Compression vs VE - Honda-Tech

Combustion Contraption · 11-02-2006, 09:27 PM

Compression goes up.... VE goes down....

Sort of an extension of a previous thought in a previous thread. Increasing compression with an adequate octane rated fuel is a good sure way to increase BMEP. Why then, can an increase in compression, also bring upon itself a decrease in Volumetric Efficiency?

hybrid_vtec · 11-03-2006, 04:05 AM

I wouldnt think this should be too difficult. Volumetric Efficiency is just a calculation of cylinder volume and the capacity to fill it in a ratio form.

By raising compression you reduce the cylinder's volume, therefore it can not fill as much as before, thus lower the point 100% volumetric effieciency is achieved.

D-Rob · 11-03-2006, 07:43 AM

Quote:

Originally Posted by hybrid_vtec

I wouldnt think this should be too difficult. Volumetric Efficiency is just a calculation of cylinder volume and the capacity to fill it in a ratio form.

By raising compression you reduce the cylinder's volume, therefore it can not fill as much as before, thus lower the point 100% volumetric effieciency is achieved.

Got to it before I could.

d16dcoe45 · 11-03-2006, 09:13 AM

Is that true? From the very small amount of decreased total volume from say 11:1 to 12:1 will decrease volumetric efficiency?

Phoenix1 · 11-03-2006, 09:35 AM

My thought also. The increase in dome volume from 11:1 to 12:1, for example, shouldn't be enough to greatly affect volumetric efficiency...

Master of the Universe · 11-03-2006, 09:39 AM

Your are assuming that the head flow can't keep up with the stonger signal. Maybe on a Shit-8 but not the case in high peformance 4V Honda's.

Combustion Contraption · 11-03-2006, 05:58 PM

But VE does drop when you keep increasing compression past a certain point, even if you arent experiencing knock or ping.

Can anyone in here answer this:

Is more heat produced from a 11:1 engine with 91 octane or an engine with 14:1 on 100 octane?

hybrid_vtec · 11-03-2006, 06:52 PM

Quote:

Originally Posted by Combustion Contraption

But VE does drop when you keep increasing compression past a certain point, even if you arent experiencing knock or ping.

Can anyone in here answer this:

Is more heat produced from a 11:1 engine with 91 octane or an engine with 14:1 on 100 octane?

The higher the compression ratio, the higher the air temperature in the cylinder at the end of the compression stroke. Higher compression ratios, to a point, lead to higher thermal efficiencies and better fuel economies.

Also, 91 octane is going to have a lower flash point than 100 octane, hence the reason for running higher octane in a higher compression engine......to prevent knock/detonation.

Now if you are not refering to temperature, b/c heat is a form of energy or work which can be measured in Joules. A higher compression engine is going to exert a greater deal of energy, but it will reach a point where mechanical effieciency will be hendered b/c it will use more energy than it will be making.

hybrid_vtec · 11-03-2006, 07:02 PM

Just found this, good info!!!

Why is a high compression engine more efficient? Because it does more work for each molecule of fuel burned. Look at the chemical reaction for the oxidation or combustion of octane:

C8H18(g) + 12.5O2(g) --> 8CO2(g) + 9H2O(g) delta Hc = -1307kcal/mole

In an ideal case in an automobile cylinder, the fuel is vaporized to a gas and ignited in the presence of air. Air consists of 20% oxygen, 79% nitrogen and 1% argon. The nitrogen and argon do not react to any great extent.

Let us ignore the temperature in the cylinder for a moment. The gases are injected, the cylinder compresses them and they ignite to form products as shown releasing 1307kcal/mole of energy. There is an increases in the number of moles of gas from 13.5 to 17 from the reaction. (Remember when we brought in 12.5 moles of oxygen we also brought in about 50 moles of nitrogen and argon for a total of 63.5 moles in the initial state). Thus, from this alone we can judge the entropy or level of disorder increases in this chemical reaction. If the compression ratio was, let us say, 5 to 1, the pressure was 5 atmospheres before the detonation; after the detonation we have 67 moles of gas, the pressure has increased somewhat from the production of new gas molecules. Disorder increases, entropy increases , delta S is positive, from the new moles of gas. And so the Free Energy (G) available to do work is reduced by the value of T delta S.

more here: http://www.chemcases.com/fuels/fuels-c.htm

Combustion Contraption · 11-03-2006, 11:04 PM

Over thinking i think.

