Compression Ratios and Forced Induction
10-06-2008, 11:09 AM
#181
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I was googling about this today because I want to build a new bottom end soon. I found this article about the PT cruiser turbo--
http://autospeed.com/cms/A_1813/arti...popularArticle
It runs 8:1 pistons but it (and I assume other) factory turbos run at stoich a/f on full boost. This isn't a direct analogue to HC/LB but I think this and other factory boosted cars are all good examples of the importance of tuning.
http://autospeed.com/cms/A_1813/arti...popularArticle
It runs 8:1 pistons but it (and I assume other) factory turbos run at stoich a/f on full boost. This isn't a direct analogue to HC/LB but I think this and other factory boosted cars are all good examples of the importance of tuning.
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10-06-2008, 11:30 PM
#183
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If you believe I am a shill because I am able to present an argument, and back my claims with empirical, experiential, and referential evidence, that would fit. It would also help explain your apparent disdain for the term, you've yet to offer anything other than an "I'm rubber, you're glue' approach. That said, my energies are better spent elsewhere.
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10-07-2008, 12:42 AM
#185
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I don't know where this thread ended up, I skimmed things pretty heavily, but I support the theory that a low compression ratio motor will spool faster. In a Carnot cycle you've got 2 phases, Isothermal compression/expansion and adiabatic compression/expansion. So that is broken up into 2 parts, P dv and Cv dt. Unfortunately humans suck so we don't get all the energy out of each chemical burn to put it to work. The Carnot cycle is more like a fairy tale but we can use it for fundamental understanding in this case.
By increasing the compression ratio we increase the capacity to do mechanical work and thus P dv is transmitted into power. Having 2 engines exactly the same and mitigating the increase in volumetric efficiency we see that we cannot spontaneously generate joules. Therefore the energy we get from P dv must be subtracted from Cv dt. That basically means the cylinder temperatures must go down. So in a high compression motor by default there is less energy to power the turbine that feeds from this waste energy. I would also wager that whoever designed the aftermarket pistons that done that compression lowering probably didn’t know nearly as much about efficient combustion systems as Mazda so it’s likely the turbulence used by the head to atomize and create favorable flame front conditions is not fully intact. This also reduces efficiency further adding to the Cv dt term, and more areas of the cylinder which will burn later in the exhaust pipe.
Also I would like to add that low compression motors using detonation limited gas should make more power because T will rise as compression goes up (duh), so the high compression motor is limited. This however becomes the low compression motors nemeses as BMEP goes very high and the gas becomes more detonation resistant. The low compression motor simply cannot convert enough of the heat in a race motor in order to keep parts from becoming molten. This is amplified by the inefficiency of any otto-cycle based motor which is already 30/70 on a good day.
By increasing the compression ratio we increase the capacity to do mechanical work and thus P dv is transmitted into power. Having 2 engines exactly the same and mitigating the increase in volumetric efficiency we see that we cannot spontaneously generate joules. Therefore the energy we get from P dv must be subtracted from Cv dt. That basically means the cylinder temperatures must go down. So in a high compression motor by default there is less energy to power the turbine that feeds from this waste energy. I would also wager that whoever designed the aftermarket pistons that done that compression lowering probably didn’t know nearly as much about efficient combustion systems as Mazda so it’s likely the turbulence used by the head to atomize and create favorable flame front conditions is not fully intact. This also reduces efficiency further adding to the Cv dt term, and more areas of the cylinder which will burn later in the exhaust pipe.
Also I would like to add that low compression motors using detonation limited gas should make more power because T will rise as compression goes up (duh), so the high compression motor is limited. This however becomes the low compression motors nemeses as BMEP goes very high and the gas becomes more detonation resistant. The low compression motor simply cannot convert enough of the heat in a race motor in order to keep parts from becoming molten. This is amplified by the inefficiency of any otto-cycle based motor which is already 30/70 on a good day.
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10-07-2008, 12:46 AM
#186
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ZX Tex, you keep insisting water has better thermal properties than PG, mainly specific heat. Nobody is denying that.
You keep ignoring the claim that PG prevents hotspotting; and you haven't tried it yourself. You also ignore the claims that the rotary crowd have about Evans.
