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> 1.8 head mods, How can I bump up the CR
Gunther
post Oct 20 2011, 02:15 PM
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Hello everyone,
I have a 1.8L motor that I want to use for my race car. I’m in a power to weight class (NASA GTS-1) were I need to get about 100hp at the rear wheels, or 115hp at the crank. I’ve got a great performance cam with race valve springs, and a set of 40IDF Webers. I would like to bump up the compression ratio by machining the heads, (I’m a machinist) and without buying a set of pistons.
The stock 1.8 puts out 80hp. + 15% from the cam = 92hp.

My 2 questions;
1- How much do I need to mill off the head to raise the compression ratio per point? (IMG:style_emoticons/default/sawzall-smiley.gif)

2- How much hp will I get per point? (roughly) (IMG:style_emoticons/default/confused24.gif)

I know that these are not easy questions to answer for the average person, and to get the exact hp has some variables to it, but I’m just trying to get in the ball park.
A little extra hp is better than less as I can always ad a little weight to the car.
Oh yea, please, no suggestions on turbos or other mods that cost money as this is supposed to be a no budget project that can be done with the parts that we have at hand. (IMG:style_emoticons/default/smile.gif)
Thanks for looking a best wishes…G
(IMG:style_emoticons/default/beerchug.gif)
My new race car.
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McMark
post Oct 20 2011, 02:30 PM
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Use this calculator to estimate the CC changes you'll need to make. You can put in your bore/stroke/deck and desired compression ratio, then click the CC button and you'll know how big of a chamber you'll need. Don't forget to include any dish/dome on your pistons. Then CC your heads and see what volume you've already got, or fill the chamber with your desired volume, and see/mark where you need to mill to.
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70_914
post Oct 20 2011, 02:38 PM
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On VW motors don't you mill the cylinder to bring the head closer to the block?
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r_towle
post Oct 20 2011, 02:40 PM
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no, you flycut the head....I have seen paper thin fins on the one closest to the cylinder.

RIch
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McMark
post Oct 20 2011, 02:40 PM
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You can, but you have to be really careful to make sure they cylinder stay perfectly equal. If one cylinder is shorter than the other you will have a combustion leak.

Also, you can only remove so much material before your cylinder fins bump into your head and then you have a big combustion leak.
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Gunther
post Oct 20 2011, 02:56 PM
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QUOTE(70_914 @ Oct 20 2011, 04:38 PM) *

On VW motors don't you mill the cylinder to bring the head closer to the block?


Either one or the other or both depending on how much you have to take off.
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Dave_Darling
post Oct 20 2011, 04:33 PM
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And, at least with the 2.0 heads, you can take off enough to blow through into one of the exhaust stud bores! I'm not sure with the 1.8s, but that might be a danger as well.

--DD
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HAM Inc
post Oct 20 2011, 06:10 PM
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~1.3cc's/.25mm fly-cut.
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jcd914
post Oct 20 2011, 07:00 PM
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According to my Porsche spec book stock 1.8 is 76 DIN HP and at 7.3:1 comp ratio you'll cut a lot off your heads to gain much ground.
Seems to me you'd be better off replacing the pistons with something less dished or even flat topped.

Jim
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sean_v8_914
post Oct 20 2011, 07:07 PM
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what is your deck height now? are your pistons dished? what size shims are under the barrels now?
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jaxdream
post Oct 21 2011, 07:54 AM
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QUOTE(Gunther @ Oct 20 2011, 12:15 PM) *

Hello everyone,
I have a 1.8L motor that I want to use for my race car. I’m in a power to weight class (NASA GTS-1) were I need to get about 100hp at the rear wheels, or 115hp at the crank. I’ve got a great performance cam with race valve springs, and a set of 40IDF Webers. I would like to bump up the compression ratio by machining the heads, (I’m a machinist) and without buying a set of pistons.
The stock 1.8 puts out 80hp. + 15% from the cam = 92hp.

My 2 questions;
1- How much do I need to mill off the head to raise the compression ratio per point? (IMG:style_emoticons/default/sawzall-smiley.gif)

2- How much hp will I get per point? (roughly) (IMG:style_emoticons/default/confused24.gif)

I know that these are not easy questions to answer for the average person, and to get the exact hp has some variables to it, but I’m just trying to get in the ball park.
A little extra hp is better than less as I can always ad a little weight to the car.
Oh yea, please, no suggestions on turbos or other mods that cost money as this is supposed to be a no budget project that can be done with the parts that we have at hand. (IMG:style_emoticons/default/smile.gif)
Thanks for looking a best wishes…G
(IMG:style_emoticons/default/beerchug.gif)
My new race car.
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Is't that Slit's pal (IMG:style_emoticons/default/smoke.gif) on the hood ????

Jack
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