it.

Flash

[brewmenn]

I've managed to put together a spreadsheet that will figure all the static numbers. What I'm still struggling with is understanding how to take the advertised cam specs and get the IVC point.