You also keep insisting that PG is PG is PG. If you just look at the datasheet of the 3 types of PG coolant that Evans sells, they have 3 different sets of characteristics (viscosity, s.g., thermal conductivity, etc). Sierra is different still...
You keep ignoring the claim that PG prevents hotspotting; and you haven't tried it yourself. You also ignore the claims that the rotary crowd have about Evans.
You also keep insisting that PG is PG is PG. If you just look at the datasheet of the 3 types of PG coolant that Evans sells, they have 3 different sets of characteristics (viscosity, s.g., thermal conductivity, etc). Sierra is different still...
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10-08-2008, 01:25 PM
#187
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Allow me to elaborate.
Hotspotting can cause knock, but not all cars with knock have hotspotting.
I understand the hotspotting issue completely as I have had this discussion with some guys I know that are heavy-hitter pros in the engine R&D business. The occurrence of hotspotting depends greatly on a particular motor's design and is not necessarily an issue on all motors. So if the motor does not have significant hotspotting creating relatively huge boiling pockets then a 100% PG solution will not be as effective as a water based mix. Period. Maybe an RX7 has this issue due to jacket geometry and the 100% PG solution helps. But anecdotal reports have to be taken for what they are.
Whether it would make a difference on a Miata can only be assessed by back-to-back, same-car, same-optimized-tune, same-fuel, same-plugs, same-test-profile, controlled condition testing, changing only the coolants, and listening for changes in knock behavior. And, even if it did indeed make a difference, then Evans PG product versus someone else's PG product would give the same results, as long as the purities are close.
Small amounts of tiny boiling points is good since the coolant phase change absorbs a tremendous amount of heat in the process. So not all phase change is bad. It just depends on the amount, the location, and the distribution.
PG is PG is PG. The variation in coolant properties on the Evans site is due to differences in the mix formulations. One is 99%+ PG (NPG), one is 66-70% EG (NPG+), and one is 86-90% EG (NPG R). EG and PG have different properties. The differences in the formulations accounts for the Evans listed differences in thermal properties, not due to differences within the PG itself. So that is what I would expect to see.
I admit I do get a bit annoyed by thing like this, and tornado intake swirlers, splitfire plugs, miracle oil formulations and fuel additives, etc. etc. that are sold as some amazing cure/performance enhancer for more then they are worth. They will even provide 'scientific' test data to back up their claim. People spend premium money on these things, and think they work, SWEAR they work, when they actually do not or they actually have worsened performance. Some people for example even believe that a K&N filter makes more power than no filter at all.
If you want to buy it, and do some controlled condition dyno testing to show the improvement in knock resistance, then great, go for it. OK that is about all I have to say on the matter.
Hotspotting can cause knock, but not all cars with knock have hotspotting.
I understand the hotspotting issue completely as I have had this discussion with some guys I know that are heavy-hitter pros in the engine R&D business. The occurrence of hotspotting depends greatly on a particular motor's design and is not necessarily an issue on all motors. So if the motor does not have significant hotspotting creating relatively huge boiling pockets then a 100% PG solution will not be as effective as a water based mix. Period. Maybe an RX7 has this issue due to jacket geometry and the 100% PG solution helps. But anecdotal reports have to be taken for what they are.
Whether it would make a difference on a Miata can only be assessed by back-to-back, same-car, same-optimized-tune, same-fuel, same-plugs, same-test-profile, controlled condition testing, changing only the coolants, and listening for changes in knock behavior. And, even if it did indeed make a difference, then Evans PG product versus someone else's PG product would give the same results, as long as the purities are close.
Small amounts of tiny boiling points is good since the coolant phase change absorbs a tremendous amount of heat in the process. So not all phase change is bad. It just depends on the amount, the location, and the distribution.
PG is PG is PG. The variation in coolant properties on the Evans site is due to differences in the mix formulations. One is 99%+ PG (NPG), one is 66-70% EG (NPG+), and one is 86-90% EG (NPG R). EG and PG have different properties. The differences in the formulations accounts for the Evans listed differences in thermal properties, not due to differences within the PG itself. So that is what I would expect to see.
I admit I do get a bit annoyed by thing like this, and tornado intake swirlers, splitfire plugs, miracle oil formulations and fuel additives, etc. etc. that are sold as some amazing cure/performance enhancer for more then they are worth. They will even provide 'scientific' test data to back up their claim. People spend premium money on these things, and think they work, SWEAR they work, when they actually do not or they actually have worsened performance. Some people for example even believe that a K&N filter makes more power than no filter at all.
