Fuel system questions

[Husqy510]

We're installing a naturally aspirated 700hp 540 in our Howard Cat. I'm getting ready to plumb the fuel system and have some questions.

The motor is running a 1050 Dominator with a -6 line supplying each float bowl. The two -6 hoses goes back to a diverter fed by a single -8 from the mechanical fuel pump.

On the other end of the fuel system we have two tanks with 3/8 nipples for the fuel line. Each fuel tank goes to a tank selector that has provisions for 1/2" fittings. The fuel then goes to a fuel filter/water separator that has 3/8" fittings. From the fuel filter it goes to the fuel pump which has -8 fittings.

I was thinking about upgrading the fuel filter mount to something that supports -8 fuel line so I can run -8 from the tank selector to the fuel pump. However, logic is telling me it doesn't matter as the tank pick ups are 3/8". What do you guys think? Here some pics for reference.

 

[rivermobster]

No one is going to answer this...

No one is going to want to be blamed, for any problems you have down the road!

I'll tell you what I would do if it was me though, get rid of the 3/8 fittings and check valves in the tank. Go to 1/2 at least with no check valves. IMCO makes a nice kit. Make sure the tanks are well vented too.

Good luck with your project.

 

[farmo83]

What is the kit IMCO makes ?

 

[rivermobster]

What is the kit IMCO makes ?

They probably make them to order depending on what tank you have.

I got a kit for the project I was doing. Worked perfectly.

Cut a hole, drop it in, plumb it up. Instant 1/2 tank fitting.

 

[Husqy510]

No one is going to answer this...

No one is going to want to be blamed, for any problems you have down the road!

I'll tell you what I would do if it was me though, get rid of the 3/8 fittings and check valves in the tank. Go to 1/2 at least with no check valves. IMCO makes a nice kit. Make sure the tanks are well vented too.

Good luck with your project.

Hopefully you're wrong. I thought this was still a forum where guys could go and discuss builds, what they did, or what they're doing. I'm hoping other guys with similar set ups will discuss how they built their fuel system.

Thanks for the suggestion on the imco kit. I was hoping I wouldn't have to do a lot of cutting, but if that's what we need to do than so be it.

 

[Tahoe540]

I too have this issue in my offshore boat. 3/8 fuel pickups in the tanks makes it a hard switchover. I did a lot of reading and I ended up running 3/8 line all the way (including the fuel valve) in my N/A 555 (not quite 700HP) but I also am running a 950 cfm 4150, not a dominator. It did not seem like the motor was starving in my situation. I would agree a -8 AN or 1/2" line would best.

 

[Husqy510]

I too have this issue in my offshore boat. 3/8 fuel pickups in the tanks makes it a hard switchover. I did a lot of reading and I ended up running 3/8 line all the way (including the fuel valve) in my N/A 555 (not quite 700HP) but I also am running a 950 cfm 4150, not a dominator. It did not seem like the motor was starving in my situation. I would agree a -8 AN or 1/2" line would best.

Thanks for the reply. I've done a fair amount of research and it seems like 3/8 fuel line will support a lot more hp than people think. I'm running a fuel pressure gauge, so that should tell me pretty quickly if I have a problem .

 

[Husqy510]

It also seems like their is no advantage to running 1/2 fuel line anywhere in the fuel system if the pickup is 3/8"

 

[Tahoe540]

It also seems like their is no advantage to running 1/2 fuel line anywhere in the fuel system if the pickup is 3/8"

That is were I was at is why I did not switch my cooler and fuel valve fittings.

Thanks for the reply. I've done a fair amount of research and it seems like 3/8 fuel line will support a lot more hp than people think. I'm running a fuel pressure gauge, so that should tell me pretty quickly if I have a problem .

That is exactly what I read. 3/8" will support some good HP. I think having a good pump will obviously help. I am running plain old Holley Marine 110Gph mechanical pump.

 

[Husqy510]

That is were I was at is why I did not switch my cooler and fuel valve fittings.

That is exactly what I read. 3/8" will support some good HP. I think having a good pump will obviously help. I am running plain old Holley Marine 110Gph mechanical pump.

I'm running the same pump. I think I'll run it as is and keep an eye on the gauge.

 

[zx14]

Ill answer
I would do 1/2”(-8an) all the way from the tank to the carb. And you are right, if you start off with 3/8” the rest getting bigger won’t help much.

 

[farmo83]

Boostpower mentioned this a couple of times in the podcast below. He suggest removing the anti siphon valves(sort of), running a bigger line after the pickup, and a bigger fuel filter or reserve tank inline to help the motor have a reserve of fuel. He gets on this topic after about 50 min into the show.

 

[Husqy510]

Boostpower mentioned this a couple of times in the podcast below. He suggest removing the anti siphon valves(sort of), running a bigger line after the pickup, and a bigger fuel filter or reserve tank inline to help the motor have a reserve of fuel. He gets on this topic after about 50 min into the show.

That was great, thanks. I'll remove the anti-siphon and run the larger fuel line with a large fuel filter. Sounds like Alexi thinks you can get away with 1000hp with 3/8 fuel line, so it should be fine for my 700hp deal. I have the fuel pressure gauge, so that should tell me if I have an issue.

 

[farmo83]

Let me know how you do. I'm about a year behind you in going with a 700hp deal for my setup and I have 3/8 lines.

 

[obnoxious001]

It also seems like their is no advantage to running 1/2 fuel line anywhere in the fuel system if the pickup is 3/8"

I would argue that the larger lines would hold more volume in them even though the pickup is smaller, so they could be an advantage. Plus, if you later decide to improve the pickups, the larger lines would already be in place.

