# TECH: How much horsepower do I gain by increasing compression?



## tdogg74 (Mar 1, 2002)

Here is your answer....








How to read chart:
Basically, the higher C/R you start off with yields you less gain for a specific target than starting with a lower C/R. 
For instance:
Going from 9:1 to 13:1, you would gain *1.2%* hp.
But going from 12:1 to 13:1, you only gain *.2%* hp.


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## WolfGTI (Jun 16, 1999)

*Re: How much horsepower do I gain by increasing compression? (tdogg74)*

Assuming you stay on pump gas.


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## tdogg74 (Mar 1, 2002)

I guess I should note..
This is for straight-up power difference excluding modification to ignition maps.


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## WolfGTI (Jun 16, 1999)

*Re: (tdogg74)*

http://****************.com/smile/emthup.gif http://****************.com/smile/emthup.gif


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## GENERAL-LEE (Sep 12, 2006)

tdogg i love your car haha


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## not SoQuick (Jan 8, 2004)

*Re: (factor11616)*

and here I thought tdogg actualy had a question


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## ghoastoflyle (Jan 21, 2003)

lol I kinda thought it was a joke :x


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## tdogg74 (Mar 1, 2002)

Im sneaky like that.


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## billyVR6 (May 8, 2000)

*Re: How much horsepower do I gain by increasing compression? (tdogg74)*

If you're going to post up a chart like that, you should at least go into the reasons on why and when one should raise their compression. There are more benefits to compression than just a power figure/gain. http://****************.com/smile/emthup.gif


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## tdogg74 (Mar 1, 2002)

They can search Google for that info....the reason I made and posted that chart was to show the decreasing percentage gain as compression increases. 
By itself, the gain isnt that much. But with supporting work, the percentages obviously will go up.


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## bearing01 (May 27, 2004)

*Re: (tdogg74)*

Interesting Chart Travis.
The equation I know gives a different result. The equation for an Otto-Cycle thermal efficency is:
TE = 1 - (CR)^-0.4
where ^ is exponent and -0.4 comes from 1-(Cp/Cv). Here, Cp is the constant pressure molar heat capacity for air as an ideal gas and Cp is the constant volume molar heat capacity for air as an ideal gas. Cp/Cv = 1.4
If you consider two separate compression ratios and calculate say TE2 for CR2=13 and TE1 for CR1=9 and calculate
Efficency Increase % = 100*(TE2 - TE1)/TE1 
then you get 9.7% increase in power. This number matches a table that is mentioned in the following article.
http://www.popularhotrodding.c....html
Just to note, this number is theoretical and there is a lot of practical stuff going on that reduces the actual improvement in performance to much less than 9.7%. One, for example, is the fact that Cp and Cv change a lot over temperature (compressing air heats it up) and the above analysis assumes the Cp and Cv are held constant. Also, that when you mix fuel with air the Cp/Cv is probably more like 1.1 insteat of 1.4. This too will affect the theoretical efficency.
There's other stuff too like increased cooling loss (more heat loss to cylinder walls because there is more heat created with higher compression). There may also be increased mechanical friction loss with increased compression ratio. There's also the fact that you got to run higher octane fuel or alcohol. Because octane increase doesn't use lead, rather now days they use MTBE or Ethanol, those additives have less energy density. Therfore, to increase the fuel octane rating you essentially reduce the energy content. Therefore, you loose some power just because you have to burn weaker fuel. All these factors reduce my number of 9.7% to something probably like 2% or so.
It's interesting that your chart has a lower prediction of power increase than the formula. How is your chart calculated? Does it account for some of the practicalities I mentioned?

_Modified by bearing01 at 6:17 PM 10-26-2007_

_Modified by bearing01 at 6:43 PM 10-26-2007_


_Modified by bearing01 at 6:43 PM 10-26-2007_


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## tdogg74 (Mar 1, 2002)

Interesting...Ill have to read into that when I get home tonite. The graph I came up with came out of an book I have. I'll look to see how they got their figures.


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## Diggatron (Sep 4, 2004)

*Re: (bearing01)*

......I think my brain just broke......


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## need_a_VR6 (May 19, 1999)

*Re: (tdogg74)*


_Quote, originally posted by *tdogg74* »_The graph I came up with came out of an book I have. I'll look to see how they got their figures.

You should definitely quote your source. Though it does look similar to the 'official' numbers published in the Bosch Automotive Handbook.


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## tdogg74 (Mar 1, 2002)

Watecooled Volkswagen Performance Handbook circa 1999 
Chapter 3
Page 46
Book gove no explanaiton of the graph...it just looks put their to support this line:

_Quote »__Raising the compression ratio does not increase horsepoer in a linear relationship. Instead, a little less horsepower is gained with each additional point of compression._


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## billyVR6 (May 8, 2000)

*Re: (tdogg74)*


_Quote, originally posted by *tdogg74* »_Watecooled Volkswagen Performance Handbook circa 1999

Those books are bit wacked, very suggestive, lacking in detail, source and results.


