Piston CR Selection Recommendations

[howelljd]

I am seeking recommendations on piston compression ratio for my pending 2.0L engine rebuild. I believe it is time for a rebuild of my carbureted 1980 Fiat Spider 2000 as it has over 90,000 miles on the stock pistons. My planning thoughts are to obtain the first-oversize pistons (to 84.4mm from 84.0mm) with increased compression to obtain a better-than-stock, street-worthy performance upgrade when using premium-grade, high-octane pump gasoline. I believe Allison's thicker (0.080in) multi-layer steel headgasket with headstuds will provide a slight reduction in "advertised" compression to keep pre-detonation in check while also dealing with the increased combustion pressures. I addressed suspension and interior issues for this car previously. I have also obtained (or will obtain as noted) many additional performance parts during the past several years that I would like to use along with the bottom-end engine work:

* New gaskets and bearings (to be purchased with recommended pistons)
* Weber 34 ADF carburetor
* 1800 single plane intake manifold
* 4-2 FI exhaust manifold with 2-1 FI exhaust downpipe to test pipe and OE center resonator and OE muffler
* 1800 head (to be ported and polished along with reground valve seats during bottom end work) with oversize intake and exhaust valves using Iskenderian springs (to be purchased)...if use of the 1800 head doesn't make sense here I can always apply the porting, polishing, and valve seat work and put the o/s valves and springs on the OE 2000 head
* Midwest-Bayless 232 performance camshafts (vendor named because the specifications for these camshafts are available online) mounted in milled cam boxes to address the reduced base circle of ground camshafts
* Adjustable aluminum cam gears
* Computronix distributorless ignition, block mounted
* Aluminum radiator
* Lightweight gear reduction starter
* 120A alternator
* Air conditioning...I know this will compromise performance but I do live in central Mississippi and I would like my wife to accompany me on rides when the top may not be able to be down...it rains here and doesn't cool down. This may mean I have to keep the OE crank pulley but I also have an aluminum crank pulley if the AC winds up being untenable...perhaps an electric water pump as a replacement for the mechanical water pump would make AC a more likely fit?

I'll be using the OE 5-speed transmission with a new clutch of course since the engine will be out. Anyway, I appreciate any feedback. Thanks for any words of encouragement and direction, too!

Cheers,
Jonathan

[vandor]

I've had ~9.8:1 compression ratio in two Spiders over the last 20 years and it provides good power/torque without the fear of detonation or having to retard timing to avoid pinging. Our 4.5mm dome pistons will give you that for a stock 1800 or 2000. If you will use an 1800 head add about 0.6 points, if using a 2mm think head gasket subtract 0.4 points (guesstimate).

One could also use the cheaper 1.25mm dome pistons, which would give 8.7:1 CR on a stock 2 liter, or ~9.3:1 on a 2-liter with 1800 head if a stock thickness head gasket is used.

[fiatfactory]

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[klweimer]

Did the folks at Midwest-Bayless give you any opinion on compression ratio for these cams?
Kirk

[So Cal Mark]

The main reason we offer thicker mls head gaskets is to compensate for heads that have been resurfaced (milled) several times

[howelljd]

Csaba,
Thank you very much! Your practical experience is invaluable!

SteveC,
I also appreciate the considerations you've presented and I've done my due diligence with the measurements and math; I'm actually an Aerospace Engineer by schooling although I practiced "destruction" engineering in the Army - ha ha! I will definitely take your caution about a maximum 10:1 static CR to heart.

Kirk,
I sent a message to Midwest-Bayless but I haven't heard anything back yet. In fairness, I don't really expect to hear back until next week.

Mark,
Thank you for the additional insight! I appreciate the clarification.

I hope to get more feedback too!

Cheers,
Jonathan

[So Cal Mark]

as an aside, we have had several customers use the thicker gasket to reduce compression when they had used the high-dome pistons and the results have been okay; no instances of increased detonation due to reduction of quench