compression ratio's and boost - Ford Mustang Forums : Corral.net Mustang Forum
 
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post #1 of 17 Old 01-24-2008, 02:26 PM Thread Starter
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compression ratio's and boost

ok so ive about made up my mind to do a 70-76mm turbo on the car in my sig. however it has 10.6 to 1 compression. i know that to be safe 9.5 or less is what the norm is however.... i know others have had 10.5-11 to 1 compression ratio's and blowers.

tuning is the key here but i wanted some of your guys thoughts on the matter.


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post #2 of 17 Old 01-24-2008, 03:40 PM
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water/meth injection


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post #3 of 17 Old 01-24-2008, 05:04 PM Thread Starter
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water/meth injection

ive got a plate kit from NX that has been flow tested and balanced.

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If it's cheap and fast it isn't reliable
If it's fast and reliable it isn't cheap

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post #4 of 17 Old 01-28-2008, 03:09 PM
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For every 3lbs of boost, you can figure 1 more compression ratio. So a motor with an 8:1cr can handle 7.5psi at 93 octane. Anything over a 10.5:1cr, and you are looking at race gas.
Alex

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post #5 of 17 Old 01-28-2008, 03:11 PM
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You can always get more cc's, and lower the cr with thicker head gaskets, and a cc port.

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post #6 of 17 Old 01-28-2008, 07:14 PM Thread Starter
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i will be running 110 race gas


im looking for about 12-15 psi boost

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post #7 of 17 Old 02-12-2008, 11:21 PM
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Alexandros: you may be right about the 1 more CR for every 3 lbs of boost, but I will have to disagree with you on only running 7.5 psi with 8:1 CR with 93 octane. brian is right when he says its in the tuning. i, personally, have not yet experienced this, but i'm definitely doing my research because I have a 10:1 setup that i haven't put together yet and am planning on opening up my combustion chambers some try to get it to 9:1, which WILL be fine. I plan on having 8-12 psi of boost and still run 93 octane. The key to any engine running its best is fuel/timing balance. With the added boost, this balance will have to be brought to order by taking away timing and adding fuel, for the most part.

With N/A, 10.5:1 is the CR limit to lookout for if you're wanting to run 93 octane because I believe retarding the timing with N/A will only hurt.

With boost applications, compression ratios after boost is added is generally not calculated, but if you wanted to take a look at a very simple, non-extreme example, plenty of people have put 10 psi on the stock 8.5:1 bottom end of a 5.0, and they have safely driven their cars for more than just 9000 miles with 93 octane. If you calculate it according to your conversion, final CR would be at 11.83:1, correct? With the correct tuning, those boost numbers have been raised even more.

brian: as long as you have the build to bare it (i haven't paid attention to your sig), i believe your ideas are possible. the 110 definitely helps, and the tune is going to be the key, so it goes right back to what you said first

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post #8 of 17 Old 02-12-2008, 11:22 PM
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btw, brian...i'm also in GA, just noticed that you were...i'm in the athens area.
good luck with everything

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post #9 of 17 Old 02-13-2008, 10:59 AM
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Invest in a barrel of 110 octane.
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post #10 of 17 Old 02-13-2008, 11:17 AM
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Quote:
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For every 3lbs of boost, you can figure 1 more compression ratio. So a motor with an 8:1cr can handle 7.5psi at 93 octane. Anything over a 10.5:1cr, and you are looking at race gas. Of course, there is always a danger in overboosting a motor without these guidelines, but what would you rather have, a torched piston and a rebuild at 9,000 miles, or a safe zone of tuning and a 100,000 mile motor rebuild, your car, you decide.
Alex
I've gotta ask, where do you get the figure of 3lbs of boost per 1 point of CR?

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post #11 of 17 Old 02-13-2008, 12:34 PM
 
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with higher compression you to setup everything properly (and i stress properly, not necessary) you should calculate out the dynamics of the expected otto cycle, here's a quote with some examples that you may be able to use:

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Based on Otto-cycle, you've got a 4% or so improvement in efficiency with higher compression. If 10 psig is your starting boost, then an extra psi will give (15 + 10 +"1") / (15 + 10) - 1 = 26/25 - 1 = .04 = 4%. So basically, your extra CR will give the same benefit as an extra psi of boost (major generalization here).

