a question about compression

935 Views 11 Replies 6 Participants Last post by Stephen Webb, Nov 26, 2001

sporter

Discussion Starter · Nov 26, 2001

alright so maybe one of you smart guys can help out a n00b:
let me get this straight: a certain PSI of boost on a high-comp motor will make more power than that same PSI on a low-comp motor. OR in other words you need more boost out of a low comp motor to acheive the same power level of a high-comp motor.
the only reason people lower the comp is to keep an "error" of margin, correct? but Im not getting something here! IF say that 8lbs on 10:1 motor is the same as 12lbs (for the sake of argument) on a 8.5:1.... I seriously dont see the point of this! why not just run lower boost on your 10:1 engine!!! (to be safe) whats the point of lowering comp if you just have to run more boost to keep up with the 10:1 guy at 8lbs? Set me straight on this please! Isnt the combustion chamber pressure the same in either case? so why would lower comp be safer or better????
tia

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Go Go Green Golf

· #2 · Nov 26, 2001

Re: a question about compression (sporter)

Sean the only advantage to low-comp I can see is to be able to run more ignition advance. I know with my setup the knock sensor can come into play pretty fast and retard way to much timing out of the equation. Now running 8 psi on a stock compreession motor vs running 12psi on a low-comp motor would the compression in the cylinder be the same? The calculator says yes right? Maybe the ECU would keep the ignition advanced longer but I think that even then the knock sensor would come into play. Having enough fuel with proper A/F is the key to the equation no matter which way IMHO.

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sporter

Discussion Starter · #3 · Nov 26, 2001

Re: a question about compression (Go Go Green Golf)

so is this the only reason people go low-comp? for how many people that do it thats seems like not such a big reason, considering we dont even know if it really does affect timing advance.

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Hardcore VW

· #4 · Nov 26, 2001

Re: a question about compression (sporter)

Your right in your thinking,but cylinder pressure is only part of the equation,with boost usually comes more airflow which is going to pick up power moreso than compression alone,it's all a fine balance with air and what fuel you have available really.

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MadGT1

· #5 · Nov 26, 2001

Re: a question about compression (Hardcore VW)

anyway of lowering compression on a 3A 2.0 block?? I don't want the double gasket idea cuase if the gasket fails then i am screwed.

Any other methods and what does it cost ..
All i know is lower compression = higher boost

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nabilsx

· #6 · Nov 26, 2001

Re: a question about compression (MadGT1)

Dish ur Pistons , or if u have the money buy JE Forged pistons

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MadGT1

· #7 · Nov 26, 2001

Re: a question about compression (nabilsx)

What are the pros and cons in dishing the pistons?
I heard it will cause knocking...

Anyone with more info on this one .

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nabilsx

· #8 · Nov 26, 2001

Re: a question about compression (MadGT1)

Why would it cause Knocking?? when u Dish the piston ur Lowering the CR and Lower CR means decreasing the chance of having knocking in ur engine. Ok lets get this straight u a dish of 6.19cc will drop it 1 point on a 1.8 16V, 6.14 is 1 point on an ABA. So between 6-7cc this I was told by VWPAT and he knows his stuff, so if u having the fueling issue taken care off and u dish the pistons to lower ur CR then unless u go really high boost "I say +16psi" then why would it knock, if u can have a mean to control ur timing alos that would be great.
hope this helps

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Stephen Webb

· #9 · Nov 26, 2001

Re: a question about compression (MadGT1)

quote:[HR][/HR]What are the pros and cons in dishing the pistons?
I heard it will cause knocking...
[HR][/HR]

Dishing the pistons is actually the preferred way of reducing compression (as compared to thicker head gaskets). In particular, dishing the pistons retains the squish band area which is important for knock control. The material is (usually) taken out of the center part of the piston, and the edge is left alone - with a double stacked head gasket (or similar hack), the you change the squish band between the piston and the head, which can lead to knocking.
-Steve

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Stephen Webb

· #10 · Nov 26, 2001

Re: a question about compression (sporter)

quote:[HR][/HR]so is this the only reason people go low-comp? for how many people that do it thats seems like not such a big reason, considering we dont even know if it really does affect timing advance.[HR][/HR]

Two things come to mind.
More advance keeps the power up and the exhaust valves cooler. Too much retard and the energy goes to heating up the exhaust valve instead of doing work on the pistons.
Also (As Hardcore pointed out), more boost / less compression isn't the same as less boost/more compression. With more boost comes more flow, and more fuel. I believe this changes the combustion properties so your burn is longer, and you average cylinder pressure is higher even if your peak pressure is the same.
If the agument that more compression/less boost made the same power as less compression/more boost, you could take it to the extreme and run 13:1 with no boost.
If I can locate the source, I will point you to it (I may have read it in "Maximum Boost", or "Turbochargers"). I read about a study of compression/boost under controlled conditions (ie same fuel, same ambient temp, etc). Compression was systematically decreased, and boost was increased to achieve maximum power for a given compression ratio. Maximum power (on this particular test engine) was achieved at ~5:1 compression (if memory serves), and some very high boost level.
Throttle response, boost threshold and overall drivability rendered such a low compression (Yet high power) engine not suitable for the application.
So, you can safely make more power if you lower compression, but you must keep in mind the consequences of lowering compression (degraded fuel economy, decreased off-boost drivability)
$0.02
-Steve

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MadGT1

· #11 · Nov 26, 2001

Re: a question about compression (sporter)

Ya i know what you mean .... i was just told dishing the piston was not good and your engine won't last aslong ... along with knocking ...
I need to hear what you guys think of this....
Any ideas how much it would cost to dish???

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Stephen Webb

· #12 · Nov 26, 2001

Re: a question about compression (MadGT1)

Two reasons I can think of (at least in principle) why dishing the pistons would be bad.
1. It weakens them.
2. It reduces the thermal capacity, making them less tolerant of brief extreme temperature conditions.
I'm thinking that the amount of material, while significant to the compression ratio, isn't significant to the overall structure/heat tolerance of the piston. I'm sure you could go too far with it (let's say 6:1) and it wouldn't be safe, but going from 10:1 to 9:1 is probably not an issue.
Just my thoughts, though. I haven't done it.
-Steve

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