# Shaving 8V heads for increased compression ratio



## kickster (Aug 15, 2000)

does any one know how much should I shave the head to get 11:1 comp? 
The block in ABA 2.0 and head is a GTI 8v. so I am starting with 10:1 (I think)
TT guys told me to shave 20 thousandths which is 0.02" to bump it to 10.5:1 
I called up a machine shop and they tell me to shave it by 60 thousandths (0.06") 
Which is right? 0.06 inch sounds very little!!


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## ABA Scirocco (May 30, 2001)

*Re: Shaving 8V heads for increased compression ratio (kickster)*

A 1.8L head on an ABA block starts off at about 10.3:1. It's been a while since I last did the calculations but IIRC you get about a 1/2 point per mm. So, .060" to get 11:1 sounds about right.


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## GDR (Sep 25, 2000)

*Re: Shaving 8V heads for increased compression ratio (kickster)*

Do a search on Google for "compresion ratio calculator". Yhere are alot out there. One I like and saved is by Eric Falgren, it already has some VW/Audi default settings or you can just plug in your own data.


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## austin neuschafer (Apr 26, 2002)

*Re: Shaving 8V heads for increased compression ratio (GDR)*

I have read that if you mill the noncrossflow 8v head to its minimum you will get a .5 comp ratio increase


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## ATS (Jan 19, 2002)

*Re: Shaving 8V heads for increased compression ratio (austin neuschafer)*

shaving 0.060" off a head can produce dangerous effect (namely pistion hitting valves, what you need to do is check the height of the deck to ensure you Can shave that much (head might of been straightened or shaved before... once you compar the height of the deck to the specs you will know how much you trully can shave off the head.
in most cases 0.040" is a safe zone. and produces about 10.8:1


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## kickster (Aug 15, 2000)

*Re: Shaving 8V heads for increased compression ratio (ATS)*

after careful calculations I shaved the head 0.021" the head was shaved before so in total 0.035" has been taken off. 
Since ABA/8v head starts at 10.5:1 I should be 11:1 or 11.2:1


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## luisgp (Oct 18, 2001)

*Re: Shaving 8V heads for increased compression ratio (ATS)*

What can you expect form the raised compression?


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## 84_GLI_coupe (Apr 3, 2001)

*Re: Shaving 8V heads for increased compression ratio (luisgp)*

quote:[HR][/HR]What can you expect form the raised compression?[HR][/HR]​Mo powa! One of the mechanics at work shaved .020" off my head (1.8L JH), but knowing him he probably took off .040" just for fun. This is going to be interesting when I put in my 10:1 pistons!


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## kickster (Aug 15, 2000)

*Re: Shaving 8V heads for increased compression ratio (84_GLI_coupe)*

no effect but it is so cool to brag about my High Comp engine








"What do you got in there? Just a high comp. racing engine" Doesnt that worth every penny


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## Andrew Stauffer (Oct 2, 2000)

*Re: Shaving 8V heads for increased compression ratio (kickster)*

Where are you guys getting your info that the ABA block topped with an 8v head offers anything other than 10:1? Replacing one 8v head with another 8v head, crossflow or not, isn't going to change anything unless it's been decked or has measureably different combustion chamber volume. I'm coming on a little strong, don't really mean to, but I'd like to know....
I've decked my head .040", bolted onto an ABA shortblock, and have calculated cr at 10.675. We'll call it 10.7:1. 
Although Bently states that the ABA is a 9.6:1 motor. Granted, that's the *only* place I've ever heard/read that, but fwiw.............


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## kickster (Aug 15, 2000)

*Re: Shaving 8V heads for increased compression ratio (Andrew Stauffer)*

Well ABA with stuck 1.8 head gives 10.5:1 According to TT guys. 
Cross flow head is different size. if you put the 1.8 head on a 1.0 it will give you a different
ratio due to larger volume of 2.0 combustion area.


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## A1Rocco (Oct 6, 2001)

*Re: Shaving 8V heads for increased compression ratio (kickster)*

Here we go again! Hasn't this been flogged to death yet.


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## Andrew Stauffer (Oct 2, 2000)

*Re: Shaving 8V heads for increased compression ratio (A1Rocco)*

That's kinda my point, A1, there's SO MUCH CRAP floating around out there on this topic. The combustion chamber volume of an 8v head is 30cc, IIRC. Someone double check me there, but the actual value isn't even important, we could call it "X". True of the 1.8 8v motors, and true of the 2.0 8v motors. In stock, unshaved form, any 8v head will yeild the same compression as ANY other 8v head on ANY stock 8v shortblock. 
When I bought the parts from TT to do my 2.0 8v build, I was assured that the compression wasn't going to change at all. I guess it depends on who you talk to and they aren't all telling the same story. 
You need taller pistons, more volume in the cylinders, or less volume in the combustion chambers if you want more compression. Different 8v heads aren't going to help you, save for the minute variations in CC volume.


