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Q-Jet tuning help (performance Merc 5.7)
Greetings,I am in the process of rebuilding both of the Merc 350's in my boat.
I have added some performance mods, so I know I will most likely need
to re-jet my Qjet carbs. I am seeking some advice for a good starting
point on the jets. Below is a list of the high performance add-ons.
Thanks! Qjet 17059280
Edelbrock performer intakes
MSD everything
Comp roller rockers (1.52)
Comp Extreme Marine cam 262/268, .462/.477
Tuned GLM manifolds, open exhaust
K&N flame arrestors Not real screamers, but should have significantly more performance
over stock. Any advice will be greatly appreciated! thanks --Ray |
Rainmann
May 22
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| Thanks everyone for the information. Your input is appreciated. As far as the noise goes, yes the sound of twin small blocks is
awesome, albeit extremely loud. But, I have Captain's Call exhaust so
it can be turned off with the flip of a switch. The exhaust manifolds have short runners cast into them. Not quite
shorty headers, but light years better than stock. Not to mention
jacketed headers start off around $2500 per set. The problem with a
twin engine boat is, you have to buy two of everything. Gets
expensive fast! I also forgot to mention that I have purchased Vortec heads with some
mild port work. The combination of all this should produce
significant more HP and torque over stock. --Ray |
Rainmann
May 23
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| Carb tuning is a long-lost art. Your cam specs(thanks for staying w/ the Marine/RV grind) staying under
..5, you may be surprised that not much adjustment is needed. Rob |
trainfan1
May 22
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| There are TONS of different Q-Jet models, but for the most part, they
all run fat on the secondaries. You may not need to jet them. If so,
you'll prolly have to pull the jets, get the nums on them and talk to
some guys at perf auto parts shops, where, if you're extremely lucky,
you can enable the 'Geezer Factor' and get some wisdom from some old
fart. Alternately, they may have a jet chart. So might the web. I would
run them first. If you can't pull WOT rpm, other things being equal,
they're lean.
JR
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JR
May 22
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| Sorry to burst your bubble, but the most significant change due to these
"performance" mods is that you are going to make a hell of a lot of noise. Eisboch |
RCE
May 22
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| I admire his industry in the rebuild - that's an undertaking worthy of
a car. I hope he's looking into props to handle the power. It would be kind
of funny to watch the froth and foam. :>) Wel, that and the noise. |
Short
May 22
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| Your carbs will probably be fine. Your mods will increase the overall flow
at higher rpms but that will probably not require a jet change. I hate to burst your bubble but there really is very few tuned marine
manifolds. To really tune an exhaust you need the individual pipes to be
longer than is practical in a water cooled marine manifold. That's why
automotive headers look like they do. Tuned headers also have the
individual lengths adjusted so that the exhaust charges line up in
succession as they are combined at the collector. That enhances the
scavenging effect of the exhaust. Which is also compromised in near-stock
marine engines because of the reduced cam overlap to prevent water ingestion
at idle in a wet exhaust. It's tough in the marine world :-) One thing
aftermarket manifolds do well though is get rid of 80lbs or so of cast iron
at the back of the boat. "Rainmann" <rwillia1@gmail.com> wrote in message |
jamesgangnc
May 23
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