I originally created this thread with the intent of educating some of the users to the effects of 'over compressioning' a motor. I see it all to often on HT.

Though i can see this discussion going somewhere good, even if its not what i intended!

D-Rob · 11-04-2006, 05:42 AM

Quote:

Originally Posted by Combustion Contraption

Over thinking i think.

I originally created this thread with the intent of educating some of the users to the effects of 'over compressioning' a motor. I see it all to often on HT.

Though i can see this discussion going somewhere good, even if its not what i intended!

So what you were originally going to teach was that a lower compression engine with more timing is better than a super-high compression engine with no timing...

(Given, of course the rest of the variables are the same -air quality, fuel octane, etc...)

*cough* CTR piston lovers *cough*

- Derek

sander · 11-04-2006, 06:36 AM

You can add compression in better ways than just piston dome too.

High compression + Flat pistons (Flattest possible anyway, provided good valve relief's) are generally a better combination than getting your compression with a big dome.

its good to pick the way you raise your compression wisely. milling > big domes.

"quench's my thirst on a daily basis"

(i know i know that was bad...haha)

-s

d16dcoe45 · 11-04-2006, 10:57 AM

Well after a certain number--compression wise it becomes very difficult to get a detonation free burn even with great gasoline. Another thing is that with very high compression the piston has to "compress" even when its not producing power--say on overrun in second gear--drive a high compression car and feel the engine braking to see what I mean. I think another issue that is lost on a lot of people who want high compression is that they will say "JoeBlow's drag car runs 13:1 on pump gas and he is fine--I should be able to run the same on my street car" A drag car can run higher compression simply because they are only running for a 1/4 mile at a time and there simply isn't enough time for everything to get hot enough to really start detonating (obvioulsy even this goes out the window after a certain point too) and also, alot of drag racers try to leave the line with water/oil temperatures colder (particularly carbureted cars) than what would normally be run on the street.

Mr.RHTuner · 11-04-2006, 11:33 AM

Good thread

Combustion Contraption · 11-04-2006, 05:29 PM

Quote:

Originally Posted by hybrid_vtec

By raising compression you reduce the cylinder's volume, therefore it can not fill as much as before, thus lower the point 100% volumetric effieciency is achieved.

This makes no sense. As long as you completely fill the cylinder, it doesnt matter if its 1cc or 100cc's, its 100%. When i said VE goes down, i didnt mean the point at which it occurs on the RPM scale, i meant the max VE value attained drops.

THEE most advanced engines in the world run around 13.5:1 compression. Some of the street guys on here are pushing close to that will fuel from their local fill er up station.

Combustion Contraption · 11-04-2006, 05:30 PM

Quote:

Originally Posted by d16dcoe45

I think another issue that is lost on a lot of people who want high compression is that they will say "JoeBlow's drag car runs 13:1 on pump gas and he is fine--I should be able to run the same on my street car" A drag car can run higher compression simply because they are only running for a 1/4 mile at a time and there simply isn't enough time for everything to get hot enough to really start detonating (obvioulsy even this goes out the window after a certain point too)

The drag car also isnt driven uphill @ 3k rpm at WOT in 5th gear. Intake valve closure has something to do with it as well.

hybrid_vtec · 11-04-2006, 08:30 PM

Quote:

Originally Posted by Combustion Contraption

This makes no sense. As long as you completely fill the cylinder, it doesnt matter if its 1cc or 100cc's, its 100%. When i said VE goes down, i didnt mean the point at which it occurs on the RPM scale, i meant the max VE value attained drops.

THEE most advanced engines in the world run around 13.5:1 compression. Some of the street guys on here are pushing close to that will fuel from their local fill er up station.

How does it not make sense? Reread the entire post.

To raise compression you have to reduce the volume by one of several methods; but the end all result will be a different volume level ratio from BDC to TDC.

