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Old 01-30-2011, 10:23 PM   #1
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Default High compression and low boost?

Hello all,

given the recent release of the 5.0l ford "coyote" motor, and the even more recent advent of supercharger/forced induction systems available for it, my mind got to working.

How is it that a factory stock 11:1 motor can withstand 10-15 psi of boost, do it on 93 octane, and do it happily? The general consensus is that the newer ford PCM is a gem in the tuning world, which is the leading/major factor, (aside from a competent tuner), that is resulting is the 550-600 horsepower five liters that are coming on to the scence.

I was reading in a popular magazine, (one that i choose to refrain from naming just to negate any personal opinons), and come across the following sentence. "The out-of-the-box high compression on these cars , (5.0 mustangs) means that they need to see very little boost to see HUGE power gains. Not to mention that it increases the overall efficiency."

So, if one were to properly build a high compression BP motor, (balanced and blueprinted 97 bottom end, carillo a-beams, wiesco 11:1 pistons, ported and polished 99 head, head and main studs, with a Hydra 2.6 running it all for instance), and then implemented a low boost super/turbo system , (5-7psi), and burn it on e85, could you then see significant gains as well?

Just some food for thought. Hoping to tap into the vast wealth on knowledge here

Thanks all
Glade

p.s. first post
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Old 01-30-2011, 10:34 PM   #2
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i am planning on that.

custom weisco pistons to be turbo spec at 10:1 compression are $145 each fyi. and they would be good for up to 30 pounds assuming the tune is good.

Matt
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Old 01-30-2011, 10:43 PM   #3
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I'm liking this topic, but what I want to get out of this is: As far as low boost, are we talking a big turbo (GT3071) with low boost, or a small turbo (GT2554) with low boost?
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Old 01-30-2011, 10:50 PM   #4
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i was plannin on the 2554.

I know that a larger turbo will generally result a lower intake temperature, but at the loss of some "spool" time. (spool time being a relative term that i am in no way hoping to start a flame war over)

However, given the availablility of intercoolers, and the fact that i would be using e85, i was hoping the the smaller housing of the 2554 wouldnt heat the intake air up enough to negate the effects of the previous mentioned, while retaining the quick "spool" characteristics.
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Old 01-30-2011, 11:01 PM   #5
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This is an interesting dilemma, because I feel that running a turbo at low boost is a waste of the turbo's abilities.
Another fact that needs to be brought out is that 7psi on a 2554 will not come close to the power of 7psi on a 3071. Same boost, but more cfm's which will also require more fuel, and the intake temps will also differ. Spooling will also be different and with the already higher compression pistons, it'll be more octane sensative and you won't be able to run as much timing. If I did a high compression piston build for boost, I would definately run a small turbo.
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Old 01-30-2011, 11:32 PM   #6
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1) There are multiple way to skin a cat.
2) I don't care to track a high-comp turbo engine
3) I don't care to pay for a competent computer that I'll need for knock control on the high-comp motor.
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Old 01-30-2011, 11:34 PM   #7
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I have been harping on this for years.

For what ever reason, it has been taboo to use high compression pistons in any kind of forced induction motors. I believe this has been from the fact that builders were trying to get the most power out of motors using some form of forced induction. To get that power, builders were using extremely high boost. However, there were problems associated with the high boost amount, and it was corrected by lowering the compression ratios.

The problem is that not everyone is attempting to make world record breaking power, but the stigma of using lower compression stuck in the motor building world.

Since I am a nitrous user, I will use it as an example.

A typical Pro Mod car will have like a 1200 hp shot of nitrous, maybe more, and in order for the motor to inhale the nitrous and not have serious detonation problems, the engine builder will use a lower compression ratio. I am not sure what they are using, but let's say it is 9:1 compression. On the contrary, a Pro Stock car that is normally aspirated, will use an extremely high compression ratio like 18:1 (I know for a fact that is the correct compression).

Most of us are somewhere in the middle of the the extremes, so does it not make sense that we should be somewhere in the middle with our compression ratio?

A small turbo motor should have more compression than a large turbo motor to maximize the potential output of that motor. The problem is there are few willing to do the development needed to see what is the highest compression allowed for a particular turbo without paying the hefty price of engine failure. There are other variables like the fuel one is using, and amount of camshaft, but somehow people got in a '9:1 ratio for any kind of forced induction', and '12:1 ratio for anything normally aspirated' without thinking of a realistic happy medium.
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Old 01-30-2011, 11:58 PM   #8
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10:1 comp + 10 psi on a pretty large turbo.

the power curve looks almost exactly like a 9:1 twin turbo miata setup.

go figure.

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Old 01-31-2011, 12:02 AM   #9
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Old 01-31-2011, 12:22 AM   #10
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Unboosted A 5.0 flows a shitload more air than a 1.8. Double your flow and double your hp. Turbo make power by flow not psi so if 5.0 or s2k or whatever heads flow more to start with, it will be easier for them to make power at a lower psi-- given the same turbo.



This is about much more than compressionratio.

