[QUOTE=KCS;12538375]Well, if you don't even want ~35 more cubic inches, then I guess this is a moot point anyhow.

However, one thing to note is that cramming the same amount of air, into a smaller space, will typically increase cylinder pressure and put you closer to the detonation threshold, requiring a higher grade of fuel for the same amount of power. The question here is, could the expense of higher octane fuel, or a water/methanol injection setup, negate the supposed fuel economy of a smaller engine? Is there better fuel economy in a smaller supercharged engine, making the same power as a bigger supercharged engine?

Also, what you are doing does not require a "high dollar" bottom end, although I wouldn't attempt it with stock pistons. You can use any stock crank, stock connecting rod combo, so why not enjoy ~35 more cubic inches? It's not a reliability issue, I assure you that. The stock LS2 crank and rod will last as long as you want. The tune and the quality of the machine work will have exponentially greater influence on the reliability of the engine than this rod ratio.[/QUOTE

I have a turbo charged truck, an mp122 blown truck (in signature), and a TVS 2300 blown vette. All run on 93 octane fuel. I factored 93 octane fuel in from the beginning. Every smaller engine I've ever been associated with went farther on a gallon of fuel vs a bigger one.

The beauty of a good f/i setup is that the power is available, but it doesn't cost you anything if you don't use it. If you can get by with a smaller engine (size wise), you can realize some savings. That's what I'm looking for. I think a 500+ rwhp rig is doable that still sips fuel.

 

You get it. Most folks equate boost with horse power. I'm hoping to make this motor breathe so well that it doesn't need a whole lot of boost to make the power I'm looking for. The motor in my sig makes 500 at the wheels at 8psi. I think that with a slightly better set of heads and a little more cam, I can be in the same power range with similar boost.

 

It's my understanding that the rods on the 4.8L are longer to make up for the shorter stroke. Pistons are the same on 4.8 and 5.3. 6L piston should work with 4.8l rod and crank in a 6.0L block. Assembly would have to be balanced.

 

well any results?

 

The deck height of the 4.8L is the standard 9.24" shared with all of the other LS engines.

 

Thoughts (no offense intended)

Hydraulic valvetrain with stock lifters and rockers, the correct springs, pushrods and cam lobes won't be a problem unless old motorhead plans rpm way north of 7000, which I highly doubt.

Each cylinder will be producing the same power no matter what the size. The shorter stroke crank with the same size rod and main journals will have more "overlap" which makes it stronger.

If one just wanted an LS this size, sure pickup a 5.3 and go for it. If the parts are cheaply available and the cool factor is a driver for you, I think the 329 will work.

I don't think cruising fuel economy will be much different. It takes a given amount of power to cruise a given vehicle at a given speed. Unless you aim for cruising at a more efficient portion of the part-throttle low-boost portion of the torque curve you probably would need OEM equipment to find the difference.

Shorter strokes, with the resultant lower piston speed do consume less friction hp than longer strokes, but in this case it's not a whole lot.


I like the idea mainly because it would be a reasonable cheap thing to try, and would have a high cool factor.

old mentioned this was for a 3500 lb. vehicle.


Jon

 

I did the math on this because I wanted to use the 4.8 flat tops in a budget 5.3 I was planning. It worked out where the 4.8 has a piston to deck height that is 2 thousandths higher than the 5.3. The 4.8 crank/rod combo will work with the 6.0 pistons, they may just come out of the hole a tad more.

 

The 4.8l has flat top pistons. The 5.3 had dished pistons. Later GM used the 4.8l flattops in the 5.3L to boost CR to get more HP out of the 5.3L. All the 5.3l are now 4.8l pistons they are the same part numbers.

Running a 4.8l crank and rod and a 6.0l flat top will boost CR. I would use a LQ4 dished piston, that will lower the CR a bit since its going to be used with a SC.

I say do it!

 

The smaller engine just may get better fuel mileage, but my point is that it will most likely require 93, and as hot and humid as it gets here in Texas, it may not even be enough then. The extra displacement gives you a larger margin of safety, and may even allow a lower octane to be used, depending on other variables. The ability to run the lower octane, ie cheaper, fuel in the larger engine may make the economic difference in the two neglibible.

 

I understand that compression ratio - timing - octane - boost is all a balancing act. I won't be trying to get by on 87 octane. I know it can be done on a boosted engine, but I'm not willing to make those concessions. I have a Snow boost cooler on the setup in my signature that works well. It comes on at a slow rate over 4psi and adds progressively more W/W fluid as the boost goes up. It's not tuned to depend on meth. It works for a living and tows on occasion. The motor I'm proposing won't tow and will be in a much lighter vehicle.

 

I considered a 5.3. That would definitely be a cheaper option. Not too easy to find aluminum 5.3l blocks though. Also, limits head choices with the smaller bore. I didn't think good heads were important on a s/charged motor until I got the results from my TVS blown LS3. I want to put this thing together mild, but want the ability to go a little wild too.

I think you may be discounting the value of the smaller motor fuel eco wise. My sons have 4.8L and 5.3L half ton trucks. I have a 6.0L in my half ton. As you would expect, the smaller motor gets the best fuel eco by a fairly large margin. I think that would hold true as long as the small motor wasn't overloaded and lugged. That won't be the case here.

 

I'm aware that GM made the switch to flat tops in the 5.3. That's what I ended up getting instead of the 4.8 pistons so I don't have to change connecting rods. The switch to flat tops was not made by GM to boost power in the 5.3. It was done to keep the same 9.5:1 compression ratio after switching over to the 243/799 heads(which have a bigger 64cc chamber). The power increase comes from the better heads, because the compression ratio is the same as before with smaller chamber heads.

