Ignition Timing Vs. Compression Ratio
Re: Ignition Timing Vs. Compression Ratio Run 25:1 compression, take the spark plugs out
and run diesel.There...problem solved. <img src="graemlins/gr_chug.gif" border="0" alt="[chug]" /> <img src="graemlins/gr_jest.gif" border="0" alt="[jester]" />
and run diesel.There...problem solved. <img src="graemlins/gr_chug.gif" border="0" alt="[chug]" /> <img src="graemlins/gr_jest.gif" border="0" alt="[jester]" />
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From: new england
Re: Ignition Timing Vs. Compression Ratio ChrisB, hit it on the head about Colonel's ability to run that high a compression. I want to bring up a misconception about using a large cam to bleed off cylinder pressure, this only lowers cylinder pressure at low rpms and this increased cam timing will actually increase cylinder pressure at peak torque. What this means is pinging can be as bad or worse at the top of each gear even with a large cam. Another point is about ignition timing, the later timing occurs as with most LS1's, the harder it is to fire plug because cylinder pressure has increased with every degree that spark occurs closer to TDC.
This problem would be extreme with a 12 to 1 compression Ls1 with very little ignition timing. Just a thought. <img src="gr_images/icons/cool.gif" border="0">
This problem would be extreme with a 12 to 1 compression Ls1 with very little ignition timing. Just a thought. <img src="gr_images/icons/cool.gif" border="0">
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From: Richmond, Va
Re: Ignition Timing Vs. Compression Ratio Also what are the long term goals of this setup? Will nitrous come into play in the future? If so, which was would be safer for a motor with say a 200 shot or so?
Re: Ignition Timing Vs. Compression Ratio YEAH - WHAT CHRISB SAID
12.5 is way too much for a car that will be driven much at all.
There is nothing better than the throttle response of a 12 or 12.5 C/R engine on proper fuel, however we dont have 100+ octane fuel like we did 25 years ago! I used to build up "real" LS6 and LS7 engine for vettes.
I have built up numerous engine types and find that aluminum heads on an engine generally can use 11 and cast heads only 10 as a max for regular street use.
If you are willing to at least blend racing fuel with premium, go to 11.5 or 12 as a maximum, but dont plan any long trips where you might not find racing fuel along the way.
If you try running even 12 to 1 ratio on just 93 octane pump fuel, the retarded timing will make it barely idle, run hot, and feel VERY sluggish, and make it undrivable in hot weather.
PS - A larger cam (longer duration) makes lower cylinder pressure at low RPM's but fills the cylinders better (creating higher pressure) at high RPM. So a big cam will allow a high compression engine to run OK at lower speed, but detonate just as bad or worse at high RPM.
[ February 07, 2002: Message edited by: GTO69JUDGE ]</p>
12.5 is way too much for a car that will be driven much at all.
There is nothing better than the throttle response of a 12 or 12.5 C/R engine on proper fuel, however we dont have 100+ octane fuel like we did 25 years ago! I used to build up "real" LS6 and LS7 engine for vettes.
I have built up numerous engine types and find that aluminum heads on an engine generally can use 11 and cast heads only 10 as a max for regular street use.
If you are willing to at least blend racing fuel with premium, go to 11.5 or 12 as a maximum, but dont plan any long trips where you might not find racing fuel along the way.
If you try running even 12 to 1 ratio on just 93 octane pump fuel, the retarded timing will make it barely idle, run hot, and feel VERY sluggish, and make it undrivable in hot weather.
PS - A larger cam (longer duration) makes lower cylinder pressure at low RPM's but fills the cylinders better (creating higher pressure) at high RPM. So a big cam will allow a high compression engine to run OK at lower speed, but detonate just as bad or worse at high RPM.
[ February 07, 2002: Message edited by: GTO69JUDGE ]</p>
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From: Troy, AL
Re: Ignition Timing Vs. Compression Ratio From another similar thread...
"Originally posted by Hydra:
Really... Very Impressive Colonel! What exactly have you done to make your motor so detonation resistant?
--------------------------------------------------------------------------------
Glad you asked!
Several factors are helping me...
Maybe the biggest is the fact that we have aluminum cylinder heads and block. Aluminum dissipates heat more rapidly than iron.
