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Discussion and questions related to the course EFI Tuning Fundamentals
Hi, so talking about compression ratios, when it comes to choosing forged pistons for a specific engine, how do compression ratios affect if it is turbo or n/a? Will it depend on the amount of psi and mm of the turbo we are running? Why with turbos the compression ratios must be lower than a n/a race application? Example forged pistons wrx sti EJ25, 9.0:1 and n/a k24 of 12,5:1
The main reason lower compression ratios are used on forced (super- or turbocharged) engines is the fuel used.
On the compression stroke the fuel-air mixture is compressed and this compression increased the temperature of the compressed mixture. You can get the idea if you have a bicycle pump - put your finger over the end and push the pump in rapidly, you will feel the end heat up from the compressive force.
If the compression heat is a high enough temperature, it can cause the fuel-air mixture to ignite - pre-ignition - and this is worsened if there are hot spots in the cylinder and/or wrong heat range spark plug which can raise the temperature around them even further. This is basically the principle behind diesel engines which are called compression ignition engines for that reason.
By using a lower compression ratio the mixture is compressed less and so the heating is less and, as the forced induction is already partly compressing the mixture, the overall compression is about the same. Less mechanical compression allows more 'boost' compression/boost.
Some fuels are more resistant to this and can tolerate more compression, so it's important you know exactly what fuel you will be using and how resistant it is to this pre-ignition when selecting the engine parts.
If you are using a fuel that allows a high mechanical compression ratio, before making your purchase check with the piston manufacturer(s) as a N/A piston may be weaker than a forced induction piston designed for the higher pressures involved.
Hi Gord, So for example built wrx sti 9:1 compression ratio using pure E85, is there a better fuel for the 10:1 compression? and what fuel will be used in case there´s no pure E85, use the pump e85? (just example), what about V-Power of shell? I´m not sure about what octane is it. Here we don´t have gas stations with pump e-85, the best I know is V-Power, octanes here are 93,95,97. Is it possible to use V-Power for the same compression ratio people use in other countries with pump E-85? How acceptable is the error range of the V-Power and pump e-85, I can import pure e-85 for special uses but V-Power should be my daily fuel?
Sorry if it is not written well at all, English is not my first language, Thank u for answering the first question.
Don't worry about your english, it's way better than my spanish - although that is a low bar.
Sorry, I can't help with a recommendation for the CR and/or fuel octane, but there are some smart people who are familiar with the engine who should be able to advise you.
To help them, it might be helpful to give the boost, power, and/or torque you're looking for, the rpm you expect to use, and the charge cooling you expect to use, etc - oh, and the highest air temperatures you expect to be operating in.
What sort of budget, resources and experience do you have to work with?
Hi, thank u for the answer I'm 19 y.o I have been reading about high performance engines maybe 2.5 years ago, I have some experience with them but not so much, I was thinking on buying a proyect car and tune it first n/a then turbo it and then go for 500whp maybe 20 psi with a big turbo depending on what mm. Turbo I choose, redline maybe 7.8k rpm depending if I can find a forged crankshaft (Wanting to make it awd), this is a toyota tercel 5E-FE engine 1.5l almost same as the 1.3 starlet, but I don't see many aftermarket parts for it such as cams, retainers, crankshaft,etc and I want to make it reliable, I saw wiseco pistons 9.2:1 C.R and H beams rods both rated to 800hp, either I don't know if this is the post to solve this questions but can crank, cams, retainers, valves,sleeves custom made for a specific engine or should I just swap it? I would like to run reliable all day, there are already some 500hp 5e fe (Don't know if whp) but I want to see more options, here some mechanics preffer to save the info. 12 metters above sea level, usually days here are at 4/25°C