Looking for advice on intake and exhaust

ShortCrew · June 25, 2003

I'm looking to upgrade the exhaust and intake on my new 2500HD, 8.1L w/ Allison. What brands and parts make proven power? Specifically looking for exhaust and intake mods for now.

Thanks

katie-bug · June 26, 2003

Shortcrew,

There was a discussion here a while ago about this. The exhaust ports in the cylinder head are purposely small and restrictive in the truck version of the 8.1 liter to improve torque in the 0 to 2500 rpm range. Marine heads have larger ports to improve torque in the 3500 to 5000 rpm range. Because of the truck engine's cylinder head exhaust port confuguration, better breathing may not be possible with air intakes, manifolds or exhaust systems. But they do sound better. There is information on this at the GM Vortec site, as well as an article in the Tech section of the July 2003 Truckin' magazine.

Richard

496Cubes · June 26, 2003

I was looking at doing the same thing, but decided it wasn't going to be worth it. The only thing I would recommend is changing just the muffler, to make the truck sound a little better. I elected to remove the muffler and just put in two short straight pipes in it's place. This is least restrictive "muffler" you can put on and didn't notice any seat of the pants difference. I might have gained 5HP, nothing you can really notice without a dyno.

I decided to go with the Wester's reprogrammed ECU. I decided it was going to be the best cost/performance ratio. There have been some recent posts about it. I'm going to install it this weekend. Here's what he told me:

"Hi Brian,

I can eliminate the torque management on the manual transmission truck, and 4L80E truck--but the Allison hates it when you start playing with the 'maximum' torque numbers. Yes--higher octane in a long stroke engine makes for more torque--however, that's great for towing. Not great at high RPM. That's why you get the best of both worlds--you can run either fuels.

I have no HP and torque graphs for a manual transmission truck. The last 2001 Allison gained 52HP and 68 ft/lbs of torque--more than enough to make this engine very responsive--but this was on an automatic. Since my dyno doesn't make charts--it's impossible to supply them. It's an electronic eddybrake system that measures at each specified wheel speed.

Lyndon"

Good Luck

8.1Texan · June 26, 2003

So, do any companies make aftermarket cylinder heads for the 8.1?

what would be the cost and what kind of gains would that get someone?

tbyrne · June 26, 2003

I have two modified 8.1 trucks ('02 Suburban and an '02 Silverado HD). Both trucks have responded very well to those modifications. Each one has a Volant induction, The Other Guys headers, Direct Hits ignition and custom programming. The main power adder for the Suburban is a Whipple supercharger with intercooler -

.

A trip to the track produced a 14.5@ 92 MPH - a couple seconds faster than a stock 8.1 Suburban.

Other performance mods for the Silverado include a ported throttlebody and spacer, Flex-a-lite electric fans and a mild comp cam. Those mods have also showed a very noticeable increase in power but it's definately not as fast as the Suburban. You might have noticed that I haven't done anything with either of the exhausts. Most of the vehicles I drive are loud so it's nice to turn the key and not hear the truck running over the radio Replacing the stock catback with one of the aftermarket systems is usually good for about 10hp on these trucks.

As mentioned above the head design is kind of restrictive on these trucks. I wouldn't say that you won't see any gains from other performance mods because those will work fine with the stock heads but there will be a gain from upgrading those. The 8.1 heads that we carry are fully ported/polished and come with new springs and retainers. The stock valves and locks are used. There isn't a big demand for ported 8.1 heads right now so there isn't alot of options available for them. The price for this is $2139.99.

Tom