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Old 09-25-2008, 06:49 AM   #61
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Originally Posted by JasonC SBB View Post
jc_rotor,

AG Bell's book "4-stroke performance tuning" warns against the common mistake of raising compression ratio too high for the octane (whether FI or n/a). The temptation is to raise c/r, then retard timing to prevent knock. The latter will lose you more power than the raised c/r will gain you. High c/r is over-rated.
Except that a 9.5:1 motor will make more power than your 8.4:1 motor from the 2000rpm to 3500rpm range, the range where you aren't at full boost. The high-comp motor will feel peppier off boost.

Jason, didn't you also play with exhaust cam timing when you redid your motor? IMO you changed too many things to provide a good example.
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Old 09-25-2008, 12:07 PM   #62
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I pointed out above that my car did NOT lose bottom end going from 9.5:1 to 8.4:1 - dyno measured. And spoolup IMPROVED.
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Originally Posted by JasonC SBB View Post
Minor DIY valve seat cleanup and 3-angle valve job.
Ok, and what else? The laws of physics don't change, so you had to change something to make it spool faster, and lower CR isn't gonna do it.

You are normally the intelligent person in these threads, but you are going in the wrong direction right now.
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Old 09-25-2008, 12:17 PM   #63
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Except that a 9.5:1 motor will make more power than your 8.4:1 motor from the 2000rpm to 3500rpm range, the range where you aren't at full boost. The high-comp motor will feel peppier off boost.

Jason, didn't you also play with exhaust cam timing when you redid your motor? IMO you changed too many things to provide a good example.
...and the truth is slowly coming out.

He changed too much to be a fair comparision, or he wouldn't be able to state that a LC will spool like a HC.
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Old 09-25-2008, 01:54 PM   #64
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Except that a 9.5:1 motor will make more power than your 8.4:1 motor from the 2000rpm to 3500rpm range, the range where you aren't at full boost. The high-comp motor will feel peppier off boost.
In theory by 4%.
How much do you think DIY headwork is worth? 4%? If so then it cancels out and that explains why I saw neither a loss nor gain at the bottom. However my spoolup DID improve.

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Except that a 9.5:1 motor will make more power than your 8.4:1 motor from the 2000rpm to 3500rpm range, the range where you aren't at full boost. The high-comp motor will feel peppier off boost.
Nope, car felt stronger below 3000 RPM.

I dynoe'd the car after the low compression pistons before mounting the turbo.
The result was very very close to when the car was near new and bone stock.

Point is none of this data supports the idea that low compression loses spoolup.
Many years ago JKav said that lower compression helps spoolup because of the higher EGT's.

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Jason, didn't you also play with exhaust cam timing when you redid your motor?
No, that came later. That was actually never dyno'ed on a chassis dyno. The effect on spoolup was very noticeable.


Ray, read above before mouthing off again.

Many people recite the mantra that "high compression helps spoolup" without an explanation.
What actually happens is torque below the boost threshold is improved, 4% per point, but that is different than spoolup.
Spoolup is improved by the higher EGT's. Low c/r has higher EGT because the motor is less efficient thermodynamically (see Otto cycle).

I also offered a possible explanation why high c/r MAY improve VE, which may explain why some people report a greater than 4% gain per point. However, I have to see data where only compression ratio was changed. My own data is the closest I've seen, because I only did minor DIY porting. My post-turbo low compression dyno was very close to original. This was my goal - that the gains from porting would offset the loss due to the lowered c/r. At least now the motor is much more ping resistant on 91 craptane.

There was another guy on the miatapower list with 8.5 (?) pistons, 4 years ago he got some other people to test drive it, none of them noticed any loss in low-end or spoolup.

Last edited by JasonC SBB; 09-25-2008 at 02:12 PM.
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Old 09-25-2008, 02:04 PM   #65
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Ok, and what else? The laws of physics don't change, so you had to change something to make it spool faster, and lower CR isn't gonna do it.
Really? Care to elaborate on exactly WHAT laws of physics you would be referring to?
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Old 09-25-2008, 03:20 PM   #66
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Compression ratio with boost

Before discussing compression ratio and boost, it is important to understand engine knock, also known as detonation. Knock is a dangerous condition caused by uncontrolled combustion of the air/fuel mixture. This abnormal combustion causes rapid spikes in cylinder pressure which can result in engine damage.

