What size carb for a Ls

[Basin_Runner355]

Looks like the ls2 has a lot less timing I guess that’s Because a factory ls1 and ls2 the ls2 has smaller combustion chamber which bumps up compression and therefor less timing to prevent detonation ?

 

[Basin_Runner355]

I’m mistaken it’s on ls2 ho

 

[Slidin Gator]

What heads?

It has trick flow heads.

Basin,

When I asked what kind of heads I was looking for Cathedral Intake Port vs. Rectangular (i.e. LS 1/2 vs. LS 3). From the ongoing discussion, I assume Cathedral port, the correct heads for a carb setup

Why don't you use the formula ...it has nothing to do with" what you think"

Because the formula don’t add up. Gm puts a 750 cfm on the zz4 motors why would more motor need less carb

This is a valid discussion for sure. The LS motors move more air (vs. old small/big block) for a given displacement. They can handle more carb vs. a similar displacement small block at similar RPM's. That said, there are plenty of LS calculators out there, but most will over carb for an airboat because they are aiming for max Hp at 6,300+ RPM (AKA zz4).

650 CFM is the small end of carb for a 408 LS turning 5,500. But the 540/540 numbers speak for themselves, that is a high torque figure. I would focus on tuning the 650 carb and your timing curve first as recommended. If it still doesn't give you what you want, try a 750 CFM, just expect less response in exchange for a bit more pitch in your prop at top end.

 

[Basin_Runner355]

Cathedral heads.

 

[Basin_Runner355]

I’m just trying to get the most out of the motor and it don’t make sense why he would put a 650 with smaller motors coke standard with 750’s

 

[Basin_Runner355]

And I see why nobody In Louisiana switched over to the ls motors they a pain in the ass . I can’t wrap my head around how this msd ignition box works because there is no map sensor to read the pressure.

 

[hdsadey]

And I see why nobody In Louisiana switched over to the ls motors they a pain in the ass . I can’t wrap my head around how this msd ignition box works because there is no map sensor to read the pressure.

With a carb you don't need a MAP sensor. Carbs are dumb. Vacuum draws fuel. MAP sensors are for fuel injection. MSD box only drives the coils by interpreting the cam position and crank position with the exception of the temp sensor which tells the box when the engine is warm enough to give it full timing. The box does have a hose port to hook up to the manifold for supercharged or turbo applications to pull timing as the MSD sees boost pressures. The LS platform is far superior to the old small block in many ways. At 540 hp just run that bitch, trying to get every ounce out of the engine shortens the life and causes potential problems. I'm gonna back the timing down on our LS2 I just built, old man don't need 511 hp on a 13 foot Hamant, I'm looking for longevity.

 

[Slidin Gator]

And I see why nobody In Louisiana switched over to the ls motors they a pain in the ass .

(Almost) Everybody in Louisiana (and Florida etc.) has already switched over to LS in their trucks, they started making then in the 90's. The only reason no one is familiar with them is because their trucks ain't broke.

Yep, it's different, but in a good way.

I can’t wrap my head around how this msd ignition box works because there is no map sensor to read the pressure.

This I don't understand.

 

[Basin_Runner355]

(Almost) Everybody in Louisiana (and Florida etc.) has already switched over to LS in their trucks, they started making then in the 90's. The only reason no one is familiar with them is because their trucks ain't broke.

Yep, it's different, but in a good way.


This I don't understand.

I don’t understand it either. That’s why I’m asking. The only two sensors it has is the one that plugs on up where a distributor would be on a sbc and behind the starter.

 

[Slidin Gator]

With a carb you don't need a MAP sensor. Carbs are dumb. Vacuum draws fuel. MAP sensors are for fuel injection. MSD box only drives the coils by interpreting the cam position and crank position with the exception of the temp sensor which tells the box when the engine is warm enough to give it full timing.

So yes and no. MAP is useful for all types of spark & fuel.

"Theoretically" (my term for "whatever"), the engine will run along the max power curve at low vacuum (i.e. atmospheric pressure) based strictly on RPM and will therefore produce power anywhere below max power just fine at a fixed timing (plus rpm).

Many (most) older tech marine engines ran fixed timing with mechanical (read RPM) advance just fine, but didn't idle worth crap. In the automotive world they added vacuum advance so the cars didn't shake themselves (and the gals) apart. On marine, the vacuum system was just too problem prone vs. normal idling assumed (they never fished with me) to be worth it.

I don't run vacuum advance on my swamp buggy engine because that's how water gets into the distributor. To make up for it I run 93 Octane just so I can have a timing setting that will idle worth a crap but no knock when I need power.

You don't need MAP to get max outa the engine, time it against WOT, RPM and assumed atmosphere and you will dyno for sure. You want MAP so the damn thing don't drink all your fuel when you froggin all night.

The part I don't understand is a modern ignition package that does not include a MAP sensor with vacuum advance? My aviation Pacemaker ignitions run MAP.

Bassin I think you runnin the drag series MSD.

 

[Slidin Gator]

MSD 60143 MSD LS Ignition Control - Black

The MSD 6014 LS Ignition controller works with 24x/1x and 58x/4x crank/cam configurations. It auto-detects the correct configuration based on the reluctor wheel pattern, so there is no need to select one. It provides six pre-programmed (non-editable) timing tables for stock engines, three customizable 3-D tables, and one customizable timing plot. The desired table/plot is selected on the fly using the rotary dial. The customizable tables and plots can be programmed via MSDView. The LS Ignition controller includes a built-in 2.5 Bar MAP sensor that can be used with Naturally Aspirated or boosted applications. This allows for timing advance or retard based on the intake manifold pressure. 8MB of internal data logging is also included so you can set up, record, and review passes at the track. As the popularity of GM LS engines continues to grow, so does the demand for a powerful, reliable, and easily adaptable ignition system. MSD has answered this demand with an updated and improved version of the popular 6LS box. Whether using a stock crate engine or building a custom powerhouse, the MSD LS Ignition provides the control you need to achieve the performance you want. Dial in the perfect tune for your LS using the easily adjustable preset timing curves or hook up to your computer and program a custom setup with our user-friendly MSD View software. New features of the LS Ignition include; high-speed data acquisition, an optional coolant temperature sensor input, idle timing control, as well as individual cylinder timing. These features are in addition to the existing programmable step retard, launch retard, and RPM rev limiters from its predecessor.