Last edited by Faeflora; 01-31-2011 at 12:30 AM.
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Old 01-31-2011, 12:29 AM   #11
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High compression and boost or power adders has been going on forever. In a street car it is a lot easier to run low compresion and deal with runinng more boost than it is to worry about a really really good tune. There are pro-mod cars and OPS 10.5 cars runing 14:1 with nitrous or boost. The major difference is the tune has to be dead on. With a low compression motor you have more leway with tuning where as with a high compression motor if something goes wrong it goes wrong even faster than normal. Higher compression with less boost equals same power with lower intake temps.
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Old 01-31-2011, 12:51 AM   #12
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Please don't take this as ignorance, this is more of me trying to learn more. Here we go: Boost is measured in PSI which is Pounds Per Square Inch. The higher the PSI, the more compressed the inlet air is, also the higher the intake temps will be. Right?
That being said, with a high comp engine you will make more power with less boost and keep your intake temps low? The higher compression from the pistons and the added boost will lead to slightly more fueling and different timing as compared to lower compression...BUT, you should make more power because of the higher compression and lower intake temps.?? Right??
If this is so, let's say I make 235 whp on my stock '99 engine with my SR20 T25 on 14psi. This is a stock engine, and it's just about to max out the efficiency of my turbo.
Now let's say I put 11:1 pistons in there and run 10psi.
Will I make more power?
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Old 01-31-2011, 12:55 AM   #13
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My budget allowed me to build an engine the first time - and no more.

My protection against not having a running engine is low compression pistons.

Ford's budget allows Ford to build many engines.

Ford's protection against not having a running engine is the next engine.
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Old 01-31-2011, 01:23 AM   #14
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Quote:
Originally Posted by fooger03 View Post
My budget allowed me to build an engine the first time - and no more.

My protection against not having a running engine is low compression pistons.

Ford's budget allows Ford to build many engines.

Ford's protection against not having a running engine is the next engine.
well put
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Old 01-31-2011, 01:58 AM   #15
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Direct Fuel Injection
Alot of these newer cars have DI, allows more pericse fuel injection directly into the combustion camber, allowing higher compression turbo setups to be acheived from the factory.

You also may noticed with it alot of the oem cars with DI\turbo have higher AFR's compared to cars with Multi port fuel injection with turbos setups.
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Old 01-31-2011, 02:17 AM   #16
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Why don't you just build a diesel and be done with it?
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Old 01-31-2011, 02:47 AM   #17
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Quote:
Originally Posted by Techsalvager View Post
Direct Fuel Injection
It saddens me greatly that it took 15 posts to point out that all of the new uber-compression+boost motors are direct injection.
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Old 01-31-2011, 02:51 AM   #18
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Quote:
Originally Posted by thirdgen View Post
Please don't take this as ignorance, this is more of me trying to learn more. Here we go: Boost is measured in PSI which is Pounds Per Square Inch. The higher the PSI, the more compressed the inlet air is, also the higher the intake temps will be. Right?
That being said, with a high comp engine you will make more power with less boost and keep your intake temps low? The higher compression from the pistons and the added boost will lead to slightly more fueling and different timing as compared to lower compression...BUT, you should make more power because of the higher compression and lower intake temps.?? Right??
If this is so, let's say I make 235 whp on my stock '99 engine with my SR20 T25 on 14psi. This is a stock engine, and it's just about to max out the efficiency of my turbo.
Now let's say I put 11:1 pistons in there and run 10psi.
Will I make more power?
It's the opposite - IAT dictates how much heat is pumped into the combustion chamber. You can remove the heat from 14psi of boost with an intercooler - it's much harder to eliminate the additional heat that comes from bumping the compression to 11:1. This is why we run less timing with higher compression motors - the added heat means they are more prone to detonation. Less timing = less power, period. If you are detonation limited, it is ALWAYS better to lower compression and increase boost until you are no longer detonation limited - this is why Trey and me and others with big hotside low compression setups can run big timing and big torque safely.

Ethanol changes the game - since it's so much more detonation resistant, you can turn the compression back up a bunch. If I ever build an E85-specific motor, it will get 11:1 slugs. I am running ~17 degrees of timing at 17psi right now - I did a pull or two at 21 and it just lost power, no ping.
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Old 01-31-2011, 09:20 AM   #19
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Quote:
Originally Posted by glade View Post
i was plannin on the 2554.

I know that a larger turbo will generally result a lower intake temperature, but at the loss of some "spool" time. (spool time being a relative term that i am in no way hoping to start a flame war over)

However, given the availablility of intercoolers, and the fact that i would be using e85, i was hoping the the smaller housing of the 2554 wouldnt heat the intake air up enough to negate the effects of the previous mentioned, while retaining the quick "spool" characteristics.
Quote:
Originally Posted by thirdgen View Post
This is an interesting dilemma, because I feel that running a turbo at low boost is a waste of the turbo's abilities.
Another fact that needs to be brought out is that 7psi on a 2554 will not come close to the power of 7psi on a 3071. Same boost, but more cfm's which will also require more fuel, and the intake temps will also differ. Spooling will also be different and with the already higher compression pistons, it'll be more octane sensative and you won't be able to run as much timing. If I did a high compression piston build for boost, I would definately run a small turbo.
This is a big thing in the subi world, the one thing i have to say is the last thing you want is a high comp and small turbo. Running a small turbo on high comp will make for high temps. Thats the one thing you dont want on a high comp build as you will be knocking out of control. I would run no smaller then a gt3071r. The high comp will spool a turbo faster then a lower comp will.
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Old 01-31-2011, 10:05 AM   #20
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Quote:
Originally Posted by Savington View Post
It saddens me greatly that it took 15 posts to point out that all of the new uber-compression+boost motors are direct injection.
Honestly...


These things are highly efficient motors. The BP is not.

Hell, the 5.0L makes 400HP on 11:1 pistons on 87 octane with zero boost. You should see how smooth the intake runners are everything is. VVT on all 4 cams. a bunch of ****.





Also, thirdgen, PSI is measurement of RESTRICTION, nothing more.
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