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Old 08-23-2010, 09:48 AM
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What's limiting my L31 5.7 Vortec?

I've got a 99 Suburban with the L31 5.7 vortec engine. These motors seem to really stop breathing after 4k RPMs. What's really holding them back? Vortec heads are known for good flow and after opening up the EFI intake, it seems to me that it should flow to 5500 RPMs. I'm getting ready to rebuild this engine and keep the factory EFI as it's the family hauler. I put close to 20k miles a year on it, with a 1/3 being intown and the rest running down the highway between 1800-2200 RPMs. I also really don't want to kill the fuel mileage because of all the miles I drive. Suggestions?

Thanks,
Robert

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Old 08-23-2010, 10:48 AM
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headers, cam/valvetrain, and a supercharger.
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Old 08-23-2010, 12:21 PM
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Quote:
Originally Posted by turbolover
headers, cam/valvetrain, and a supercharger.

i agree (not the forced induction thought)

long tube headers would help a great deal, lower restrictive exhaust, new cam but if you want to keep the EFI then you will have to be careful how much you go with that. but the biggest is the stock EFI system. with a reflashing and the other stuff mentioned you should be able to make power up to 5000-5500.

and this is for Turbolover; i always find it more respectable when i guy does it Naturaly Aspirated. but that is just my opinion, and we know what opinions are like
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Old 08-23-2010, 12:28 PM
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I love the sound and smell of na engines, but its hard to not have respect for thee guy passing you.
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Old 08-23-2010, 12:50 PM
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Quote:
Originally Posted by turbolover
I love the sound and smell of na engines, but its hard to not have respect for thee guy passing you.

you do have somewhat of a minor point, but what is cooler a 9 second na car or a 9 sec forced induction car. in my opinion it would be the na car cause he didn't need any help to get it done.

when it comes to running serious numbers i can understand the forced induction especially when you want to have it street legal and run on pump gas but running anything slower than mid to low 10's on forced induction to me is just a waste as you could have built a na motor on pump gas to run that.

i'm certainly not saying that i have the fastest car around but for a street/strip car that runs on pump gas and actually hooks on the street similar to how it hooks on the track and runs 7.20's (1/8) and 11.50's (1/4) aint too bad, but i know that a forced induction would have me in the mid-low 10's
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Old 08-23-2010, 01:17 PM
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Quote:
Originally Posted by bobbyv
I've got a 99 Suburban with the L31 5.7 vortec engine. These motors seem to really stop breathing after 4k RPMs. What's really holding them back? Vortec heads are known for good flow and after opening up the EFI intake, it seems to me that it should flow to 5500 RPMs. I'm getting ready to rebuild this engine and keep the factory EFI as it's the family hauler. I put close to 20k miles a year on it, with a 1/3 being intown and the rest running down the highway between 1800-2200 RPMs. I also really don't want to kill the fuel mileage because of all the miles I drive. Suggestions?

Thanks,
Robert
If you want to retain all the OEM equipment including the chip, the cam needs to make good vacuum.

One option might be 1.6 ratio rocker arm swap. Or maybe a recalibrated chip. A chip and cam swap- but now you're starting to spend some money and given the use and RPM range you want to run, a cam might not be that much help at all.

Stock L31 cam specs:
191/196 @ .050"
0.414"/0.428" lift
111 LSA

As a comparison, LT1 cam specs:

1992 Y-body:
205/207 @ 0.050"
0.451"/0.450" lift
117 LSA

1993-1995 Y-body, F-body:
202/207 @ 0.050"
0.450"/0.460" lift
116 LSA

1994-1996 B-bodies (including L99 4.3L):
191/196 @ 0.050"
0.418"/0.430" lift
111 LSA

1996 Y-body, 1996-1997 F-body:
200/207 @ 0.050"
0.447"/0.459" lift
117 LSA

1996 Y-body, F-body LT4:
203/210 @ 0.050"
0.476"/0.479" lift
115 LSA

Obviously there are aftermarket cams that may well provide adaquate vacuum for the OEM PCM calibrations.

But that said, w/the RPM range and use you have for it, the OEM spec cam will be tough to beat. The best seat of the pants improvement may come from a rear gear swap, depending on what you currently have.
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Old 08-23-2010, 01:53 PM
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a gear swap helps with acceleration but it reduces mpg's.

A cam swap can increase power and mpg's if done right and you get a rather small cam.

A supercharger has the highest initial cost but it will give the most return and still be VERY streetable.



Headers will help all around.


To 87Z I like turbos because they can make a 500hp engine tame enough to teach you 15 year old daughter how to drive in. And get you 25+ mpg's in a decent car.

Nothing sounds like a nasty big block with a tunnel ram and open pipes though.
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Old 08-23-2010, 03:32 PM
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the stock intake maniflld with its 19 lb/hr poppets and fuel supply parts that are inside the plenum(retsricting flow), small cam and 1 7/8" piping(that crimp down much smaller in the bends) that goes from the manifolds to the cats, combined with the very weak PCM tuning(low timing advance, very rich WOT a/f ratios) all contribute to good off idle mid rpm torque, but cause upper rpm power to suffer.
I switched to the GM/Mercruiser marine intake manifold that uses a conventional fuel rail that is located outside of the plenum. Power/torque was improved at all rpm ranges. I also am running Hooker 2462 1hkr headers with a 1 5/8" primaries and 2 1/2" collectors. PCM tuning brings the whole package together.

With exhaust, headers, cold air intake, 4.10 gears from 3.08's and PCM tuning (TM delete, leaned WOT A?F ratio) I went from a stock 15.9 in the 1/4 down to a 14.8 in a 1997 reg cab shortbox 2wd 4l60e 1/2 ton.

A PCM tune and headers/exhaust improved acceeration markedly.
I havent made a pass with the marine intake and L35 V6 Vortec 4.3 torque converter installed yet, but I assume low-mid 14's woudl be in order.

FAstchip http://www.fastchip.com/
PCMforLess http://www.pcmforless.com/
Westers GArage http://westersgarage.eidnet.org/

This forum is full of very experienced L31 Vortec 350 guys and is home of the 411 PCm swap and the marine intake manifold retrofit.
http://www.pacificp.com/forum/index....e805d5d626670f
peace
Hog
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