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I have a 99 3/4 ton suburban with 5.7, I have a 5.3 I'm lookin at ls strokers. I'm boring it to 3.905 regardless, I want off idle tq. I'm wondering how the torque curve compares to one another. (I understand it varies wildly with choice of cam) I'm lookin for stock reliability, 454 tq and v6 mpg. Just goin by stock vs stock the 5.7 makes more tq sooner then 5.3 witch I find surprising due to 5.3s longer stroke. Can I pick a cam that will get me more hp and more tq? Can the 3.905x3.9 ls match torque of the 4x3.75 vortec?
On of the BIG WHINES back in 1997-1999 was the Gen 2 Small Block Chevy LT1 & Vortec cousin all felt more powerful and torquey down low in the rpm range than the LS1/Gen III engines.
Half a dozen people thought my 91 RS with a 305 TBI would whip any LS1 because low end torque was good (ROFLMAO). The old 305 TBI made 170 hp at 4,000 rpm and 255 lb⋅ft at 2,400 rpm.
There was a lot of butt hurt handed out by the new aluminum Gen III engine to the old guard Even Dyno Day's were fun. The aluminum engine cars were ridiculed as "muffler cars not muscle cars" by some of the old timers. Seeing a dozen "450hp" 454/455 cube old school engines fail to make 250 whp with a manual transmissions :was hilarious Then the old skoolers cried like spoiled Millennials when a stock LS1 made 326 whp There weren't any more muffler car comments
1998 5.7 Vortec in Silverado The Vortec generated 255 horsepower at 4,600 rpm and 330 foot-pounds of torque at 2,800 rpm and compression ratio of 9.4 to 1.
1998 Corvette LS1 5.7 rated at 345 horsepower (257 kW) at 5,600 rpm and 350 pound-feet of torque (470 Nm) at 4,400 rpm
Notice the LS1 makes 20 more foot pounds but it 1,600 rpm higher than the Gen 2 Vortec.
My experience typically the old school stuff feels like it and has a fatter torque curve down low.
I think this typically holds true due to the different style of cylinder heads - the LS can breath better and peaks higher the SBC with siamesed exhaust ports and other handicaps is less able to breath and peaks sooner.
Notice the peaks on the newer Gen3/4 Vortec 5.3's. They make more than the old Vortec but it's higher in the rpm range. The new engines make good torque down low but it's really less critical to make a ton of torque very low because of all the newer 6 speed, 8speed , 10 speeds etc automatic transmissions
For stupid low end torque a SBC 400 would be my budget choice. For live with everyday a 6.0/6.2 Gen III/IV would be my truck engine of choice.
Last edited by 99 Black Bird T/A; Apr 30, 2019 at 03:14 PM.
Not having a ls1 I never looked up there hp/tq I do have a pile of 5.3 stuff not being sure what I had I compared 99 5.7 to 99 5.3 were in the 5.3 has 15 more hp but 20lbs less tq much higher. This is were my question stems from. I also have several superchargers "laying around" from Half a dozen teflon lined m90s, m122, to a few 6-8-71s. If I can find a reasonable way to mount the m122 I'd just bore the 5.3 to 3.905 and run stock crank/rods at 9-10lbs of boost. A 6-71 on top of a 6.0-408 would be great lol
Small ports for velocity and longer runners promote low end torque. I suspect the older fuel injected Vortec engines have longer runners.
Those old Vortec engines basically don't have runners; it's a two piece intake manifold, with an upper half and a lower half. It's hollow on the inside, just a big, open chamber, and the injectors are inside of it.
It blew my mind the first time I ever took one apart.
To the original poster: An old-school 383 will probably be cheaper, and won't disappoint you on power.
But this is an LS board, and most people will try to talk you out of it.
Those old Vortec engines basically don't have runners; it's a two piece intake manifold, with an upper half and a lower half. It's hollow on the inside, just a big, open chamber, and the injectors are inside of it.
It blew my mind the first time I ever took one apart.
To the original poster: An old-school 383 will probably be cheaper, and won't disappoint you on power.
But this is an LS board, and most people will try to talk you out of it.
Old school Vortec 383 gets my nod.
