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American 451 Turbo Kits

Mod Description
Info about the American turbo kits for the 451
Mod Details
PremiumNo Difficulty Mod ID824 Creditevilution Cost£$4K + tax For451 Fortwo Link Copy to Clipboard
Props to Rob at S2smarts and Tony at Tune4Performance for some of this info.

Are These Kits Any Good?

If you are in America then this is the best option for getting the most power out of your 451.
Of course, when I say best option, I mean only option. It's not ideal especially for the price.

And If You Aren't In America?

Don't waste your time. If you want a turbo you can chop your naturally aspirated smart in and buy a turbo charged version and still have money left over for mods and a remap. All of which will be more reliable, cheaper, better build quality and far better on fuel. All with the same BHP as the US turbo.

Will These Kits Fit My Non US 451?

Fit: yes
Work: not necessarily
Work well: no.

American companies will happily sell you this kit for 2 reasons.

1 – They are a company, they sell stuff, it's not up to them to ask questions.
2 – They don't know any better, as far as they are concerned their version is the same as ours.

Why Don't They Work?

There are several reasons why these kits shouldn't be fitted to a non US 451.

Lambda Sensors

Looking at the data produced by the piggyback controller for the turbo system, the US 451 uses wideband lambda sensors which creates a data voltage between 0v and 5v. All non US smart 451s use standard lambda sensors which create a data voltage of 0v to 1v.

Wideband lambda sensors have a greater range so emissions can be controlled more efficiently.

The controller tricks the ECU into adding more fuel when the turbo kicks in by making the ECU believe the engine is running lean. Unfortunately the 0-5v output of the controller in incompatible with the 0-1v input of the ECU. As soon as the controller gets to 1v (on its way up to 5v) the ECU believes that the car is running rich and cuts the fueling right down. The engine is suddenly running very lean because of the turbo so the ECU tries to correct it and you are soon back at 1v again.

The flip flop of the fueling happens over and over. If the fuel isn't added, you don't get the power.

ECU Pin Out

Because of the use of wideband sensors, the wiring to the ECU is slightly different. That means
that you will be several wires short when you come to wire up the piggyback controller.

Compression Ratio

Another difference not considered by the American turbo kit sellers it that the
American 451 has the same compression ratio as the non American turbo version.
But the Euro style non turbo 451 has a much higher compression ratio.

USA non turbo  10 : 1
Euro non turbo  11.4 : 1

The American 451 had the compression ration lowered in an attempt to reduce the detonation
(knocking/pinking) caused by the lower quality (RON) of petrol available throughout the country.

Because the quality of petrol in Europe is far higher, smart could increase the compression
ratio of the Euro non turbo car and therefore increasing the efficiency and power of the engine.

The compression ratio of the Euro turbo smart was lower because it's a turbo engine.
The air is already compressed as it enters the cylinders from the
turbo so it doesn't need as much compression from the pistons.

Compression ratios of turbo engines is very rarely over 10.5 : 1
So by adding a turbo to a non turbo Euro model 451 you have a compression ratio of 11.4 : 1

With that amount of compression you will:

*lose power as more is needed to compress the air in the cylinder,
*increase detonation which is picked up by the knock sensor and the power is reduced,
*increase the chance of piston ring damage,
*increase the chance of valve damage,
*almost certainly blow the head gasket.

Rotational Balancing

The Euro turbo engine is balanced far better than the non turbo versions and early 451 smart
documentation even mentioned a balance shaft that wasn't installed on the non turbo version.

Nearly doubling the power of an unbalanced engine can cause power loss through
engine movement and will cause the knock sensor to cut the power also.


If you are in America, you will have to pay a load of cash for not a great system to get about 114bhp.

If you aren't in America, the turbo kit will cost you about £2500 + approximately £500
import charges and tax. A proper Euro turbo version of the 451 costs about £1000 more
than the non turbo version. With the £2000 you saved by not buying a turbo kit you can
have a performance exhaust, filter, silicone hoses, sparkplugs, lowered springs and a remap
at tune4performance and still have money left to put towards some nice alloys and tyres.

A remapped 451 turbo with a few choice mods like exhaust and filter can break the 120bhp mark.

So, unless you are in America with an American 451, forget the American turbo kits.
They aren't worth the hastle that they cause on the Euro cars.