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Big Bore Pistons: Torque vs. Horsepower

September 27, 2010 | Troubleshooting & Repair

By Alan Klinger and John Wilhoit

[Editor] Dave Brenny asked a question on 356Talk wondering if rebuilding a 356 engine using big bore pistons produce more low-end torque at the expense of horsepower at high RPMs. Two 356 engine veterans responded:

Alan Klingen, The Stable: No single item will make a motor"scream": it is a function of all of its parts. You can make a 1200cc motor feel real good to 10,000 rpm and you can make a 2000cc motor be a very good low RPM motor.

The basic items that make a motor rev freely are the design of the cam, the compression ratio, and porting. There are a few others that come into play like timing ignition curve, fuel management, etc. I have found the biggest limit to a 356 to make good horsepower is the design of the heads: they are in a word, bad. A 911 can deliver 100HP/liter quite easily and be drivable, but to get a 356 engine over 75hp/liter takes some serious effort (and expense).

Of all the dyno testing I have done it seems the maximum street horsepower is around 130 HP and that requires a lot of refinement. 100HP (DIN) is not hard, but to go beyond that you will need to rev the motor well over 6000 rpm and then there are a whole bunch of issues with rocker failure.

John Willhoit: We did considerable work with engine simulation software when developing our 1925cc engines. The stock 356C/SC heads and exhaust system are the limiting factors when going to a larger displacement, especially over 1720cc.

If you're just changing the pistons and cylinders, you will gain torque by increasing the bore but you won't gain much horsepower because your power in the upper revs will be limited. Your horsepower curve will get lower because the larger engine needs more air at any given RPM.

Most people like the torque but if it's upper RPM pulling power you like (say from 5,000 to 6,500 RMPM) you'll need to work on the heads and exhaust. You can look at the horsepower and torque curves for our fuel injected and carbureted 1925cc engines.