Looking for a little input to either confirm or convince thoughts I’ve had about after-cat exhaust size. I’m running a fairly mild 383 with Sniper EFI and by looking at fuel learn tables I can see that even at low engine speeds and load my 3” Hooker catback is a restriction when comparing open and closed cutout datalogs. I have Dyno Don’s headers on it and what I’m really asking is whether a 4” exhaust is just extra weight and noise over 3.5” going behind the 2.5” y-pipe that comes with those headers? I want the power the cutout affords without all the extra noise and attention of an open cutout at the passenger door. The engine in a perfect state of tune should make an honest 450-470 hp. Thanks for the input.

 

I am DEFINITIELY staying tuned in for this thread. I also run the DynoDons headers and his y-pipe to my 3" hooker cat back (with no cat) and a 3" cutout. My engine makes 531 crank hp and I know that an exhaust upgrade is in my future when I put the nitrous to the engine soon. I have wondered about a 3.5" or 4" setup, but also am thinking that I might want to sell the dyno-dons headers and get a set of real long tube headers. This may not happen until fall of 2021, but I do want to learn.

I'm staying tuned.

 

I’m betting you guys see gains as long as you use a good muffler. I went from a 3 inch spin tech to a 4 inch with a dynatech muffler. No dyno numbers but made a huge sotp inference. I’m running 1 3/4 long tubes though. Both cat backs

 

I know I’ll see a benefit from a larger after cat exhaust for certain and I know my absolute best bet would be long tubes into 3” Y into 4” exhaust but packaging those longtubes presents its own challenges. Because I have the Alston inner SFC’s welded onto the car I’ve had to modify every aftermarket trans x member I’ve ever installed on the car. I have a T56 in there now, want the torque arm off the tail shaft while maintaining exhaust clearance and the blackheart swap x member was the only one that checked every box. Problem was that it had the worst interference with those SFC’s of any of them. The others a little material removal, maybe welding and all was well. The Blackheart needed the mounting completely reworked to fit so I may just have to build my own which will be tough to do right.
The math says the 2x2.5” pipes are a near equal volume to a 3.5” single pipe. I’ve seen lossless hp values limited as low as 400 for that diameter pipe though, others as high as 460-70. The 4” more closely matches my engine’s output but flows more than the 2.5” y-pipe which would be counterproductive because it comes after the y. That’s where I am now.

 

I gotcha. makes sense to me! I also have some tight clearance issues and obstacles under my car, specifically the bars involved with the S&W subframe connector kit and its crossmember which is where the torque arm mounts and driveshaft loop. All that stuff is great and needs to stay, but it will be tough for my exhaust project when that time comes along.

 

I made my own system fwiw. I have UMI connectors, not on yet though. I had to make my own trans crossmember, so i built in as much clearance as possible. I have the aftermarket t56, couple inches longer than standard LT1 t56. I have a thread on here with the exhaust build if it helps.


https://www.thirdgen.org/forums/exhaust/765658-my-homemade-4-inch.html

 

2Slow that trans mount looks fantastic! I hope mine comes out at least as well and I’ll be building my exhaust too. $600+ shipped for a mufflex system doesn’t seem attractive when I can build a better fitting exhaust for half that

 

I think hawks sinister may be the way to go. I had about as much in my own with piping, not to mention fab time. Hawks is about 450 bux

 

DO IT!

Here is the thing....

No matter what you do in the future, long tubes, etc, the 4" will handle (ANYTHING) you can throw at it, i an genuinely surprised that you think Dyno Dons shorty's and y pipe isn't "ENOUGH" with a 3.5-4" cat back... Those should support a (LOT) of power...

I plan on going 4" sinister, dyno dons headers and y pipe on my formula... Well, if everything goes the way i want.

 

How much power are you making? A 3.5" cat back will support gobs of power. A shorty header and y-pipe is what is limiting you.

 

I ended up building a 3.5” exhaust out to a single exit. It’s louder than I wanted but the muffler is tiny so the extra noise was expected once I held the muffler in my hands and saw its size. Sounds good though. Power is listed in my first post but that will be going up with a cam swap, went way too small first time. The power noses over hard at a little over 5k and I’ve got more spring than the cam requires with the stock AFR 8017’s so it isn’t float. I really don’t know what the DD headers are good for. We’ll see if I’m still wanting more once the new cam is in, Sniper tuned more closely to optimal and probably new gearing. I think my 10 bolt is already dying behind this engine, pinion seal is weeping oil with less than 5k miles since it was rebuilt.

 

Some out there with my headers are putting out 500 @ the crank and 440 @ the rear wheels.
This with 350-362 cu. in.

 

I would be one of those people. It is a 406 sbc with the holley sniper efi. Made 531 hp at the crank 513 tq. The guy who was tuning my car did rough calculations using datalogs and said I am about 455 hp to the wheels.
-my problem for the future with the exhaust will be that I am absolutely definitely certainly going to be running nitrous on this engine, probably up to a 200 hp wet shot. So, that's my future dilemma....

 

IROCZman, what intake manifold are you using under your Sniper? I’ve run mine with both a Vic JR and RPM AG. The dual plane didn’t seem to gain much down low over the single plane but judging by MAP readings in datalogs the Vic peaks at about 101kPa and the air gap 95kPa. I did change my target afr tables between manifold swaps though. Only difference I’ve even been able to note between the 2 is cruise vacuum but I still have to stay with 5th gear at highway speeds (T56)with the dual plane. But also the car has developed a 5k+rpm misfire too so hard to tell anything about top end performance right now.

 

currently running an Edelbrock Victor Jr, which is what the race shop built the engine using. Some people have recommended that I switch to a dual plane, but I haven't had the desire to pull the manifold. if I ever do, I will then have plenty of datalogs to compare results. I don't think its in my game-plan for this year either though. My rear gear-swap in January gave me a big seat-of-the pants gain on acceleration and the rest of my 2021 budget will be for some street/autoX tires. Then money will have to be budgeted for a 6-point roll bad as I am pretty sure this car will be below 11.50 when I get to the track in april.