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Discussion Starter #1 (Edited)
Intelligent Engine Modifications

With so much misinformation and BS out there in the performance aftermarket world, we have decided to offer the reader some real tips based on 20 years of performance engine building and turbocharging experience.

Street or Race?

This is probably the biggest question related to successful mods and the most often ignored. Many people just don't understand why you can't drive a race spec engine on the street. Let's examine the differences in the 2 different worlds:


A good street engine should have a smooth idle, have lots of low end torque, a wide powerband, long life and good fuel economy. To get these characteristics, most street engines have relatively moderate camshaft timing, small turbos, small diameter intake ports with long runners and usually cast pistons. They are designed to run on gasoline with an octane rating of 87 to 92 RON in most cases and usually produce less than 100 hp/liter in naturally aspirated form and 120 hp/liter in turbocharged form


Ideally, a good race engine should have all of the same characteristics that the street engine has above but since high power output is one of the primary concerns, many compromises in those other desirable traits must be made to achieve this power level. To achieve higher power, ports are opened up for increased flow at high rpm and camshaft timing and lifts are increased, both of which kill off low rpm torque, power, fuel economy and that smooth idle.

The rpm capabilities are upped to permit higher airflow rates. This is usually done by changing to stronger parts such as connecting rods, pistons, crankshafts and valve springs. If the engine is turbocharged, a larger turbo and intercooler along with forged pistons and stronger rods are fitted to handle the loads. Raising the redline will not make any more power in most cases unless the engine components are modified to efficiently pass that increased airflow.

On naturally aspirated engines, the compression ratio is often raised substantially to boost torque and power. This is possible when using high octane race fuel. On turbo engines, the compression ratio may either be raised or lowered depending upon fuel octane allowed, maximum boost pressure and possible fuel limits for the race.

As you can see, the two engines vary considerably in requirements and execution. The problem comes in when someone wishes to increase the power output of a street driven engine beyond reasonable limits while expecting no major degradation in "streetable" qualities.

Naturally Aspirated Engines for the Street

On atmo engines for street use, there are only a few ways to substantially increase airflow and thus power.

Porting the head will improve airflow if done correctly. If the ports and runners are enlarged greatly, low speed torque will suffer considerably.

Higher duration and lift cams are the main modification for increasing power. The more duration and valve overlap a cam has generally, the worse the low end torque, fuel economy and idle will be. Of course, top end power should be better. On most 4 cylinder engines, going with more than 285 degrees of duration at 0 lift will result in truly gutless bottom end power. With too much cam, the effective powerband becomes so narrow that the car is just plain miserable to drive in traffic. Most street engines spend the majority of their time in the 2000-4500 rpm range. Engines which are heavily cammed may not begin to produce substantial gains until above 4500 rpm and you are paying for this 95% of the time while being able to enjoy that top end only 5% of the time. We see more problems and complaints with people fitting race type cams in street type engines. It makes the EFI hard to tune and the car annoying to drive in many cases. Don't overcam!

Increasing the compression ratio is another way to increase power. It also increases fuel mileage. Unfortunately, the pump fuel available in most areas limits the compression ratio useable on the street to under 10.5 to 1 on most engines. The difference in power is minimal going from say 9 to 10.5 to 1 and it is a lot of work to shave the head or install new pistons. Again, if you get stupid and try to run an 12 to 1 CR on 92 octane fuel, you will suffer with lots of pinging and eventual failure. Many high compression street engines must have their timing severely retarded to avoid detonation which reduces the power right back to stock levels. Don't raise the compression ratio too high!

Raising the redline to achieve higher airflow through the engine is another way of increasing power. To do this effectively, you will likely need to install a hotter cam with stiffer valve springs, port the head and possibly install stronger bottom end parts like connecting rods. The factory redline is there for a reason. If you exceed it repeatedly by a large margin, you may eventually have a catastrophic failure.

Installing a header and free flowing exhaust along with a cold air induction system may free up a few more hp on certain engines. Don't expect gains of over 10% with these mods on most engines.


