I'm sending my B18a block out October 1st to get resleeved (Dart). Also I'm getting the crank balanced, new bearings installed, Eagle rods, and JE Pistons for 1800 Bucks....not a bad price eh? I'm getting it resleeved for two reasons, 1: I'm going LS/Vtec and what better way to strengthen the block (low rod ratio) than resleeving it, and most importantly 2: I plan on going turbo. Now my question, what's a good compression ratio to shoot for. Like I said, I'm going to go Turbo, but not until the middle of next year....so low compression pistons are no good at the moment. Any opinions will be much appreciated. :thumbup:

 

i went all motor on my b16. its going to be running helluh high compression like 11.6:1. if you are going turbo ive heard to try to shoot around 9 or 8:1 just to be safe. really depends on how much boost yah want to push..

 

9.5:1 will be decent for your NA days. It'll be on the upper side of turbo duty CRs, but still workable. You just won't have to run as much boost to get crazy power.

 

Why VTEC and Turbo? VTEC wont help you much once you turbo (cam overlap baaaad)

 

A good compression ratio for turbo is like harry mentioned. 9:1~9.5:1 is a good power maker, and it doesn't feel quite so sloth~like out of boost. I much perfered how my stock 9.2:1 LS engine felt out of boost as opposed to my 2.0 8.5:1 engine now.

edited by jaje: play nice no names please

 

Adjustible cam gears can be used to close up overlap, but overlap in a turbo application is not an entirely bad thing. Your fuel economy won't be great, and your emissions will probably go up, but you will spool sooner, not fall out of boost as easily, and still make just as much power.

Superchargers are where overlap absolutely kills power since the lost cylinder pressures don't contribute to making boost.

 

You say you will spool sooner with more overlap, but the cams with overlap or on the high rpm cams of vtec. By the time you engage the vtec and switch to the high rpm gearing with over lap you should already be in full boost.

Also, please explain what the point of letting the precious positive manifold pressure leak out by having the exhast valve and intake valve open at the same time????

 

Starting from the top.

A supercharger has nothing to do with the exhaust gases, or how they're scavenged. For that reason, the exhaust on a supercharged engine can be optimized in a similar fashion that a high output NA engine would be.

A turbocharger will always introduce some back pressure into the exhaust system between the head and the turbine. The extent of added back pressure is going to vary with the size of the turbine and housing, downpipe size/shape, and the diameter of the exhaust system. For clarity: back pressure is the impedence of flow of the exhaust with respect to the desired flow direction. This simply means that when comparing a supercharged engine with a turbocharged engine, the turbocharged engine is going to need more time to evacuate the exhaust from the cylinders [the difference is small, but there]. There's a couple ways to do this, and one of them is to allow the boost in the manifold do a little extra pushing during overlap.

The goal in any engine is to introduce the most fresh air and fuel into the cylinders per cycle as possible while retaining the least amount of exhaust. I'm not talking about running high output NA race cams on a turbo'd engine, but still, a moderate to mild amount of overlap on a turbo engine will respond better than cams optimized for a supercharged application, which should have a very small amount of overlap.

Another way to think about it is that a turbo will maintain a preset boost level once spooled [if properly sized], and even during overlap the exhaust valve will close first and the boost in the cylinders will be the same as the boost in the manifold. However, with a supercharger, it will only move a fixed amount of air per cycle, so wasting any amount of pressure is more costly to the performance, especially given that maximum boost on a supercharger is only attained during a given RPM window.