Originally Posted by David Mills
VVC already has 52mm throttle body so shouldn't need that.
Nope. Stock F VVC has the 48mm SU plastic throttle body. Later MY2K VVCs switched to the 48mm aluminium throttle body. Only F to get the 52mm Trophy throttle body as standard was the Trophy 160 itself.
Hmmm, with a grand to spend, then you don't want to be messing with the head. VVC port and polish job is great, but with the head off you should replace the gasket with a heavy duty one, and convert to steel dowells. The head itself is nearly half your budget, factor in between £2-300.00 more on labour and you are close to your budget and still aways from 180bhp.
Mods to consider:
1 ) Induction kit / K+N 57i
MGF has a very restrictive air intake tract, in total it is approaching 2½ metres in length and folds around on itself in a convoluted manner bending though at least 810 degrees. This coupled with an air intake sited within the engine bay, thus drawing at best 'warm' air severley hampers power. The K+N 57i system is a hybrid, sitting somewhere between an exposed cone filter and an induction system - this has been independantly tested and produced +15bhp on a VVC F. You will note that only some of these gains are attributable to the K+N, a large part of them come from ditching the rubbish OE set up. The K+N, whilst excellent, is still an exposed cone siting in the hot air of the engine bay - it has a pair of cold air ducts feeding it cold air from outside, but these only provide cold air when the car is moving quickly, at other times the filter can draw hot air from the engine bay resulting in poorer performance. Cold air is denser = more oxygen = more powerful burn. To overcome this the cone filter needs to be isolated from the hot air of the engine bay, this can be done cheaply with a Rover 820 airbox, or you could buy an induction system like the ITG Maxogen or Mike Satur's Vader / MGF Centre's Taipan system (i do not recommend the Piper Viper as it is supposed to be restrictive at 160bhp plus). These are only worth at most +1 or +2bhp more than the K+N top end, but will eliminate the heat soak problem, thus giving you better performance across the entire rev range as opposed to only when the car is moving quickly.
2 ) 4:2:1 Manifold
Looking at the air path as a whole, the 'choke points' are as follows (in order worst to best):
Air intake tract
Air filter box
We have ruled out head porting due to budgetary constraints, so the next area of concern is the OE exhaust manifold. This has short primaries, short secondaries and a long tertiary pipe arrangement, this is not conducive to efficient power. Also, the manifold has quite a narrow bore and where the flanges are welded on, the welds project into the bore. Some gains can be had by simply grinding off the internal welds and re-welding the flanges on the outside, but to really improve matters here there are a couple of 4:2:1 aftermarket manifolds available. The best one (theoretically) is the E.B.D. manifold, used extensively by the Elise boys. However, to get the maximim length in the primary and secondary pipes the EBD doesn't have a flexible section. On an Elise, with solid engine mounts, this is OK, but on an F, where the engine is on flexible mounts, a rigid exhaust manifold can cause problems. To date, to the best of my knowledge, no one has fitted an EBD manifold to an F successfully.
The 4 pipes immediately out of the head are called primaries, these become 2 pipes, called secondaries, finally they become 1 pipe, called a tertiary - long, straight pirmaries maintain a fast, laminar flow of exhaust gasses, aiding thier expulsion from the engine, likewise long, straight secondaries help too - the F is the wrong way round!
The other alternative is the 4:2:1 manifold by Janspeed. This is also available direct from MG under thier X-Power banner as well as from Mike Satur. This is a compromise on the EBDs perfect solution, the secondaries are shorter to make way for a short flexible section in the initial part of the tertiary pipe. Unfortunately this is set at an angle to the engine and is subjected to large shear forces - i have heard of two flexible sections failing so far - bear in mind that very few people have these at the moment as they are fairly recent.
Clearly both solutions available have 'issues', but the Janspeed/X-Power/Mike Satur one is the closest to a good solution so far and in most cases seems to be effective and robust.
3 ) Trophy throttle body
The limited edition MGF Trophy 160 had a VVC engine with number of engine modifications to up the power from 143bhp to 158bhp. The main one is the air filter set up, the Trophy has a much simpler system. It also had a new aluminium throttle body with a 52mm internal bore as opposed to the plastic version which only has a 48mm internal bore. This doesn't sound like much (only 4mm difference), but we are talking surface area rather than linear dimensions here so it actually equates to around a 15% improvement. It is supposed to only be worth +2bhp, but given that it is directly related to pedal pressure on the accelerator (i.e. for a given amount of pedal travel the TB allows more air into the engine) it makes the car feel more sprightly, with a more instantanious throttle response. Being aluminium as opposed to plastic it also does not suffer from minor warping (which the OE one can do resulting in a sticky throttle, or high idle speed), so there are other reliability benefits to fitting one too.
4 ) Rear silencer
The standard rear silencer isn't actually that bad from a power perspective, likewise the catalyst is capable of handling increased flow rates pretty successfully. An aftermarket silencer is worth probably no more than +5bhp (independantly tested - don't believe the hype!), but is much better made, using better quality materials and both looks and sounds better. Despite being a rather large purchase financially, there are clearly other valid reasons to consider one, not least of which the grin the noise induces on your face!
1 ) K+N 57i - £100.00 ish - +15bhp
2 ) Janspeed exhaust manifold - £350.00 ish - +10bhp (estimate)
3 ) Trophy throttle body - £115.00 ish - +2bhp
4 ) MS Daytona / Miltek Supersports rear silencer - £350.00 ish - +5bhp
£915.00 / +32bhp = 175bhp
Obviously the bhp gains are subjective and based upon individual items being tested separately from each other - you can't simply add bhp gains together to produce a result as there are losses and gains when the items are used in conjunction with each other. But this is a theoretical ball park figure and is +/-10bhp achievable within your budget.
Hope this helps