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Why not have a Fox?
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So i've got some questions about turbo and superchargers. I know the basics of how they work, but id like some things cleared up. im not thinking of adding either of these to my car, (at least not for a while) just curious. here goes...
1 Why do people lower compression when they use either of the 2? I thought higher compression = more power. Does it have to do with detonation?

2 i thought a supercharger crams more air/fuel into the cylinders, which increases compression, doesnt it? if so, why lower the compression just to gain it back?

3 Why do you retard/advance the timing?

Thanks for everyones imput! guys here are great!
 

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Exhaust fume addict
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1. Detonation
2. Detonation
3. Detonation, and you retard the timing when adding boost

This is grossly oversimplified of course but its pretty much what it all comes down to
 

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So i've got some questions about turbo and superchargers. I know the basics of how they work, but id like some things cleared up. im not thinking of adding either of these to my car, (at least not for a while) just curious. here goes...
1 Why do people lower compression when they use either of the 2? I thought higher compression = more power. Does it have to do with detonation?

2 i thought a supercharger crams more air/fuel into the cylinders, which increases compression, doesnt it? if so, why lower the compression just to gain it back?

3 Why do you retard/advance the timing?

Thanks for everyones imput! guys here are great!
Adding boost does add compression. I don't know about purposefully lowering compression just because you are adding boost but at least take whatever compression you have and account for it when adding boost.

You can do it how you want but the end result in power is effectively the same.
Low static compression and high boost, med static compression and medium boost, or high static compression and low boost.

Advantages of low static compression (or normal average compression) and high boost is you can use "cheaper" valve train components that can handle the lower compression and stress and regular gas station octane fuel. I don't know if cheaper is a good term but there is less stress on the valve train with lower compression.

If you are going to pay the prices for a reliable high compression high revving engine, it makes less sense to also by a turbo or a supercharger. A boost device can overcome the low static compression by you adding more boost. It results in a different type of power curve but it still is power.

You reduce the timing to prevent detonation. The act of compressing air in a turbo or SC heats the intake air temperature, so does a high static compression. The combination of the two is not good. These put you closer to pre ignition. Backing off on the timing reduces the risk of pre ignition. Cool your intake air or have a lower compression and you can back off less timing.

I've never had, built, or maintained a high compression engine so I dont know if the disadvantages, stess, and extra heat generated are actually real world issues or not but I do know coupled with boost, fuel reqirements and preignition would be.
 

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Vey well put^^, saved me a bunch of typing. Yes you can supercharge high compression, look at all the coyotes with turbos and blowers, they are 11 to 1 stick. Same with ls1's. Detonation is prevented by: low compression, higher octane, cooler air charge temps, or less timing. You get high comp and boost, you need some of these to keep out detonation. Make sense?
 

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In a lot of cases, the extra boost will make more power than higher compression. So people use lower compression pistons in order to run more boost = more power. Boost doesn't make the compression ratio higher, the only way to do that is to physically change the parts that compress the air/fuel mix (pistons & combustion chambers).
 

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I would guess not much more than 5 psi on pump gas without pulling a bunch of timing
 

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I'm sure more could be done with careful R&D and lots of tuning
 

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There's a lot of things that decide how much boost you can run so it's kind of hard to say. For example in the vw world 15lbs of boost isn't uncommon at all. On pump gas.

The amount of boost pressure also doesn't automatically equal power. The best example is a straw and a large tube. If you blow through the straw there will be high pressure but not a ton of air flow. Now blow through the larger tube. And you'll have lower pressure but more air flow.


Sent from my iPhone using AutoGuide.com App
 

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There's a lot of things that decide how much boost you can run so it's kind of hard to say. For example in the vw world 15lbs of boost isn't uncommon at all. On pump gas.

The amount of boost pressure also doesn't automatically equal power. The best example is a straw and a large tube. If you blow through the straw there will be high pressure but not a ton of air flow. Now blow through the larger tube. And you'll have lower pressure but more air flow.


Sent from my iPhone using AutoGuide.com App


An engine is a big air pump. The goal is to get a higher volume of air through the engine. With more air volume passing through, you can add more fuel and then get more power. Adding more fuel is the easy part, larger injectors and a pump and your done. How do you get more air in the engine? Some combination of more cubic inches, higher rpm, less restrictive intake/exhaust, higher compression, or force it in there via NO2 or with turbo/supercharger. The cam selection along with timing and specific A/F ratios are what ties the whole combination of parts together so you get the desired max/average/flat/low/high torque and HP curves you are looking for.

About that boost.. more boost or pressure in the intake = more airflow into the cylinders but on the same hand, more total airflow results in less measured boost. Does that make sense? Boost is the build up of what is left over that did not make it into the cylinders. Here is one way to look at it. With your mouth, blow through a narrow coffee stir straw, your mouth has a high pressure (boost) but there is a low flow at the end of the straw. If you could blow harder [insert joke here], your mouth pressure will go up and more airflow will go through the straw. Now blow through a paper towel roll. You have less pressure in your mouth but more air flow goes through the tube. Another way.. A car with stock E7 heads might gnerate 10 psi of boost, put on a nice set of heads and a cam and you may now only see 7 psi of boost but you will be making a lot more power because air flow went up. This is the reason you see small 4 cylinder engines running 15-25 psi and they are about equal in HP gain as a larger v8 only running 5-10. More cubic inches therefore less boost pressure needed to get the same gains in airflow. The 4 clyinder is the coffe stir and the v8 is the paper towel tube.
Pulled this from another thread.
 

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so if you had, say 10:1 compression could you add a little boost and get the same result as having 8:1 and adding more boost?
No. All things being equal, if you ran both combos to their safe volume of air/fuel they can handle the 8:1 would dominate. It would ingest way more air/fuel mix resulting in more power.
 

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Why not have a Fox?
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Pulled this from another thread.
Thaks. That makes sense! Now, i have no clue how nos works, anyone want answer that one? Sorry for the noob qestions, ive only worked with 40-50's cars that are stock so im not very bright when it comes to super/turbo/nos
 

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No. All things being equal, if you ran both combos to their safe volume of air/fuel they can handle the 8:1 would dominate. It would ingest way more air/fuel mix resulting in more power.
Ive had different experience with this. Unless you are saying on 93 oct, then I would tend to agree.
 

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Thaks. That makes sense! Now, i have no clue how nos works, anyone want answer that one? Sorry for the noob qestions, ive only worked with 40-50's cars that are stock so im not very bright when it comes to super/turbo/nos
Nitrous works by the nitrous liberating extra oxygen in the cylinder allowing more fuel to be burned, making more power. Very simple aanswer, your lucky Im on my phone and not by a keyboard or the answer would have gotten windy, lol.
 

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Why not have a Fox?
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Discussion Starter #19
Nitrous works by the nitrous liberating extra oxygen in the cylinder allowing more fuel to be burned, making more power. Very simple aanswer, your lucky Im on my phone and not by a keyboard or the answer would have gotten windy, lol.
haha, im ok with windy answers! i like reading as long as it has to do with fast cars, drums, and women! lol!
 
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