Our Top 10 “Bolt On Horsepower” Engine Performance Upgrades

Fancy adding a bit more oomph to your stock aircooled motor?

  • What is bolt on horsepower?
  • Can I do it myself?
  • What bolt on upgrades are available and where do I start?

What is bolt on horsepower?

Bolt on horsepower is as it sounds, it’s basically removing an external stock part that you “bolt on” a replacement aftermarket part with the intention of it increasing the motors horsepower without splitting the crankcase and modifying or upgrading the internals.

Obviously you’re not going to suddenly triple the horsepower of your motor with bolt ons but you can make subtle improvements bit by bit and enjoy a modest return in performance and power gain the more you do.

The best part about bolt ons is that they can be removed and returned back to stock if required.

Most of us dream of a professionally built powerful motor, but they’re incredibly expensive and for good reason as you’re paying for precision hand finished parts, years and years of experience and skill and a staggering amount of man hours that go into a top quality build.

The old saying you get what you pay for is generally true, but unfortunately we don’t all have that kind of money or mostly we can’t warrant spending more than the value of our daily drivers or weekend getaways on just an engine. So what is worth saving up for and spending a few quid on to give you a satisfying extra oomph on your beloved aircooled that you can install at home?

Can I do it myself?

Most “bolt on” performance upgrades don’t generally require a high level of skill or specialist tools and can be installed at home with a decent enough tool kit and some basic skill, but there are some upgrades that will benefit from additional expertise with specialist tools for final settings, adjustments and tuning, but we’ll make sure to mention those.

Don’t forget if you’re not sure which modification or upgrade will work and perform the best to suit your set up, year, model and engine, please don’t hesitate to give us a call, send us an email or jump on our website LIVE CHAT and we’ll walk through all the options available to suit your requirements.

What bolt on upgrades are available and where do I start?

As with all bolt on and most performance upgrades it’s a topic that’s been discussed for decades as to what works best and is or isn’t worth it and opinions vary hugely normally down to experience, knowhow, technical ability and skill to get the most out of them and bang for your buck as it were.

With this list we’ve decided to not get unnecessarily or overly technical (another list for another day) or go into specific details regarding oil and air cooling improvements, manifold and fanshroud swaps, upgrading from single to twin port and basic swaps including changes like stock 1600cc heads on a 1300cc and compression ratios etc, this list is the best performance bolt on upgrades for aircooled motors, mainly for 1300cc – 1600cc engines as there are not as many performance upgrade parts available for 1200cc engines.

If you’re new to an aircooled engine and have a thirst for knowledge, we highly recommend doing some research before tackling any engine upgrade and the below books are an invaluable resource that you’ll keep referencing and coming back to and will really help in understanding the more technical side of upgrading so you can get the most out of it. Don’t forget to search online for internet groups, clubs and specialist websites. These books will make a great addition to your library next to your workshop manuals (click the books to see them on the website).

Don’t forget to ask your VDub mates around and get involved with your upgrade project, many hands make light work and nothing beats working and learning together on your cars over tea and biscuits or a beer, right?

Ok to the top ten…

1. Exhaust

We always recommend the first upgrade to any standard engine is an aftermarket exhaust if you’re looking for an instant and noticeable bolt on power gain. The advantage is much better flow over the standard high back pressure systems, so typically your engine will rev quicker and the power output will be increased further throughout the rev range. Not only do they look good (you will be going round the back of the vehicle for at least a week checking it out going yesssss to yourself) but they sound better and are quite simple to install. Instant oomph, style and sound in one!

There are absolutely loads of choices when it comes to aftermarket exhausts to suit your style, look and budget, from mild steel high flow OE look, to fully merged stainless steel 4 into 1 systems, so whether you’re going for that sleeper look or all out mega motor noise-fest, there’s something that will fit. Big bore J-Tubes are also available for even more flow if you’re doing away with heating.


Spray some Plusgas or other dismantling lubricant on your exhaust studs and nuts the day before you attempt removal of your stock exhaust, it always make life a lot easier.

Click on your model below to see the performance exhausts available:

2. Carburettor

Bigger carburettor upgrades allow more fuel mixture to enter the combustion chamber, creating a bigger bang and improve the power potential.

Available in all shapes and sizes from slightly larger single barrel stock fitment like the 37 or 39 Pict Big Bore, or the 32/36 Weber Progressive carburettors, perfect for a quick bolt on upgrade on smaller motors, to a multitude of choices of complete twin carb kits. Starting with the small 34 ICT’s and Kadrons which are spot on for 1641cc + 1776cc (although not a bolt on displacement increase), moving up to the larger IDF, D, EPC + HPMX, all available with linkage, manifolds and filters as a complete kit, mainly for bigger displacement motors, but can work with smaller engines when configured correctly.