Flat top pistons. My extra compression is gotten from my shaved LS6 heads and my thin metal head gaskets.
Quench area. My quench area is very small for the same reasons that my C/R is high. This creates some major turbulence in the combustion chamber which is condusive to having a more even fuel/air mixture throughout the combustion chamber (less chance for a lean spot.)
A camshaft with lots of overlap makes for less dynamic compression at rpms when you are "off the cam." 244/244 112 makes for some pretty good overlap.
160 degree thermostat.
The ability to lower the timing advance and enrichen the fuel mixture by cranking up the MAFT. Running the mixture rich helps to prevent detonation. I can also lower my timing advance through use of a resistor plugged into the IAT sensor. A 5.2K ohm resistor gives the ECM a reading of 92 degrees, for example. The ECM uses a less aggressive timing scale for hot weather.
Practically no backpressure in the exhaust system. This one has been debated but testing at MTI has shown that having cats installed on a car makes for more detonation.
[ February 07, 2002: Message edited by: Colonel ]</p>
"Originally posted by Hydra:
Really... Very Impressive Colonel! What exactly have you done to make your motor so detonation resistant?
--------------------------------------------------------------------------------
Glad you asked!
Several factors are helping me...
Maybe the biggest is the fact that we have aluminum cylinder heads and block. Aluminum dissipates heat more rapidly than iron.
Flat top pistons. My extra compression is gotten from my shaved LS6 heads and my thin metal head gaskets.
Quench area. My quench area is very small for the same reasons that my C/R is high. This creates some major turbulence in the combustion chamber which is condusive to having a more even fuel/air mixture throughout the combustion chamber (less chance for a lean spot.)
A camshaft with lots of overlap makes for less dynamic compression at rpms when you are "off the cam." 244/244 112 makes for some pretty good overlap.
160 degree thermostat.
The ability to lower the timing advance and enrichen the fuel mixture by cranking up the MAFT. Running the mixture rich helps to prevent detonation. I can also lower my timing advance through use of a resistor plugged into the IAT sensor. A 5.2K ohm resistor gives the ECM a reading of 92 degrees, for example. The ECM uses a less aggressive timing scale for hot weather.
Practically no backpressure in the exhaust system. This one has been debated but testing at MTI has shown that having cats installed on a car makes for more detonation.
[ February 07, 2002: Message edited by: Colonel ]</p>
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From: Houston, TX
Re: Ignition Timing Vs. Compression Ratio Good replies so far!
1. This isn't my only vehicle now. I might drive it 100 miles per week maximum, less when bad weather is around.
2. I am planning on buying a trailer, makes it much more convenient for track visits (can prep the car the night before instead of in the pits). I've also spent more on tow trucks than a trailer would have costed me haha.
3. I am looking primarily for performance first, drivability second. It used to be the opposite.
4. No, I do not want to require race gas.
Another idea: What if I build the motor to 12.0:1 CR and try it out, and if I am having detonation issues I can always pull the heads and install a thicker pair of headgaskets. We will be testing the Holley intake vs LS6 intake when they arrive, and I'm going with whichever one flows better, hopefully the Holley since it can be port-matched to my new LS6 heads.
This should be fun <img src="gr_grin.gif" border="0">
Tony
1. This isn't my only vehicle now. I might drive it 100 miles per week maximum, less when bad weather is around.
2. I am planning on buying a trailer, makes it much more convenient for track visits (can prep the car the night before instead of in the pits). I've also spent more on tow trucks than a trailer would have costed me haha.
3. I am looking primarily for performance first, drivability second. It used to be the opposite.
4. No, I do not want to require race gas.
Another idea: What if I build the motor to 12.0:1 CR and try it out, and if I am having detonation issues I can always pull the heads and install a thicker pair of headgaskets. We will be testing the Holley intake vs LS6 intake when they arrive, and I'm going with whichever one flows better, hopefully the Holley since it can be port-matched to my new LS6 heads.
This should be fun <img src="gr_grin.gif" border="0">
Tony
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From: Troy, AL
Re: Ignition Timing Vs. Compression Ratio Another idea: What if I build the motor to 12.0:1 CR and try it out, and if I am having detonation issues I can always pull the heads and install a thicker pair of headgaskets.
