| externals (turbos, manifolds, SMICs, piping, etc) and it was determined through dyno testing that the setup would only allow ~8psi of boost on 93 octane fuel and proper tuning (i.e. a LOT of ignition timing was pulled). While pound for pound of boost the NA engine made more power and torque (300RWHP and 300RWTQ @ 8psi), it was severely laggy and was at the ragged edge of safety. The high compression (10.5:1) NA engine was never designed for a forced induction application and because of its higher compression, it will not allow for a lot of boost pressure. Additionally, because of the higher compression, it is critical that the setup be properly tuned and limits religiously adhered to. Eg, you cant get crazy with the boost and you need to have a good ear for detonation. You dont always get good fuel every time you fill up. Something that my previous endeavours with this configuration revealed ties directly to the compression ratio. I mentioned it above, about being really laggy. It is a common misconception that higher compression will yield better spoolup and hence, better power/torque. This is an incorrect assumption. The two vehicles I've had the opportunity to work with both showed severely compromised spoolup... and it makes a lot of sense if you think about it. If you take two identical engines and configure them with different compression ratios, the engine with the higher compression will produce more power and torque. This is because the higher compression produces an air/fuel mixture density that is greater than the lower compression engine. The greater the air/fuel density is at the time of ignition, the hotter the combustion reaction is. This increase in temperature results in an increase of pressure. This additional pressure is the force which pushes the piston down the bore. The greater the pressure, the greater the power/torque. This is all easy to understand, admittedly. However, it appears that some stop at that point and conclude that higher compression is ALWAYS better. This is not the case, and of course, I'll explain why: Go back to these two hypothetical engines: same identical engines with only the compression ratio being different. They both have the same volumetric displacement, which means they are both moving identical volumes of air. Going further to keep apples to apples, both of these engines would have to be running the same air/fuel mixtures. Each gallon of fuel contains around 118K BTUs - for those that dont understand what this means, this is the total amount of heat energy available to perform work. Both of these engines would be running on the same exact fuel, consuming the same exact quantity for this comparison. Even with this being the case, the higher compression engine will produce more usable power than the lower compression engine while consuming the same amount of fuel. What it boils down to is efficiency. The higher compression engine operates at higher efficiency. It is capable of converting more of the fuel energy into actual work than the lower compression engine. But if there is only so much energy within the air/fuel mixture, and you more efficiently transfer that energy into the piston to drive the crank, this obviously means there will be less energy left in the exhaust gases being blown out of the engine on the exhaust stroke. Turbochargers are awesome devices for the fact they use what is normally wasted energy and convert it into useful work - pressurizing air and forcing it into the engine to increase its effective displacement. This increased air mass means more fuel can be added to create a more intense combustion reaction. But, they are driven by exhaust gas energy. They require a specific amount of energy input through exhaust pressure and temperature in order to drive them. It may be obvious where I'm going - the higher compression engine may convert more of the fuel energy into work on the crankshaft, but because it has less energy in the exhaust gases, the turbochargers' performance will suffer. A lot of times it is difficult to weigh out drawbacks and benefits from a theoretical position. At first glance, most people know that a higher compression engine will make more power/torque than an identical engine with less compression. Where the problem comes into play is when one tries to apply that fact to a turbocharged system. The problem is, compression ratios directly influence the performance of the turbocharger in a very significant way. I humble myself to this observation - it never occurred to me that changing the compression ratio by only 2.0 points would have such a significant negative effect on turbo spoolup. My explanation above is just that - it is only an explanation that makes sense of the observation. If someone had asked me proir to the experiences, I would have been inclined to say that it wont make much of a difference. But the dyno results speak for themselves. In a comparison between two Z32's, one with the NA engine and one with the TT engine - both having nearly identical mods, the high compression NA engine took nearly 1000RPM more to reach 8psi of boost. Another thing that'll cook your noodle when looking at these charts is that the high compression setup was using automatic turbos, which have a slightly smaller turbine housing. The smaller turbine housing benefits spoolup - even with these turbos, the high compression setup was still very laggy. Native TT engine:
 High Compression setup:
 Bottom line in all of this: if you want more power, collect the parts and do a NA-TT conversion using a native TT engine. Would be interesting to see an all-out drag setup put together using ~7.5:1 compression ratios with some really big hair dryers. :)
Enthusiasts soon understand each other. --W. Irving.
Are you an enthusiast? If you are out to describe the truth, leave elegance to the
tailor.
Albert Einstein
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I've worked with the NA engine equipped with the TT - 