Of course with all performance upgrades you need to make all sure all the upgrades you are doing are compatible with each other and the engine itself to achieve the performance gain you are expecting. For example there’s a reason why you don’t see a pair of Weber 51 IDA’s on a 25hp motor, that’s not just overkill, it’s never going to work.


After installing twin carburettors we always recommend you book an appointment with your local rolling road or tuning shop specialist to balance and fine tune them. The number one reason for badly running twin carbs are they have never been set up properly in the first place.

Click on your model below to see the carburettor upgrades available:

3. Cylinder Heads

A massive increase in power can be made when fitting the correct compatible cylinder heads and manifolds.

Whether you choose to fit out the box or go on a full port and polish mission, big valve cylinder heads are where the power is and can make all the difference. When configuring with your other upgrades, if you can match the mixture quantity coming in with your big carb(s) to your big valve heads, to then your larger diameter exhaust, what you have there is bigger bang, better flow, then ultimately higher horsepower.

Depending on your engine you’ll typically have 35.5mm x 32mm valves, but a straight bolt on “044” style or GTV-2 head will have 40mm x 35.5mm, that’s an increase of almost 15% on the exhaust and almost 10% on the inlet.


Don’t forget to tighten your cylinder head studs in the correct order and at the correct torque settings both times! Consult a workshop manual or How to Rebuild your Aircooled Engine book

Click on your model below to see the cylinder head upgrades available:

4. Ratio Rockers

Ratio rockers are a very popular bolt on accessory and are perfect for mildly tuned performance engines.

Basically they increase the camshaft lift without the need to split the crankcase case and change the actual camshaft.

 Rocker arm “ratio” refers to the amount of movement on the valve side of the rocker arm in comparison to the pushrod side. Increasing the ratio, or rocker lift, increases the amount the valves are open, this ultimately means your engine can move more air and exhaust through it.

So a 1:25 rocker arm for example will move a valve 1.25 times the lift of the camshaft.

We sell a range of ratio rockers from 1:1, 1:25 to 1:4


Tall ratio rockers will require a longer pushrod than stock to accommodate the taller rocker lift, but we do sell a large range of cut to length pushrods and measuring tools to suit all budgets and applications or qualities required HERE.

Click on your model below to see the ratio rocker upgrades available:

5. Barrel and Piston Kits

There’s only so much displacement increase you can achieve without machining the crankcase to accept much larger diameter barrels on all aircooled crankcases, but there are “slip in” options available for a slight increase, some more so than others.

  • 1200cc engines with 87mm crankcase holes, there’s only one option, but it’s a good one, the big bore 1384cc kits, which give quite a healthy 15% increase in displacement.
  • 1300cc and 1500cc engines you can install 1600cc barrels straight on, with no machining required.
  • 1600cc engines the only option is 1641cc

Check out our money saving top end rebuild kits as well, a cheap way of overhauling everything from the crankcase out, available in 1600cc and 1641cc:


Check the crankshaft endfloat of your engine is within tolerance before increasing the displacement. If you add more power to worn out main bearings, they will likely fail in the near future and a full rebuild or worse case even new engine may be required!

Click on your model below to see the barrel and piston upgrades available:

6. Lightened Flywheel

If your engine is out already then a lightened flywheel is a great addition to your engine. While not technically adding horsepower, they will increase acceleration and are designed to help engines rev faster with less rotating weight taking less horsepower to get going, offering a smoother clutch action and they can prolong the life of the engine due to less weight.

The lighter the vehicle the more noticeable the effect is, so a Buggy for example will benefit the most and as they are available with up to a 6kg weight saving they are definitely noticeable.

The heavier the vehicle, like a bus for example, lightened flywheels are generally only suitable for larger displacement higher torque motors as the lighter flywheel means pulling away will require more revs to get going, once you’re cruising then they come into their own with quicker mid range acceleration, but it all really depends how you drive.

Available as lightened, chromoly forged and lightened to fit 25hp and 30hp, Type 1 and Type 4 aircooled engines.


The heavier the vehicle, like a bus for example, lightened flywheels are generally only suitable for larger displacement higher torque motors as the lighter flywheel means pulling away will require more revs to get going, once you’re cruising then they come into their own with quicker mid range acceleration, but it all really depends how you drive.

Click on your model below to see the lightened flywheel upgrades available:

7. Electronic Ignition

Electronic ignition are a solid state unit meaning no more points and condenser (2 less things to worry about failing or replacing as part of your scheduled maintenance), they deliver twice the voltage to the spark plugs, increasing horsepower, fuel economy and spark plug life. Moulded in epoxy eliminating deterioration from dirt, oil, grease and moisture, with no complicated wiring cluttering your engine bay, plus they work with stock HT leads and coils.

