Ok guys, I need some help here. I am pretty new to the LS motor thing and could use some advice. I am currently putting together a LS motor for my boat. It's going to be a 6.0 based motor with LS1 fuel injection. The ProCharger is a M1SC head unit with a AW-504 water to air cooler. These are the specs on the head unit

Max Flow 1400cfm
Max RPM 57,000 rpm
Internal Step-Up 4.10:1

As you can tell by the RPM's, I'm not gonna be spinning the motor very hard. I figure about 5700-5800. Which will work fine since the big block that's in it now only spins to 4800, and I am going to have a custom cam ground to keep my power band in that range. I plan on running 7-10psi.

Now, here is my main question. I have been looking for a LQ4, and at my local machine shop I came across one that is a complete fresh rebuild that seems to be a really good deal. It is a stock crank, stock rods with ARP rod bolts and has been bored .030 over with forged pistons. The only issue is, instead of dishes pistons, they used flat tops. I'm a little concerned about the compression with running boost. I this a motor I should consider? Or should I keep looking for a lower compression LQ4?

If I do go with this motor, I have two sets of heads setting here that I can use. A set of 806's and a set of 799's, which ones would you use?

Just an FYI, I'm not looking to make HUGE power with this motor. I would like to see 550-600 at the flywheel, which from reading on here, doesn't seem like to much of a reach.

Any help or suggestions are greatly appreciated!!

 

Nobody? Really?

 

With 10.5:1ish, I would keep the boost low and make sure to have the timing pulled properly as air temps rise. Sustained high rpm runs with high compression and low timing can be problematic as EGTs will be very high.

 

I would go with that motor and try to get a set of 317 heads. They can be had fairly cheap used Usually $150 to $300. They have a 72cc Combustion chamber and should be around 10:1 with flat tops. Where I live we can only get 89 octane fuel on the water unless i tow my boat to the gas station to fill up with 93. I dunno if 93 octane is an option for you or not, But will help with that amount of compression. The cool thing about forced induction in a boat is you can run an air to water intercooler and pump 60 degree river/lake water through it all day which will help tremendously.

 

I'm on Lake Cumberland in Kentucky and Premium fuel is no problem. Plus I don't leave my boat in the water so towing to a gas station to get 93 is no problem. Heck, we have a marina that has 110 at the pumps, but at $14.99 a gallon and a 72 gallon tank, I think I'll pass on that.


My main fear was what HellbentS10 was referring to on long runs. 95% of the time the motor will be running at 2500-3500 RPM's just cruising and not really seeing much, if any boost. But when I do decide to lay the throttle down, it won't be for just a few seconds. It could be for a mile or more. That's a lot of heat to build up. I plan on running dry exhaust so I can run a wide band and an EGT gauge. My hope was to keep the motor below 10:1. But I will just have to see. Thanks for the replies guys.

 

Like I said a good air to water intercooler would be your best friend. With you pumping 60 degree lake water through it your intake temps would probably never get over 100 degrees.

 

I will be running the big Air to Water intercooler, but the water in the summer is usually in the 80 degree range. Which I know will still do a great job of cooling the air, but I just want to be sure that the engine will do exactly what I want with no worries.

 

Add a meth injection kit to the party, that with the air to water cooler you should have some low air temps.

 

here's the bad thing about asking this question on a car board, everybody probably only has car experience I know everybody means well in their advice but high performance automotive combos will fail 95% of the time in a marine application.
most car guys don't understand that a marine motor it under 100% load all the time, something that can't even come close to being duplicated in the automotive world.
detonation in a marine motor comes MUCH, MUCH sooner in a boat.
you really need to be 9-9.5 to 1, stay under 8 psi even with 93 pump gas and the A/W intercooler, don't go leaner than 11.3 to 11.4, be very conservative with the timing, and I would also try to keep you max RPM around 54-5500.
a good reliable boat motor has a strong low flat torque curve that tops topping out at a reasonable RPM, don't search to squeeze out every last HP out of the combo.

 

I see what you're getting at and its pretty good advice, but a marine motor is not under 100% load all the time. It depends on the propeller curve, much like a car engine depends on gear ratios. This guy just wants to do the equivalent of road racing.

 

If your road race tracks have 10 mile straightaways then yes. A good marine engine is much different then a road race engine!

 

Yeah, this is quite different from a road race engine. Even just cruising at 3500 RPM's, you are still under a heavy load. In fact, the only time you aren't under a load is when you are idling.