VE is just a ratio or percentage of your volume and how much you can fill it, and at what point you are able to achieve 100% VE if you are truly able to achieve it at all!

So to break it down barney style, if you have a 16 oz glass and a 12oz glass and the rate of flow of water being poured into the glasses remains the same, which will fill first?

Combustion Contraption · 11-04-2006, 11:09 PM

No need to break anything down 'barney style' for me. i understand what you are trying to convey, but raising the compression on a pair of 'over valved' heads could raise VE.

Maybe i misunderstood, YOUR intent.

hybrid_vtec · 11-05-2006, 04:36 AM

Quote:

Originally Posted by Combustion Contraption

No need to break anything down 'barney style' for me. i understand what you are trying to convey, but raising the compression on a pair of 'over valved' heads could raise VE.

Maybe i misunderstood, YOUR intent.

HAHA!!! I like using that phrase since it was so common in electronics school. And from working on electronics over the past few years, I can say theory is theory for a reason: it isnt totally proven fact, and has not become a law for laws are the only true guarantees. There are many views, a lot of reaseach, and logic input from many different many people. From what I am researching, I am not finding much difference in statements for both engine designs: OHV and OHC. What I have found is that there is a difference in opinion when it comes down to a 4 stroke engine compared to a 2 stroke engine. I am almost certain that other variables are in play into this, but I do not know how far we really want to stray away from the original concept. Now what I thought you where asking is why peak torque is achieved sooner when compression is raised and how it affects volumetric effiency. I stated what I know and what I found from searching. If there is another question that I am not answering, maybe I am mis-reading what you are trying to ask.

Definitely a good thread!

I love talking about theory and design. I am always out to learn as much as I can.

hybrid_vtec · 11-05-2006, 01:37 PM

I do not want to stray too far from the topic, but after doing some reading today, I came across an article that talked about compression.

It claimed that to get the most effective power from increasing compression that a piston with no dome were to be used and that the piston to deck clearance should be minimized as much as possible (as close to 0.000" as possible, and even claimed that piston out of the deck was okay as long as it didnt interfer with valve to piston clearance). It also claimed that the combustion chamber volume should be minimized to satisfy compression needs (to properly mate a specific camshaft with the engine).

What are your thoughts? Really worth the difference?

91SiZ6 · 11-05-2006, 03:31 PM

Be careful bringing up camshaft and compression in the same sentence around here. :D

11-02-2006, 09:27 PM	#1
Combustion Contraption Senior Member Join Date: Jan 2003 Location: So Cal Posts: 8,534	Compression vs VE Compression goes up.... VE goes down.... Sort of an extension of a previous thought in a previous thread. Increasing compression with an adequate octane rated fuel is a good sure way to increase BMEP. Why then, can an increase in compression, also bring upon itself a decrease in Volumetric Efficiency? __________________ EARL LASKEY, YOU WILL BE MISSED CCCylinderheads.com Check out the INDUCTION SPECIAL Matco Doublebay Toolbox FOR SALE Click Here!

11-03-2006, 04:05 AM	#2
hybrid_vtec Join Date: Jul 2004 Location: VA Beach, VA, USA Posts: 4,640	Re: Compression vs VE (Combustion Contraption) I wouldnt think this should be too difficult. Volumetric Efficiency is just a calculation of cylinder volume and the capacity to fill it in a ratio form. By raising compression you reduce the cylinder's volume, therefore it can not fill as much as before, thus lower the point 100% volumetric effieciency is achieved. __________________ Dyno #1 Dyno #2 <FONT SIZE="1"><FONT COLOR="red">NON</FONT>VTEC.COM</FONT> Rick <FONT COLOR="navy">US NAVY</FONT> 98 Prelude - H22 92 Civic HB - B18B 179 WHP, 142 FT-LBS. TQ 99 Firebird

11-03-2006, 09:13 AM	#4
d16dcoe45 Senior Member Join Date: Jul 2005 Location: Armonk, NY, USA Posts: 3,729	Re: Compression vs VE (D-Rob) Is that true? From the very small amount of decreased total volume from say 11:1 to 12:1 will decrease volumetric efficiency? __________________ Cusco EF master cylinder brace-- brand new, unopened, still in package $75 shipped obo (RHD ONLY!!!) WTB: 88-89 Civic hatch driver side door w/ glass and regulator. Losi xxx4 G+ Associated Rc10 t4 FT Losi xxx BK2 JConcepts BJ4 WE

