About different fuels (for Genesis Coupes)
Hello my fellow Genesis Owners and not-owners-but-I'm-in-the-group-ers.
In today's
AlphaSpeed
PSA, we are going to talk about fueling, and two topics inside the broad range of things the word "fueling" encompasses.First, let's talk about octanes and the different types of fuel:
I'm not going to explain what octane is because you can google this easily ( https://en.wikipedia.org/wiki/Octane_rating ), I'm going to talk about the (somewhat) power limits on different ratings and fuels ON THE GENESIS COUPE 2.0T
91 octane: typically can hold do about 16-18psi without knock (emphasis on typically).
93 octane: about 22-25psi without knock
pump E85: about 35psi without knock (maybe more? I haven't tested more than this since we locally don't have E85 pump stations, and no remote tune has been able to supply enough fuel to go past this on E.)
These numbers TYPICALLY don't change when you change turbos, since for all effects and purposes, having the same intake manifold pressure means that on a larger turbo, there will be less heat at the same psi ( since it is more efficient), mitigating the increased cylinder pressures (on this part, I'm sure a mechanical engineer can explain better than me). This is why you can get a GT35 and you will still "only" run the same PSi if you use the same fuel.
The power you get out of changing turbos is because of the increase in raw proportions of the turbos, which you can kind of visualize as the famous straw example. You get a bigger straw, you push more air at the same amount of pressure.
Now, why do we need so much more fuel when running E85? Why don't we need that same amount of fuel on Q16?
This is becaause E85 is way less energy-dense than normal fuel, meaning you need more of it to be able to acheive the same amount of power. And no, you don't make more power on it BECAUSE you use more fuel (more units of fuel do not create more power ON THEIR OWN), you make more power out of it because it has two very interesting properties. It has a much higher octane rating (usually 100+, and yes, 7 points is a ton considering you can push about 7 more psi of boost on 93 than on 91, a difference of 2), and because of the amount of fuel you have to inject, it creates a slight "cooling" effect inside the chamber, removing the heat that causes broken pistons and ringlands (remember, we're using about 33% more fuel to create the same amount of power we would on a petrol based fuel). This is what makes E85 so good, but it's also the reason you need significant fueling upgrades when planning to use it.
BK2's can do this without modification because the fueling system (on both engines) is overengineered by a gigantic margin, seeing as stock fuel system 2.0Ts can break 400whp and 3.8Ts can break 500 without having to change anything. Some simple math tells us that if we can make 420whp on pump gas MAX, then we can make 280whp on E85 on the same fueling system.
420*.33 = 138.6 -> 420 -138.6 = ~280
This is obviously a conservative estimate, since the laws of thermodynamics are quite complicated and the fact that combustion temps drop so much help it make a bit more power, but it gives you a ballpark.
Now remember, percents make changes bigger as numbers get larger, so sure, you can make 300whp on a fuel system that on normal fuel would support 420whp, but what if you want 550whp on E85? You then have to do this math:
550*.33 = 181.5 -> 550 + 181.5 = 731whp
You need to have a fuel system that flows for 731whp to be able to do 550 on E85, so, in laymans terms, you need an ENOURMOUS fuel system to support that.
"Then why can't I just run 93 pump?"
You seem to have forgotten 93 pump can only support about 24psi without knocking (and when I say about, I mean tending downwards, 22-24), while E85 can do WAAAAY more boost.
You just need to be able to supply the fuel.
I hope this helps some people understand the how and why's of E85 vs regular petrol-based fuels, and that there is no such thing as free lunch (for non-BK2s, or for BK2s looking to add mods to their current car that would put them over the fueling threshold of the stock BK2 fuel system (so, anything that is not FBO needs a fueling upgrade).
My fingers hurt.
bye.
**POST-OP EDIT**
I forgot to talk about returns vs returnless fuel setups which is another subject I want to discuss.
Having a boost-referenced return on your car allows your fuel system to compensate for changes in manifold pressure in the feuling system itself. How does this work/how can I visualize it?
Well, a BK1 runs 55psi of static fuel pressure.
Imagine you have a hose and you have a fan blowing equally into the hose, for water to come out, you need to open the faucet more and more,, or the air from that fan is going to keep pushing most of the fuel inside the hose itself.
Well this is basically what happens as manifold pressure raises and fuel pressure does not, the boost itself pushes harder and harder onto the injector, making the injector effectively smaller and smaller as boost raises (since the boost is pushing the fuel into the injector).
"Then why can't I just run a higher fuel pressure, like a BK2's stock 85psi?"
You can, for sure, the problem is that when you increase fuel pressure statically (without boost reference), you injectors now are technically assisted by manifold vacuum at idle and cruising, effectively trying to suck the fuel out of the injector (in the hose analogy, this would by like sucking the hose with the faucet on), and now the injector needs to open and close at a speed it physically can't (if the injector size is too large). By having it boost referenced, you can have it at a low pressure at idle and vacuum conditions, where the injector can open and close at a normal speed (typically about 2ms), then raise pressure as boost raises.
Now, personally, I've been able to idle 1000cc injectors at stoich on our cars, but when adding E85 to the equation (specifically flex tunes, since they need to be able to idle on 93 or E85), the fact that it needs to support gobs of power on E85 means you need a very large injector, but then, when you switch to 93, the injector is too large, and the idle will either be too rich, too high or it will stutter, since the injector cannot open as fast as the ECU needs it to, to supply the tiny amount of fuel you need on idle with 1700CC injectors on 93.