Three primary factors that influence engine knock are:

Knock resistance characteristics (knock limit) of the engine: Since every engine is vastly different when it comes to knock resistance, there is no single answer to "how much." Design features such as combustion chamber geometry, spark plug location, bore size and compression ratio all affect the knock characteristics of an engine.

Ambient air conditions: For the turbocharger application, both ambient air conditions and engine inlet conditions affect maximum boost. Hot air and high cylinder pressure increases the tendency of an engine to knock. When an engine is boosted, the intake air temperature increases, thus increasing the tendency to knock. Charge air cooling (e.g. an intercooler) addresses this concern by cooling the compressed air produced by the turbocharger

Octane rating of the fuel being used: octane is a measure of a fuel's ability to resist knock. The octane rating for pump gas ranges from 85 to 94, while racing fuel would be well above 100. The higher the octane rating of the fuel, the more resistant to knock. Since knock can be damaging to an engine, it is important to use fuel of sufficient octane for the application. Generally speaking, the more boost run, the higher the octane requirement.
This cannot be overstated: engine calibration of fuel and spark plays an enormous role in dictating knock behavior of an engine. See Section 5 below for more details.

Now that we have introduced knock/detonation, contributing factors and ways to decrease the likelihood of detonation, let's talk about compression ratio. Compression ratio is defined as:



or



where
CR = compression ratio
Vd = displacement volume
Vcv = clearance volume



The compression ratio from the factory will be different for naturally aspirated engines and boosted engines. For example, a stock Honda S2000 has a compression ratio of 11.1:1, whereas a turbocharged Subaru Impreza WRX has a compression ratio of 8.0:1.

There are numerous factors that affect the maximum allowable compression ratio. There is no single correct answer for every application. Generally, compression ratio should be set as high as feasible without encountering detonation at the maximum load condition. Compression ratio that is too low will result in an engine that is a bit sluggish in off-boost operation. However, if it is too high this can lead to serious knock-related engine problems.

Factors that influence the compression ratio include: fuel anti-knock properties (octane rating), boost pressure, intake air temperature, combustion chamber design, ignition timing, valve events, and exhaust backpressure. Many modern normally-aspirated engines have well-designed combustion chambers that, with appropriate tuning, will allow modest boost levels with no change to compression ratio. For higher power targets with more boost , compression ratio should be adjusted to compensate.

There are a handful of ways to reduce compression ratio, some better than others. Least desirable is adding a spacer between the block and the head. These spacers reduce the amount a "quench" designed into an engine's combustion chambers, and can alter cam timing as well. Spacers are, however, relatively simple and inexpensive.

A better option, if more expensive and time-consuming to install, is to use lower-compression pistons. These will have no adverse effects on cam timing or the head's ability to seal, and allow proper quench regions in the combustion chambers.
Turbo By Garrett

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Old 09-25-2008, 03:55 PM   #67
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Turbo By Garrett

The question answered by the experts.

/Thread
nice article on knock resistance. no info on spool or power that i saw.
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Old 09-25-2008, 04:02 PM   #68
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nice article on knock resistance. no info on spool or power that i saw.
+1.

ray_sir_6, YOU SAID:

Quote:
The laws of physics don't change, so you had to change something to make it spool faster, and lower CR isn't gonna do it.
So you copy and paste an article on knock? Why don't you just outline the underlying physics that formed the basis for your statement I quoted?
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Old 09-25-2008, 06:10 PM   #69
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Originally Posted by y8s View Post
nice article on knock resistance. no info on spool or power that i saw.
Generally, compression ratio should be set as high as feasible without encountering detonation at the maximum load condition. Compression ratio that is too low will result in an engine that is a bit sluggish in off-boost operation. However, if it is too high this can lead to serious knock-related engine problems.

In bold so noone misses it. That is what this thread was asking, so that is the CORRECT answer.

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So you copy and paste an article on knock? Why don't you just outline the underlying physics that formed the basis for your statement I quoted?
I'm not gonna try to get into formulas trying to prove how it works, cause there are too many variables that have to be included that we are purposely removing in order to get to a simple answer for a simple question:

"With all other things being the same, which is faster, a HC low boost or a LC high boost engine, both making the same peak hp?"

Even the "fact" that Jason C is trying to say that he tested his engine after the LC pistons were installed and got almost the same power, yet had faster spool, only shows one thing...that his HC pistons were fucked, either his rings were shot, or a piston was cracked, etc.

