Injectors

[matt]

The FSE is a fuel pressure regulator....... adjustableish  

 

Ash

You're a lucky man ..........bolt-on is the only option 4 most of us ...........

[mark brown]

Ash is right about the weakness re AIC fuelling, but I will in the short term accept this. I would prefer to fit 720cc injectors now controlled by an AFC or similar to give margin for future mods. I know the proposed set up is not going to be perfect and I will not maximise the power available, but I guess on the proposed set up there will be more power than can reasonably be used., although I always say that after each mod.

 

The FSE is just an adjustable fuel pressure regulator. Increasing fuel rail pressure will richen the mixture, but the amount of fuel that can be injected will be limited by the injector capacity. Previous experience of increasing fuel rail pressure too far on another vehicle resulted in leaks from pipe joints etc

 

Ash, as I'm determined to go for a turbo upgrade from my current hybrid set up, which set up will be better, twins or one large single?

[Guest Ash]

 

Increasing fuel pressure is not really a practical way of getting any significant extra fuel into the engine. Okay, if you are going a tad lean at WOT then you might decide to increase pressure a little bit just to bring the A/R down a little.

 

But the key phrases in the above paragraph are "tad lean" and "a little bit".  

 

Problem with increasing pressure is you need 4 times the pressure to double the flow. Consider that the system pressure runs at 36 psi stock, and the upper limit of the fuel injectors is in the region of 65 psi, then you don't have much pressure headroom. Especially when running increased boost.

 

At 20 psi boost pressure, for example, the fuel line pressure will be at 56 psi. So you are already pushing the boat out quite a bit. Push it out much further and you can run into problems with things like injectors sticking and or leaking, leaking pipes, burst flexy hoses, etc.

 

Keeping the stock line pressure and increasing the injector size is a far better way of doing things; as in order to double the flow, you only need to increase the injector outlet diameter by 1.4. Chances are, though, the fuel pump will have to be substituted for one that can pump a higher volume of fuel.  

 

Yours,

J

 

[Guest Ash]

 

 

Mark.................

 

Personally, I've gone right off turbo-charging so I'm biased against twins or a single. But if I were you I'd fit the twins. Chances are, equivalent twins will be slightly less laggy than a big single. Plus the twins' install looks much better IMO.

 

Yours,

J  

[Paul Booth]

Quote: from Ash on 9:57 am on July 18, 2001[br]

 

Mark.................

 

Personally, I've gone right off turbo-charging so I'm biased against twins or a single. But if I were you I'd fit the twins. Chances are, equivalent twins will be slightly less laggy than a big single. Plus the twins' install looks much better IMO.

 

Yours,

J  

 

Interesting point: which has most net effect, the lower inertia in the twin system and therefore faster spin-up rate, or the lower energy availble in the exhaust gases and hence slower spin-up rate.

[Branners]

I dont think we ever really got a proper answer on this when it was discussed on the big list.

 

The twins obviously have smaller internals which can spin up quicker, but then the air flow is split between them meaning that the overall performance gain is very similar to a single turbo which will have all the airflow but larger internals.

 

Either way having seen Leons car in various states of tune I can tell you that fueling, timing and airflow are the vital ingredients. On a low compression engine lag is even more noticeable and you need to do everything possible to eliminate that lag.

 

And as Ash says, the twins do look lovely. In the gallery on mkivsupra.net theres a couple of pictures of Stew Baileys single turbo and a couple of Leons twin set up and the twins do look very nice indeed.

 

JB

Shop!!

[Guest Ash]

 

 

I looked into the big-single or two half-sized twins argument when I was researching the possibility of fitting an uprated sequential turbo system.

 

Which I subsequently decided was simply not possible to do. Well, what I mean is, it was not possible to do *and* design items such as the exhaust manifold, for example, with maximum flow efficiency in mind. It simply became a question of too many pipes in too small a space.

 

On the surface it would appear there can be no difference between two twins or a single. Simple explanation being, one big turbo gets all the exhaust and two half-sized turbos get half the exhaust.

 

But, like many things in life, all is not as it appears on the surface.  

 

For the benefit of anyone who wishes to follow it, I'll run through the basic calculations.

 

The time required for the turbo to spool to maximum speed is a function of the overall turbo efficiency and the Polar Movement of Inertia of the rotating mass.

