Compresion ratio ?
- Thread starter TrentNell
- Start date
This might be a little off subject but how much does the compression ratio change when going with over sized pistons?
Troy Wakeman claims he runs cut lbz pistons. Truck smokes pretty heavily at an idle like its decompressed quite a bit, i dont know if his trucks 1000hp, but i dont think he has any trouble competing.
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I am deffinately not saying you can't run 1000hp on a decompressed engine, I am saying that the a 16-17 or higher compression ration will make it a lot easier to do.
FWIW, I have seen a guy that run a stock charger, stock engine that pulled better before than after his built slightly decompressed 2.6 chargered motor. And the dyno is shows different, but on the pulling track, it doesn't work as well. With a bigger turbo, I am sure that would change as far as how the truck works with the sled.
I too know that you can over power some tracks. I have seen stock trucks with a great setup beat 2.8 trucks.
FWIW, I have seen a guy that run a stock charger, stock engine that pulled better before than after his built slightly decompressed 2.6 chargered motor. And the dyno is shows different, but on the pulling track, it doesn't work as well. With a bigger turbo, I am sure that would change as far as how the truck works with the sled.
I too know that you can over power some tracks. I have seen stock trucks with a great setup beat 2.8 trucks.
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Troy's truck runs great.. He has some of our parts on it
Didn't intend to start a pissing match here.. Somebody with an engine dyno could prove the theories but alas I don't have one
Hmm? So I guess I can scrap my goal of 1500hp with a low compression setup
Sooo, again, IMO, a low compression engine will and is capable of well over 1000 hp on a single. My single at that time was small, IMO.
Matt,
I don't think it merits a
but yeah I think that on an electronically controlled engine there's no reason to decompress it. That is just my opinion. Your truck ran good. Not detracting from it a bit.
I guess to digress a bit.. A stock displacement 6.6 liter duramax with water cooling is going to be very hard pressed to make a 1000 hp on a 3.0" single with anything lower than 16:1.
Matt,
I don't think it merits a
Your truck ran good. Not detracting from it a bit.
I guess to digress a bit.. A stock displacement 6.6 liter duramax with water cooling is going to be very hard pressed to make a 1000 hp on a 3.0" single with anything lower than 16:1.
There is a needle that pokes the large head swells on this site......how both of you avoided it amazes me?
Both of you have excelent points,,,,However it is just your opinion!
PlEASE DONT RUIN A DISCUSSION WITH INFLATED HEADS:hug:
Troy's truck runs great.. He has some of our parts on it
Didn't intend to start a pissing match here.. Somebody with an engine dyno could prove the theories but alas I don't have one
I thought i seen something of the sort on his dash.
I wasnt trying to prove anyone wrong either, i like reading both sides of the discusion. This is how improvements are made.:thumb:
Something that has bothered me when looking at the pressure plots for normal and reduced compression engines is that the pressure increase from the fuel burn does not appear to increase in proportion to the compression ratio of the engine. I would expect to see a good 15% or more difference between a stock compression LLY and my 14.5 compression engine, but I am not seeing anything near that.
After looking at some temperature plots, I think I know why. The chamber temps in the higher compression engines are significantly higher. Since Temperature difference is what drives heat transfer from one object to another, I think the higher compression engines are transferring more heat to the block and pistons, thus sucking away some of the heat that should be increasing the chamber pressure.
So when theory meets reality here, it looks as though the great efficiency increase is minor, if any, from having higher compression engines. I am sure there is an optimum CR, someplace, but it is not strictly a matter of higher CP is required to make higher HP.
I could be all wet here, just what I've been seeing in the plots.
But, what do I know............
After looking at some temperature plots, I think I know why. The chamber temps in the higher compression engines are significantly higher. Since Temperature difference is what drives heat transfer from one object to another, I think the higher compression engines are transferring more heat to the block and pistons, thus sucking away some of the heat that should be increasing the chamber pressure.
So when theory meets reality here, it looks as though the great efficiency increase is minor, if any, from having higher compression engines. I am sure there is an optimum CR, someplace, but it is not strictly a matter of higher CP is required to make higher HP.
I could be all wet here, just what I've been seeing in the plots.
But, what do I know............
I've actually run, raced, and dyno'd the same charger, fueling and tuning on a 17.5 and 15.5 engine. But I'll just let the experts tell us how it all works. I'm no expert when it comes to diesel compression ratios.
I'm all ears. (eyes) I am interested in correlating what I have logged and what the actual results are. My engine configuration database is limited.
I am def no expert but i can say that the experience i have seen with the built motor trucks around here , the best example i can think of as far as keeping HP on a decompressed engine is Matt Larsons truck , 15.9-1.......... i think.......... and he managed to run 11.70 @ 117 on a single cp3 and a gt42 i doubt he would have run much faster with stock compression . I will have a good base line on my truck because no extra power adders are going on from before build and after build ,and i think i will be going with 15-15.5:1 so if it runs slower than 11.0 i will know why :rofl:
So the combustion of diesel in the chamber gives us heat energy along with work in terms of pressure-volume. This pressure-volume work is what makes the engine make torque. We know the heat energy produced is a lot more than the work produced.
But Ive never understood how a compression ratio effects efficiency. People have mentioned an increased in burn rate due to the smaller volume during combustion, but this does not directly relate to converting more total energy produced by combustion into work rather than heat.