I base this on several things:

Common Sense I understand how an engine works, and how a turbo works. I am not an EXPERT by any means, but it doesn't take one to figure out the answer to this question.

Experience I work with a man who has been making his living building turbo cars. He is an EXPERT, and I try to get as much info out of him as I can. He is the one who said the question is not answerable due to all the different variables that can play into it. But he knows that, making all variables the same, a LC motor will make less power under the curve, and thus be slower overall. Maybe only .1 sec slower in the 1/4 mi, but still slower.

Hmmm....another idea...Forced Performance...they should be a good source of info on this.

"Higher CR will make it more responsive, have faster spool, and more power out of boost."

Anyone have some "turbo" books they can quote from? They will all say the same thing.

HC works so long as you have adequate knock suppression, or you have to lower the CR till you have adequate suppression.

So if you want to make a 300whp Miata, you CAN do it with 11:1 pistons, but you'll be running 120octane gas to do it.

If you want to make a 300whp Miata and use 93 octane gas, 9.5:1 or lower CR.

This isn't including the options for running alcohol injection, or anything else like that. Again, trying to limit the variables to make for a simpler answer. The complex answer is, "If you have enough money, you can do whatever you want."
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Old 09-25-2008, 07:46 PM   #70
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This thread is about power and compression ratios, not knock. We all know what knock is. Your big post from Garrett addressed compression ratios and knock. Nobody is debating about knock.

Quote:
The laws of physics don't change, so you had to change something to make it spool faster, and lower CR isn't gonna do it.
So you would agree that for a given amount of air/fuel that higher compression results in more HP to the crankshaft, right? It's more efficient? So in other words, more chemical energy is converted into mechanical energy that turns the crank instead of being wasted in some form of energy, be it into the block as heat or out the exhaust as heat? Is energy conserved?

Consider the following. What if the engine was 100% efficient? All of the chemical energy was converted into mechanical energy to turn the crankshaft. 100% efficient. There would be no heat in the exhaust, only fluids exiting at ambient temperature and pressure. We could not drive the turbine of the turbocharger now could we?

What if the engine was 1% efficient? Then there would be quite a bit of wasted energy going out of the exhaust wouldn't there?

I will say this though in your defense: There are several questions being asked and debated. Your last post is mainly right. jc rotor kept insisting that 30% more air 30% more HP and that an 11:1 motor at 15 PSI is gonna make MORE hp than an 8:1 motor at 24PSI because the 11:1 motor is so much more 'efficient'. Do you agree that he was wrong in that regard, or do you think he is correct?
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Old 09-25-2008, 08:34 PM   #71
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Originally Posted by patsmx5 View Post
This thread is about power and compression ratios, not knock. We all know what knock is. Your big post from Garrett addressed compression ratios and knock. Nobody is debating about knock.



So you would agree that for a given amount of air/fuel that higher compression results in more HP to the crankshaft, right? It's more efficient? So in other words, more chemical energy is converted into mechanical energy that turns the crank instead of being wasted in some form of energy, be it into the block as heat or out the exhaust as heat? Is energy conserved?

Consider the following. What if the engine was 100% efficient? All of the chemical energy was converted into mechanical energy to turn the crankshaft. 100% efficient. There would be no heat in the exhaust, only fluids exiting at ambient temperature and pressure. We could not drive the turbine of the turbocharger now could we?

What if the engine was 1% efficient? Then there would be quite a bit of wasted energy going out of the exhaust wouldn't there?

I will say this though in your defense: There are several questions being asked and debated. Your last post is mainly right. jc rotor kept insisting that 30% more air 30% more HP and that an 11:1 motor at 15 PSI is gonna make MORE hp than an 8:1 motor at 24PSI because the 11:1 motor is so much more 'efficient'. Do you agree that he was wrong in that regard, or do you think he is correct?
Why do you think people lower the CR? To reduce the chances of knock. That would be asking people about swimming, and not wanting to mention water. If there is no water, than there is no swimming. If there is no knock, than there is no reason to lower the CR.
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Old 09-25-2008, 09:17 PM   #72
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Why do you think people lower the CR? To reduce the chances of knock. That would be asking people about swimming, and not wanting to mention water. If there is no water, than there is no swimming. If there is no knock, than there is no reason to lower the CR.
Let me rephrase what I was trying to say. We were not discussing the topic of compression and knock resistance. Well, we were inadvertently, but not specifically. We were trying to discuss the physics of compression ratio and boost with respect to power. Of course in either case knock is of concern. But it's not the "topic" that was being debated. Is that a little clearer? We went like 60 post deep into discussing the topic without mentioning knock, as we are assuming it is not present. Granted it's of concern, and so are a million other things. Just saying we weren't having a discussion on knock.