 

[farmo83]

I would argue that the larger lines would hold more volume in them even though the pickup is smaller, so they could be an advantage. Plus, if you later decide to improve the pickups, the larger lines would already be in place.

How much would you be comfortable running on 3/8 line ?

 

[obnoxious001]

How much would you be comfortable running on 3/8 line ?

I would always chose to err to the side of excess given the chance. If you dyno an engine, you should learn how many pounds of fuel it is using at a given rpm and horsepower. If you are running your engine at maximum output (assume 700 HP since that's what you stated), it will require that your system delivers that much fuel reliably so you don't lean the engine out.

 

[Husqy510]

I would argue that the larger lines would hold more volume in them even though the pickup is smaller, so they could be an advantage. Plus, if you later decide to improve the pickups, the larger lines would already be in place.

My brother and I were just debating this. I'm not sure a bigger filter and fuel lines will help under full throttle load. In a proper fuel system there is no air in the system. Fuel going into the motor must be replaced in the fuel system, which is being drawn through the 3/8 fuel line. Some people believe the extra volume of fuel in the lines and filter will act as a safety, but I disagree. For example the big filter doesn't provide a safety because it can only flow fuel as fast as it can be replaced through the 3/8 feed, it will never draw it down.

What do you guys think? I do like the idea of larger fuel lines in case we upgrade the pickup later.

 

[obnoxious001]

So circle track guys do this why?

 

[Husqy510]

I honestly have no idea what circle track guys do, nor do I know much about building engines, but I do know a little about hydraulics and I'm hoping to spark some healthy debate. Does anyone think the fuel pump will run away from the supply and draw down the fuel in the lines and seperator?

 

[Riverbottom]

I don't have any performance engines in boats anymore, but inspect the AN fittings you are using. I would find that some of the cheaper fittings were very restrictive compared to high quality fittings. I have found #8 elbows that did not even measure 3/8 of an inch on the inside.

 

[farmo83]

Do you have saddle tanks ? If so I'm assuming you could run 3/8 lines into one larger line through a Y or T fitting.

 

[Lavey 29]

the stock filter mounts can be your cork in the bottle. Some of the internal holes can be smaller than 3/8 s so check that out real well. Also I would not use the Merc fuel filters as the internal quality is junk. I know I cut open filters everyday. I would use the biggest fuel filter / water separating as possible. Racor makes some really nice filters with the metal bowls at the bottom to check for water in the fuel. I like the racor S3232TUL. Innovation marine uses them a lot and did for the older DPX Volvo 600 as well. Those engines in their day were runners

Also some of in the gas tank pick up tubes can have a filtering screen at the end of them. Sometimes that screen can be restriction from contaminants stuck all in them. Also I have seen cracks in the pick up tubes especially at the treaded areas. Also seen pick up tubes were a rubber hose extension was added to make it longer and that rubber hose extension was collapsing that caused a fuel delivery restriction especially at WOT. Actually seen that more than expected. Anyways just throwing out some things to chew on about boats and their fuel systems.

 

[Lavey 29]

Forgot with some of the racor filters especially the one I mentioned above -- a bigger mount is needed. I like the biggest filters one can run. Just saying.

Also wanted to ask just in case I missed something in the OP pic why a duel stage water impeller pump ? NA motor correct ? Are you going to feed one side direct to the headers or I must have missed something in the pic ?? THANKS

 

[Husqy510]

the stock filter mounts can be your cork in the bottle. Some of the internal holes can be smaller than 3/8 s so check that out real well. Also I would not use the Merc fuel filters as the internal quality is junk. I know I cut open filters everyday. I would use the biggest fuel filter / water separating as possible. Racor makes some really nice filters with the metal bowls at the bottom to check for water in the fuel. I like the racor S3232TUL. Innovation marine uses them a lot and did the older DPX Volvo 600 as well.

Also some of in the gas tank pick up tubes can have a filtering screen at the end of them. Sometimes that screen can be restriction from contaminants stuck all in them. Also I have seen cracks in the pick up tubes especially at the treaded areas. Also seen pick up tubes were a rubber hose extension was added to make it longer and that rubber extension was collapsing cause a fuel delivery restriction. Actually seen that more than expected. Anyways just throwing out some things to chew on about boats and the fuel systems.

Awesome stuff. I'm going to pull the pick ups and inspect everything closely. Im also going to run -8 from the tank switch through a quality fuel filter/separator to the fuel pump.

 

[Husqy510]

Forgot with some of the racor filters especially the one I mentioned above -- a bigger mount is needed. I like the biggest filters one can run. Just saying.

Also wanted to ask just in case I missed something in the OP pic why a duel stage water impeller pump ? NA motor correct ? Are you going to feed one side direct to the headers or I must have missed something in the pic ?? THANKS

We got a great deal on the accessory kit which included the dual stage pump. We're going to plumb one for the exhaust and one for the motor.

 

[Lavey 29]

So I was correct about the headers one side of the dual stage goes to that. I thought so but was not sure if the set up in the pic was going to change. THX

 

[wzuber]

one of the first things I do on any boat build that has the potential for hi h.p. etc. is pull the tanks and have a couple larger bungs welded in so I can run about any size p/u that may be necessary as well as allow for a return line at the front in the event the fuel system should require it some day. If their glassed in tanks I set them with brackets and bulk heads for easy removal service down the road. It also allows for a thorough inspection and cleaning. You never know if a seam weld is cracking or some damage somewhere etc. that could lead to a problem down the road. I've pulled alum. tanks out of boats that were rotted thru on the back side from leaves and debris trapped between the hull and tank, hardware dropped between etc. rubbing and chaffing their into the glass and alum. etc. It can be pretty surprising sometimes what ya find back there.