If you want to buy it, and do some controlled condition dyno testing to show the improvement in knock resistance, then great, go for it. OK that is about all I have to say on the matter.
Last edited by ZX-Tex; 10-08-2008 at 01:53 PM.
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10-08-2008, 08:18 PM
#190
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I can respect your pessimism a little more now that you have more adequately explained yourself, however a lot of the posts below are crap...
Your understanding is only as sharp as the folks that you listen to; does that sound scientific to you? You just mentioned anecdotal evidence, ironic. While I can appreciate you know some folks 'in the biz', I know a ****-ton of engineers that could not dig their way out of a wet paper sack without a calculator and a slide rule. IOW, this is fallacy, 'Appeal to Authority'.
I would assume that these folks you mentioned have used it? If not, I would suggest that the degrees they hold have just de-valued.
Note the change to the emboldened section; Knock is not necessarily audible, just as your posts are not necessarily laudable. Also, no one else recommends, directs, or suggests that you use it straight w/o water. It would stand to reason that they will therefore not work as well as Evans because if they did, they would go after that market.
This is the largest nugget of *** gold here; as I mentioned to Jason, while the coolant can indeed absorb that heat, it cannot do anything with it unless it is vented to the atmosphere. Steam cannot transmit heat as quickly or as efficiently as a liquid. Put more simply; what is a more efficient intercooler, air to air, or air to water (all things being equal)? You should say A/W. Liquid will -always- convey heat better than air, which is why you won't see an Air-Cooled Bug running 11-1 compression and having a long service life (yes, on pump gas).
None of the other companies you alluded to belong to SEMA; they won't put up with claims that aren't reasonable and backed by tests. And I hate Splitfires, Tornados, and all the rest of it. However, I have learned through experience that experience is the best way to learn.
---This reply is for those interested in Evans, nor for the unscientific wobbler quoted above. Okay, I admit it, I keep posting to **** him off.---
The occurrence of hotspotting depends greatly on a particular motor's design and is not necessarily an issue on all motors. So if the motor does not have significant hotspotting creating relatively huge boiling pockets then a 100% PG solution will not be as effective as a water based mix.
Whether it would make a difference on a Miata can only be assessed by back-to-back, same-car, same-optimized-tune, same-fuel, same-plugs, same-test-profile, controlled condition testing, changing only the coolants, and measuring the changes in knock behavior. And, even if it did indeed make a difference, then Evans PG product versus someone else's PG product would give the same results, as long as the purities are close.
I admit I do get a bit annoyed by thing like this, and tornado intake swirlers, splitfire plugs, miracle oil formulations and fuel additives, etc. etc. that are sold as some amazing cure/performance enhancer for more then they are worth. They will even provide 'scientific' test data to back up their claim. People spend premium money on these things, and think they work, SWEAR they work, when they actually do not or they actually have worsened performance. Some people for example even believe that a K&N filter makes more power than no filter at all.
---This reply is for those interested in Evans, nor for the unscientific wobbler quoted above. Okay, I admit it, I keep posting to **** him off.---
Last edited by fahrvergnugen; 10-08-2008 at 08:53 PM. Reason: See above.
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10-08-2008, 09:41 PM
#192
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I don't think two examples from two people is "proof", especially since neither case is documented. omaharma has a good story anyways. He's just stating a real world example. Nothing wrong with that. But I wouldn't say combining his post with ray sir 6's means it's the only way and it defines the answer. I mean, I've seen a single turbo 302 mustang pick up spool by dropping compression.
Originally this thread started out with jc rotor claiming that a 11:1 motor at 15PSI would make MORE power than an 8:1 running 24PSI because his 11:1 motor is more than 43% efficient at turning chemical energy into mechanical energy. I think it's understood that something more like 11:1 at 15PSI is ~= 8:1 at 20PSI. Something like that. The argument now is which spools faster. All the people I know that build boosted motors are looking for maximum power, not maximum efficiency (since they run 1/4 mile or a track that does not have a fuel factor in it like having to pit). They drop static compression very low and turn the boost way up. Makes more power. How much more, I dunno, but it puts the numbers down at the 1/4 mile.