As far was knock tolerance goes, it is strongly related to pressure (and therefore temperature) in the cylinder. P2 = P1 * CR ^ gamma

Base engine - CR 8, 10 psi boost (25 psia)

P2 = 25 psia * 8 ^ 1.4 = 459 psia

Next config - CR 10, 10 psi boost (for 4% extra power)

P2 = 25 psia * 10 ^ 1.4 = 628 psia

Last config - CR 8m 11 psia boost (for 4% extra power)

P2 = 26 psia * 8 ^ 1.4 = 477 psia
this should help you calculate on what ocatane or othe rmethods you can use to manipulate the flash point of the fuel

Last edited by selfinfliction; 02-13-2008 at 12:36 PM.
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post #12 of 17 Old 02-13-2008, 03:00 PM
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You can buy small 25 gallon barrel's of race gas from most speed shops.
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post #13 of 17 Old 02-13-2008, 03:44 PM
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and i think i can get 93 at the pump, right?

selfinfliction: that's some good stuff...i've actually thought about calculating things like this before, but wasn't really sure where to start and haven't researched enough to learn about the otto's cycle.

i guess i also need to do some searching on the flashpoints of different octane ratings...so the 15 psia conversion would more precisely be 14.73 psia (atmospheric pressure)...where is the gamma value coming from? i guess i could do some searching and actually figure it out, but just throwing the ? out there to see if you knew...

and the results from multplying the atm pressure x CR ^ gamma are what exactly? i'm thinking pressure in the cylinder? is this correct?
thanks

brian: hope i'm not robbing your thread, we can both learn from this

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post #14 of 17 Old 02-13-2008, 04:13 PM
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brian: i was just reading your sig, and if it hasn't been done you can have your afr's combustion chambers ported to lower the compression some also, if that's what you want to do

that's what i plan to do with my TW's...they are 61cc now, fox lake takes them to 65cc with their stage 2 porting...that's what i'm thinking about but all these things are months away, so i'll keep thinking

selfinfliction: what kind of air/fuel/spark parameters are those calculations assuming? i'm guessing these are all assumed static equations that don't take in the dynamics of tuning (adding fuel/taking away timing, etc)???

these are all thoughts running through my head, so sorry if anything seems really confusing

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post #15 of 17 Old 02-13-2008, 04:15 PM
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brian: i was just reading your sig, and if it hasn't been done you can have your afr's combustion chambers ported to lower the compression some also, if that's what you want to do

that's what i plan to do with my TW's...they are 61cc now, fox lake takes them to 65cc with their stage 2 porting...that's what i'm thinking about but all these things are months away, so i'll keep thinking

selfinfliction: what kind of air/fuel/spark parameters are those calculations assuming? i'm guessing these are all assumed static equations that don't take in the dynamics of tuning (adding fuel/taking away timing, etc)???

these are all thoughts running through my head, so sorry if anything seems really confusing

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post #16 of 17 Old 02-13-2008, 06:02 PM
 
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selfinfliction: what kind of air/fuel/spark parameters are those calculations assuming? i'm guessing these are all assumed static equations that don't take in the dynamics of tuning (adding fuel/taking away timing, etc)???
yeah those are static equations. they would only be good to get a baseline of what to work with. in short i would say just go with a 50/50 meth injection and 93 pump gas, depending on what the charge temps are after the meth, and my egt's are, i would work my boost levels around that setup.
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post #17 of 17 Old 02-15-2008, 07:25 PM
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I think there is a very important missing factor here...

Boost vs. rpm

What is the main factor that limits boost?
Answer: detonation, right?

Well, when is detonation more likely to occur?
Answer: Lower rpms, right?

What are some of the significant contributing factors to detonation?
Answer: Intake air temp, hot spots (in the cylinder), timing, fuel octane, and combustion efficiency (chamber shape, cylinder filling/evacuation, etc).

So, if you limit your boost at lower rpms (and the other contributing factors for detonation), you can safely run higher boost levels.

I know I haven't given any direct answers...every situation is different. For example, I have successfully run 27 psi on my DD street car (wastegate set at this level 100% of the time) with ~8:1 CR & 91 octane. Also, I'm doing a turbo build on my 96 Mustang with a 10.5:1 460 and plan on running it on pump gas.

Think outside of the box and you'll find a good solution. Finally, keep in mind what your goals are: how fast do you want to run? How fast can your car certify to? How much hp do you really need to run these numbers?

Have a good day!
Michael

2003 Ford F150 Supercrew--------1996 Ford Mustang
Stroked BBF------------------------2.3t
Garrett 98mm turbocharger------12.10 @ 114 mph

Last edited by 96mustang460cid; 02-15-2008 at 07:27 PM. Reason: Edit: Don't forgot about cylinder pressure bleedoff (camshaft specs)
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