Induction Kit Review

One of the first modifications done to the Skyline is to fit an induction kit to rid the engine of the restrictive stock air box, which helps the engine breathe more easily. This is a very simple modification, and there are many different induction kits available. The next question is, "Which one is best?". This article relates to the 4 most popular kits available, being HKS, Apexi, Blitz and K & N.

What are the benefits of an Induction Kit?

The primary function of an Induction Kit is to clean the air before it enters the engine, but also to provide better air flow to improve the engines power output. The power output of an engine is proportional to the volume of air it can ingest at any given time, assuming that the fuelling is also set up correctly. The most minute restriction in the air induction system will cause an engine's optimum power output to drop. Maintaining optimum, unrestricted air flow becomes a problem when the air must pass through a filter element. There are many different designs of filters on the market, and each manufacturer will make claims that their design is more effective than their competitors. Generally, the greater the air flow restriction, the greater the power loss. Therefore, the ultimate acheivement is to choose as filter that will flow as much air as possible, witthout sacrificing filtration efficiency.

Blitz Sus Filter
The BLITZ 'Sus' Power air filters are made from T304 stainless steel which is totally unique to Blitz. The entire filtration element is made from a fine stainless steel mesh which means the filters never wear out and Blitz claim they have substantially less restriction than any other filter. Some of the other aftermarket filters are made from paper or foam elements, and some do require oiling to maintain their filtration capacity. According to Blitz, this oil gradually disperses into the inlet tracts of an engine, and coats the walls and in particular the air flow sensor, slowly reducing the performance of the vehicle. The Blitz air filters also have the same fine mesh in the top of the filter, which improves the amount of air they can flow, and because the element is 100% metallic, Blitz claim the induction note it produces is similar to a Touring car or group 'A' rally car.

Blitz Claim:

"The Blitz air filter is by far the best performing and best looking automotive air filter money can buy..."
HKS Mega Flow Filters
HKS state that engine power is directly related to the amount of air an engine can take in and expel. Therefore, one of the easiest and least expensive ways to increase the horsepower of any engine is to improve the airflow into the cylinders. However, many products in the marketplace sacrifice air filtration capability for improved air flow characteristics. HKS claim that their High Performance Air Filter Systems offer the ultimate in both air filtration and air flow for maximum engine performance. The elements use a special oil to stop unwanted dirt particles from passing through the multi-layer polyurethane filter. Filters are thrown away when their lifetime has expired and are replaced with new filters, which are inexpensive and easy to replace.

HKS Claim:

"The ultimate in both air filtration and air flow for maximum engine performance..."
Apexi Power Intake Filter
The Apexi dual funnel Power Intake has evolved from the Apexi Super Intake filter with some subtle refinements in the design and filter element. According to Apexi, this resulted in a 10% reduction in pressure drop by utilizing an upper and lower funnel. The Power Intake reduces air turbulence within the filter and creates smooth air flow into the throttle body. Maintenance is kept to a minimum as this filter is a dry-type air filter and will alleviate the problem of filter oil from "wet" air cleaners, interfering with the air-flow meter.

Apexi Claim:

"Accept No Imitations..."
K & N Filtercharger
K & N use their own unique design unlike others which use a conventional paper or foam material. K & N state that because the dirt particles are trapped inside the passages of foam or paper filters, that they eventually clog, and when this occurs, airflow restriction increases dramatically. The K & N Filtercharger uses a special cotton fabric which is sandwiched between aluminum screen wires. K & N claim that the pleated design provides five times more filtering surface over the element circumference. The filter is then saturated with an air filter oil which attracts particles of dirt and debris which build up on the outside of the filter.

K & N Claim:

"...Doubles the air flow...adds up to 15 extra horsepower..."

How they were tested.

The filters were tested on the quality of the filtration, simulated by using a vacuum cleaner and millions of microscopic dirt particles. The particles that penetrated the air filter, were collected on a secondary paper filter, so that a fair comparison could be performed, and the results could be compared like for like. As well as the fine particle test, the performance gains were also measured using each of the induction kits in the test. Therefore, in the quest to find the ultimate induction kit, the filters must not only filter the particles out efficiently, but must maximize the amount of clean air flow, through the filter. The winner would be the induction kit that filters the most dirt particles, whilst acheiving the maxmimum power output of the vehicle.

Technical Comparison

Filter Type Filtration Maintenance Cost BHP Gains
Blitz Sus

Dry Metallic Mesh Type Filter

Zero Maintenance - Lifetime Use. 200.00 13.9 bhp
HKS Mega Flow

Wet, Foam type filter.

Very Poor
Throw away. Change Filter every 10,000 Miles. Replacement filters cost around 25. 200.00 13 bhp
Apexi Power Intake

Dry, Mesh type filter.

Zero Maintenance - Lifetime Use. 200.00 14 bhp
K & N Filtercharger

Wet, Mesh type filter.

Lifetime Usage - Inspect every 30,000 Miles or 12 Months - Clean and re-oil, if appropriate. 200.00 13 bhp


The comparison clearly shows that the Apexi Power Intake filter wins the test, not only on filtration but also by providing the maximum power gains, with K & N, Blitz and HKS coming in second, third and last, respectively. From a filtration point of view, the photographic evidence shows that various induction kits had poor filtration properties, leaving high levels of dirt and carbon on the paper filters. Any deposits that are allowed through the filters, can potentially cause problems with the Air Flow Meters on the car, which are attached to the ends of the induction pipes. It is a fairly common fault with the Skyline, to 'blow' an A.F.M, and a large amount of carbon can be found coating the sensor in the A.F.M.

It is questionable how accurate the original source of data is, as HKS are proven to sell more induction kits than any one of the others featured in the test, and yet there induction kit rates as the worst kit in the test. This suggests that either HKS rely upon their brand name to sell their 'poorly' manufactured air filters, or that the original Japanese test was nothing more than a PR stunt to promote the lesser known equipment. HKS and Blitz continue to be the best selling induction kits in the UK, and are used by Middlehursts, Hiteq and Abbey Motorsport as their first choice when the induction kit modification is carried out.

One thing that does stand out is the fact that all the induction kits seem to offer similar performance gains, of around 13 - 14 bhp. This would seem consistant with actual power gains that have been confirmed by using a rolling road.

So the question still stands, "Which Induction Kit is the best?". Well, if you can trust the original source of data, then the clear winner is the Apexi Power Intake. For those of you that are more sceptical, the choice is probably more to do with the cosmetic looks of the induction kit, as the performance gains are very much the same.

Important Notes regarding the comparison:

- Extracts of this article have been transcribed from a Japanese article featured in Revspeed Magazine, August 1999.
- The original test was based upon the results found when using filters specifically made to fit the Toyota Supra. However, the filtration is unlikely to differ, as all the filtration materials used are the same for every model of car.
- The cost given for the filters are based on a the kit for an R33 Skyline GTR, except the K & N kit which only currently exists for a Skyline GTS, and are inclusive of VAT.