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Old 03-22-2012, 12:09 PM   #41
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By "baller" I meant he's not running a log with a 2.5" exhaust lol
And its funny you should say that, because somebody is soon going to try
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Old 03-22-2012, 12:58 PM   #42
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Originally Posted by Full_Tilt_Boogie View Post
Its BMEP, and yes that is very important, but revs are important too.
An engine at 300 psi BMEP at 4000 RPM and another engine making 300 psi BMEP at 8000 RPM, which one has more stress on it?
If the 2 motors have the same displacement, the motor making 300 BMEP at 8000, will make twice the power of the other one.

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Now if were strictly talking about knock limitations, then yes, between equal engines, the BMEP is going to indicate said limit.
V.E. is the limit. Around 210 psi BMEP for an n/a motor, or 75 ft-lbs per L per atmo, is 100% VE. You can very well have maxed out on your BMEP and be nowhere near the knock limit, by say, using E85.

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The problem is that were not talking about a simple comparison, such as different CR or boost with all else being equal.
Changing boost changes compressor efficiency,
No it doesn't.

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and changing compression changes quench.
Yes but quench affects knock resistance (which may not be an issue), and can increase burn speed (which can increase combustion efficiency).
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Old 03-22-2012, 02:03 PM   #43
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Interesting read.....can anyone elaborate on this?


Combustion Chamber Dynamics
A cool charge may be the first step toward utilizing a higher CR, but what happens in the combustion chamber can make or break any such efforts. A prime factor here is never to loose sight of the fact that the faster the charge can be burned the higher the compression the cylinder will stand. Chamber cavities between the piston and the cylinder head between about .060-inch - .0120-inch appear most likely to be the site of detonation. Speeding up combustion mixture motion/agitation is vital. This means maximizing the quench action. On a small-block Chevy with a stock block height, a stock compression height piston is typically .025-inch down the bore. With a .040-inch gasket this makes the static quench clearance .065-inch, which is way too wide. By cutting the quench clearance the burn rate and quality improve to the point where the motor gains compression and is less likely to detonate even at the higher ratio involved.

So how closely can the pistons approach the head face? Although it comes under the heading of "don't do this at home" I have run the static piston/head clearance down to as little as .024-inch in a 350 with stock rods and close-fitting hypereutectic pistons. The pistons just kissed the head at about 7,000 rpm. As far as power is concerned, an associate of mine ran some tests in a nominally 450-horse 350 and found that each 10 thousandths of quench reduction was worth approximately 7hp. If you are building from scratch, make maximizing the quench your number one priority toward achieving compression and avoiding detonation.



Read more: http://www.popularhotrodding.com/tec...#ixzz1prkxGm41

Edit: Based on engine builder's specs my piston to head clearance should be .045"-.05". Hope I quench with 10:1

Last edited by Boost Joose; 03-22-2012 at 02:55 PM.
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Old 03-22-2012, 06:29 PM   #44
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Quote:
Originally Posted by thirdgen View Post
Because 8psi on a big turbo is not the same as 8 psi on a small turbo. PSI is compressed air measured in pounds per square inch. CFM is cubic feet per minute. Although the psi's are the same, the amount of cfm's with a bigger turbo will be greater.
The higher the psi's (compressed air), the higher your IAT's should also be. Hence, a big turbo will move a higher volume of air (CFM's) with less boost because of turbo size, plus IAT's should be lower.

This answers a question I did not ask. So let me rephrase: Measured at the intake manifold, 10psi at +10 degrees F ambient is 10psi at +10 degrees F, regardless of what compressed the air. There may be differences in exhaust backpressure that will affect measured power output levels, but keeping all other hardware the same, the engine doesn't care what/how the PSI is created.

Note I am specifically not talking about compressor outlet temperatures, nor compressor wheel efficiencies.
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Old 03-22-2012, 06:37 PM   #45
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The air is more dense, and there is more of it. Restriction (ie: boost) is just 1 part of the equation. The other is flow. Would you be able to push more air through a garden house or a coffee straw? Assuming same pressure?
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Old 03-23-2012, 12:24 AM   #46
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...

I understand the difference between flow and pressure. 10psi through a garden hose != 10psi through a coffee straw. But, we are not discussing dissimilar flow rates - we are discussing a comparison of the same engine with different turbos.

Please note the initial conditions of the problem. 10psi measured at the intake manifold. IAT's equal. Same engine. Same RPM. Given those constraints, there are no variables left to change. Flow rate cannot be different unless different laws of physics are in effect. Any power differences between the two setups must be due to differences in the power lost driving the turbo.

I understand that a bigger turbo will flow more air at a lower boost pressure than a smaller turbo. I can read a compressor map. But that is not a part of this discussion. The claim is that for the same engine, with nothing else being changed, 8psi from a bigger turbo will generate more power than 14psi from a smaller turbo, because there is actually more CFM going through the motor. This reasoning is broken.


