8.1 Power Adder

Nikto · August 24, 2003

Has anyone tried this cam? I guess I'd have to reprogram the ECM etc. etc.

The GM part number for the cam

is 12558912.

ShortCrew · August 25, 2003

Any idea what the specs are? I know on the LS1 you can throw some pretty big cams in without reprogramming. If the LSA is simillar between the cams, then it should idle nice. Does it work with stock valve train?

Nikto · August 25, 2003

Not sure of the specs.

I found the engine on GM POWERTRAIN's site and emailed INDMAR MARINE ENGINES for the pn.

http://www.gm.com/automotive/gmpowertrain/...arine/index.htm

CMNTMXR81 · August 25, 2003

Comp Cams also has three grinds available for the 8.1.

You techincally don't need any re-programming for a cam change unless you're doing something radical. It's is highly recommended though in order to take advantage of the new cam's profile.

ShortCrew · August 26, 2003

What all goes into making these marine engines so much more powerful. Looking at the GM specs, they have one version that makes 525hp and 560tq, all at the stock 9.1:1 compression ratio. The heads appear to be the same ones on the trucks. Is this all coming from cam and open exhaust?. How can this be?

Nikto · August 27, 2003

From what I can gather, they have bigger injectors and throttle body, and the cam.

496Cubes · August 27, 2003

GM is offering the HP3 8.1 engine as a crate motor that makes more than 525 horsepower and more than 560 lb-ft of torque. This was announced last year and I haven't really heard much more about it. This is made for automotive applications. I don't see why you couldn't order the parts that are different and have the same engine. Article:8.1 Crate Motor

Differences:

Fuel Rail

Throttle Body

Camshaft

Valve Springs

Roller Rocker Arms

Intake/Exhaust Pushrods

Harmonic Balancer

Ignition Coils?

Programming

I think this would be a good alternative to supercharging. You could still run on regular octane. I was hoping there might be a kit from GM or the aftermarket to upgrade the regular 8.1. Maybe that was just wishful thinking.

Nikto · August 27, 2003

Thanks,

I remember that announcement but nothing has happened since.

Big Secret, I guess.

ShortCrew · August 27, 2003

Quotes from GM "75-millimeter throttle body with an HP3-specific 400 kPa fuel rail, providing a pressure increase of more than 30 percent over the standard Vortec 8100 production engine"

Isn' the stock TB 75mm? I know they are on the LS1.

"The Vortec HP3's valvetrain system is performance matched throughout, with a high-lift/high-duration camshaft, premium race-style valve springs, 1.7:1 ratio roller rocker arms, dedicated rocker arm studs, and HP3-specific intake and exhaust pushrods"

Mmm'kay. That should be less than $1000 for all the hardware.

"with its 9.1:1 compression ratio, is designed to run on regular (87 octane) unleaded fuel."

To me, that says the heads and pistons are the same as in the truck

So let me get this straight. Take a stock 340hp truck motor. Throw in a cam and some better valve train. Raise the fuel pressure, reprogram the computer, and boom.........you gain 180hp!!!!!!!!!!!!

Holy Chit Mang!!!!!!

Since the compression is still low, I get the impression that this cam is not that radical. So how the hell can you gain all this power with such minor mods?

americanmuscleman · August 28, 2003

Just some food for thought, the "420hp marine cam" everyone is talking about is the cam GM puts in the Mercury Marine 496 HO motor and the 425hp 8.1L Volvo Penta. They run the factory valvetrain with that motor (stock GM heads, valvesprings, etc..) but they swap the cast crank (in the truck 496) to a forged steel crank.

On a positive note though, the factory cast crank can take a lot of power. The steel crank is used due to the sustained high rpm abuse a marine engine gets.

The only differences between the Mercury Marine 375hp 496 Mag and 425hp 496 Mag HO are the steel crank in the HO and the bigger camshaft. Of course the computer calibration is adjusted for the HO cam, and the HO runs slightly higher rpm at WOT.

I've seen 496 HO's dyno over 440hp and 520tq on a water-brake dyno, at or under 5000rpm.

ShortCrew · August 28, 2003

On a positive note though, the factory cast crank can take a lot of power. The steel crank is used due to the sustained high rpm abuse a marine engine gets.

Last I read, GM did away with the steel crank because the cast crank was more than up to the task. I believe they now use the cast crank even in the 520 HP motors.

Any idea how the HO cam would idle? I would love to know the specs and compare it to the new cams from Comp Cams.

Nikto · August 28, 2003

So, how do we put all this info together to come up with a plan?