Originally this thread started out with jc rotor claiming that a 11:1 motor at 15PSI would make MORE power than an 8:1 running 24PSI because his 11:1 motor is more than 43% efficient at turning chemical energy into mechanical energy. I think it's understood that something more like 11:1 at 15PSI is ~= 8:1 at 20PSI. Something like that. The argument now is which spools faster. All the people I know that build boosted motors are looking for maximum power, not maximum efficiency (since they run 1/4 mile or a track that does not have a fuel factor in it like having to pit). They drop static compression very low and turn the boost way up. Makes more power. How much more, I dunno, but it puts the numbers down at the 1/4 mile.
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10-08-2008, 09:43 PM
#193
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This thread is meant to be a general information thread, please think before you post and post only logical information/arguements/opinions. Thanks.
The arguement of Static and Dynamic compression ratios has long been raging in the world of turbocharging. Some say run 8.0:1 or even as low as 7.5:1 and crank up the boost. Others say that 11:1 CR with conservative boost is the way to go.
Right answer? There isnt one. It all depends on your long and short term goals for the car. Building an all out drag car? Dish out those pistons and crank the boost up! Want a reliable street car with low down umph and considerable top end as well? I would stay north of 9:1 and run less than 15 psi.
My personal preference is to raise the static CR. To me, if you are going through the trouble of overhauling the motor to make more power, its better to increase your engines performance and efficiency off boost. Most tuners will recommend 93 octane to turbo'd cars anyway, so you arent really sacrificing anything on that end. The upside to this is that you make more power off boost, your engine has higher egvs and will spool the turbo quicker, allowing you to run a larger one and pick up even more HP/L. The downside to this is that its difficult to tune and it gets more and more difficult as you head north of 15 psi, sometimes requiring 100 octane or greater to do so.
However, an 8:1 car at 20psi is not going to make the HP of an 11:1 car at 15 psi. And even if they were making the same peak HP to the wheels, the 11:1 car is going to have a better powerband and is going to be faster in the 1/4 mile.
Really, the biggest issue with static CR is the margin of error. It is much easier for the average Joe to run a lower CR and high boost because there is a much larger margin for error than there is with high Static CR. But you will lack the efficiency and the HP on low boost.
When advising others in their turbo builds, I normally advise that the stock CR is kept, simply because of the information/number of people running that setup allows for a wide knowledge base and less of a blind spot. And you arent sacrificing that performance down low, you will see improvements all the way across the RPM range and still be moderately safe.
Any imput/arguement is welcome. Keep it civil, we arent ricers or muscle heads, we are Miata owners.
The arguement of Static and Dynamic compression ratios has long been raging in the world of turbocharging. Some say run 8.0:1 or even as low as 7.5:1 and crank up the boost. Others say that 11:1 CR with conservative boost is the way to go.
Right answer? There isnt one. It all depends on your long and short term goals for the car. Building an all out drag car? Dish out those pistons and crank the boost up! Want a reliable street car with low down umph and considerable top end as well? I would stay north of 9:1 and run less than 15 psi.
My personal preference is to raise the static CR. To me, if you are going through the trouble of overhauling the motor to make more power, its better to increase your engines performance and efficiency off boost. Most tuners will recommend 93 octane to turbo'd cars anyway, so you arent really sacrificing anything on that end. The upside to this is that you make more power off boost, your engine has higher egvs and will spool the turbo quicker, allowing you to run a larger one and pick up even more HP/L. The downside to this is that its difficult to tune and it gets more and more difficult as you head north of 15 psi, sometimes requiring 100 octane or greater to do so.
However, an 8:1 car at 20psi is not going to make the HP of an 11:1 car at 15 psi. And even if they were making the same peak HP to the wheels, the 11:1 car is going to have a better powerband and is going to be faster in the 1/4 mile.
Really, the biggest issue with static CR is the margin of error. It is much easier for the average Joe to run a lower CR and high boost because there is a much larger margin for error than there is with high Static CR. But you will lack the efficiency and the HP on low boost.
When advising others in their turbo builds, I normally advise that the stock CR is kept, simply because of the information/number of people running that setup allows for a wide knowledge base and less of a blind spot. And you arent sacrificing that performance down low, you will see improvements all the way across the RPM range and still be moderately safe.
