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> piston question, flat or dish
nycchef
post Jul 17 2008, 09:27 PM
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i have a set of mahle dish pistons from my 2.0. they seem perfect. i will be putting carbs and a web cam on it. what do i lose by staying with dish as opossed to flat. i'd like to save the $400. or so bucks.
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SGB
post Jul 17 2008, 09:38 PM
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It would be lower the compression, but IDK how much. Seems like I 'member the US 2.0 was 7.6/1 (that may have been the CA model) and I'm pretty sure it used dished pistons. The euro flat topped were 8.1/1 or thereabouts. This is all of the top of the ol' head, and I may be halucinating AGAIN, so don't quote me.
Conversly, as gas prices go up, lower compression would allow use of lower grade gas. Might be a good thing...
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hydroliftin
post Jul 17 2008, 10:13 PM
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QUOTE(nycchef @ Jul 17 2008, 08:27 PM) *

i have a set of mahle dish pistons from my 2.0. they seem perfect. i will be putting carbs and a web cam on it. what do i lose by staying with dish as opossed to flat. i'd like to save the $400. or so bucks.


When I built my 2.0 the machinist cut the dished mahle pistons down on top to make them flat, then machined the bottom of the cylinders to raise the compression to 8.5 to 1. Works great. This was done by Jay of Jay's Machine Shop in Santa Clara, CA. He is an old school machinist and has worked his fair share of Porsche engines in his time.
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SGB
post Jul 17 2008, 11:06 PM
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QUOTE(hydroliftin @ Jul 17 2008, 11:13 PM) *

...machinist cut the dished mahle pistons down on top to make them flat, then machined the bottom of the cylinders to raise the compression to 8.5 to 1. Works great.

I'm guessing he made the (shorter) pushrods for your hydroliftin cam? I don't guess it is enough to really see, is it?
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nycchef
post Jul 18 2008, 08:28 AM
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so am i correct in thinking less compression = less hp? will the difference be significant? i don't want to build a 2.0 that pulls like a 1.7. i am not (obviously) well versed in these matters (IMG:style_emoticons/default/unsure.gif)
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jmill
post Jul 18 2008, 08:42 AM
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You are correct. Less compression = less HP. Doing some quick math the difference between 7.6 and 8.1 is about 9% less HP.
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Bartlett 914
post Jul 18 2008, 08:46 AM
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There are also Bus pistons that have a deeper dish. 914 dished pistons would be fine but I would stay away from bus pistons.
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nycchef
post Jul 18 2008, 09:22 AM
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QUOTE(Bartlett 914 @ Jul 18 2008, 06:46 AM) *

There are also Bus pistons that have a deeper dish. 914 dished pistons would be fine but I would stay away from bus pistons.


can anyone tell me what i have?


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Bartlett 914
post Jul 18 2008, 09:36 AM
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Looks like 914 to me. My bus pistons have a smaller dia dish but a lot deeper. I don'r remember the cc's but about double IIRC
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nycchef
post Jul 18 2008, 09:50 AM
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QUOTE(Bartlett 914 @ Jul 18 2008, 07:36 AM) *

Looka like 914 to me. My bus pistons have a smaller dia dish but a lot deeper. I don'r remember the cc's but about double IIRC


thanks i'll clean em up and see what i have. this is my first rebuild ( unless you count a 68 triumph spitfire 35 years ago) so i want to avoid putting in pistons and then blowing em up.
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Bartlett 914
post Jul 18 2008, 02:08 PM
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914 on left bus on right
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Bartlett 914
post Jul 18 2008, 02:08 PM
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914 on left bus on right


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hydroliftin
post Jul 18 2008, 02:47 PM
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[/quote]
I'm guessing he made the (shorter) pushrods for your hydroliftin cam? I don't guess it is enough to really see, is it?
[/quote]

I bought the Isky "torquer" hydralic cam with matching lifters and push rods. As far as I know it did not require modifying the push rods, so the hydro lifters must be taking up the slack.

In terms of compression ratio, the jugs were modified to sit deeper in the case, so compression ration was increased rather than decreased. Without modifying the cylinders, you are correct that a loss in compression ration would result.
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