Newbie w/ 8.1

jbone · November 27, 2006

Hi,

I got my 2002 GMC Sierra 2500HD RCLB w/ the 8.1L/Allison just about a week ago. Since then ive been looking all over online and couldnt find much about them until tonight when I found this site. I am still just as excited about the truck as the day I picked it up!!! I cant wipe the smile off of my face or take my foot off the gas Im not even sure if this is the right place for this thread but I couldnt find any other place. I have lots of questions but ill ask just one for now. This is a great site by the way!!!

Jason

Has anyone ever tried going to an 80mm throttle-body?

THE BIGPUNN · November 27, 2006

congrats on truck the only negative i have is low mpg.

ABQFirefighter · November 27, 2006

My 8.1 is a pulling monster! It has no problems pulling whatever I ask it to. The downside, of course with any big block, is the gas mileage. But you don't buy a big block for economy, just for raw power. Enjoy it and keep that foot to the floor!

Shaners · November 28, 2006

One of the first and easiest mods that you'll notice right away is a custom tune (PCM). These tunes will really wake up your throttle response, improve you shifting and in some cases improve your MPG's too.

Most people seem to think that a custom written tune from "Wester's" or one of the other companies is far superior to a handheld programmer.

Schmids5.3l · November 28, 2006

Yeah Tbyrne has an 80mm for the 8.1, BBK

http://www.tbyrnemotorsports.com/truckcatalog.html

Its under 'induction' on the left, I was thinking about getting one. Part # 1756

$429.99

Cheekster96 · November 28, 2006

congrats on the truck. I have a 2006 RCLB with the BIGBLOCK. I love it! I didnt buy it for the mileage (is the only downsize). No problem merging on the turnpike, getting pushed back in my seat or the nice idle that only a big block produces!

Post some pictures.

Later

Lee

ChrisNJ · November 28, 2006

Congrats, I'm on my 3rd 8.1, currently in a 06-CC-ally. Gets almost same mileage (better hwy, better towing, worse mix), than my 02-6.0 I traded. It's the best all-around do-everything tow-whatever family-hauler bike-hauler me-hauler looker, truck/engine on the road, IMO. Love it and use it.

Hey Cheekster, I noticed you have the 4:10's with a 6sp Ally. Mines got the 3.73's and am wondering if it's worth a change to 4:10's? I searched for months for this truck since I couldn't order one, and learned the new HD won't get the 8.1, so I had to take what I could get. Drove 4 hrs to get it. My only issue is at 70mph it's running 1800rpm, which is too low, gotta put it to the floor to pass, and it takes a couple seconds for the ally to downshift, gotta plan a pass, unlike my older 8.1-4sp-4:10 which would run at 2500rpm at 70 and would pass like a freight train without a downshift. Bonus is it gets 15mpg hwy vs 12-13 for the 4sp-4:10, but I'd rather have the performance than the mpg. I like the feeling of the throttle connected to the road. What's yours running (70mph) in 6th, 5th, 4th? Mines 1800, 2100, 2900. I guess I could keep it in 5th in manual mode, but even 2100 is a little low too, and 2900 is too high for cruising. Just curious. Thx.

jbone · November 28, 2006

thanks for the replies!! So does anyone here ahve the 80mm tb, and noticed any gains?? With a tune on a stock truck what exactly would you change, or does westers, or others know what to do? Does anyone know of any good "free mods" for the 8.1? Has anyone heard of getting a larger "maf"(?) sensor???

Haulin' 8.1 · November 28, 2006

Jbone, I have a 80mm BBK TB and a Granetelli MAF along with some other mods, free and pay. I'm at work now but I'll tell more about them later. In the mean time don't rush off and order a TB yet.

Vernon

Cheekster96 · November 28, 2006

Howdy Chris. Yup, mine has the 4.10 gears with the six speed Ally. Cruising at 70 in sixth, tach is reading at 2000 even. I havent tried putting it in manual in 5th and 4th gears, will try and let you know.

I do know this: I used to live by this hill, and my '03 Sierra (4.8L, ECSB 4x4) had to go down to 2nd gear to maintain 60mph. Had the cruise control on and wanted to see what would happen.

Well now, with the big block and 4.10s, that same hill with the cruise on, it stays in sixth gear and has no hesitation of wanting to downshift whatsoever.

I dont do alot of passing, just usually set the cruise for 70 and let her do her thing. Averaging about 12 mpg with about 85% turnpike commute.