The L31 intake does have runners. They are crossram oriented. Combined with the cylinder head port length the total intake runner length is close to 12" long. The plenum is choked with the spider system. The Marine L31 intake has external injectors. The L31 marine manifold breathes much better. I have one that I heavily ported sitting on top of aluminum heads ported to 210cc intake ports. I am making 395 rwhp @ 6,000 rpm. Over 300 ft/lbs of torque @ 1,500 rpm with a peak of 380 @ 3,800. I am swapping the marine crossram out for a marine dual plane with a 4bbl to 102mm intake elbow and 102mm throttle body. My next engine is going to be a Dart block with a 4.125 bore × 3.875" stroke.
Comparing stock "ratings" is WORTHLESS. I wouldn't bother with that.
Torque comes from CID and compression. PERIOD. The brand, shape, age, etc. of the castings makes essentially ZERO difference to the peak value. Since the max compression you can run is pretty much a function of the fuel you are using, then on pump gas, about the highest peak torque you can get, is somewhere around 1.25 - 1.3 times the CID. For a 383, that's something around 450 ft-lbs at the crank, which would be in the 375-400ish range at the wheels in most vehicles, as Fast355 claims for his.
Now, that said, the RPM that the peak torque occurs at, can vary WIDELY. And then since HP = Torque * RPM ÷ 5252.11, if the torque peak occurs at a higher RPM, then HP will also be higher. I suspect that a SBC 383 will not be able to be forced to have its peak at as high a RPM as a LSx would, but either can be made to have plenty of low-end grunt with the right cam choice.
Seems to me like it would be easier, given your situation, to stick with the old-skool SBC, since that's what's already there. I don't think either type will produce materially better "performance" for your application. Reliability, weight, cost, effort, risk, etc. might be different, but just pure push-the-truck-around, not so much. Go with what's easier and costs less.
I know that I’ve told this story on here before, but here goes again. My 94’ Silverado with a 5.7.... which I just sold last summer, bought it in 95’....should have kept it!....would pull my EMPTY trailer so much easier than my 12’ Silverado with the 5.3. Night and day torque difference. I know the torque ratings for both trucks, but real world experience trumps what’s written on paper. Having said that, sure you can build the 5.3 to produce torque, but it would be simpler and much cheaper to just throw a supercharger on the 5.7. I’m a pro-LS guy and I approve this message.
Comparing stock "ratings" is WORTHLESS. I wouldn't bother with that.
Torque comes from CID and compression. PERIOD. The brand, shape, age, etc. of the castings makes essentially ZERO difference to the peak value. Since the max compression you can run is pretty much a function of the fuel you are using, then on pump gas, about the highest peak torque you can get, is somewhere around 1.25 - 1.3 times the CID. For a 383, that's something around 450 ft-lbs at the crank, which would be in the 375-400ish range at the wheels in most vehicles, as Fast355 claims for his.
Now, that said, the RPM that the peak torque occurs at, can vary WIDELY. And then since HP = Torque * RPM ÷ 5252.11, if the torque peak occurs at a higher RPM, then HP will also be higher. I suspect that a SBC 383 will not be able to be forced to have its peak at as high a RPM as a LSx would, but either can be made to have plenty of low-end grunt with the right cam choice.
Seems to me like it would be easier, given your situation, to stick with the old-skool SBC, since that's what's already there. I don't think either type will produce materially better "performance" for your application. Reliability, weight, cost, effort, risk, etc. might be different, but just pure push-the-truck-around, not so much. Go with what's easier and costs less.
Even though it fits right in your example. My setup probably makes about 450 ft/lbs but it is from a 9.6:1 350. Somewhere in the 1.28 ft/lbs per cubic inch range. A 383 cammed properly of equal compression and build would likely make over 500 ft/lbs. My next build being 4.125 x 3.875 is 414.27 cid. Should be able to make about 540 tq if not a little more. Would love to have 550 tq from a mild build.
There is a guy around town that has a 98 or 99 Chevy crew cab dully with a 5.3 swapped in he uses to pull his lawn equipment with. He says it works wonderful and gets around 11-12 mpg average LOADED compared to the 454 that was getting around 8. BUT it has a 70mm turbo, I dont know much about the tune but the engine is completely stock he says except the injectors. He claims it will build 7psi by 2000rpm and max boost at 3000 11psi. I was very curious about it and have even thought about doing something similar on my 5.3 suburban.
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Ls 383 torque vs vortec 383 torque 
Even Dyno Day's were fun. The aluminum engine cars were ridiculed as "muffler cars not muscle cars" by some of the old timers. Seeing a dozen "450hp" 454/455 cube old school engines fail to make 250 whp with a manual transmissions :was hilarious 