Any imput/arguement is welcome. Keep it civil, we arent ricers or muscle heads, we are Miata owners.
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10-08-2008, 09:49 PM
#194
"So wait. Which would make more power? The higher comp motor or the lower comp motor?"
At the same Dynamic CR, the high CR motor should make more power because of its higher efficiency.
the High CR will make more power per PSI than the low CR all day long.
"Makes since. I agree. I think this idea would be better applied to out of boost scenarios though. I'm arguing I'll be at more like 350 whp (arbitrary number, but one higher than yours) when I get to 24 psi and have the same dynamic compression ratio as you"
Not likely, no offense. At a given dynamic CR, the more efficient motor will make the most power. However, as boost is added, the gap in efficiency will get smaller.
At the same Dynamic CR, the high CR motor should make more power because of its higher efficiency.
the High CR will make more power per PSI than the low CR all day long.
"Makes since. I agree. I think this idea would be better applied to out of boost scenarios though. I'm arguing I'll be at more like 350 whp (arbitrary number, but one higher than yours) when I get to 24 psi and have the same dynamic compression ratio as you"
Not likely, no offense. At a given dynamic CR, the more efficient motor will make the most power. However, as boost is added, the gap in efficiency will get smaller.
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10-08-2008, 10:02 PM
#195
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So you can see I never said anything about being 43% more efficient. You pulled that number right out of your ***. At the most I said 12%.
And now I will once again cease to argue because you are overlooking the fact that I have experience building and tuning 11:1, 1.8L motors at 15 PSI that make over 400HP.
Tell me it cant be done and Ill slap a dyno chart up here so quit itll shut you up for a week (if thats possible). Ill even post pictures of the build.
I picked up 30 WHP from switching from 9:1 to 11:1 pistons on a miata. Naturally aspirated. Granted some of that power was from the lighter forged rotating assembly but that cant account for all of it. And that was on stock cams. Stock timing. Only adjusting the fuel.
The general consensus on every other car forum in the world out there is, HC is better unless you are running ridiculous amounts of boost. Ive already said raising the CR isnt for everyone. But IMHO its flat out STUPID to lower the CR any lower than 9:1 on a BP. But go ahead, after all, I cant possibly know anything about it. Whens the last time I built a turbo car? Oh thats right, today.
And now I will once again cease to argue because you are overlooking the fact that I have experience building and tuning 11:1, 1.8L motors at 15 PSI that make over 400HP.
Tell me it cant be done and Ill slap a dyno chart up here so quit itll shut you up for a week (if thats possible). Ill even post pictures of the build.
I picked up 30 WHP from switching from 9:1 to 11:1 pistons on a miata. Naturally aspirated. Granted some of that power was from the lighter forged rotating assembly but that cant account for all of it. And that was on stock cams. Stock timing. Only adjusting the fuel.
The general consensus on every other car forum in the world out there is, HC is better unless you are running ridiculous amounts of boost. Ive already said raising the CR isnt for everyone. But IMHO its flat out STUPID to lower the CR any lower than 9:1 on a BP. But go ahead, after all, I cant possibly know anything about it. Whens the last time I built a turbo car? Oh thats right, today.
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10-08-2008, 10:11 PM
#196
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Ive already agreed with you that changing nothing but the pistons will not reach higher numbers than the lower CR. But if you are BUILDING A MOTOR, does that make sense to go forth on such a path? NO.
There are other factors, that result from higher efficiency, but are not DIRECTLY related to it that can result in higher power. But apparantly that does not matter. According to you the only thing that matters is replacing the pistons.
How about this scenario? You do everything you can on a 8:1 motor to make more power and Ill do everything I can on a 11:1 motor of the same displacement and who do you think will end up on top in that situation?
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10-08-2008, 10:24 PM
#197
So you can see I never said anything about being 43% more efficient. You pulled that number right out of your ***. At the most I said 12%.
And now I will once again cease to argue because you are overlooking the fact that I have experience building and tuning 11:1, 1.8L motors at 15 PSI that make over 400HP.
Tell me it cant be done and Ill slap a dyno chart up here so quit itll shut you up for a week (if thats possible). Ill even post pictures of the build.
