fuel pump.

Re: fuel pump.

Postby smokey15 » Tue Aug 27, 2013 7:28 am

Luke,

That fuel pump, I think, was installed in 1996 just before I went to Pikes Peak. I never had trouble with it feeding the engine and never cleaned the screen. From your pictures, it certainly needed your expertise and attention. As far as spark plugs, the Autolite R135s worked best at hill climbs because of the need for a hot spark over a short period of time. When I ran Pikes Peak and/or NHMS, I used a colder plug - R133 or 34.
You missed a great event at Burke II this weekend. The weather was perfect and, for those who wanted or were able to, got 6 runs on Sunday. Hope the fuel pump and carb issue works out for you at Ascutney.

Smokey
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Re: fuel pump.

Postby Chief Geek » Tue Aug 27, 2013 12:57 pm

I don't know much about carb-amur-ators, someone please educate me.

Q: Wouldn't Luke's engine have gone scary lean when the fuel level in the bowl fell? Functionally, Luke is describing the mechanical analog to an EFI system that didn't maintain full fuel pressure @ full load and high revs. That situation in an EFI car, especially those with forced induction, has caused many pistons to meet firey deaths.

Also, what does the collective NEHA brain-trust say the recommended fuel-tank-cleaning interval should be? I'm scared to think of what can accumulate in a small car's fuel tank after 145k miles with, best I know, the original fuel pump.

Paul
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Re: fuel pump.

Postby Mopar 151w2 » Tue Aug 27, 2013 2:35 pm

Basically, naturally aspirated engines are a good deal more tolerant of tuning error than those with "power adders", whether turbosupercharged (the correct term), supercharged, "sprayed" with nitrous oxide, or fueled with an oxygenate like nitromethane.

It is actually the imperfection of carburetors that is a saving grace in situations like this. In a dynamic enviroment, a fuel supply problem will rarely lean the engine out evenly, so that cylinders will tend to misfire randomly when the fuel level in the float bowls go down. Luke's DaVinci/Holley has 2 seperate float bowls - so they are not apt to go down evenly. And it holds about a pint of gas in those float bowls - so if your supply is marginal, it'll run hard 'til it depletes the inital supply of gas - lift, and it'll refill the bowls.
Is it possible to burn a piston in an N/A race engine? Oh yeah, it is! You see a lot less of it these days, as modern ignition systems will fire a richer mixture, particularly with the larger plug gaps that are possible. But, in the old days, the margin between "Max power lean" and burning pistons was pretty thin,and, in some cases, virtually nonexistent. Throw in some detonation - rich mixtures are often used to prevent detonation, and several factors make lean mixtures prone to it - and it does'nt take long to toast a couple. Another saving grace is the tendency of "high aromatic" gasolines - like race gas or Luke's avgas - to not load plugs when rich like pump gas (particularly unleaded).
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Re: fuel pump.

Postby Challenger392 » Wed Aug 28, 2013 12:47 pm

After cleaning the pump and re-installing it in the car it sounds much healthier now. It must have been noisy because it was struggling to draw out of the tank. Concerning spark plugs, Fr now im going to replace them like I should have at the beginning of the year, The gap on the old ones is pretty large due to erosion, what should I be setting the new gap to? I also started playing around with the rear suspension, one problem being that the forward bolt on the RH link was quite loose, Im blaming that for slight change in direction that the car had when changing from acceleration to deceleration. That also could have been a factor in my wheelbase measuring shorter on that side. After tightening that up and adjusting that link a little, My wheel base is now square. Well see how it handles at Ascutney, Ill make sure to bring wrenches to make adjustments if need be.
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Re: fuel pump.

Postby Chief Geek » Wed Aug 28, 2013 1:48 pm

Mopar 151w2 wrote:Basically, naturally aspirated engines are a good deal more tolerant of tuning error than those with "power adders"


Can I assume that the culprit is the higher cylinder temps, thus hot spots, caused by higher compression ratios? In this respect, isn't forced induction effectively same thing as higher compression?