You reduce compression to reduce knock. You also do it so that you can run enough timing so that peak cylinder pressure occurs at a certain instance during the power stroke, usually ~14* ATDC. With a high compression motor and boost, you will reach a point where you can not add advance timing to maintain peak cylinder pressures at 14* ATDC without knocking. After this point, you will be retarding timing and moving peak cylinder pressure further away from ATDC. You would start loosing HP with your "more efficient" setup, even though you are not "knocking". If you tried to maintain peak cylinder pressures at 14* ATDC, you would encounter knock.

Anyways, what do you think of post 70 with regards to efficiency?
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Old 09-25-2008, 09:50 PM   #73
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Anyways, what do you think of post 70 with regards to efficiency?
The reason a HC/LB would make more power off less boost is cause it is more efficient. This also helps spool the turbo earlier, since it will have more heat and velocity sooner. This will make more power sooner than a LC/HB setup.
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Old 09-25-2008, 09:59 PM   #74
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The reason a HC/LB would make more power off less boost is cause it is more efficient. This also helps spool the turbo earlier, since it will have more heat and velocity sooner. This will make more power sooner than a LC/HB setup.
I agree. And I believe it will therefore be a much more streetable car. Seems like to me JC Rotor is dancing around this notion, that a HC/LB car will be much more likely to win a race on the street, if you condone that sort of thing.
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Old 09-25-2008, 10:06 PM   #75
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Originally Posted by ray_sir_6 View Post
The reason a HC/LB would make more power off less boost is cause it is more efficient. This also helps spool the turbo earlier, since it will have more heat and velocity sooner. This will make more power sooner than a LC/HB setup.
Well, I guess I whole heartedly disagree with this. I could be wrong though, but to me, it doesn't make sense. However, I'm no engineer. But from what I do know, I think that's wrong. You saw my example in post 70 to support what I think. Care to explain how the higher comp motor makes higher EGTs and EGV's? I'm all ears.

Here's another bit to support my reason of thinking. How does antilag work to spool a turbocharger? By cutting spark, the engines efficiency takes a ****, but the energy in the exhaust skyrockets and spools the turbo.
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Old 09-26-2008, 01:09 AM   #76
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Originally Posted by ray_sir_6 View Post
Generally, compression ratio should be set as high as feasible without encountering detonation at the maximum load condition. Compression ratio that is too low will result in an engine that is a bit sluggish in off-boost operation. However, if it is too high this can lead to serious knock-related engine problems.

In bold so noone misses it. That is what this thread was asking, so that is the CORRECT answer.
Notice they dont explain this at all. It's mentioned casually in an article about something else entirely. I'm still dubious.

dont get me wrong though. I have a 10:1 comp turbo motor at low boost making good power. Maybe for ***** I'll put my current head back on the motor when I build it with the 8.5:1 pistons and see what happens. No other changes. As long as you agree that changing rods wont affect power measurably.
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Old 09-26-2008, 02:53 AM   #77
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Note that they talk about OFF BOOST operation, NOT spoolup.
I still say it's 4% per point. If you lower it 2 points, that's 8%, and that WILL be noticeably more sluggish.

I think the myth came about because people confuse off boost operation and spoolup.

patsmx5, that's a good point about anti-lag.