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## ABA Scirocco (May 30, 2001)

*Re: Shaving 8V heads for increased compression ratio (Andrew Stauffer)*

Actually, the 2.0L head do have bigger chambers than the 1.8L head, 32cc for the 2.0l as opposed to 30cc for the 1.8s and the 1.6 and 1.7l heads had some smaller chambers



[Modified by ABA Scirocco, 10:21 PM 5-18-2002]


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## A1Rocco (Oct 6, 2001)

*Re: Shaving 8V heads for increased compression ratio (ABA Scirocco)*

Does this feel like a deja vu?


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## ABA Scirocco (May 30, 2001)

*Re: Shaving 8V heads for increased compression ratio (A1Rocco)*

Could be worse, we could be talking about Group A gaskets


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## luisgp (Oct 18, 2001)

*Re: Shaving 8V heads for increased compression ratio (84_GLI_coupe)*

quote:[HR][/HR]What can you expect form the raised compression?
Mo powa! One of the mechanics at work shaved .020" off my head (1.8L JH), but knowing him he probably took off .040" just for fun. This is going to be interesting when I put in my 10:1 pistons!







[HR][/HR]​Sorry, I know you get more power, but how much, is it really worth it? Also, you get more pinging, how much pinging can you get?


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## 84_GLI_coupe (Apr 3, 2001)

*Re: Shaving 8V heads for increased compression ratio (luisgp)*

Sure you get pinging, if you do it improperly. If you get a knock sensor system, it will prevent the pinging. Or you can do what the knock system does and retard the hell out of the spark timing. But that's no good if it's not pinging. Basically, don't raise the compression too much if your system can't handle the pinging, or you can't splice in a system to deal with it. I just got a knock system from an A2 Jetta Carat, and it should splice into my existing system very easily. I grabbed the distributor, knock box, knock sensor, ignition control module, and the harness. I'm not sure what the MK3 cars have to handle pinging though. High comp cars are fun!


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## A1Rocco (Oct 6, 2001)

*Re: Shaving 8V heads for increased compression ratio (ABA Scirocco)*

quote:[HR][/HR]Could be worse, we could be talking about Group A gaskets[HR][/HR]​D'oh, eww you had to say it, please don't anyone ask what he means.


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## citat3962 (Oct 15, 2001)

*Re: Shaving 8V heads for increased compression ratio (A1Rocco)*

So it sounds like we have two sides of the story here.....
one says the ABA Combustion chamber is bigger than the 1.8l heads
the 1.8l heads are 2cc's smaller. this equals higher compression. This makes sense to me because the compression of each stock motor is a result of both the combustion chamber volume and shape and height of TDC position in the cylinder.
I figure this means the only way to tell is to measure the volume of Each block at TDC then measure the volume of the combustion chamber in the various heads.

The other side says the 8v heads are the same compression on any block.
What about low compression rabbit GTi motors at 8.5:1. Was that a function of the combution chamber volume or the height of the piston at TDC? is the JH head a low compression head or is the block low compression?
I don't see how the second one is possible unless ALL 8v heads have the same volume combustion chambers...........in which case why are we shaving the heads......why not add material to the combustion chamber to decrease volume?


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## ABA Scirocco (May 30, 2001)

*Re: Shaving 8V heads for increased compression ratio (citat3962)*

FWIW, I have cc'd both the ABA and JH heads. The 1.8L engines all have the same head chamber volumes, the difference between the total chamber volume between the high compression and low compression 1.8L engines is the deck height volume, the low compression engines have a bigger dish in the pistons and hence larger chambers. This is very apparent even by a simple visual comparison of the pistons.


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## kickster (Aug 15, 2000)

*Re: Shaving 8V heads for increased compression ratio (ABA Scirocco)*

Ok, back to group A gaskets


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## citat3962 (Oct 15, 2001)

*Re: Shaving 8V heads for increased compression ratio (kickster)*

No we're onto stock turbo pistons for 1.8L cars.








