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.
Nitrous oxide injection is used quite extensively in drag racing for a substantial power gain. When adding large amounts of nitrous, engine components may have to be upgraded to withstand the higher pressures involved. This is not usually a great mod for street use as everything must be just right as far as fuel and nitrous flow goes and of course the major disadvantage is that the tank runs dry after only a few minutes of use and must be refilled.
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.
Turbo Race Engines
I will use a Toyota 2TC engine which I prepared for road racing use a few years ago as an example of what can be done with properly applied engine modifications and turbocharging. The stock engine starts out as a 1588cc, 2 valve per cylinder, pushrod, crossflow hemi. The stock hp is rated at 70 at 6000 rpm.
The block was bored out from 85mm to 88mm to fit Mahle VW forged pistons. This mod brings the displacement out to 1702cc and drops the compression ratio from 8.6 to 7.2 to 1. The rest of the block is totally stock as is the crankshaft.
The connecting rods were polished and shotpeened. They were converted to a full floating pin arrangement to suit the new pistons and Ford SPS big block bolts were fitted to withstand the higher anticipated rpms.
The camshaft selected was the same cam we used on our race atmo 2T engines with .430 valve lift/ 284/222 degrees duration at 0 and .050 lift respectively on 108 degree lobe centers. Valves were enlarged from 41 to 44.5mm on the intake via Ford 6 cylinder ones and from 36 to 38mm via Nissan 200SX ones. The head was extensively ported on the flow bench taking intake flow from 82 to 122 cfm and the exhaust from 66 to 86 cfm. Valve guides were shortened and bronze bushed for increased flow and heat dissipation. Exhaust seats were widened to .080 for better heat transfer. Norris triple valve springs and aluminum retainers were also used.
A stock oil pump was used and an HKS 1mm metal head gasket was fitted.
On the externals; A custom, equal length header was made using 1.625 inch ID thick walled tubing , a custom intake manifold was made fitted with a 70mm Mercedes throttle body and eight Bosch 490cc injectors. The turbo was a Garrett TO4 with H-3 compressor and a .58 turbine. This blew through a massive Spearco intercooler measuring 17 X 21 X 3 inches and 2.5 inch mandrel bent tubing. The exhaust was 3 inch mandrel bent tubing open. Fuel was M-85.
This engine produced 358hp at 7700 rpm at only 15 psi boost. The stock hp was quintupled! Engine life was approximately 6 hours at this power level and about 15 hours at 12 psi and 310hp. Eventually, the main bearing caps cracked from the power output but this was caught before major damage occurred. The effective powerband was 5000 up. Redline was limited to 7700 rpm mainly for valvetrain longevity although hp was still increasing at this point. This engine was used for road racing so the life expectancy had to be about a full season or 15 hours.
Turbocharged race engines can produce staggering hp numbers given strong enough parts however engine life goes down as power is increased. A narrow powerband may be acceptable on a race engine because close ratio gearboxes are usually fitted to minimize rpm drop between shifts.
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.
For street use, you want as many cubic inches as you can get. Torque on the street is king. Always go for as many cubes as you can if you have a choice of engines.
Performance EFI Considerations
When increasing airflow through your engine for more power, you must also increase fuel flow to match. At some point, the stock injectors and possibly fuel pump will not supply enough fuel. Larger injectors will have to be fitted. As soon as you do this with the stock ECU, the engine will no longer run properly. You will have to either rechip or install a different EFI system.
If your engine uses a vane type airflow meter, you are losing a substantial amount of power potential through its restriction. It is foolish to spend a lot of time and money improving engine airflow, then strangling it with a door type meter on the front. Engines fitted with this type of meter will usually gain at least 10% when changed to a large hot wire or speed/density type system. It is important to note that when the airflow flap bottoms out at high airflow rates, it is no longer capable of sending a proper signal to the ECU. The fuel mixture will no longer be correct.
Some companies offer rising rate fuel pressure regulators with their turbo kits to allow increased injector flow rate over stock pressure. Instead of adding 1 psi of fuel pressure per psi of boost as in a conventional FPR, they will ramp up at 2-5 psi per psi of boost. Some of these work OK at low boost but the fuel delivery curve is now in the hands of a mechanical device, not the ECU. This is crude at best. It takes 4 times the fuel pressure to double the fuel flow. If your stock fuel pressure is 45 psi, you will need 180 psi to double your fuel flow.
Two things happen here. First, many injectors become non-linear in fuel delivery above 60-70 psi differential or may not even open, leading to a possible lean out condition under boost. Secondly, the fuel pump is not designed to do this. It either can't produce the pressure or volume needed or will burn out quickly due to the massive increase in current draw. These are a bad idea at high boost pressures.
Use the right tool for the job. You don't normally use pliers to turn a screw in. It works, but not well. The same thing goes for performance EFI applications. Sure, you can trick an old L-Jetronic system with a resistor on the water temp input and get some more fuel out of the system but the method has serious limitations past a point and will not really supply the correct mixture across the operating range.
Hopefully I have touched on some of the major points here and saved you some money and time on your project.