Fuel Pressure Relating To Hose Size 1

[StephT]

I'd really appreciate some help with the design of a fuel supply for a fuel injection conversion of an engine currently fuelled by twin carburettors. The engine is a 4 cylinder 1.2 litre Honda from the 1970s. It currently has 6mm ID fuel lines and hoses feeding twin 34mm CV carburettors. For the conversion I will be fitting 4 x 38mm individual throttle bodies that were sourced from a 2006 Honda CBR600 motorcycle. The use of these types of throttle body assemblies has become commonplace on the later series 4 cylinder Honda engines, but to my knowledge only one person has done such a conversion on the 1.2 litre engine, and with great success. Fortunately I have a good relationship with this person, but his engine was developed for full race conditions alone, whereas my application will predominantly be for the street.

My problem is the selection of fuel line and hoses. I was hoping to retain the original fuel line from the fuel tank to where it enters the engine bay. From that point on I plan on using a 6mm ID to 8mm ID fuel line adapter, and run 8mm ID hoses from there. In the first instance it will feed into a large FRAM HPG-1 fuel filter, which is often used in this application as it doubles as a surge tank, and from there the fuel line will lead directly to the fuel rail attached to the throttle bodies and injectors. My question is, how much of a restriction will the 6mm fuel line be to fuel pressure supplied to the throttle body rail? The volume of fuel is not an issue, as the engine itself will not use much more fuel than it did with the twin carburettors fitted, it's whether the pump will be able to supply a minimum of 50psi to the rail. The fuel pump is to be located next to the fuel tank, at the lowest point possible, while still safely positioned, where it will push the fuel up to the fuel filter and fuel rail. There will also be a small pre-filter immediately before the pump to protect it from 60 micron and above sized particles, whereas the aforementioned FRAM HPG-1 filters down to 10 microns.

Hopefully this makes sense. It'd be ideal if there was a formula, or some way to calculate the pressure reduction that occurs due to a reduction in fuel line size? For example, if a pump were capable of supplying 90psi through a 10mm ID pipe, what pressure would it be capable of supplying through a 6mm ID pipe, or an 8mm ID pipe?

Thanks,
Steph

 

[patprimmer]

TTF

I am fully aware of the wank factor. If I m honest I am trying to bait the OP into admitting it so we can stop wasting time on faux technical questions.

Norm.

I know OEM EFI has baffles, but I still got surge from sucking a bit of air despite the baffles and till never had a pump fail.

I do agree a better tank would be good and I think trying to fix the OEM tank would be difficult and dangerous.

Despite my comments to TTF, my thoughts run to a maybe one gallon tank close to the front centre and slightly below the bottom of the OEM tank with a couple of well placed large bore pipes with non return valves connecting them as a gravity feed also with a breather connecting to the filler neck. Install the pump into this tank.



Regards
Pat
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[izzmus]

My personal car has an unbaffled tank and EFI. (RX-7 converted from carburetion) Fuel pump is a Bosch unit intended for an older turbo Subaru.

The world has failed to end in the four years since the installation. The only problem is that, when the fuel is down to the last two or three gallons in the (wide, flat) tank, hard left-hand cornering sloshes the fuel away from the pickup and the engine will cut out momentarily. The fuel pump does not seem to care about running dry for that brief period of time, and to be honest, if it did have a problem with that, I'd start looking for a better pump!

The problem with running low on fuel is mainly an issue with in-tank pumps, which tend to be far underbuilt relative to external pumps. Modern cars have an internal surge tank to always keep the pump immersed in fuel so even this isn't much of a problem anymore.

 

[LionelHutz]

TTF - I'm not sure what your issue is. I never posted there was any other flaw with the proposal. The OP had posted, before it was edited, in the 21 Jul 11 9:59 post that the Fram filter works as a surge tank by running a second pump after the filter, something he clearly didn't describe in his opening post and the reason why I then posted that he needs to get his story straight.

If I had known that not bothering to describe a second irrelevant filter to issue of the Fram filter being a surge tank would be such an issue then I would have included it. The configuration changing from;

tank -> pump -> Fram "surge tank" filter -> fuel rail

to

tank -> filter -> pump -> Fram "surge tank" filter -> fuel rail

Makes absolutely no difference in my comment of the Fram filter not funtioning as surge tank.

Apparently, I should have just followed my first instinct and hit the red-flag link and reported this post as a hobbiest post.

 

[thruthefence]

"Apparently, I should have just followed my first instinct and hit the red-flag link and reported this post as a hobbyist post."

Rodger that!

Instead we get rage and vindictive over a non-issue, from you, and the OP.

Of course it's not a surge tank.(I've also heard surge tanks referred to as "header tanks")

It's a filter. This is simply a matter of (as you say) a hobbyist being confused over technical terms he has heard from others in his peer group, whom he respects for their accomplishments, and became defensive when someone attempted to correct him.

At the risk of starting more controversy, I submit that a "coarse filter" at the pump inlet is not a "second irrelevant filter", but one intended to protect the high pressure pump from FOD, and is good practice whether you are "Ricer" or an OEM.

If I have offended you, or anyone else, I apologize.

 

[potteryshard]

I have a related (and similarly outlawed) question:

I'm working on an EFI system that uses three TBI (GM throttle body injection) units.

Each TBI contains it's own fuel pressure regulator. I could feed fuel to all three units in parallel, but getting the FPR settings to match seems like an ordeal. I could remove two FPRs and series the string of TBI to let the last one control pressure. Or, because the fuel return passages are pretty small on the TBI, I could parallel the two non-FPR units and series the return to the third unit for regulation.

Most systems I've seen use one FPR to control the entire system. For those of you who actually know something about EFI, what seems to be the least troublesome?

 

[LionelHutz]

The only controversy about what you posted is the fact that I was quoted out of context.

 

[thruthefence]

I apologize once more if I missed an original post, and misconstrued what you were replying to.

I was not aware, that a post could be edited, "once out there" .

http://www.eng-tips.com/viewthread.cfm?qid=249557

Such edits do in fact "confuse" the readers, as stated in answer #3 in the above link, (which explains why posts cannot be edited).

Gosh, I'm REALLY confused now!

How does one edit a post?

 

[LionelHutz]

pottyshard - Interesting project. I would remove all the stock regulators, plumb all 3 in parallel and then put a single regulator on the return line. But, I would think 2 in parallel running to the 3rd stock regulator would also work.

TTF - I would suppose the post was red-flagged and asked to be changed.

 

[patprimmer]

I red flagged the thread and it was then edited. I am surprised it did not disappear completely on the grounds of it being clearly not work related and the OP clearly is not any kind of even amateur engineer.

potteryshard.

There is an outside chance your thread might remain if you ask intelligent questions (which it seems you have) and you show some ability to understand the answers and some appreciation for them AND if you start your own thread rather than hijack someone elses thread, especially one of very low quality.

Regards
Pat
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[Lou Scannon]

'nuff said.

I forgot what I was going to say