How much power will my engine make?
This is of course the most common question that we get asked in relation to our kits, and it is of course the main reason that most people look into supercharging in the first place. However the answer is not straightforwards. For example installing Kit A does not equal X horsepower. The actual answer is far more nuanced and will depend a lot on the specification and condition of your engine. Strap yourself in, here's the low-down...
The basics
Before you can get an understanding of how much power you will make, you really need to understand how boosted engines make power and how that differs from normally aspirated engines. All internal combustion engines make power by burning fuel. Fuel needs air to be able to burn. To make the optimum amount of power for any given amount of air it needs exactly the right amount of fuel. You can make more power, by getting more air into the engine, which in turn allows you to add more fuel. It is actually airflow that is the determining / limiting factor on how much power your engine can make.
In an normally aspirated engine, the engine sucks the air (and appropriate amount of fuel) into the engine. The smoother and less restrictive the pathway for the air into the engine, the more air will be sucked in. Hence why cylinder head work, camshaft choice and valve jobs make power - they help to smooth the flow path into the engine and remove restrictions. Forced induction setups on the other hand PUSH the air into the engine. They care less amount the smoothness of that pathway, because what you push in one end WILL come out of the other. Think of it like drinking water from a hose pipe. You can stick one end in a bucket and try to suck the water through it like a straw, which is really hard work, or you can attach it to the tap / faucet and voila - you've just supercharged your drinking experience. (You'll never look at beer taps the same way again - lol)
In a normally aspirated engine the baseline force that needs to be considered when calculating air flow and fuelling requirements is the barometric pressure. This is the pressure of the air around you - essentially the weight of all of the air above you all the way up to the edge of the atmosphere. You don't notice it, but there is an immense pressure there. That pressure is nominally 14.7psi at sea level or 1 bar (1 atmos) in the metric system. This is the pressure that on a normally aspirated engine is pushing down into your engine inlet. if it increases you need more fuel, or if you're at the top of a mountain, you'll need less
The maths
Lets assume your (normally aspirated) engine is making 50hp. It's making that power with 14.7psi on the inlet. Now lets say we add another 6psi to that inlet pressure in the form of boost, so now you have an increase of 6psi / 14.7psi x 100 = 41%
lets increase our baseline power by 41% - So now you have 50 x 40% = 70hp
The amount of power is (simplistically) a ratio of how much boost you can add to your engine. There are also some losses to factor in, but it's a good overall general guide.
So, if you want to double the power of your engine, just double the pressure at the inlet. Add an additional 14.7psi (1 bar) of pressure and you'll have doubled the amount of air entering your engine. Easy eh. or is it?
Supercharging is not just a case of adding boost until you get the horsepower figure you want. There are drawbacks... 5 or 6 psi is absolutely safe with low risk of detonation and is approximately what our budget kits with a stock AMR500 pulley and stock crank pulley will give you. Move up to 8-10psi and your detonation risk increases and you ideally need to look at need some form of boost referenced ignition control. 10-12psi and beyond and engine design and detonation management becomes essential. Running 1 bar on an aircooled engine takes some serious detonation strategies to keep things reliable.
So the question you should ask, is not just much power will I make, but how much boost can my engine reliably handle?
Ok, so how much power do your AMR500 based kits make?
As outlined above, it basically comes down to how much boost you are making, which is a product of your drive ratios, and how restrictive the kits are.
Generally speaking an increase of about 40% is a good estimate for our budget kits on a stock engine running moderate boost of 5-6psi (stock 70mm AMR500 pulley and crank pulley). This will give you a reliable increase in power without putting too much strain on your engine. It is also a bolt on modification which gives you a good hp per $ return as you do not need to make internal modifications to your engine. When you consider that a stock 2.0 Type 4 engine makes 71hp you can see that this setup is a worthwhile addition.
Our performance kits ship with pulleys to make 8-10psi which give a typical increase of 60% out of the box. These kits flow more and so generate less heat and have less sacrificial losses than round port based systems, which opens up the possibility of targeting higher boost by swapping out pulley ratios. Our dyno testing matched this with a power output of 83hp from a stock 1600 50hp engine, which is comparable with dyno figures measured by TDS on one of their EFI powered AMR500 setups which made pretty much the same. If you are seeing other competitors claiming higher figures than this in their sales blurb, they are basically inflating figures to get sales. Ask to see their dyno runs.
For more power you can easily change the drive ratio to increase the level of boost. However there are limitations on how far you can go related to the maximum speed of the blower and the capacity of your engine (More on that later), as well as power transmission related to belt slip.
If you want to maximise the power gains, you can also make other sympathetic modifications to your engine. The following modifications will help you to make the most from your kit
- Improved head work – We recommend something like the CB performance Panchito 35.5 x 40mm heads
- Increased capacity - We recommend max 1776
- Proper merged collector header like a sidewinder
- Boost referenced ignition control. We recommend using one of our crank fired distributor-less ignition systems but a CB black box, MSD or similar boost referenced controller will also work.