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Originally Posted by ray_sir_6 View Post
Even the "fact" that Jason C is trying to say that he tested his engine after the LC pistons were installed and got almost the same power, yet had faster spool, only shows one thing...that his HC pistons were fucked, either his rings were shot, or a piston was cracked, etc.
Nope. The bone stock dyno was with ~5000 miles on a brand new stock car, and the lowered c/r was with ~1000 miles on a rebuilt motor. That stock motor went on to make 240 whp at 10 psi.
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Old 09-26-2008, 03:14 AM   #78
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Even the "fact" that Jason C is trying to say that he tested his engine after the LC pistons were installed and got almost the same power, yet had faster spool, only shows one thing...that his HC pistons were fucked, either his rings were shot, or a piston was cracked, etc."
That or you are wrong.
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Old 09-26-2008, 03:27 AM   #79
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Just throwing this out there, but a motor with lower compression will be flowing more air under boost, yes? (if we're discussing similar or identical power output) The higher comp. motor is simply doing more with the available air, thus the need for less boost (as we're assuming). Coming off vacuum to boost may or may not favor the high comp motor. When approaching boost, the higher comp motor deals with higher cylinder pressures, but there may be less gas pressure in the turbine, due to the lesser amount of air being consumed (maybe not initially, but as the boost builds). The lower compression motor will demand more air as it spools, which may result in lower cylinder pressures, but there is more gas pressure in the turbine housing from the higher amounts of airflow being demanded.

So spool-up with lower compression could be quicker when it comes to pressurizing the turbine, which is what matters when you want to make boost. The higher volumes of gas in the lower compression motor will cause quicker spool once the turbo begins to spin up.

The bearing that cams have on all this, is the amount of air let into and out of the cylinder. You need to flow more air so the motor works more efficiently on spinning your turbo. In turn, the turbo will give you the boost you need to make the power you want. So if someone changes cams and lowers compression, I think they'd be introducing more pressure in the turbine housing, which causes more boost in more of a hurry. Duration and overlap play on this a great deal, but my point is that if you have the right cams, then you can better spool the turbo on a lower compression motor vs. a higher compression motor. I think...

Unless I missed something. I mean. Its 1:30 in the morning and I've been up for almost 24 hours. Maybe I just pulled all that out of my ***... :shrug:
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Old 09-28-2008, 11:26 PM   #80
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Just throwing this out there, but a motor with lower compression will be flowing more air under boost, yes? (if we're discussing similar or identical power output) The higher comp. motor is simply doing more with the available air, thus the need for less boost (as we're assuming). Coming off vacuum to boost may or may not favor the high comp motor. When approaching boost, the higher comp motor deals with higher cylinder pressures, but there may be less gas pressure in the turbine, due to the lesser amount of air being consumed (maybe not initially, but as the boost builds). The lower compression motor will demand more air as it spools, which may result in lower cylinder pressures, but there is more gas pressure in the turbine housing from the higher amounts of airflow being demanded.

So spool-up with lower compression could be quicker when it comes to pressurizing the turbine, which is what matters when you want to make boost. The higher volumes of gas in the lower compression motor will cause quicker spool once the turbo begins to spin up.

The bearing that cams have on all this, is the amount of air let into and out of the cylinder. You need to flow more air so the motor works more efficiently on spinning your turbo. In turn, the turbo will give you the boost you need to make the power you want. So if someone changes cams and lowers compression, I think they'd be introducing more pressure in the turbine housing, which causes more boost in more of a hurry. Duration and overlap play on this a great deal, but my point is that if you have the right cams, then you can better spool the turbo on a lower compression motor vs. a higher compression motor. I think...

Unless I missed something. I mean. Its 1:30 in the morning and I've been up for almost 24 hours. Maybe I just pulled all that out of my ***... :shrug:

I agree that it appears this way on the surface, but there are things that you are leaving out. #1 The LC/HB application is moving more air volume, but once the piston hits TDC, the total pressure in the cylinder is identical. This is because of the dynamic CR directly( HC/LB vs LC/HB). So, if you have two tubes, with equal pressure and one has 30% more volume, the lower volume is going to travel through the tube faster than the higher volume. However, the higher volume will be spinning the turbine for a longer duration. This would be if the gases in the cylinders were identical in behaviour, but because of the differences in efficiency and burn cycle you cant exactly compare them. The reason the higher CR motor spools faster simply has nothing to do with that though. It has to do with making more power off boost, and revving faster than the LC motor. If you were to put them both in the same gear and stomp it, the HC motor is going to reach a higher RPM in the same timeframe, because of the increased power and efficiency, therefore having physically more exhaust volume than the LC motor, no matter which way you slice it. If after 2 seconds, the LC motor is at 2500 rpms, and the HC motor is at 3000, the HC is making more power, more boost, and overall is just much faster.

Does that make it clearer Pat?

Like I said before, there are other factors such as cam timing and ignition timing that can make the effect even more dramatic, but this is the basic principle behind the reason that HC/LB cars are faster.
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