 

Moment of Inertia is the resistance of a rotating body to a change in speed, represented by the letter I.

 

I = Ksquared x M

 

Where K is the radius of gyration and M is the mass of the body.

 

The Radius of Gyration is the distance from the centre of the rotating mass to the point where all the rotating body could be said to have the same "I" as the body itself.

 

Basically, because the outer of the turbine wheel has blades with an air space (obviously) inbetween. Calculating K is a little different than if the rotating mass were a simple, solid circular body. In which case, calculating the Radius of Gyration would be a simple act of measuring from the centre axis to the outside edge.

 

But as we have a body with vanes on the outside, you have to calculate what would be the equivalent-sized solid body with the same Moment of Inertia as our bladed wheel.

 

For example, a turbine wheel with a total diameter of 2 inches, may be represented by a solid body, of the same material, with a diameter of only 1.2 inches. Each would give the same Moment of Inertia. Therefore, "K" in this example would be half the diameter of the equivalent-sized body, i.e. 0.6 inches.  

 

For fast spool-up, you need a turbine wheel with the lowest possible Moment of Inertia. That is why turbine wheels are designed with the absolute minimum of material near the *outside* diameter: to reduce "K" as much as possible.

 

Incidentaly, that is also why when lightening a flywheel you always remove material as close to the outside diameter as you can. In effect, what you are doing is the very same action of reducing "K".

 

Now this is the crux of the whole thing............

 

The Moment of Inertia is proportional to the *square* of "K".

 

Which means, reducing "K" by half does not cause a corresponding reduction in Moment of Inertia by half: it reduces "K" by a quarter of its previous value.

 

So say we have a big-single turbo with a "K" of 2 and a rotor weight of 1 lb. So "K" squared x 1 equals an "I" of 4. Compare this with a half-sized turbo with a "K" of 1 and a rotor weight of half a lb, gives an "I" equal to 0.5. Double this, because we have two of the half-sized turbos, gives a total "I" of 1. Which is a quarter of the value of the big turbo.  

 

And that's basically why two half-sized turbos will generally spool faster than one big one. All things being equal, the Moment of Inertia of the rotating mass will be a quarter of the big-single.      

 

Yours,

J  

[Phil Wall]

A brilliant post J.

 

Which I understand!

 

Thanks for the information.

[Paul Booth]

Aren't there any issues around volume x surface area ratio with respect to the exhaust gases and pipe sizes, i.e., two smaller pipes have a much lower volume x surface area ratio and there's a much higher heat loss with 2 tubines.

 

[Digsy]

Quote: from Paul Booth on 9:08 pm on July 16, 2001[br]But to chuck fuel into the intakes on an already injected engine seems to be a direct route to an unmanageable power profile; or a straight line dragster whereby the throttle is a binary switch.

 

Doesn't look like it has any elegance, but maybe I'm being a bit of a purist.

 

Just my twopenn'th worth.

 

Additional fuel injectors are sometimes used in production engines. The Lotus 3.5l V8 has eight sequential "normal" injectors hovering over the intake valves and an extra two injectors in the plenum just downstream of the throttle body.

 

Interestingly, these are actually there to keep the intake air cool (through evaporation) and they only come on at full boost when the intake air is hottest and there is a risk of knocking. They are of course correctly mapped into the ECU.

 

Even more interestingly, you can get big increases in power if you put the fuel injectors BEFORE the turbine compressors. The reason has something to do with therodynamics which my brain is far too small to understand, but it does work, at least in a "development" environment. No idea what it does for emissions and the durability of the turbos though, although I can hazard a guess...

 

Regards all,

Darren

[Paul Booth]

I seem to remember when F1 cars were turbo'd, they experimented with burning fuel in the exhaust turbine to create a low pressure burn there and keep the turbo spinning at a reasonable speed, under certain conditions.

 

I'm seriously thinking about going back to what I understand: polished ports, high lift cams with loads of overlap and a BIG bunch of bananas.

 

[Guest Ash]

 

 

There's a big difference between slapping on some universal-fit thingy, and a fuelling method that has been carefully considered and thoroughly tested by a commercial car manufacturer.

 

All manner of things can be different, i.e. the ratio of fuel between the two injectors, the direction in which they point, the actual injector spray pattern, fuel pressure, etc., etc. All of which would have been seriously tested under all kinds of conditions.

 

Yours,

J