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1.8 heads and P&Cs on a 1.7 with stock d-jet - any power increase?

After a search of the PP archives, it looks like the 1.8 P&Cs are of a larger displacement and the 1.8 heads have larger valves. Thinking of a weekend project, it looks like I should be able to switch both of these onto my 73 1.7 case without much expense or difficulty. IEHOs, will this result in a noticeable power increase with the stock d-jet? I have no desire to split the case and I sold my carbs but I could spring for larger P&Cs and head work as long as I stay within the parameters of the 1.7 FI. All I want is improved acceleration and great around town driveability - I never drive faster than 100 and my budget can't support track time this year. Any thoughts besides "you cheap bastard buy a 2.0?"

Thanks,
Rich
Old 03-22-2002, 08:06 AM
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If you are going to increase to a 1.8l (93mm) why not go to 96mm P&c's. That way you would have a 1910cc motor and with the larger valves you would see a hp increase. I would also set the compression to 8to1.
You would need to fine tune the FI to run with the set up.
Geoff
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Old 03-22-2002, 09:30 AM
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You will get less power with the 1.8 P&Cs and heads. The reason? Lower compression. The US-spec 1.8 ran 7.3:1 compression and made 76 DIN HP. The 1.7s ran 8.2:1 compression and made 80 DIN HP.

If you flycut the bejeesus out of the 1.8 heads you could probably get the compression back up to 8+:1--use the compression ratio spreadsheet in the "914 tech specs" section of this site to find out how much you have to cut. You may start running into issues with the valve train geometry at that point...

The 1.7's D-jet should at least run halfway decently with the 1795cc "1.8" displacement. You may want to tweak it, though.

96mm P&Cs are another option, as was already mentioned. The ones that fit 1.7 heads are a bit thin, and seem to not last as long as they might. The ones for 1.8 and 2.0 heads are better, but some folks are still reporting quality problems. (Raby is making his own 96mm cylinders out of 94mm ones--you could have it done to 93s as well.)

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Old 03-22-2002, 03:30 PM
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Dave,

I have been all over the shop talk forum and the PP archives and I am still a little confused. I have located a set of 96mm pistons for a 1.8. Will I still need to flycut the "bejeesus" out of the stock 1.8 heads to get the compression ratio up to a useful level or are the pistons somehow different (wrist pin location?) so that they correct for this? I'm going to use the 1.8 cylinders and have them bored out to the 96mm spec and I would like to use the 1.8 head "as is" (unless you think they need modification) to maximize the package.
Old 03-27-2002, 12:16 PM
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I am not sure about the 96es. I'm guessing you'll have a relatively reasonable compression ratio with those, as I think it's the 1.8L pistons that give you the really low compression.

The only way to be certain, however, is to test-fit everything and measure all of the pertinent volumes. Jim Thorusen put together an Excel spreadsheet that crunches the numbers for you, but you have to do the measurements accurately to get good results. A small measurement error in some cases can result in a good-sized change in compression ratio... Look under "914 tech specs" for the spreadsheet.

--DD
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Old 03-27-2002, 01:15 PM
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