On street driven atmo engines, there are minimal gains to be had on most small engines without sacrificing a lot of driveability. If you need more power, you need a larger engine usually. Expecting your 18 second car to do 13 seconds while retaining good idle and fuel economy when modified is unrealistic most of the time.

Turbocharged Engines for the Street

Turbos are a different ball of wax but many of the same mistakes are made when modifying them. Most of the same power increasing methods from above can also be applied to turbo engines. Because turbo engines usually have lower compression ratios than atmo engines, they do not take kindly to hot cams on the street. The gain in top end will almost always be offset by a huge loss in the lower powerband and more turbo lag. Stock cams are the way to go on most turbo street engines. Don't waste your money on so called "turbo cams" for 4 and 6 cylinder engines. These may boost economy slightly but they almost always lose power. Most of these were designed by guesswork rather than by actual turbo experience. 4 Valve engines in general when turbocharged do not need hotter cams for the street.

Porting a turbo head will make the same type of gains as on an atmo head despite what some people say. You can make the same power with less boost or more power with the same boost.

To obtain higher than stock outputs, the compression ratio should be LOWERED on a street turbo. This will permit higher boost with optimized timing on low octane fuel. Forged pistons are an excellent idea on turbos as they have 2-3 times the strength and heat dissipation of cast pistons. Forged connecting rods, colder spark plugs and stronger head gaskets are also recommended.

Stock turbos are usually sized for mid range torque and are undersized even for stock top end power. Compressor and turbine size upgrades are needed to realize substantial power gains. Going too large on turbos will lead to poor low end response. Turbos need to be properly matched for the application and primary intended usage. A couple of rules of thumb can be used if you have access to a compressor map. HP X 1.62 = airflow in CFM, HP divided by 8.07 = airflow in lbs./min. Avoid matching for efficiencies of under 65% at full power and operation near the surge line also.

Intercooling is extremely important. Stock intercoolers with a few exceptions are total crap when used for performance applications.They offer low efficiencies and high pressure drop. Install a properly matched core from Spearco. The closer that your charge temperature is to the ambient temperature, the higher the HP potential will be.

Finally, boost pressures can be raised to increase engine airflow and power. This can only be done within the limitations of the fuel octane rating and ignition timing. Read the other tech articles relating to combustion and fuel for a better understanding. In any case, running 20 psi on the street is relatively meaningless. High boost pressure does not necessarily mean high HP. If you are running this kind of boost on the street, you probably have a host of mismatched or restrictive parts on your engine. With properly matched components and an efficient intercooler, one rarely needs to exceed 15 psi on the street. With these in place, you will be at the safe mechanical limits of most stock based engines and HP will be doubled or tripled over stock. Check out some of the cars on our project page prepared at Racetech if you don't believe this. Since engine life will plummet once you exceed this type of output, it is not a viable option for most people to be rebuilding an engine every 10,000 miles. You don't have a streetable engine in my opinion at this point.


Power may be increased substantially through turbocharging on the street but reliability will suffer unless it is applied correctly.

There seems to be two types of people preparing turbo race engines for import drag racing. One school uses small, stock based turbos for quick spool up. These engines run super high boost but don't make any power. School two fits turbos which are way too large. These have poor turbo response and a super narrow powerband. They produce very high hp across only 1000 rpm on the top end and as a result are not very quick. Bigger turbos don't necessarily mean quicker times. Turbos must be properly matched on the compressor as well as the turbine end.

Some people really know what they are doing and some don't. 450 hp out of a 16 valve 1900cc Acura drag motor at 25 psi is just not impressive when years ago Jack Roush was producing in excess of 700 hp out of 8 valve 2.3 and 2.5 liter Ford Pinto engines for road racing events running from 2 to 24 hours.

since 1996
05 Impreza RS 2.5
1,772 Posts
I can say from experiance that having no low rpm power sucks for daily driving. There should be nothing really done to my car strangly enough, but I find I can't let it drop below 5,000rpm or I will have absolutly no acceloration.

27 Posts
haha, let's all go back to Lafora's signature now

Cheap and reliable won't be fast
Fast and Cheap wont' be reliable
Fast and reliable wont' be cheap

that's why M3's and Porsche are so expensive

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