Cylinder Head Porting
This is one of the most important factors to consider (along with camshafts) when turbo matching your engine as it can have one of the biggest impacts on turbocharger performance. Many people believe (incorrectly) that bigger is better when it comes to port size and design when modifying a cylinder head… this could not be further from the truth when fitted to a turbo car.
A turbocharger relies on exhaust gas speed and expansion to operate correctly so if the exhaust ports are too big the gas expands too quickly, cools and slows down… The same can be said (to a lesser extent) on the intake side. A big port needs more air and time to fill so the net result can be more lag. It is common knowledge these days that the manufacturers are getting better and better when it comes to cylinder head design and manufacture… so much so that some tuners have actually lost power from “porting” a cylinder head… the biggest gains are usually made from carefully tidying, matching and blue-printing of the ports.
Most modern engines are now fitted with at least one knock sensor; some even have one per cylinder. The device can sense internal pinking (not audible to the human ear and relay this information back to the ECU, which can then immediately adjust the ignition timing to eliminate it. A must on any high performance engine.
This is again one of the most important factors to consider (along with cylinder head porting) when turbo matching your engine as it can have one of the biggest impacts on turbocharger performance. Again, many people believe (incorrectly) that bigger is better when it comes to camshaft size and design when modifying an engine… this could not be further from the truth when fitted to a turbo car.
The bottom line is big camshafts with wild lift and overlap work very well on naturally aspirated engines but don’t work on turbo engines. If you want good response and no lag they simply don’t work… end of story! Sure you will get huge power increases but at the very top end of the rev scale, (remember the Robin Reliant at the lights again!) but the bottom end response will be very poor. This sort of cam design can also have a more serious side effect on the turbo as it can increase the chances of inducing compressor surge from the turbo, again due to the gas speed. We have many situations where we have had to down spec a hybrid turbo to fit an engine with a “Ported & Polished Head” and “Fast Road Cams” to gain the same response as a larger turbo on a standard engine.
Camshaft Valve Timing
This is not as critical as the porting and camshaft design but again it can have a big influence on the matching and performance of the turbocharger you select. Cam timing will not have much of an effect on overall performance but there are gains and loses to be had… for instance, retarding the cam timing can often help with response and lag, just the same advancing the cam timing will give more top end power… again it’s the word compromise. Most of the time we find the manufacturers have got it pretty well sorted from the factory (not allowing for mass-production variance). Obviously you can’t beat adjustable cam timing to set it to the optimum position.
This is a difficult one to pinpoint for absolute power and torque gains. A low compression ratio will be of benefit when you are planning to run high boost, if the boost is to high for the compression ratio this will lead to “Detonation” or “Pinging” when under load. A low compression ratio – say 7.5:1 will be of benefit for big horsepower but again the trade off is the response or lag (gas speed again). It is very important that we get back to application and decide what the car is used for and what you are trying to achieve.
In our experience a compression ratio of 8.5:1 is around the mark for the best compromise between response and power, it is also important to remember that you can always cheat a little here and use different fuels (super unleaded/ octane booster) to over come the problems of detonation associated with higher boost levels. The main manufacturers are actually going higher and higher with compression ratios to reduce lag and increase torque, this is mainly due to the really sophisticated management systems now available and the general improvement in the fuel we get from the pump.
There are many more factors that come into play when choosing a compression ratio and one type of engine can be quite different to another.
We have a particular component here which divides the tuning fraternity down the middle… some people believe a drain pipe exhaust system (3.5-4.5”) is the way to power nirvana, where as there is a camp that believes in a well made average size (2.5-3.0”) with flowing mandrel bends will be of more benefit… we tend to sit in that camp for a couple of reasons.
- The engine/turbo combination on any vehicle was never designed to run with a 4” ‘drainpipe’ exhaust with one straight through box at the back.
- These big exhausts often have very detrimental effect on the turbocharger, the turbo oil seal system is designed to run as a pressure differential seal and the lack of back pressure is one of the first reasons a turbocharger leaks oil (smokes at idle and on overrun). How many turbo cars do you see with 4” exhausts smoking at the traffic lights?
- The other important fact is that the waste gate system (on integral waste gate units) are designed to have back pressure to equalise the gas pressure before & after the turbine… this differential can be so distorted by these exhausts that the waste gate can no longer control the boost which leads to boost spikes, boost creep and the worst effect can be over speed of the turbine rotor which will cause premature failure of the turbo.
This would encompass the low end version of over adjustment of the wastegate actuator (to much pre-load) so much so the wastegate can no longer control the boost which leads to boost spikes, boost creep and the worst effect can be over speed of the turbine rotor which will cause premature failure of the turbo, drilling small holes in the actual waste gate actuator or sensing hose (yes it is true!) to bleed off pressure (actually wasting precious boost pressure) bleed valves (crude but better than drilling holes) which is at least adjustable, to various aftermarket electronic bolt kits.
The advantages and disadvantages are fairly obvious (and down to budget) but there are two glaring examples that you should stay well clear of… wastegate actuator adjustment and drilling holes!
The intercooler part of the system is probably the least important in the majority of applications unless you are running very high boost (1.4 bar and above) however, there are things to keep an eye out for, this is the pressure drop across the intercooler (pressure difference from the inlet to outlet side) and the charge air temperature difference again from the inlet to outlet.
The figures for pressure drop should be no more than 1-3 psi if you have anymore than that there is a mismatch somewhere in the system, as for temperatures it is important to keep an eye on the temperature going in to the cooler (this will vary according to boost levels) but on the outlet is where it is critical and you should be aiming for a temp range of 30-40 degrees C… anymore than that and the intercooler isn’t doing the job.
A word of warning here… this doesn’t mean you need a front mount intercooler (you may just need a more efficient standard size intercooler) as we have said before the compromise comes in again, remember that adding 12 feet of 2.5-3” pipe work and an intercooler the size of block of flats you are going to need to fill this extra area and will give you more lag.
The mapping is really what is all about, get it wrong and at best you will have a real thirsty handful to drive and at worst a blown up melted engine! Get it right and difference can be amazing…. there are various methods employed to achieve the best map for the application…the best solution is a live map (all cars are different) on a rolling road with part load capability going down to a mass produced “chip” that is a fix all for all vehicles (not great).
There are some tuners who actually map on the road with wide band lambda sensors and associated test kit via a lap top…this is a great system as it involves real world driving and loads which a rolling road or engine dyno can some times lack, but it is a little risky as it’s hard to concentrate on a map parameter when you are doing 150mph on the open road and if something goes wrong you can’t just stop and turn off the engine!
The best method again will be down budget, but don’t let this final and most important part of the tuning process become the common “close enough is good enough” fix, after all you have spent a lot of money to get to this point why wreck all the good work with a poor state of tune!