Paul
"Christina", New #13/#55x, '90 Miata: In progress
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Re: fuel pump.

Postby Mopar 151w2 » Wed Aug 28, 2013 2:52 pm

No, not really. Higher compression ratios tend to "spike" cylinder pressures, due to the phenomenon of adiabatic heating - air heats when compressed, raising it's pressure, and the cycle kinda feeds itself - It's what a diesel depends on for ignition (at around 20/1 static compression). Forced induction and a lower static compression will have less adiabatic heating than high static compression, for a similar bmep (Brake Mean Effective Pressure, which is what makes power)thus making the "spike" lower - making detonation less likely.

The increased heat with "Power Adders" come from having more oxygen and fuel in the combustion chamber - simple as that. At some point, enough free oxygen and heat may be available to burn metal. Increased cylinder pressure makes the "torch" more aggressive. A lot of getting engines to live with power adders has to do with not giving the torch a place to start cutting!
Morway's Junkyard has an old cutting torch which runs on gasoline and oxygen, and it cuts just fine - they hid it away deep before some fool got hold of it and left a crater.
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Re: fuel pump.

Postby Chief Geek » Wed Aug 28, 2013 5:25 pm

Adiabatic heating is clear, it is why intercoolers exist.

I'm asking if a N.A. engine (1 bar absolute) w/ 12:1 compression would be different than a turbo engine w/ 14 psi of boost (2 bar absolute) and 6:1 compression. The mass of air into the cylinder would be the same so the max output would be the same. Also, relative to the outdoors, the compression ratio is the same. All other things being equal, would either of these engines be more likely to pre-ignite than the other?
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Re: fuel pump.

Postby Mopar 151w2 » Wed Aug 28, 2013 8:00 pm

In practice, there are several factors that mitigate the abiabatic heating inherent in supercharging - and a couple that exacerbate it.
Almost any forced induction setup will radiate some heat from the intake tract, even without a nominal intercooler (aftercooler in diesel parlance). "Remote" turbos are an excellent example.
Second is fuel evaporation, which is significant even with gasoline, and huge with alcohol - the turbo IRL V-8 "oldsmobile" engines used NO seperate intercooler, but a single main injector at the turbo outlet and a relatively large intake plenum. Carbureted N/A alky burners will have ice on the intake runners after a race. :shock: Most who make serious power with a Roots blower use alcohol fuel for just this reason - and a lot of it goes in before the blower.
Third, in many applications, forced induction's air intake location is optomized for temperature and density - this is harder to do with N/A engines, and fuel vaporization/emission issues with carburetors often dictate exactly the opposite.
And there is the time element - at 5000 rpm, the compression stroke takes 6/1000 of a second, so there is no time to dump any of the heat created in the cylinder, and little place to dump it to.
Exacerbating factors include heat transfer from the "hot side" of turbos, and considerable friction in Roots blowers.

ATTN : webmaster - can we make this discussion it's own thread, lie "Engine Theory" or something? We ran over Luke's fuel pump back around turn 14 or so......
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Re: fuel pump.

Postby Challenger392 » Thu Sep 05, 2013 7:31 pm

Bought a new set of plugs what's a good gap to use for an engine like mine? The plugs came with an average of about .025. Is that too small?
Luke Moultroup
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Pratt & Whitney Engine Services

http://moultroupracing.blogspot.com/

2012 Dodge Challenger SRT8 392
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2006 Jeep Grand Cherokee 5.7
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Re: fuel pump.

Postby Mopar 151w2 » Thu Sep 05, 2013 9:13 pm

.035 is usually a safe figure with modern ignitions. The paperwork that came with my HI-6 box (similar to yours) says that .045 is a starting point. It did not work well for me when I tried it, but I likely had other issues, and the car ran fine with .035. I've since resolved the issues, and had not gotten curious enough to A/B plug gaps since.
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