Beefing up my 620

[adams620]

I'm just looking to give my 620 a little bit more speed. What would the best way to do that be? I'm leaning towards L18 pistons and then i'm open for suggestions. I know alot of stuff has already been posted about this subject but just not seeing exactly what I need.

[datzenmike]

Assume this is an L20B? L18 pistons will bump compression to almost 9.3 or almost one point of compression. This will add maybe 5 hp at full RPMs and hardly worth the trouble and you probably will have to run a higher expensive octane gas to make use of it. 

 

However if you combine this with a low restriction muffler and a custom 2" exhaust pipe, a larger carb... maybe a 32/36 Weber, and a slight cam increase you may get 2 or 3 times that. Port match the intake and exhaust manifolds to the head by using a new gasket. Un-shroud the head around the valves. These things work better together rather than one thing by itself. Engines are just big air pumps. The more air you can get in and out the more power it will make.

[adams620]

Yep, that's what I wanted thanks.

[datzenmike]

The port matching and unshrouding you can do yourself and cost pretty much nothing. To add to this you can also make small reliefs at the top of the block near the valves to further unshroud the valves, but stay above the top ring and don't go outside the metal ring on the head gasket. With 9.3 compression you have compression to spare for this. This will improve high speed breathing and is permanent and does not wear out over time.

[Stoffregen Motorsports]

If you're tearing into the motor to do some head porting, why not throw in a $200 set of pistons while you're at it? I would not worry about the bump in compression ratio. Have your distributor recurved and set your timing properly and it will work fine.

 

10:1 is what I shoot for in an L street motor.