11-03-2006, 09:35 AM	#5
Phoenix1 Join Date: Sep 2002 Location: El Paso, TX Posts: 709	Re: Compression vs VE (d16dcoe45) My thought also. The increase in dome volume from 11:1 to 12:1, for example, shouldn't be enough to greatly affect volumetric efficiency... __________________ 2006 Honda CBR1000RR : Track rig 1998 Toyota Tacoma : Track rig hauler 1993 Yamaha TW200 : Daily rider

11-03-2006, 09:39 AM	#6
Master of the Universe Join Date: Mar 2002 Posts: 6,493	Re: Compression vs VE (Phoenix1) Your are assuming that the head flow can't keep up with the stonger signal. Maybe on a Shit-8 but not the case in high peformance 4V Honda's. __________________ Semi-Retired as of April 25, 2007. Don't bother to PM me. I am tired of internet fools. Contact via RM Dealer or e-mail.

11-03-2006, 05:58 PM	#7
Combustion Contraption Senior Member Join Date: Jan 2003 Location: So Cal Posts: 8,534	But VE does drop when you keep increasing compression past a certain point, even if you arent experiencing knock or ping. Can anyone in here answer this: Is more heat produced from a 11:1 engine with 91 octane or an engine with 14:1 on 100 octane? __________________ EARL LASKEY, YOU WILL BE MISSED CCCylinderheads.com Check out the INDUCTION SPECIAL Matco Doublebay Toolbox FOR SALE Click Here!

11-03-2006, 07:02 PM	#9
hybrid_vtec Join Date: Jul 2004 Location: VA Beach, VA, USA Posts: 4,640	Re: (hybrid_vtec) Just found this, good info!!! Why is a high compression engine more efficient? Because it does more work for each molecule of fuel burned. Look at the chemical reaction for the oxidation or combustion of octane: C8H18(g) + 12.5O2(g) --> 8CO2(g) + 9H2O(g) delta Hc = -1307kcal/mole In an ideal case in an automobile cylinder, the fuel is vaporized to a gas and ignited in the presence of air. Air consists of 20% oxygen, 79% nitrogen and 1% argon. The nitrogen and argon do not react to any great extent. Let us ignore the temperature in the cylinder for a moment. The gases are injected, the cylinder compresses them and they ignite to form products as shown releasing 1307kcal/mole of energy. There is an increases in the number of moles of gas from 13.5 to 17 from the reaction. (Remember when we brought in 12.5 moles of oxygen we also brought in about 50 moles of nitrogen and argon for a total of 63.5 moles in the initial state). Thus, from this alone we can judge the entropy or level of disorder increases in this chemical reaction. If the compression ratio was, let us say, 5 to 1, the pressure was 5 atmospheres before the detonation; after the detonation we have 67 moles of gas, the pressure has increased somewhat from the production of new gas molecules. Disorder increases, entropy increases , delta S is positive, from the new moles of gas. And so the Free Energy (G) available to do work is reduced by the value of T delta S. more here: http://www.chemcases.com/fuels/fuels-c.htm __________________ Dyno #1 Dyno #2 <FONT SIZE="1"><FONT COLOR="red">NON</FONT>VTEC.COM</FONT> Rick <FONT COLOR="navy">US NAVY</FONT> 98 Prelude - H22 92 Civic HB - B18B 179 WHP, 142 FT-LBS. TQ 99 Firebird

11-03-2006, 11:04 PM	#10
Combustion Contraption Senior Member Join Date: Jan 2003 Location: So Cal Posts: 8,534	Over thinking i think. I originally created this thread with the intent of educating some of the users to the effects of 'over compressioning' a motor. I see it all to often on HT. Though i can see this discussion going somewhere good, even if its not what i intended! __________________ EARL LASKEY, YOU WILL BE MISSED CCCylinderheads.com Check out the INDUCTION SPECIAL Matco Doublebay Toolbox FOR SALE Click Here!