Edit: I understand that the exhaust backpressure caused by the different turbos will be different. There are two consequences for the turbo with higher backpressure - a) more power is required to drive it, to overcome the backpressure, and b) that higher backpressure will also reduce the CFM at any given boost pressure. So it's a double whammy to overdrive a smaller turbo.

Last edited by Mobius; 03-23-2012 at 12:37 AM.
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Old 03-23-2012, 01:03 AM   #47
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Originally Posted by JasonC SBB View Post
No it doesn't.
Yes, it does. Anyone who's spent 30 seconds with a compressor map can see that the compressor efficiency varies with pressure ratio.
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Old 03-23-2012, 01:08 AM   #48
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Quote:
Originally Posted by 18psi View Post
The air is more dense, and there is more of it. Restriction (ie: boost) is just 1 part of the equation. The other is flow. Would you be able to push more air through a garden house or a coffee straw? Assuming same pressure?
The asterisk to this is that the coffee straw is the turbine housing on the turbo. The reason that big turbos make more power at similar boost than small turbos is because the turbine inlet pressure is lower. Lower TIP = lower chamber pressure as the exhaust valve closes = more pressure delta between the IM and the chamber as the intake valve opens = more flow = more power.
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Old 03-23-2012, 03:27 AM   #49
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I only read the first post. I deleted my nice long reply since I'm drunk an dp figured no one would understand.

Essentially, the F20c enigne has Vtec. I bet you're laughing at this statement. When Vtec engages, it greatly lowers the dynamic comrpession ratio by having crazy overlap that most engines wont have unless they have crazy aftermarket camshafts. In effect, the static 11:1 commpression means dick on a F20c. Add in a nice high octane fuel such as E85 with a nice free flowing tubular manifold, and your worries about an 11:1 compression ratio go out the door. Hell, I'm running 28psi on my stock F20c and it runs great with Vtec set at 4,800 rpm.

How this compares to a Miata engine, I do not know. I just wanted to clarify why a stock 11:1 compression ratio F20c is a capable engine and that it doesn't necessarily correlate to other engines.
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Old 03-23-2012, 04:11 AM   #50
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Quote:
Originally Posted by Savington View Post
The asterisk to this is that the coffee straw is the turbine housing on the turbo. The reason that big turbos make more power at similar boost than small turbos is because the turbine inlet pressure is lower. Lower TIP = lower chamber pressure as the exhaust valve closes = more pressure delta between the IM and the chamber as the intake valve opens = more flow = more power.
Isn't this what I said? Although said better. My 11.5% Abyss Stout has had its way with me, and I'm going to bed.
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Old 03-23-2012, 01:28 PM   #51
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Quote:
Originally Posted by Savington View Post
Yes, it does. Anyone who's spent 30 seconds with a compressor map can see that the compressor efficiency varies with pressure ratio.
Yes, you are correct. I was thinking along the lines of small changes and as a first approximation, efficiency doesn't change with small changes in boost.

Efficiency can go up or down with increased boost depending on where you are initially on the compressor map.
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Old 03-23-2012, 01:32 PM   #52
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Quote:
Originally Posted by D1sclaimer View Post
I only read the first post. I deleted my nice long reply since I'm drunk an dp figured no one would understand.

Essentially, the F20c enigne has Vtec. I bet you're laughing at this statement. When Vtec engages, it greatly lowers the dynamic comrpession ratio by having crazy overlap that most engines wont have unless they have crazy aftermarket camshafts.
You are not using the concept of dynamic compression ratio properly.

What lots of overlap means for fixed-cam-timing motors means is that the cylinder pressures at torque peak are lower, reducing chances of detonation.

If you are comparing VTEC with non VTEC, a VTEC motor at 6000 RPM will be more detonation prone *at 6000 RPM* than a non VTEC motor, due to greater cylinder pressures from the greater VE.

There is also the factor of time (RPM). For the same cylinder pressure, a motor is less prone to detonation at higher RPM than at lower RPM.

So a motor with a higher peak torque RPM, for the same peak VE (and thus peak torque), will be more detonation resistant than a motor with a lower peak torque RPM. Ergo the former can run a higher CR.
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Old 03-23-2012, 01:34 PM   #53
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Quote:
Originally Posted by D1sclaimer View Post
I only read the first post.
...
...
How this compares to a Miata engine, I do not know. I just wanted to clarify why a stock 11:1 compression ratio F20c is a capable engine and that it doesn't necessarily correlate to other engines.
I'll tell you how: it makes us sad.
Miata motors are in every way inferior to the f20c, so we either
1) be happy with sub 300
2) invest insane amounts of money to go higher
3) sell miata and buy s2k
4) swap f20c into miata
5) bitch about it on the forums and cry ourselves to sleep at night.
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Old 03-23-2012, 01:35 PM   #54
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Quote:
Originally Posted by 18psi View Post
The air is more dense, and there is more of it. Restriction (ie: boost) is just 1 part of the equation. The other is flow.
The equation is actually *only* flow. We are attempting to estimate flow from boost.

The first approximation is that flow is proportional to MAP (and to VE).

If VE goes up a lot, and boost goes down a little, you will have more flow (and thus power)
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