How much hp left on the table?.........

[fred cook]

I am planning a 3.2ss rebuild of my SC engine in the near future. If I go with the 9.5:1 pistons and stock 3.0 heads with single plugs how much horsepower would I be leaving on the table compared to 10.5:1 pistons with ported twin plug heads? Assume the use of 964 cams and 46mm PMOs in both cases just to keep it simple.

Thanks,

[racer]

50hp?

[cgarr]

Originally Posted by racer

50hp?

and about 3k

[boxermania]

The rule of thumb is 4% per 1 point increase in compression, everything else the same.

[longhornchris04]

You are probably leaving more HP on the table by using 964 cams than by going 1-plug and lower compression.

Of course, to get those HP you have to go to higher RPMs, which may be counter to your goals.

Do you already have the 964 cams, or are you intending to get a regrind.

Also (and I can't answer in response but someone else might be able to) are you starting with your '80 SC heads, or are they 78/79 or ROW heads - i.e. the large port SC heads? This will make a difference with the gains from porting - but I can't give you any numbers.

[fred cook]

Ok, reasonable questions, so here are some answers:My plan A is to rebuild the 3.0 in my '80SC coupe using 98 or 100mm 10.5:1 pistons and cylinders for either a 3.2 or 3.3 Ltr. short stroke engine.* I will also use early S cams and 46mm PMOs. Of course, with this compression ratio, the conversion to twin plug heads is highly recommended if not required. If I go the twin plug route, I am planning on using 3.2 large port heads with the twin plugs and the XDI direct fire ignition system.* However, there is always the question of the funding being there so for plan B, I am considering the 98 or 100mm cylinders with 9.5:1 compression ratio, 964 cams and keeping the CIS injection system (which is working well at this time). For plan B, I would still need to do a rebuild of the cylinder heads but would not have to spend the dollars on the 3.2 heads,* twin plugging nor have to buy the carbs or XDI ignition. Also, with the CIS, I would expect to get better fuel mileage than with the carbs. The exhaust in either case will be SSI exhausts with a 2 in/1 out muffler which are already installed on the engine. Thanks,

[longhornchris04]

Well (and this is based on me asking similar questions here... hopefully Steve will drop by and comment) going >98mm you will get more benefits from twin-plug. Short version is the >98mm cylinders have a "huge" combustion chamber and having dual spark helps to ensure that you don't have a pocket of unburnt fuel that can detonate.

I'm not saying you can't do a single-plug Big Bore, there are folks in Cali driving them right now (emissions laws there prevent twin-plugging), but to be safe you will have to retard your timing appropriately (don't ask me what that is).

CIS will get you better mileage, at the obvious expense of performance both in its induction and the limited-performance cams.

Both plans look good to me. I'm looking to do something similar, using a 79 SC motor to start (large-port SC heads ). I too am concerned about the financial implications, because none of this is cheap.