My plan is to build a motor that can easily handle 800+ hp and tune it very conservatively. I was just curious about how much the compression would be on a motor like that with the heads I have, and it seems like it will be higher than I originally had thought. I'll probably just start searching for a good block and build what I need to get the compression down where it's safe. Thanks for the responses guys. I know it's a weird question to ask on a car board, but I didn't really have any other place to go. Not too many people are doing this yet, and definitely not in boats my size. Once I get everything going, I'll start a build thread.

 

Right, road racing was a bad example... just trying to equate it to something a car would do.

 

I would equate it to putting the car on a load bearing dyno and clicking the trans into 4th gear. Then hold the motor at 2500-3500 RPM's for maybe an hour or so. And during that hour you make a few WOT pulls, but hold it at WOT for 5-10 minutes. Lol. That's about as close as I can get to a description.

 

The stock crank will handle the power but I'd add Compstar rods to the engine, I wouldn't go with stock rods. Bob

 

Got any details on the boat or what kind of racing you will be doing? I'd love to see this build. Even though I'm a marine engineer, I've never been interested in boat racing (all I've worked on are the big ones, so I don't know what translates). As you probably already know, there is so much more than the engine design required to make a boat perform. I'm curious about things like how you are having the prop spec'd out and what kind of drivetrain/ hull design?

 

like was pointed out, while under way it is.
the only way that a prop wouldn't fully load an engine is if the boat is ridiculously under propped. when you prop a boat correctly it will hold the motor back.


ok
load your race car on the trailer. take the tire and wheels off put the truck in first. hold it to the floor and drag the trailer up Pikes Peak.
that's what my 502's go through on the way to the first card stop on a poker run.

 

if 8 bills is your goal you need to start from scratch and build it right. if your wanting a budget built 550-600 hp search for an LQ4 to keep it easy.

what boat do you have and what are you going to use for a drive?

 

My boat is a 98 Checkmate 253. It's 25'3" long with a 8'6" beam. The only "racing" I'll be doing is beating up on a few local boats that used to pass me. In car terms, it'll be a 11sec street car, faster than most, but far from being the big dog. My goals are to run it in the 80mph zone for now. What is holding me back is my out drive. It's a fresh mercruiser bravo one. They can handle about 650hp as long as you are easy on them. No slamming the throttle down out of the hole, just get it up on plane easily, and be mindful when the boat goes airborn to back off the throttle until the props re-enter the water. That's why I'm only looking to build around 600 for now. That should get me to the speeds I want. I have a buddy running a mildly built hp500 502ci mercruiser in the same hull I have. He sees low to mid 80's and is in the 580hp range. I figure if I make around the same power as him, but save over 300 pounds of weight in the transom with the smaller motor, I should be good to go. Prop selection is going to be kind of a crap shoot. I know my hull, with the notched transom, really likes a 4 blade prop. Right now I run a 24p Bravo. The 4 blade gives a lot of transom lift to get the back of the boat out of the water. But with the weight reduction out of the transom with the LS motor, I might not need all that lift any more. So I might be able to go to a 3 blade prop like a Mirage. I actually hope that is the case because the 3 blade will actually push my boat faster and will be easier on the lower unit. Here is an analogy for the car guys, 3 blade prop is like street tires, 4 blade is like drag radials and 5 and 6 blade props are like slicks. The more blade the prop has the better it "hooks" which cause more strain on the rear end, or in my case the lower unit. I'll probably start with a 28-30p bravo or a 27-29p mirage. And go from there. Here is what is meant by pitch, the number is a measurement in inches that if your prop is hooked up perfectly with no slip, that's how many inches forward in the water that prop should travel in one revolution. So a 28p should go 28" in one revolution, a 24p, 24". So the bigger the pitch prop is like going to a taller gear.

I'm going to have a lot of fun trying to set this boat up when it's all done because I'll have nothing to compare it to. Nobody has ever done this swap into this hull or any similar hull that I can find. That's the main reason I want to do it. I have always liked my stuff to be different. Plus I can't wait to see the look on peoples faces when I wax their *** on the water and then raise my hatch and they realize their big block just got smoked by a mouse motor!!!

 

Hellbent brings up a good point regarding high EGT's - will stock valves hold up to the heat a long WOT pull brings on?
HP Mercruiser's use inconel exhaust valves...

Just a thought. Sounds like you're doing what I've thought about doing with my Formula. Keep us posted on your progress!