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## 2.0mk3jetta (Jul 17, 2013)

i know this isnt a machining thread but i think if anyone would know the answer to this question it would be here ... any idea on the measured amount that the head can be decked without valve/piston interference on an aba 8v ? im in the process of building an all motor aba and this is the foundation of all motor power lol high compression so want to deck it as much as possible but safely...want to try collecting as much info as possible before sending out the block and head (dont even know of a good machine shop to send it to...the search continues for me here in nyc lol) 

thanks for any help in advance


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## tdogg74 (Mar 1, 2002)

*FV-QR*

Depends on your cam profile. (mainly your lift/timing events)


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## 2.0mk3jetta (Jul 17, 2013)

911_fan said:


> Depends on your cam profile. (mainly your lift/timing events)


thanks for the quick response...
im going with the TT cam thats the "schrick copy" for VW its a 272degreee and .449 lift, figured its the safest cam for a daily setup without giving up low end torque for high end power or vice versa...hoping with this cam i could get a away with more decking as well because the lift isnt major


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## 2.0mk3jetta (Jul 17, 2013)

2.0mk3jetta said:


> thanks for the quick response...
> im going with the TT cam thats the "schrick copy" for VW its a 272degreee and .449 lift, figured its the safest cam for a daily setup without giving up low end torque for high end power or vice versa...hoping with this cam i could get a away with more decking as well because the lift isnt major


sorry forgot to add im sticking with the stock timing...im putting the lightweight non adjustable gear on


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## tdogg74 (Mar 1, 2002)

Ok. Here is the profile info for that cam measured at both .040" and .050".

272 camshaft measured @ .050"/1mm
Advertised Duration: 272*
Duration @ .050" 228.7*/228.7*
Duration @ 1mm 243.6*/243.8*
Valve Lift: .44835"� / .44902"
Centerlines @ .050": 109.1* / 110.5*
Centerlines @ .040": 108.6* / 110.8*
Lobe Center @ .050": 109.8*
Lobe Center @ .040": 109.7*
Valve Timing: 4.6/44.1 - 44.2/4.6
Valve Timing: 12/51.6 - 51.8/12
Valve Overlap: 9.1* @ .050"
Valve Overlap: 24.1* @ 1mm"

How much are you planning on taking off the head?


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## 2.0mk3jetta (Jul 17, 2013)

911_fan said:


> Ok. Here is the profile info for that cam measured at both .040" and .050".
> 
> 272 camshaft measured @ .050"/1mm
> Advertised Duration: 272*
> ...


great info thanks alot again :beer:

i would like to deck the block and head just enough to get it perfectly straight and true for a perfect seal...but im pretty sure you cant deck the block at all because it will affect the stroke of the motor and possibly lead to valve/piston interference ...if you know if that can be done id greatly appreciate ur opinion on it and all in all id like to deck off as much as possible while keeping this a safe motor lol


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## tdogg74 (Mar 1, 2002)

*FV-QR*

I would stay away from machining the block and stick with the head. Taking off .050" should put you right up to 11:1 and the edge of what 93 octane can handle on an aggressive tune. If you really want higher compression than that, I could suggest changing pistons.


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## 2.0mk3jetta (Jul 17, 2013)

911_fan said:


> I would stay away from machining the block and stick with the head. Taking off .050" should put you right up to 11:1 and the edge of what 93 octane can handle on an aggressive tune. If you really want higher compression than that, I could suggest changing pistons.


yea im on intergratedengineering often(any better sites for pistons?) i want the je 10.5 pistons but il probably settle with the wiseco 10.5 with an 83.5bore and yea i just wanted the block straight and true for a perfect seal (whatever little i take off of the block to clean it up i will just minus it from the .040 and take whats remaining off the head for a good solid seal with great compression[deff over 11 at that point]) i have a junked aba laying around i wana see how much space the piston has at TDC (measuring top of piston to top of block difference)on the block to see if machining it down is even reasonable...il get back to you when i try that out


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## tdogg74 (Mar 1, 2002)

Unless your block is destroyed, a typical shave for trueness is very very little material removed. Like .001-.003". Thats nit going to do much in terms of raising your compression. And remember, larger bore pistons will drop your compression.


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## 2.0mk3jetta (Jul 17, 2013)

911_fan said:


> Unless your block is destroyed, a typical shave for trueness is very very little material removed. Like .001-.003". Thats nit going to do much in terms of raising your compression. And remember, larger bore pistons will drop your compression.


yea thats what im hoping is all that has to be taken off wouldnt say i want to "deck" it juss clean it up and deck the head....and il be honest i did not know that a bigger bore lowered compression, do you think a bigger bore is a good idea in the pursuit of all motor power? i like the idea of being able to get more air and fuel in there but now im not 100% on it, being that it lowers compression


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## leon whalen (May 28, 2007)

2.0mk3jetta said:


> yea thats what im hoping is all that has to be taken off wouldnt say i want to "deck" it juss clean it up and deck the head....and il be honest i did not know that a bigger bore lowered compression, do you think a bigger bore is a good idea in the pursuit of all motor power? i like the idea of being able to get more air and fuel in there but now im not 100% on it, being that it lowers compression


bigger pistons yes....more displacement means more power. (bigger pistons lowers comp.) that is why you deck the head or block....to get the comp. back up to where you want it.


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## 2.0mk3jetta (Jul 17, 2013)

leon whalen said:


> bigger pistons yes....more displacement means more power. (bigger pistons lowers comp.) that is why you deck the head or block....to get the comp. back up to where you want it.


with the wiseco 10.5 pistons being taller do you think its still safe to take .050 off ?


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## leon whalen (May 28, 2007)

2.0mk3jetta said:


> with the wiseco 10.5 pistons being taller do you think its still safe to take .050 off ?


No....you need to put them in the holes and measure to see how much has to come off. maybe .050 maybe more, maybe less. you need to measure to be sure.


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