- One of our custom camshafts. – Our cams are developed specifically for use in blown applications and are available in a few different grinds depending on your setup.
- Using our performance kit with higher flowing manifolds
By employing modifications such as these our customers have managed to see massive improvements. The best results reported so far are 138hp from a 1776.
Belt slip is a big issue when targeting higher boost levels. The amount of force needed to push the air through the system increases exponentially. So for an increase from 6psi to 10 psi, the force required to drive the blower increases by 100%. So thin PK4 belts, or vee belts like those offered by our competitors will simply not cut the mustard. This is why we offer PK6 belts as standard on our performance kits with the option to increase the belt width to PK8 or even move to a Gilmer setup.
If you are chasing ultimate power, the other question to consider is how much boost can your engine handle? You can increase the drive ratio to give you more boost, but the more boost you push into the engine, the more supporting modifications need to be done. Higher boost applications require additional modifications such as boost referenced ignition control, water methanol injection and forged pistons to help manage detonation. 138hp is also getting towards the top end of what the stock crank and rods can handle.
If you are looking at a true performance solution and larger capacity engine then one of our centrifugal setups is the way to go. These will easily make more boost than your engine can use, which provides the opportunity for boost regulation and management and so are perfect for high boost and larger capacity applications.
As the old adage goes... "how fast can you afford to go?" (Cheap / Fast / Reliable - pick any two !!)
Capacity Limitations
Traditionally blowers are selected based on the engine specs and application. Just like with a turbo, you need to calculate the air consumption of the engine, your target boost and then select the blower accordingly. But with the AMR500, what happens is essentially backwards. The blower choice is made first and the spec is therefore fixed, so you need to design the engine to suit the blower. This also means that if you have an existing performance engine it may not be suitable for an AMR500 despite what you may wish or read on competitors sites.
At 16,000 rpm the AMR is at its maximum design speed and will move around 200-220 cfm of air. To put this in perspective, this is about the same as what a 1500cc engine running 12psi consumes, or a 1915 running 6psi. This is also measured at max engine speed.. So if you want to increase capacity, or boost or engine rpms past this (i.e by fitting a larger camshaft), you will exceed the maximum rpm limit of the blower which will increase the risk of blower failure. This is also with the blower running at 100% duty, which is not the ideal place to run a Rootes style blower as the efficiency drops off considerably. At rpms that exceed the design limit, the centrifugal force also has a tendency to seperate the rotor coatings from the rotors, this in turn causes the rotors to touch, which then sends shards of shattered rotor coating into your engine. Add to this that 99% of blowers on the market are all pre-loved with unknown provenance and the risk of rotor failure increases even more. So an important consideration is to operate the blower within its design constraints.
As a result we generally recommend 1776 as the maximum capacity for performance applications, which takes into consideration the increased engine rpms you will see from a performance cam and head work, the optimum operating range of the blower and also the boost versus capacity tradeoff. So if you want to use the AMR500 on a non stock engine in a performance application, the setup will be dictated by your engine capacity and max rpms, what boost you want to target and the application. You can trade some points off against each other, like running a larger capacity at a lower boost level, or overdriving the blower slightly for a street application which will rarely see max engine rpms, but we do not recommend moving too far from this combination. The further you move away from this, the more one of the design constraints will be compromised. Also, the more performance you are building your engine for, the more attention you need to pay to the details. Building a performance blown engine is not simply a case of bolting a blower on.
We do not generally recommend targeting engine capacities much larger than this as the AMR500 cannot push enough air to make meaningful boost without overdriving the blower and running it outside of its optimum range. Of course the internet experts and even the other copy-cat blower kit manufacturers will tell you that the AMR500 can support ridiculously large capacity engines up to 2.1 litre but the truth of the matter is that it cannot. At 6k rpm with the blower maxed out you will only see 1.8psi on a 2100cc engine, which most likely is not enough to overcome the sacrificial losses of driving the blower.
If you are interested in building a performance engine based around the AMR500 (or AMR300), we are happy to help. Feel free to reach out or start a conversation in the Blown VW group on Facebook..
Snake Oil Warning
When shopping for supercharger kits, don't get sucked in by those competitors who claim to be making 100hp+ on a stock 1600 with an AMR500 and a skinny PK4 / vee belt and whacky / angular manifolds, using essentially the same pulley ratio's and round manifold ports as our budget kits. They are lying to you. The maths just doesn't add up. As with everything on the internet; sex and hp numbers sell products. If they were dishonest enough to copy our products and pass them off as their own, they won't stop at lying to you about how much power their products make. Caveat Emptor.
Online Boost and Power Calculator
If you are interested to find out what kind of power your engine can make we've put together a boost calculator which pumps drop down selections into the maths outlined above to give you an easy to use overview of different engine combinations.