Running a 4.8 crank in a 6.0 will not boost compression, it will actually lower it. Decreasing cubic inches also decreases the amount of air compressed in each stroke, thus reducing compression. A flat top 6.0 has a 10:1 CR with a 3.622 crank and swapping to a 4.8L 3.28 crank lowers that to 9.2. If you use the LQ4 7cc dish pistons with the 3.28 crank the compression will be an anemic 8.5:1. You would have to be running some serious boost to require going that low on the compression.

I do however agree with you that it should be done. It has long been an ambition of mine to destroke an LS engine just for fun.

 

Hello

I would like to share my opinion/experience with forced induction motors.
I have build destroked 95 cid motor from 122 CID engine (stroke was reduced from 3.54 to 2.81 inch). Engine is 2 valves per cylinder. I used stock head (home ported) with all stock valvetrain, stock cams, valves, stock cast 122cid turbo engine pistons (122cid had 7.5-8:1 and this gave me compression ratio of ~6.5-6.8 if I remember correctly). Rod ratio increased from 1.61 to 1.79. I made 360fwhp at 30+psi boost at 7800rpm on pump gas without water injection. If we multiply everything by 4 I should have had 1440fwhp 380 CID engine With no forged components.
My theory is that hp depends on
1. Head flow (it is the same as the same heads are used).
2. efficiency (or comp ratio)
3. amount of mixture (or combustion chamber volume).
If You put destroked 4.8 crank to 6.0 block with the same flowing heads You should make the same amount of power as You keep the same combustion chamber volume. So You only need the same dynamic comp ratio to reach same hp number. You use higher boost safely and You will have the same dynamic comp ratio.
So I would like to encourage You and to make such engine and I think You will have great results. The only downfall I see in Your case is valve train (as You will need to go higher in rev range with smaller engine if You use the same heads/cams).
Another point for non believers is amount of power made on stock block. I thinks somebody here made over 700whp with stock 4.8 long block and turbo. I have seen highest of 780whp here with stock 6.0 long block also. And if You have lower compression and higher boost I think You will have more than that with stock components in 4.8.
Good luck with the build if You decide to try different route.

Jonas

 

Fuel econmy comes down to many things, some of which you can control as a driver, some as a designer, and many things you can't control. Individual driving habits are a big part of overall fuel economy over a period of time. If one drives in any kind of urban/suburban traffic just anticipating acceleration and braking has a big effect on fuel used.

We all see folks ahead of us who keep up their cruising speed until they approach the stop light, than brake fairly hard to stop. If you see that the light is going to change a few hundred yards ahead of you, back off and let her coast. If your car/truck has a instant fuel economy readout you will see that closed throttle coasting uses very little fuel, and on some engines the fuel is shut off. I could go on, but you get the idea.

Two identical trucks, but with one carrying around a few hundred pounds of "necessary stuff" like took boxes, a few cases of beer, etc. will get lower average fuel economy than the empty one. It's all about the energy needed to accelerate the mass.

If fuel economy is a biggie, design and gear the engine/vehicle package so you are using the engine in a very efficient part-throttle low rpm range. That may not equate to best for acceleration. I'm not sure if the blower you have has a bypass like a Maggie, but that feature is a pretty good help for MPG.

Bottom line, IMO, is that driving style probably is a bigger factor than the difference in displacement (364 vs 329) on your blown engine. Design of the ports and valve events and gearing willmake even a bigger difference. Remember that the better the port, the less cam the engine will need to produce a given amount of power at a given rpm...NA or Forced Induction.


Jon

 

[QUOTE=Old SStroker;12546517]

We all see folks ahead of us who keep up their cruising speed until they approach the stop light, than brake fairly hard to stop. If you see that the light is going to change a few hundred yards ahead of you, back off and let her coast. If your car/truck has a instant fuel economy readout you will see that closed throttle coasting uses very little fuel, and on some engines the fuel is shut off. I could go on, but you get the idea.


If fuel economy is a biggie, design and gear the engine/vehicle package so you are using the engine in a very efficient part-throttle low rpm range. That may not equate to best for acceleration. I'm not sure if the blower you have has a bypass like a Maggie, but that feature is a pretty good help for MPG.




When you take your foot off the go pedal, folks behind you lose their minds. I've pulled over and let them by only to be right behind them at the light. They don't get it. Same thing when approaching a sharp curve. Unless someone is bleeding in the car, it makes no sense to have to brake hard going into the curve. Brakes last a lot longer too.

The blower I'll be using is probably a Magnacharger MP122 (unless I can find a TVS cheap). The MP 112 and 122 are getting pretty common on the used market. It's amazing the deals you'll come across when you don't have to have it RIGHT NOW. They, as well as all of the Magnuson blowers I'm familiar with, have the bypass valve. When you're not needing boost, the blower is just along for the ride using little if any power. I think the Magnuson boys quote a third of a horsepower in no boost mode.

I'll be shooting for somewhere around 10.5/1 compression ratio. I think I can use all of the air an MP122 will pump effeciently and still keep the compression on the high side. I know I can with an MP112. A good flowing set of heads, boost cooler, and cam choice will be keys to making this thing stay together and make the power and economy I'm looking for.

 

if your that worried about fuel economy, then hot rodding is not the right hobby for you. lol

 

I want it ALL.

 

This is why your building an boosted ls engine, you can make over 600 rwhp and still get 20+ mpg. I have built a few in the high 600 and still get 22 mpg. Have cake and eat it too.

 

EXACTLY. My vette does 670+ and still gets 24 on the highway. I know it's easily doable mpg wise with a smaller motor. Not shooting for that much in the power department, but would like to do 25+mpg.

 

Since the 6.0 is the only LS motor with the 4 inch bore right out of the box, spend a little extra on a 3 inch crank and make a 5 liter trans am motor.