Re: TBI, was Re: Hornet status

[Tom Jennings <tomj@xxxxxxx>]

On Mon, 13 Jun 2005 farna@xxxxxxx wrote:

> Scratch the booster pump idea.

Umm well the technique is immaterial if you acheive....

> EFI needs a steady high pressure
> fuel stream for the injectors.

... the goal!

I assume the problem is that whateverthepumpandregulatoris, that
it will "pump up" to it's highest-possible value when there's no
or little fuel being consumed; as soon as you "get on it" and fuel
is taken out the pressure will drop towards the lower end of the
regulated range, meaning the fuel pressure fluctuates with demand.
Not Good.

The circulation business is basically a big "leak" that causes
"demand" all the time, so that the regulator can stay close to the
middle of it's range.

So I'll just circulate back to the front of the booster pump, or
to the front of the mechanical pump. I see no reason why it must
go all the way back to the tank. It would help lower fuel
temperatures, but that's not a problem I see in my application.

> So you need a pressure regulator and return line or an
> in-tank fuel pump with pressure bleed. The only way to get the
> later is to use a 97 or later XJ/TJ pump and tank, or convert your
> tank to mount the pump (not sure how big the hole needs to be,
> might be doable without cutting the tank itself).

Well, the problem is that electric pumps are not very good at
sucking, they cavitate, they prefer to push, plus OEMs made them
physically small to save pennies and stick them in the tank to
cool them too. Ordinary motor-mounted mechanical pumps suck just
fine (big diaphragm) and would deliver 3 - 5 psi to a small
fender-mounted boost pump. Volume isn't an issue for a stock motor
(a concern for high-performance motors).

> There is no problem with using an HEI distributor or knock sensors.

Oh I know HEI works -- but I spent a lot of time and effort on my
TFI and I want to keep it! I suppose I could bolt on a knock
sensor though.  Then there's that nice hack to replace the
Duraspark box with an HEI module -- but also I want to change one
thing at a time now that the car is on the road.


> You may not need an adapter for the throttle body. You might be
> able to do a little port matching and drill/tap the intake. That
> you'll have to see. Since you're using a 2V carb now you'll need
> the small 2V throttle body from a 2.8L V-6 (Holley lists the
> replacement as 400 cfm for 1985.5-1989 S-10/T-10 Chevy/GMC 2.8L
> equipped trucks, I'm sure Camaro/Firebird and other cars used the
> same, but may have direct port injection).

Yeah, the mechanical issue should be the easiest to deal with.  I
can mock that up on the bench before install.

I had a crazy idea this weekend -- wouldn't it be nice to have a
crappy old Carter YF turned into a throttle body, for all those
dead-stock older Americans? You'd need to weld an O2 sensor into
the tailpipe, but you could use a clamp-on coolant temp sensor
(I've played with this and found you get nearly no error outside
to inside the water hose if the outside is insulated.) It would
look stock(ish), accept stock aircleaner and throttle linkage, and
except for the O2 sensor, cable to a little box of electronics,
and an added fuel pump would pass as stock.


(I'm not seriously proposing this, but for really small demand
engines, like a stock 196 or 232, I bet that with a large-bore
injector and software that monitored fuel pressure, you might get
TBI to work at 5 psi, eliminating the added pump. It wouldn't be
hard to work out of you could get the fuel flow needed. The
variable and low fuel pressure would be the problem. A huge
injector might minimize the duty-cycle.)