908ssp

The STROKE is the distance the piston moves.

908ssp

A positive displacement blower always pumps the same amount of air for every revolution. Therefore the blower doesn't care what the stroke is. The change in displacement does not give you more hp with a PD blower. My tuner says he can tune a 4.6 3v to get the same hp as a 5.0 3v. He has never seen any evidence that the added stroke did anything but add stress to the motor. The rules for a NA motor just don't apply when a PD is the source of the air. A larger bore on the other hand opens up the space around the valves making it easier for the blower to pump in air lowering the boost pressure and lowering IAT. If you want to do something that helps increase the bore leave the stroke alone.

siggyfreud

Isn't that assuming that the blower is always pumping ONLY the amount of air the engine can use? I would imagine that blower can compress more air than the engine needs per rotation, and that by increasing the displacement, you're allowing more air into the cylinder than you did previously. Unless that blower is providing the exact amount of air needed, and no more, more cylinder space has to choice but to accept more air (and more fuel, and more power).

908ssp

Yes it is putting more air in the manifold or there wouldn't be any pressure. With the larger displacement you'll see a slight drop in pressure. If you don't put a smaller pulley on the HP stays the same but you do have that option. But you also have all the negatives to contend with. The smaller pulley means faster rotation and higher IAT.

siggyfreud

I guess I don't see why there would be a drop in pressure. Are you saying there is less pressure because of the increased stroke? I don't see why that would happen. Isn't the pressure building when the valves are closed anyways? The manifold size is staying static.

908ssp

If you visualize a one piston engine you can see that when the valve opens the total volume of the space down stream of the SC is larger and the pressure drops at the instant of opening. With a multi cylinder motor the total volume the SC fills is the manifold and combustion chamber space from open valves. Since the engine volume is larger the space is large and a slight drop in pressure will result. The total air gas volume and compression ratio determine the engines ability to make power. Obviously there are lots of losses so increasing efficiency can help. But a longer stroke actually works against efficiency as it increases side loads on the pistons and decreases maximum allowable piston speed.

By the way this applies to mechanical driven SC a turbo automatically supplies more air since the waste gate doesn't open as early since the pressure is lower.

The advantages of a longer stroke on a NA motor don't translate to a SC motor. Given X amount of money to spend I would not pay extra for a longer stroke.

jyna06gt

im going with the procharger f1a as my F/I. still unsure if stroking it or going with the big bore short block would be best yet seems to be a bunch of mixed reviews.

908ssp

Bigger bore opens up space around the valves allowing more air to get in faster a plus on any motor.

jyna06gt

right but what about the reliability difference? Is there really a difference? and arent big bore motors usually sleeved?

908ssp

I'd pick the bore over the stroke for reliability. I don't know what they have to do to over bore the 3v block.