Available as just the ignitor unit for the distributor to replace the points and condenser, or as money saving upgrade bundle kit which includes 8mm Flamethrower HT Leads in a variety of colours and a High Output Flamethrower performance coil in either black or chrome. Also see our range of pre installed electronic ignition plug and play distributors, or the super hi tech CSP USB plug in or Bluetooth remotely controlled and adjustable and fully lockable for security!


When installing the often very tight and fragile magnetic collar on the distributor shaft, we recommend lubricating the collar and using a wide socket to ensure equal pressure and tap it very lightly until it seats correctly on the shaft (check alignment first).

Click on your model below to see the electronic Ignition upgrades available:

8. High Output Coils

If you’re upgrading to electronic ignition a compatible high output coil is an absolute must, the higher voltage allows for larger spark plug gaps for added power, smoother response and better fuel economy, should you also be going to a higher compression set up, you’ll need a bigger output coil.

Flamethrower 1 coils are an uprated 40,000 volt, perfect for use with Pertronix electronic ignition and Pertronix Flamethrower HT Leads, but can be used with standard points and condenser and standard 7mm HT leads. 3.0 Ohm primary resistance, 9.0 Ohm secondary resistance, 80:110.0mH primary inductance

Flamethrower 2 coils are an uprated 45,000 volt, perfect for use with Pertronix electronic ignition (Ignitor 2 only) and Pertronix Flamethrower 8mm HT Leads, 0.6 Ohm primary resistance, 9.0 Ohm secondary resistance, 100:16.0mH primary inductance

Flamethrower 3 coils are an uprated 45,000 volt, perfect for use with Pertronix electronic ignition (Ignitor 3 only) and Pertronix Flamethrower 8mm HT Leads. 0.32 Ohm primary resistance, 7.0 Ohm secondary resistance, 110:12.5mH primary inductance


Flamethrower coils are not backwards compatible and cannot be used with a lower number Pertronix Ignitor electronic ignition as they will overload and cause premature failure. Ignitor 1 must be fitted with a Flamethrower 1 coil, Ignitor 2 must be fitted with a Flamethrower 2 coil, Ignitor 3 must be fitted with a Flamethrower 3 coil.

Click on your model below to see the High Output Coils available:

9. Flamethrower 8mm HT Leads

Upgrade your flimsy stock 7mm HT leads to these suppression style dual current path HT Leads. These will be a great upgrade on a stock motor but if you’re running electronic ignition then it is vital that you use a good quality set of leads to get the most out of your electronic ignition so as not to affect the running of your vehicle. All Pertronix Flamethrower leads are compatible with all Flamethrower coils and ignitors.

Most modern electronic ignition systems require suppression style wires to preserve sensitive components.  Flamethrower 8mm HT Leads provide enhanced RFI suppression and dual current paths for more power delivered to the spark plug.

These HT Leads have a thick outer insulation around a central conductor core and the central core is manufactured from copper.

Available in Black, Blue, Red and Yellow for both Type 1 and Type 4 aircooled engines


Please Note: These are supplied with an extra long unfinished coil lead designed for performance engines that may have the coil fitted in a non standard location. Requires finishing and crimping before installation.
We do sell and rent the correct tools to get this job done, click HERE for the lead stripper and HERE for the crimp kit.

Click on your model below to see the Flamethrower HT Leads available:

10. 30hp Okrasa Engine Kits

This whole kit goes on our list because it’s just too cool not to. What we have here is proper original old school power for 30hp motors from 1955 – 1960. If you have an earlier model and want to retain the period performance and look, this kit is the business.

Okrasa or “Oettinger Kraftfahrtechnische Spezial Anstalt” was the name of the company founded by Gerhard Oettinger in 1951, a company that specialised in performance parts for the early VW engines This performance Okrasa top end engine kit, is to fit 30hp engines fitted to Beetles, Splitscreen Buses and Karmann Ghia`s from 1955 to 1960.

Available with standard or upgraded CSP Linkage.


We gave this a 4 in difficulty mainly because of the amount of work and time required to remove the engine, strip and rebuild, then install the linkage, set up the carbs and then reinstall the engine.

After installing twin carburettors we always recommend you book an appointment with your local rolling road or tuning shop specialist to balance and fine tune them. The number one reason for badly running twin carbs are they have never been set up properly in the first place.

Please note that whilst every attempt is made to ensure that these instructions are as clear and correct as possible, we cannot be held responsible for misinterpretation of these instructions or for any subsequent accident or damage caused through misfitted parts.