Tony, read my above comments on quench area. If you install thicker head gaskets you'll have less compression, yes...but you'll have MORE chance for detonation.
Tony, read my above comments on quench area. If you install thicker head gaskets you'll have less compression, yes...but you'll have MORE chance for detonation.
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Verdad Gallardo Re: Ignition Timing Vs. Compression Ratio You might consider building your engine with 12:1 using 5.3 truck heads which have smaller chambers. That way when you realize 12:1 is too much for pump gas, you can bolt on your 5.7 heads which will give you about 11:1 which you will probably be happier with. EXPERIMENT!!
Re: Ignition Timing Vs. Compression Ratio [quote]This is how nascars work - the run ungodly amounts of compression, but because they are pulling it in through that little tiny restrictor plate their dynamic compression is nothing like the calculated static. <hr></blockquote>
Right, they were running 20:1 + a few years ago on the restrictor plate motors. since then, they have a CR limit. one thing, with that high CR, WOT they were fine, but then coming in and out of the pits, they would blow head gaskets.
-Louis
Right, they were running 20:1 + a few years ago on the restrictor plate motors. since then, they have a CR limit. one thing, with that high CR, WOT they were fine, but then coming in and out of the pits, they would blow head gaskets.
-Louis
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From: College Station, Tx
Re: Ignition Timing Vs. Compression Ratio Louis - yep, good point on the low rpm/load situation also!
I would say 12:1 is pushing it still for a streetable ride. Maybe with reverse cooling (would be curious to see what kind of extra compression the EWP allows). Honestly though there is not going to be a huge hp difference between 11.5:1 and 12:1 but there is a big impact on the wallet if you have to fill up on 104 vs. 93.
Either that or I am just trying to slow you down <img src="gr_images/icons/wink.gif" border="0"> <img src="gr_images/icons/wink.gif" border="0">
Chris
I would say 12:1 is pushing it still for a streetable ride. Maybe with reverse cooling (would be curious to see what kind of extra compression the EWP allows). Honestly though there is not going to be a huge hp difference between 11.5:1 and 12:1 but there is a big impact on the wallet if you have to fill up on 104 vs. 93.
Either that or I am just trying to slow you down <img src="gr_images/icons/wink.gif" border="0"> <img src="gr_images/icons/wink.gif" border="0">
Chris
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From: NJ
Re: Ignition Timing Vs. Compression Ratio [quote]Originally posted by Nine Ball:
<strong>Lets say you are designing a new LS1 motor combo (I am!). What route would you choose between these two, keep in mind that I will be running 93 octane pump gas 90% of the time:
A. 11.0:1 Compression ratio, and higher timing values
B. 12.5:1 Compression ratio, and less timing
Both should run fine on pump gas when tuned correctly, but which is the better method of making the power, and WHY?
Tony</strong><hr></blockquote>
Tony, here is case B on my own 85 Vette.
National Dragster Article on my 85 Vette
Julio
<strong>Lets say you are designing a new LS1 motor combo (I am!). What route would you choose between these two, keep in mind that I will be running 93 octane pump gas 90% of the time:
A. 11.0:1 Compression ratio, and higher timing values
B. 12.5:1 Compression ratio, and less timing
Both should run fine on pump gas when tuned correctly, but which is the better method of making the power, and WHY?
Tony</strong><hr></blockquote>
Tony, here is case B on my own 85 Vette.
National Dragster Article on my 85 Vette
Julio
I am wondering that if optimum cyl pressure is at 14 deg ATDC what the relationship between cyl compression is and the time it takes for the flame front to develop to burn the compressed mixture and build max pressure
at the 14 deg? This time is probably compression dependent? Once this were known, one could translate this into igntion timing for each rpm.
Gert
at the 14 deg? This time is probably compression dependent? Once this were known, one could translate this into igntion timing for each rpm.
Gert
Gert,
You are correct. I know 1 man that is working on this at this time and he has developed a device and program to measure cylinder pressure in 1/2 degree crank increments. Once developed fully this tool can then be used to properly tune an internal combustion engine.
Chris
You are correct. I know 1 man that is working on this at this time and he has developed a device and program to measure cylinder pressure in 1/2 degree crank increments. Once developed fully this tool can then be used to properly tune an internal combustion engine.