I picked up 30 WHP from switching from 9:1 to 11:1 pistons on a miata. Naturally aspirated. Granted some of that power was from the lighter forged rotating assembly but that cant account for all of it. And that was on stock cams. Stock timing. Only adjusting the fuel.
The general consensus on every other car forum in the world out there is, HC is better unless you are running ridiculous amounts of boost. Ive already said raising the CR isnt for everyone. But IMHO its flat out STUPID to lower the CR any lower than 9:1 on a BP. But go ahead, after all, I cant possibly know anything about it. Whens the last time I built a turbo car? Oh thats right, today.
And now I will once again cease to argue because you are overlooking the fact that I have experience building and tuning 11:1, 1.8L motors at 15 PSI that make over 400HP.
Tell me it cant be done and Ill slap a dyno chart up here so quit itll shut you up for a week (if thats possible). Ill even post pictures of the build.
I picked up 30 WHP from switching from 9:1 to 11:1 pistons on a miata. Naturally aspirated. Granted some of that power was from the lighter forged rotating assembly but that cant account for all of it. And that was on stock cams. Stock timing. Only adjusting the fuel.
The general consensus on every other car forum in the world out there is, HC is better unless you are running ridiculous amounts of boost. Ive already said raising the CR isnt for everyone. But IMHO its flat out STUPID to lower the CR any lower than 9:1 on a BP. But go ahead, after all, I cant possibly know anything about it. Whens the last time I built a turbo car? Oh thats right, today.
But will you now admit that you were wrong before about the dynamic compression ratio BS?
I know high comp is more efficient. How much is what's questioned. You originally said (what you were saying back on page 1) that high comp motors are soooooo much more efficient that at the same dynamic compression ratio, you'd make more HP even though I am moving 30% more air. This BS is where the 43% more efficient number came from It's no more valid than saying high comp motors can move 30% less air and make more power.
If you want to post pics and dyno plots open your own build thread.
I know high comp is more efficient. No ****. But it's not so ******* more efficient that it will make more power at the same dynamic compression ratio as a low comp motor. That's what we fought over forever. You finally quit defending this I think. Good for you.
You say people shouldn't lower static comp lower than 9:1. Ok. Maybe you're right. Not saying you aren't. Seriously. However, that's ASSuming a perfect tune, good fuel, great tuning, and a great tune. The higher comp motor has less margin for error. For a street car, you need some margin to account for BS like variance in octane of the fuel you are buying, IAT's, etc. You can't tune it on the verge of knock where adding in 1 degree of base timing would cause it to knock in every cell of the map. Maybe on a race car where that 1 degree means the difference between wining and losing, but not on a street car.
My argument is that with lower comp, you can make more HP SAFELY. Note, I didn't say more efficiently. I said safely. I'll also say you can make comparable power that a high comp motor would make more reliably. That is, the motor will be less likely to fail. Less heat. Less chance for knock. Motor should last longer. I would venture to say that 99% of engine failures in miatas are NOT caused directly by too much power exceeding the strength of the factory components. It's more like something happened and it detonated for a few seconds and something broke. Anything will break when detonation occurs. Granted detonation is more likely to occur at higher power levels. Hence lower comp pistons.
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10-08-2008, 10:40 PM
#198
Well, I'm not building a motor. But an 11:1 motor will be knock limited trying to reach peak cylinder pressure at ~14*ATDC at some level of boost and the 8:1 motor will be able to run considerably more boost before it too is knock limited to reach peak cylinder pressure at ~14*ATDC. Should easily make more power. Approximately 16% more IIRC. If I wasn't a poor college student I'd take you up on your offer. For now I'll have to settle for ~350whp on my stock block if I ever finish my GT3271 turbo setup. But if I ever build another motor it will be a low comp/high boost engine. So maybe the day will come one day.
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10-09-2008, 02:23 PM
#199
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Oh, snap, they are a member of SEMA?! Why didn't you say so before? Well then, they must know what they are doing...
Well then you are going to have to try harder because it's not working LOL. However if you are trying be amusing, you are doing a fantastic job
In fact this whole thread is full of amusement.
Well then you are going to have to try harder because it's not working LOL. However if you are trying be amusing, you are doing a fantastic job
In fact this whole thread is full of amusement.
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