I tried looking up the part # for the cam on the Pace and SDPC parts lookup and they come back "not found."

Tjwong · August 29, 2003

The reason for "part not found" is that GM Marine parts are restricted to marine OEMs only. In retrospect marine dealers can't access automotive only parts. As for the HP3 there is a difference in the cylinder heads. The intake and exhaust ports are 100% CNC ported. That and the new cam profile will give you the extra ponies over the old HP2 variant.

In our trucks the fuel rail pressure is already set at 400kPA. I use the marine injectors in my truck along with the Whipple blower to get my fuel enrichment for boost. The HP2 fuel injectors was rated at 300kPa and flowed more at 300 than the truck injectors at 400kPA. The HP3 injectors are the same as the HP2 but running at a higher fuel pressure.

Don't go nuts over the HP ratings of these marine engines. The 375Hp marine engine is identical internally to the truck engine we have in our trucks. The difference in HP ratings is because GM uses a different test standard for marine vs automotive engines. Using the same automotive test standard for the HP2 420 engine will only give it about a 370 Hp rating. The difference in test standards is basically that the marine standard uses a very low exhaust restriction for the test.

Making a cam swap into your new 8.1L truck maybe more of a headache than what the power gains are worth. For one the PCM will not like the cam change. Unless you have a tuning tool like LS1 edit (about $600) or you are willing to take chances with a mail order tuner which is another PITA. Namely because he won't get it right the first time and most likely not the second or the third.

The new OBD2 PCMs in our trucks has a myriad of tables. GM spent millions in R&D. They have mapped air flow, fueling and timing curves to a nats a**. Even when Whipple released their 8.1L blower kits way back last year in May or June. They ran into a lot of problems, the kits caused a DTC because the PCM saw more air flow than expected at a given throttle, and RPM. It would throw the engine into reduced power mode which basically means that it would only rev to about 1500 RPMs about enough to get you to the side of the road, that is if you didn't get killed trying to get there off the freeway in lots of traffic. I know this as it happened to me.

I was involved with tuning a 8.1 that was installed into a C5 Corvette. It ran fine with the stock cam, but the owner wanted to install that marine cam into the engine which was done. They spent hours and hours tuning it in. In time they got it to run right but it took a lot of time and they had the editing tools to do it with.

It would be a nice to drop in a HP3 into a truck. But it would most likely take some considerable tuning before it would run right. It can be done just like anything else all it takes is time and lots of cash. No one really knows what this new HP3 crate motor will cost. But I am in line for it at my buddys dealership. There is no telling when it will be available. The new 572 crate motor introduced over a year ago is still not available.

For the gains, one is better off getting a decent exhaust system. And if that isn't enough get a blower. If you guys really want that cam I can get if for you, I know the marine OEM here in Portland, I also explored this installation over a year ago.

Nikto · August 29, 2003

Thanks for the insight.

Case closed.

Any thoughts on an exhaust set up ?

Tjwong · August 29, 2003

I don't think anyone offers a complete system for the 8.1 trucks. B&B offers a rear sections but no headers. There is a company that makes a set of nice headers, they are called The Other Guys Headers

Here is a link to them:

The Other Guys Headers

I have a set of them in my Suburnban. But here is a word of warning. They told me that they was a drop in fit with "minor welding" meaning it was just to weld the mating peices to the stock exhaust where one would cut off to fit with the new system.

Well the pieces that was sent in my kit didn't even come close to mating to the factory tubes. I ended up having to bend a couple new peices and some other fab work to make it all fit. Their explanations was that GM must have used a different exhaust vendor for my production run. Now I know that is pure BS even if GM used a different vendor they sure as heck would be using the same exact templates to manufacture the exhaust piping with!

That was my experience otherwise fit and finish is fine as far as looks and the way it fit on the engine. I don't know if anyone else out there had problems like mine. I do know that several people has used them on this forum.

My system is a total custom setup. I once used a Magnaflow dual 3" inlet with a single 4" outlet muffler. It was way too loud in my Suburban. I removed it and installed a Banks SS dual 2.75" inlet single 3.5" outlet muffler, the noise level is acceptable and still with an authorative bark as well. I retained but modified my 4" tail pipe. Banks doesn't normally sell just their mufflers but we are a dealer for them and if there is interest I can get them for this mod. It is a drop in fit and an easy installation for any competent exhaust shop that can bend a 3.5" tail pipe. I can also explore the single purchase of just a 3.5" tail pipe from Banks for perhaps a PU or a Suburban that may fit with little or no mods.

Here is a picture of the business end of my truck:

Suburban 5" exhaust tip