Chris
A quick backward calculations assuming an LS1
with known set-up:
6000 rpm
opt pressure at 14 deg ATDC reached
igntion timing 28 deg
spark to opt. pressure = 1.16 milliseconds (gives an idea of this time parameter)
This would be for NA, 10.1 compression, 93 fuel etc.
Gert
with known set-up:
6000 rpm
opt pressure at 14 deg ATDC reached
igntion timing 28 deg
spark to opt. pressure = 1.16 milliseconds (gives an idea of this time parameter)
This would be for NA, 10.1 compression, 93 fuel etc.
Gert
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From: Fort Collins, CO
It's never made sense to me to be over-aggressive in one area of the engine build (lots of compression) and conservative in another part of the engine (low timing). Kind of like over-camming an engine because it's got a little bit too much compression and you want to bleed some of it off.
Building the combination to work together has always seemed to be the best way to go.
Building the combination to work together has always seemed to be the best way to go.
Interesting old thread... Welp, I think this is a hard thread to tackle.
I mean, why not build a 9:1 LS1 that uses 35 degrees of max timing, or a 13:1 engine that uses 22 degrees...
Tony's original parameter was 93 pump gas.
Based on things I have seen over the years, I don't think I can come up with a generalized response to this thread. Some cars that I know are running really well are running more than stock timing thru stock bottom end cars (using 93).
I suspect that timing is more important than compression, but someone would have to show that for example adding 4-5 degrees of timing was better than adding say a full point of compression. The added power from adding timing would need to be more than the 3-4% increase from adding a point of compression.
I mean, why not build a 9:1 LS1 that uses 35 degrees of max timing, or a 13:1 engine that uses 22 degrees...
Tony's original parameter was 93 pump gas.
Based on things I have seen over the years, I don't think I can come up with a generalized response to this thread. Some cars that I know are running really well are running more than stock timing thru stock bottom end cars (using 93).
I suspect that timing is more important than compression, but someone would have to show that for example adding 4-5 degrees of timing was better than adding say a full point of compression. The added power from adding timing would need to be more than the 3-4% increase from adding a point of compression.
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From: Texas
See my thread on optimal timing. When you stop making power, don't add more timing. If you have good chamber that doesn't need spark lead, then I won't add it. On some of the race cars we run a high speed retard which takes out timing in high gear, and the car goes faster and picks up mph.
You want as much compression as you can safely use without predetonation, and you want as much timing as it takes to properly light off the whole mixture in the proper ammount of time. To little timing and the burn isn't done in time for peak power. Too early, and it done and you're working against your motor, not with it.
You want as much compression as you can safely use without predetonation, and you want as much timing as it takes to properly light off the whole mixture in the proper ammount of time. To little timing and the burn isn't done in time for peak power. Too early, and it done and you're working against your motor, not with it.
I've seen folks run 30-32 degrees of timing with stock compression, is that better than running 11:1 and say 28 degrees of timing? Only a shop with a lot of dyno time with various combos might have that answer.
Sure when you stop making more power you should stop adding timing, I figured that we would also assume that. My old ARE 422 picked up like 5rwhp with 5 more degrees of timing, so I went back down to 25 degrees of timing. When we last dynoed it, I tried different amounts of timing, and it was not worth running more timing given the negligible increase in rwhp.
Additionally, LSx heads are more efficient than Gen II and I stuff, where folks would hit stuff with a lot of timing, like LT1's. At least, that is what I have read.... of folks running a lot more timing advance with LT1's.
I would say, with a stock bottom end, that I'd mess with timing more than milling heads for compression. Doesn't seem like the added compression is a verified true gain all things being equal.
Sure when you stop making more power you should stop adding timing, I figured that we would also assume that. My old ARE 422 picked up like 5rwhp with 5 more degrees of timing, so I went back down to 25 degrees of timing. When we last dynoed it, I tried different amounts of timing, and it was not worth running more timing given the negligible increase in rwhp.
Additionally, LSx heads are more efficient than Gen II and I stuff, where folks would hit stuff with a lot of timing, like LT1's. At least, that is what I have read.... of folks running a lot more timing advance with LT1's.
I would say, with a stock bottom end, that I'd mess with timing more than milling heads for compression. Doesn't seem like the added compression is a verified true gain all things being equal.