Features:​

• Fits most LS Engines
• No PC required for basic functionality
• Six preprogrammed timing tables for easy set-up
• Rotary selector switch allows timing changes on the fly
• Nitrous and boost compatible
• High speed data acquisition
• Fully Sealed direct plugin connectors
• Programmable step retard
• Launch and max rev limiters
• Optional coolant temperature sensor input
• Launch retard
• Idle timing control
• Individual cylinder timing
• Compatible with all MSD, Accel and GM LS coils

 

[90chevy396]

All the money in a carb intake, new carb, med management system, and you could’ve had a fuel infected boat that you don’t have to rejet in the winter. I stepped outta my comfort zone and went injected on my new boat and I will tell you this only maintenance between seasons so far is just verify oil level and hit the key love it

 

[Basin_Runner355]

All the money in a carb intake, new carb, med management system, and you could’ve had a fuel infected boat that you don’t have to rejet in the winter. I stepped outta my comfort zone and went injected on my new boat and I will tell you this only maintenance between seasons so far is just verify oil level and hit the key love it

Ida liked fuel injected i bought the motor it was already built and ready to go with a carb.

 

[hdsadey]

It's the standard MSD LS Series box. It's designed to only drive the coils. The carb has to be properly tuned, the box is your distributor. The timing tables are the same as mechanical weights just infinitely more adjustable when using 1 of the 4 custom tables. Like I said it does monitor temp so internally it determines how much timing to allow based on temp. What it also does is allows for a little less timing due to the fact that there are 8 coils, dwell time for the coils are increased because of 8 of them providing a much stronger spark than a single shared coil there by producing more horsepower. By far a much better and stronger system. They now offer coil on plug kits for older distributor style engines because it is superior. Pacemaker system for aircraft designed the same way.

 

[hdsadey]

All the money in a carb intake, new carb, med management system, and you could’ve had a fuel infected boat that you don’t have to rejet in the winter. I stepped outta my comfort zone and went injected on my new boat and I will tell you this only maintenance between seasons so far is just verify oil level and hit the key love it

No offense to anyone one but if you having a hard time understanding this carb/MSD setup then fuel injection is not for you, it doesn't get much simpler. Just sayin! I'd just assume running injection but the boat came with a carb so that's what we're using. OEMs didn't ditch the carb just for giggles, injection is more accurate (again properly tuned) and self adjusting. I run a self learning on my Windsor. Dad doesn't really care as long as it starts and goes. He quit wrenching for a living when fuel injection was coming to market, he didn't want to learn the new system. Old dog new tricks deal.

 

[Basin_Runner355]

I understand how it works just don’t see why msd couldn’t give you a straight timing curve instead of them jacked up timing tables. Why not have a set curve. To me they should give you a timing chart based on engine speed.

 

[hdsadey]

The box is designed to work with multiple engine combos. To keep prices low they make them run several different setups. Those timing tables I assume are based on dyno tuning at MSD with what can be considered safe for standard engines. What you see are set curves. A custom tune is up to the customer/builder/tuner. The timing tables are simple. Without boost pressure you use the horizontal line between 15.2 and 11.6. Since atmospheric pressure is 14.5 psi I would error to the 15.2 side. ALL OTHER HORIZONTAL VALUES ARE IRRELEVANT UNLESS YOU RUN BOOST. As boost comes in timing has to be retarded or catastrophic engine damage is inevitable.

 

[Basin_Runner355]

Yea when you told me that earlier I understood. But if you wouldn’t have told me that Idd have no clue.

 

[twb119]

What intake is on it? Single planes can get away with a smaller carb. The 650 will probably be snappier down low. Manifold velocity is key. A larger carb will make more HP up high, as long as it's not too big. When we dyno any engine we pay a lot of attention too the vacuum readings. It tells us if the carb is too big, too small or just right. 3 bears scenario. LS engines do move a ton of air.

After you get the engine on boat with prop, air filter on and throttle blades vertical with gas pedal all the way down measure manifold vacuum with vacuum gauge one of the best bang for the bucks tool out there, with prop set how you are going to run it, should be 0 vacuum after it has run for 10-15 seconds at WOT or very very close to 0, if not a little larger carb could help max HP, but torq at lower rpm could go down some. Dual Plane manifold usually needs a little larger carb than single plane. A dyno is great to see the potential of a engine, but a engine at work is the test.

I'm not a engine builder or mechanic, I'm just a farmer with a small tool box that has always tried to have the best mechanics do work for me over the years, there are more qualified people on this board than me.

 

[Basin_Runner355]

This My second airboat first one was a 14x7 diamond back with a !!HOT!! 350 440 hp. The guy who built the motor I have now is friends with them guy who built my last motor. They are an actually shop who builds 99% airboat motors.
Guy that built my 408 used them do dyno it. And they told him it needed a bigger carb and when I went to visit them the other day for something we got to talking and he recommended me putting at least a 750 carb on there because he dyno all his motors and he said unless you running a stock 350 a 650 isn’t enough carb in a airboat application. That is how this hole thing started.