Rahmoune
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« Reply #90 on: February 22, 2014, 10:29:31 AM »
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i agree turbo engine does not need high CR pistons as much as NA engine but i think most of us would like to talk about static CR. Although the most important thing is dynamic CR which depends on many factors (as cams profile , cc head chamber , engine dilpacement etc......)
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SixSeven
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« Reply #91 on: February 25, 2014, 01:46:51 PM »
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Well....I just stumbled on this little gem of a thread poking around for injector info, and while I didn't find much of that, what a scintillating discussion on compression ratio! Anyway, I was a little disappointed that no one answered prj's question, so here goes.
What makes power in an internal combustion engine? Converting the chemical energy contained in a fuel into the most useful work possible, as fast as possible.
Or it could just be the bangity things.
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prj
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« Reply #92 on: February 25, 2014, 02:01:01 PM »
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What makes power in an internal combustion engine? Converting the chemical energy contained in a fuel into the most useful work possible, as fast as possible.
Or it could just be the bangity things. Exactly, and the more knock restricted we are, and the later the ignition timing, the more work is wasted and blown out of the exhaust as heat. Thus dropping CR has only ever a penalty in those areas where MBT is attainable before encountering knock. |
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« Last Edit: February 25, 2014, 02:02:41 PM by prj »
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NOTORIOUS VR
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« Reply #93 on: February 25, 2014, 02:25:12 PM »
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I'm going to have to ask nokiafix and ibizacurpa to calm down a little here.
Both of you are taking this too far, have provided no proof and making some odd claims. If you guys want to play the game, post up logs.
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prj
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« Reply #94 on: February 25, 2014, 02:32:23 PM »
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If you look at the post dates, then you will see this is quite old.
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SixSeven
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« Reply #95 on: February 25, 2014, 02:41:26 PM »
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Thus dropping CR has only ever a penalty in those areas where MBT is attainable before encountering knock.
This is 100% true. If MBT cannot be reached with a given fuel before hitting the knock limit, reducing the compression ratio can only help. It is important to remember that there's a balance to strike between the timing gained and the thermal efficiency lost by reducing the compression ratio, but in support of your point, I think in most forced-induction port-injected engines running pump gas, it pays to reduce CR. Also, sorry prj - didn't mean to get you in trouble! |
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NOTORIOUS VR
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« Reply #96 on: February 25, 2014, 02:44:14 PM »
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If you look at the post dates, then you will see this is quite old.
You're right, last post from them was Dec 2012. The posts on this last page were from this year so I didn't think of it much (I've never read this thread before today). My post was entirely towards the other two and not you PRJ FWIW. |
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prj
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« Reply #97 on: February 25, 2014, 02:57:39 PM »
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This is 100% true. If MBT cannot be reached with a given fuel before hitting the knock limit, reducing the compression ratio can only help. It is important to remember that there's a balance to strike between the timing gained and the thermal efficiency lost by reducing the compression ratio, but in support of your point, I think in most forced-induction port-injected engines running pump gas, it pays to reduce CR. Exactly, to get the same power as before dropping CR you will need more timing, but as long as you are knock limited in the area you are interested in, you can not really lose power by dropping CR. Higher CR is beneficial when not knock limited, such as when using E85 or other exotic fuels. Or indeed in a high revving naturally aspirated engine. |
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nyet
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« Reply #98 on: February 25, 2014, 03:19:12 PM »
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Higher CR is beneficial when not knock limited, such as when using E85 or other exotic fuels.
Or indeed in a high revving naturally aspirated engine.
As usual, I agree with prj. My real life experience also backs up theory; with crappy gas, dropping CR and bringing timing up from the basement is a HUGE gain, just about ... everywhere you aren't near MBT. Which, on pump gas and a lot of boost, is basically everywhere power matters. |
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NOTORIOUS VR
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« Reply #99 on: February 25, 2014, 03:36:18 PM »
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As usual, I agree with prj.
My real life experience also backs up theory; with crappy gas, dropping CR and bringing timing up from the basement is a HUGE gain, just about ... everywhere you aren't near MBT.
Which, on pump gas and a lot of boost, is basically everywhere power matters.
Absolutely I agree |
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ABCD
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« Reply #100 on: February 25, 2014, 09:12:03 PM »
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Hi guys,
I've never worked with forced induction engines. But I couldn't agree more with prj's theory wrt lower revs for forced induction engines.
My exp wrt naturally aspirated engines tell me that increasing CR has to be backed with suitable fuel, else it is a waste.
But, one doubt still bothers me : increasing CR and advancing timinng will both take u towards knocking. So how are 2 things diff. Maybe the dynamics are diff for forced induction engines. Is it so prj?
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ddillenger
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« Reply #101 on: February 25, 2014, 09:18:24 PM »
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They may both take you in the same direction, but not at the same speed!
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Please, ask all questions on the forums! Doing so will ensure the next person with the same issue gets the opportunity to learn from your experience!
Email/Google chat:
DDillenger84(at)gmail(dot)com
Email>PM
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ABCD
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« Reply #102 on: February 26, 2014, 07:54:32 PM »
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Another thing which I have noticed is that increasing the CR gives more torque at high revs (close to peak power band) and not much improvement at low revs.
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bomby
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« Reply #103 on: February 28, 2014, 01:33:03 PM »
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Hi i just came a cross this topic, and got interested about the info about the CR… What I don’t get is how it is possible to make a lot of power lets say 300~400hp with high CR and not have trouble with knock. On normal fuel not Race or E85. Because my own engine has a lot of timing pull due to knock and I’m only making like 260-270hp at 2400mbar(1.4boost) at +-8~10 degree this is with stock pistons of my AUM engine 1.8T Cr 9.5. So I was thinking of dropping it to 8.5 and at the same time go bigger bore and stoke. But if a then read te post about the hi power witk “high” CR I think that is maybe is not woth all the work and that maybe there is more to get out of the engine without all the work  Or is it possible my engine is not realy knocking yet but he ECU is a bit “to soon” in recognizing knock and that there lies the difference with other tunes with higher power on the same CR. My knock control is a bit changed but not a lot want it to be safe not killing the engine ofcourse |
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prj
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« Reply #104 on: March 01, 2014, 04:42:56 PM »
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If you want to make power you need to drop CR.
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