MUCH more interesting than a stupid gasket that could in theory add 2 hp and 2 torque......whoopty doo. even if it works.....now turbo pistons......now we're getting somewhere


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## ATS (Jan 19, 2002)

*Re: Shaving 8V heads for increased compression ratio (citat3962)*

Hate to advise you all but the GTi 8v head, the audi 3a Cross flow head , the ABA and the vw JH head are not equal folks.
if it was true they are equal, then do explain why a 1.8L Gti engine produced 20HP more than a 1.8L JH Engine? and the comporession ratio according to VW information is 9.5:1 for GTi and 8.5:1 for JH ? (I do not have info about ABA and 3A engines compressions)
I am not flaming anyone here, trying to get a intelligent conversation (I know it is hard to do sometimes on this board) about engine specs and possibilities of hybrid engine and the benefits of ABA/JH or 3A/JH or with GTi head (personally I prefer non-hydraulic heads).
My view is the following, I have a JH engine with 90 HP and 8.5:1 compression, as per the details I have (and they could be faultyy because we all know the internet can provide faulty information at times) is that a combinaision of a 3A block with a JH port/polish head and a GTi cam, (not that the cam will change the compression in this case) will with proper intake and exhaust upgrades (audi 5K TB and Header + Exhaust from Supersprint for example) produce 9.5-9.8:1 compression and 130-145HP.


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## A1Rocco (Oct 6, 2001)

*Re: Shaving 8V heads for increased compression ratio (ATS)*

A JH head on a 3A will be 10.5:1 comp. as will any 8v VW head either solid or hydro accept the crossflow which will be a little lower. I have a 3A and several JH heads.


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## Man&Golf (Oct 2, 2001)

*Re: Shaving 8V heads for increased compression ratio (kickster)*

thanks guys i just learnt heaps reading that.......even though mines a 1.6L carby engine.


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## citat3962 (Oct 15, 2001)

*Re: Shaving 8V heads for increased compression ratio (A1Rocco)*

So you're saying thst if I swapped the block from my 10:1 RD 86 Hydro onto an 8.5:1 JH Head I'd have a 10.0:1 compression JH....
alternatively I could use the JH or A2 GX pistons in virtually any 1.8 block and have the same 8.5:1 comp??? I just bought a Callaway stage one....I'll be needing a rebuilt block for peace of mind. I want lower compression for reliabilitys sake. Any body know where to get HIGH QUALITY ring, gasket and bearing kits for VW 1.8 blocks?


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## A1Rocco (Oct 6, 2001)

*Re: Shaving 8V heads for increased compression ratio (citat3962)*

As was stated earlier in this thread, your JH and RD engines have the same combustion chamber volume. The blocks use different pistons resulting in the compression difference. 
When rebuilding any 1.8 block, target compression is only a matter of putting in the right pistons, whether that's 8.5, 9.0, 10.0 or custom. There are also three choices for bore size from the factory, stock first oversize, and second oversize which is I believe .5mm larger.
An HT or RD motor with 10.0:1 pistons and a JH head with a 2mm shave will give around 11.3:1 compression.


[Modified by A1Rocco, 6:10 PM 5-21-2002]


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## luisgp (Oct 18, 2001)

*Re: Shaving 8V heads for increased compression ratio (A1Rocco)*

I am at 10.0, 100 hp right now, What power increase can I expect from shaving the head to 11.0?


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## Atlantonius the IInd (Aug 9, 2001)

*Re: Shaving 8V heads for increased compression ratio (A1Rocco)*

i was also told by a machine shop that the most you can shave a 1.8l 8V head is .5mm . any more than that would be risking clearence issues.


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## ABA Scirocco (May 30, 2001)

*Re: Shaving 8V heads for increased compression ratio (Atlantonius the IInd)*

Sounds like your machine shop may have been confuing the 8v head with the 16v head. The 16v head has clearance issues ( Bentley recommends that 16v head not be resurfaced at all) whereas you can safely take at least 2.0mm off of an 8v head.


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## Man&Golf (Oct 2, 2001)

*Re: Shaving 8V heads for increased compression ratio (ABA Scirocco)*

hmm i think u know too much
*hand the brain over* please!!


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## Atlantonius the IInd (Aug 9, 2001)

*Re: Shaving 8V heads for increased compression ratio (ABA Scirocco)*

no, that definetly wasn't the case here. he pulled out a huge book which told him all the clearences that can be safely shaven off just about every car out there (was a pretty big book). it clearly stated .5mm on an 8V 1.8l. i'm gonna call around to see what other shops say too, the last thing i want to do is get it shaved too much and have the pistons bend the valves, first time over.


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## ATS (Jan 19, 2002)

*Re: Shaving 8V heads for increased compression ratio (A1Rocco)*

quote:[HR][/HR]A JH head on a 3A will be 10.5:1 comp. as will any 8v VW head either solid or hydro accept the crossflow which will be a little lower. I have a 3A and several JH heads.[HR][/HR]​question a little off topic, have you ever seen dual carbs on an JH head?
I have a possible purchase on a dual carbs setup.. wondering if it worth doing it


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## ABA Scirocco (May 30, 2001)

*Re: Shaving 8V heads for increased compression ratio (ATS)*

About I saw two months ago a JH with twin Webers on an engine dyno, 145 bhp


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