11-04-2006, 06:36 AM	#12
sander Senior Member Join Date: Feb 2003 Location: Maryland Posts: 1,741	Re: (D-Rob) You can add compression in better ways than just piston dome too. High compression + Flat pistons (Flattest possible anyway, provided good valve relief's) are generally a better combination than getting your compression with a big dome. its good to pick the way you raise your compression wisely. milling > big domes. "quench's my thirst on a daily basis" (i know i know that was bad...haha) -s __________________ K20A Short Block, AEM EMS 30-1070 For Sale. http://www.hondamarketplace.com/zerothread/2407249 sander dash tuned dot com.

11-04-2006, 10:57 AM	#13
d16dcoe45 Senior Member Join Date: Jul 2005 Location: Armonk, NY, USA Posts: 3,729	Re: (sander) Well after a certain number--compression wise it becomes very difficult to get a detonation free burn even with great gasoline. Another thing is that with very high compression the piston has to "compress" even when its not producing power--say on overrun in second gear--drive a high compression car and feel the engine braking to see what I mean. I think another issue that is lost on a lot of people who want high compression is that they will say "JoeBlow's drag car runs 13:1 on pump gas and he is fine--I should be able to run the same on my street car" A drag car can run higher compression simply because they are only running for a 1/4 mile at a time and there simply isn't enough time for everything to get hot enough to really start detonating (obvioulsy even this goes out the window after a certain point too) and also, alot of drag racers try to leave the line with water/oil temperatures colder (particularly carbureted cars) than what would normally be run on the street. __________________ Cusco EF master cylinder brace-- brand new, unopened, still in package $75 shipped obo (RHD ONLY!!!) WTB: 88-89 Civic hatch driver side door w/ glass and regulator. Losi xxx4 G+ Associated Rc10 t4 FT Losi xxx BK2 JConcepts BJ4 WE

11-04-2006, 11:33 AM	#14
Mr.RHTuner Senior Member Join Date: Nov 2003 Location: 519 by the 401, Canada Posts: 8,984	Re: Compression vs VE (Combustion Contraption) Good thread

11-04-2006, 11:09 PM	#18
Combustion Contraption Senior Member Join Date: Jan 2003 Location: So Cal Posts: 8,534	No need to break anything down 'barney style' for me. i understand what you are trying to convey, but raising the compression on a pair of 'over valved' heads could raise VE. Maybe i misunderstood, YOUR intent. __________________ EARL LASKEY, YOU WILL BE MISSED CCCylinderheads.com Check out the INDUCTION SPECIAL Matco Doublebay Toolbox FOR SALE Click Here!

11-05-2006, 01:37 PM	#20
hybrid_vtec Join Date: Jul 2004 Location: VA Beach, VA, USA Posts: 4,640	Re: (hybrid_vtec) I do not want to stray too far from the topic, but after doing some reading today, I came across an article that talked about compression. It claimed that to get the most effective power from increasing compression that a piston with no dome were to be used and that the piston to deck clearance should be minimized as much as possible (as close to 0.000" as possible, and even claimed that piston out of the deck was okay as long as it didnt interfer with valve to piston clearance). It also claimed that the combustion chamber volume should be minimized to satisfy compression needs (to properly mate a specific camshaft with the engine). What are your thoughts? Really worth the difference? __________________ Dyno #1 Dyno #2 <FONT SIZE="1"><FONT COLOR="red">NON</FONT>VTEC.COM</FONT> Rick <FONT COLOR="navy">US NAVY</FONT> 98 Prelude - H22 92 Civic HB - B18B 179 WHP, 142 FT-LBS. TQ 99 Firebird

11-05-2006, 03:31 PM	#21
91SiZ6 Join Date: May 2006 Location: California, United States Posts: 961	Be careful bringing up camshaft and compression in the same sentence around here. :D __________________ ArtieTuned D16Z6 135.6whp 111.8tq. http://www.LSDMotorSports.com http://www.RyWire.com http://www.CTRDynoSports http://www.ArtieTuned.com http://www.J-K-Tuning.com