Maybe I am trying to seperate work and heat to much. But there is a reason the temp in the chamber cools way down when the piston comes down.
It looks to me like when diesel is burned in the chamber, it is released in heat. So it looks like the goal is to not let the heat get absorbed into the surroundings in the cylinder (to not put in work to heat up the surroundings)
So it looks like a cold engine becuase of the difference in temp. of the combustion and metal in the chamber is inefficient. Because more work goes into heating the metal in the chamber than it would if the chamber and combustion were closer to the same temp.
So is it right saying you could get a 100% efficient engine if your chamber and all surrounding metals were the same temp. as your combustion?
It kind of makes sense know becuase higher compression ratios give you more temp. but then again that temp needs to go all the way cool again when the piston is at BDC.
But Ive never understood how a compression ratio effects efficiency. People have mentioned an increased in burn rate due to the smaller volume during combustion, but this does not directly relate to converting more total energy produced by combustion into work rather than heat.
Maybe I am trying to seperate work and heat to much. But there is a reason the temp in the chamber cools way down when the piston comes down.
It looks to me like when diesel is burned in the chamber, it is released in heat. So it looks like the goal is to not let the heat get absorbed into the surroundings in the cylinder (to not put in work to heat up the surroundings)
So it looks like a cold engine becuase of the difference in temp. of the combustion and metal in the chamber is inefficient. Because more work goes into heating the metal in the chamber than it would if the chamber and combustion were closer to the same temp.
So is it right saying you could get a 100% efficient engine if your chamber and all surrounding metals were the same temp. as your combustion?
It kind of makes sense know becuase higher compression ratios give you more temp. but then again that temp needs to go all the way cool again when the piston is at BDC.
Close. There are unobtainable things in your "ideal" world. First, the surrounding metals would have to raise and lower their temps in harmony with the chamber gasses. Two, there is a hard limit on how much work you can get out of a gas. This shows in the ideal case as excess pressure at BDC. You have two "buckets" that the energy can flow into in your ideal case. Mechanical work and temperature increase(thermal work). The energy will split evenly between the two given enough time.
Hotter chamber walls are more efficient. There are limits on how much heat the engine stand.
Hotter chamber walls are more efficient. There are limits on how much heat the engine stand.
The best i can figure Trent, is that the drop in compression on my motor made me lose about twenty horsepower and 175 lb/ft of torque.
Dmitri's truck with an almost identical setup with 17.5:1 compression made 599 hp 1150 lb/ft. Whereas mine with ~16:1 put down 575 hp 976 lb/ft on the same dyno in similar conditions.
I don't have any scientific data for track times but it didn't seem to hurt me.
Dmitri's truck with an almost identical setup with 17.5:1 compression made 599 hp 1150 lb/ft. Whereas mine with ~16:1 put down 575 hp 976 lb/ft on the same dyno in similar conditions.
I don't have any scientific data for track times but it didn't seem to hurt me.
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Yes, just our opinions. Didn't intend for head swelling.
Pat, I would be very interested in your one on one comparisons, no matter the outcome.
Ok thanks for the answer, understanding thermodynamics really helps to understand the way these engines work. So I guess your saying even if you could have a chamber temp that matches combustion temp through the stroke, you could still only get half the total energy.
It dosent look like engines have really made advances in this area, it seems like their more concentrated on emmisions, fuel delivery, and turbulence and fluid dynamics in the heads and chamber. Do the aluminum heads in the D-max give any efficiency advantages or disadvantages?
Aluminum = .900J/(g*K) Iron = .450J(g*K)
It looks like aluminum is a disadvantage because it would take more heat away from the chamber, but then again I guess the head is a small part of the total surroudings(block, pistons).
I really didn't see a change in performance outside normal variation by dropping compression. Guess it could have been 20hp drop, but maybe 20hp rise, dunno.
Gasoline burns slow, and diesel burns slower. Anything that can allow for faster burning is capable of increased HP.
Ideally, you want max average pressure on the piston from about 30 deg ATDC to 120 ATDC. Earlier or later has no big leverage on the crankshaft.
Much like gas engines, we have problems getting the pressure early enough. With gas engines, if you run high compression and run the "correct" advance to get the pressure in the sweet spot, you often detonate, so you must retard the timing. With the diesels, if we want the pressure in the sweet spot, we might have to start spraying the fuel in before the air is hot enough to ignite it. Compression helps us advance the timing but hurts the gas guys, and it's one of the reasons that many timing issues are "backwards" on turbo diesels. We can run MORE timing with more boost, but the gas guys run less.
Gasoline burns slow, and diesel burns slower. Anything that can allow for faster burning is capable of increased HP.
Ideally, you want max average pressure on the piston from about 30 deg ATDC to 120 ATDC. Earlier or later has no big leverage on the crankshaft.
Much like gas engines, we have problems getting the pressure early enough. With gas engines, if you run high compression and run the "correct" advance to get the pressure in the sweet spot, you often detonate, so you must retard the timing. With the diesels, if we want the pressure in the sweet spot, we might have to start spraying the fuel in before the air is hot enough to ignite it. Compression helps us advance the timing but hurts the gas guys, and it's one of the reasons that many timing issues are "backwards" on turbo diesels. We can run MORE timing with more boost, but the gas guys run less.