I do want to get rid of the stock Bridgestones and put on a Magnoflow exhaust system. Want to see what a big block really sounds like!

Later

Lee

Haulin' 8.1 · November 29, 2006

Jbone, the stock throttle body is 78 mm. The main flow limiter of the stock throttle is that it necks down considerably behind the blade. In fact the BBK unit comes with a adapter plate that necks it back down to match the opening on the intake. I scored a 80MM on Ebay for $100 so I can't complain too much but I even went through the trouble to hog out my intake flange to 80mm to mate up with the new throttle without an reducer plate. The gains were a slight gain on the top end of the rev range. Nothing spectacular. If I were you I woud port my stock throttle body like LS1 guys do and port match the intake like I did. This would give you any gains a 2mm bigger throttle would get you for a lot less loot.

The Larger MAF helped a bit. But that along with my Hypertech programmer and Ethanol blended gasoline leaned out my fuel trim to the point it was surging at WOT and would detonate on 89 octain. I installed a set of Delphi Marine 8.1 injectors and lowered to fuel pressure down to 42 PSI. This set my fuel trim to 11% rich and the truck hauls again but a bit faster due to the larger MAF. It's a lot of trouble to start stacking mods so be prepaired to either work around it like I did or have your truck retuned every time you add a significant part. $100 on Ebay for the MAF and $275 for the injectors to a board member on performancetrucks.net

My Volant air box with a parts store air duct routed to the front of the radiator core provides a nice HP boost. So much so that I could tell the difference with the cold air box and then again with the home made ram air hose. Once the hose fell off on a trip and it made such a difference in power I started worrying about the engine until I found the hose off and replaced it. Power jumped right back up to what I was used to. $125 on Ebay.

A low restriction muffler is worth decent speed. I've tried two kinds of Flowmaster on the truck and they were too loud at cruise for me. I've got a Magnaflow now and love it. The factory exhaust is dual 3" stainless so only a muffler(s) and another tail pipe is needed for power. Don't spend extra for a full cat back that won't change anything as far as pipes cat to muffler go.

The Hypertech is better than nothing but won't touch a decent custom tune. Mine came with the truck so I use it until I need a full on tune.

Don't neglet a good tune up. You'll need good plugs and fuel filter before you see any gains. Don't expect these bolt ons to set the truck on fire. It'll be torquey and fun but the cam, heads, and intake are a major bottle neck to real power. I'm going to tackle that obsticle with HP3 Marine heads, a Comp Cam, and Whipple Supercharger. I don't think I'm going to find the right cam and the headers I want on Ebay for a song though so I probably buy those new next year and start on the build-up.

Vernon

ChrisNJ · November 29, 2006

Howdy Chris. Yup, mine has the 4.10 gears with the six speed Ally. Cruising at 70 in sixth, tach is reading at 2000 even. I havent tried putting it in manual in 5th and 4th gears, will try and let you know.

I do know this: I used to live by this hill, and my '03 Sierra (4.8L, ECSB 4x4) had to go down to 2nd gear to maintain 60mph. Had the cruise control on and wanted to see what would happen.

Well now, with the big block and 4.10s, that same hill with the cruise on, it stays in sixth gear and has no hesitation of wanting to downshift whatsoever.

I dont do alot of passing, just usually set the cruise for 70 and let her do her thing. Averaging about 12 mpg with about 85% turnpike commute.

I do want to get rid of the stock Bridgestones and put on a Magnoflow exhaust system. Want to see what a big block really sounds like!

Later

Wow, only 200rpm difference in 6th! Probably not worth a gear change? Would love to see what 5th and 4th are. Thx, you may have saved me a ton of bread.

Lee

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linda · November 29, 2006

enjoy the cubes man, there is no replacement for displacement. There are plenty of things to make that heavy truck faster.......just make sure you read up and learn as much as you can before blindly adding parts to your truck. You AND your truck will be better for it.

jbone · November 29, 2006

thanks for the tips. I probably will just port the stock tb. I would buy the volant but I hear good things about the K&N as well and its almost $100 cheaper!! Although I would love to do heads, intake manifold and a cam it would be too risky considering this is my DD and work vehicle. I cant afford to have it down for a long time or worry about something going wrong. Has anyone done done just a cam? No heads or intake manifold. I understand that I will probably need new springs, rocker arms, or whatever which is fine. I love the speed it already has but im mainly looking to make more power down low for towing, and plowing.