con

moderate comp like 9.0to1 and moderate boost will own low comp high boost on the street. :P

ob1eg

that would suck I was already fighting lag with a stock z6.I think the compression on a d15z1 is 9.3:1 wich is barley higher than the z6 so hopfuly between the two I can get decent compression.

baldur

No, that's a complete myth. Compression ratios don't make a huge difference to bottom end torque or exhaust volume. If anything the lower compression would give higher exhaust volume per engine power. I'd never sacrifice boost for compression, high boost owns everything.
There are lots of factors in picking the right compression ratio however. Rod to stroke ratio, fuel octane, chamber design, desired power band. Higher compression gives a tighter power band because there isn't room for optimum spark timing without detonating, and unoptimal timing gives bad thermal efficiency and thus increases heat and further adding the danger of detonation. This is why top speed runs are so dangerous. Quarter mile drags are usually ok because the engine is never run for long enough to build up much heat.
And about tuning, you must always get the tuning right. But at a certain point you just won't be able to make any power due to detonation, which is where you need higher octane fuel or lower compression.

baldur

an 8:1 motor on 20psi makes more power than a 9:1 motor on 10psi. And pick the right turbo for the job.

ob1eg

I just finished my car and I realy was impressed by the way it drove but it needs to be tuned and mabey more boost wouldnt hurt,but it revs up alot quicker..thanks for all the imput..

HMT-Admin

Balder, you've obviously never drove a honda with a 7.8c/r They are slow as ---- and take alot of boost to even get moving. Its just like a stock DSM.

Me personally if you have a good wideband and know how to dial in fuel and run a conservitive timing map you should have no problems running 9.5~10.0 c/r. I tuned a 11.3c/r Twin turbo ls/vtec just fine, and had a little trouble tuning a 11.5c/r SBC 350.

9.5 is the sweetspot

ob1eg

I seen the build up on the tt ls/vtec I think on nwsr it was a crx right?my brother realy like the chev truck he has the same one but 4wd so no turbo,but props on both setups.you are right this motor is slower than my stock z6 but with 10psi,and new managment it has the potental to be faster.I noticed this engine goes from vac. to boost alot faster,but I dont know ---- about tuning if you know any places localy that can do it please let me know..

baldur

No that's where you're completely wrong.

Higher boost is squeezing more air into your cylinder, the static compression comes later.

When the engine is drawing in air via the inlet valve, it will draw as much as 400cm^3 of air into the cylinder on a 1.6L engine. The air is much more dense if the pressure is 250kpa (about 21psig) than if the pressure at the port is 170kpa (about 10psig) for example. At 250kpa you are getting a lot more oxygen into the cylinder.
If your volumetric efficiency (engine's ability to draw in air) was 100% the pressure at bottom dead center would be 250kpa. You compress this air charge 8:1 and the pressure at top dead center would be 2000kpa if there was no combustion at that point.
Now let's say the pressure at the inlet port was 170kpa and the volumetric efficiency was 100%. Compress this 170kpa 11.7:1 and you'll get the same theoretical peak pressure but the amount of oxygen in the cylinder is much, much less so there isnt going to be as much of a combustion.
Now this is oversimplified, assuming 100% VE which is a little much, not taking into account thermal efficiency or the combustion process. The case with the higher boost and lower compression gives roughly the same cylinder peak pressure which happens right after TDC. However with the bigger combustion chamber and the same mixture density you have a lot more explosive mix packed at the top of the piston to push it down, giving a higher mean cylinder pressure on the power stroke. The higher compression, lower boost engine may have the same peak pressure around TDC but the smaller chamber when charged to the same density just doesn't pack the same punch to follow the cylinder on the way down.
I am well aware of the fact that higher compression gives a little higher volumetric efficiency due to less residual gases after the exhaust stroke, and the higher compression gives a little higher thermal efficiency by packing the mixture into a smaller space so it gets to expand more. (higher peak pressure raises the mean pressure a little bit)

So overall high compression is better, but this is only true on a diesel engine where the combustion is 100% controlled. On an otto engine (piston engine that draws in premixed combustibles and ignites with a spark) you have the problems of detonation, being where the combustion goes out of hand and instead of the mixture burning and expanding steady and controlled in a single direction it will detonate where there is no longer a single steady flame and the mixture is spontaneously igniting from the high pressure and temperature in the chamber. This happens if the mixture is expanding too fast and the pressures and temperatures go too high for the fuel mixture to handle without going out of hand. Hot spots in the combustion chamber can also do sabotage when it comes to combustion stability. Of course with higher pressures the mixture will burn faster so less ignition advance is required to get the peak pressure to the sweet spot in the engine cycle.
Now if your fuel isn't up to the job you will have to retard timing and move the point of peak pressure later in the cycle, where there is more room for the combustion and effectively a lower peak pressure. Guess what, your thermal efficiency is gone at that point (No point in compressing the mixture into a small space if you can't ignite it until the space is getting bigger again). This also means the gases will be expanding more on the exhaust stroke, meaning you're blowing more heat out the exhaust. This is good, right, since we're using the gases to spin a turbine? Sure but we got more than enough gases and the wastegate takes care of the extra pressure to keep the turbine at a steady speed. Of course engine speed plays a part here, when the engine is turning faster there is less time for the combustion in the measure of crank degrees so more advanced spark timing can be run and the precision of the spark timing becomes a little bit less critical in regards to detonation because each degree is a shorter elapsed time in the engine cycle for the combustion to happen, but we still want to have the peak pressure at the right place for maximum power delivery.

So yes, higher compression is better but it requires more exotic fuels to behave right in a spark ignition engine. On a diesel the combustion is controlled but go too high on the compression and you're just increasing the mechanical stress on the engine internals really, but not as catastrophic as the detonation in the gasoline burner.

Now don't try to tell me you can make the same torque at 10psi with 12:1 as you can with 20psi and 8:1 compression. If that was true then why bother with turbocharging at all if we could just jack the compression up to the 20's and make double the torque as we could with 10:1. Doesn't happen like that.