Howdy Moparts!

I have a tight engine bay and don’t want full custom headers. Gonna try hedman 1 3/4” shortys but wondering if the 2.5” collector will be a major restriction on a 512. I can neck up to 3” quickly after the collector, or cut it off and replace with 3”. Looking for opinions & advice. More details below:

I built a 512 for my ‘54 pickup truck a few years ago. It’s got a gen1 dak front clip and it’s real tight in there.

Running low on budget I installed a set of 1 5/8” primary chinese stainless block hugger headers- more like 318 size than 512, and terrible pipe routing. They did fine for street duty but the motor predictably felt corked up when you really give it the beans. I’m sure 1 7/8” long tubes would be the best but there’s no room. It made 525tq and 365hp at the wheels on a conservative dyno, and hadn’t nosed over at 5500. Not trying to rev over 6k- it’s a street truck.

Anyway, I have EFI on it which doesn’t like exhaust leaks, and even though the tune is locked in the collector welds are failing on the internal star/crotch areas.

So I got a set of Hedman 78070 shortys. They have 1 3/4” primarys which should help some but neck down to 2 1/2” collectors. I’m considering cutting them off and removing the taper and going with 3” and doing v-bands instead of 3-bolt flanges. With driveline losses I’m right around 500hp at the crank, and hoping to wake it up upstairs some by unchoking the motor but really what I want is ease of maintainance and no stinky power robbing leaks.

Thanks Moparts!
Radar

I would definitely cut off the 2.5" collector and step it up to 3". And if possible, maybe step up to 3.5" somewhere in the system.

My mild 512 lost 40 peak hp on a chassis dyno going through 2.5" exhaust. Headers are 1 7/8" into 3" collector mid length. If you were going to neck it down to 2.5" then best to do farther down the line where the exhaust gases have cooled down.

Thanks fellas. I knew there was a reason I bought the ones with temporary paint instead of coated!

The current system is the $150 chinese 1 5/8” tiny primary block huggers with short center tubes and longer sides. The star welded in the collector center was poorly welded and failed after a thousand miles or so but the head flanges are really flat and very thick which is nice. The corner tubes ended up needing special short sparkplugs AND a hex adapter slipped over them so you can access them with an open wrench. All 8 have fiber boots to avoid burning.

You can find the same chinese stainless headers all over (google, ebay, amazon) but with 2.5” collectors. The ones I got of ambazon have 3” collectors and came with necked down 2.5” collector bolt ups that I didn’t use.

My system goes from 3” downpipes, curves to head back under the cab, and increases to 3.5” then into dual 3.5” flowmaster 40s. I always hated the flowmaster sound and the drone but I got these used off a racecar. They’re cartoonishly big, kinda heavy, and honestly a little too thunderous for what’s barely a quick street machine these days. I don’t have room to go over the axle and out the back so I did turnouts from the mufflers that aim at the tire contact patch for burnout glory (very rare but works great)

I’m thinking of re-doing the whole system with something a little more civilized- possibly 3” borlas or magnaflos or something with a pleasant bass and a little rapp on the throttle.

Any suggestions are welcome. I think it’ll be easier to fabricate a whole new system and just re-use my rubber hangers from thw old system.

Definitely going to cut off the 2.5 collectors and replace the 3bolt flanges with v-bands. I have high hopes for no leaks and less maintainance, or at least maintainance that doesn’t require me to have tiny hands or extra arm joints!

Radar

My 512 uses short headers, but they are 2-1/8 to 2-1/4 step headers to 4" collectors....I would not even consider anything below 1-7/8 tube size.

They'll work fine on a street car. What you want with a street car is something that is trouble free. Thick flanges, good material, solid connections, etc. is what is important. Size isn't really very important since most street cars spend 90% of their time at less than 50% throttle.

I wouldn't even bother changing the collector.

Either build a set of headers or run what you have.

IMO, the block hugger aspect is your first/biggest restriction.

If you can build something with larger radius bends and matched to your exhaust port, even without increasing primary size you may find some benefit in a larger collector.

Hi Andy! Even with how small the mopar community is I’m always a little starstruck to get your input! Your book was a major inspiration. You hit the nail on the head as usual. Even the terrible chinese shortys felt like I had a snarling beast most of the time- my ‘2400’ converter isn’t mushy off idle but runs up to 3k like I sidestepped a clutch when you whack it, and the 512 is a neck snapping monster at any rpm.

But the rare instances that I run WOT and shift at 5500 it feels boring like a 13 second car. I’m not sure how sensitive to exhaust my motor is but I’m really looking forward to the possibility of few more ponies and really looking forward to no leaks and access to the collectors for maintenance.

If you really want to know then get the truck on a chassis dyno and do a before and after test. Everything else is just a guess.

Mopar BB large C.I. motors don't like small pipes, especially on the exhaust side of the collectors
I saw the difference on my old BB 1969 Dart GTS at LACR years ago, it had a set of Hooker 1/78 primary pipe with 3.5 inch collectors, It had a set of exhaust pipes and mufflers that were 2.5 inch from the collectors into s set of Thrust Corvair Turbo mufflers 18 inches back from the collector and then around 20 inches out from the mufflers to the edge of the car in front of the rear tires. The car ran 3 runs in the 12.31 to 12.23ET at 109.4 to 109.7 MPH by using different RPM shifts with that exhaust system on.
I remove it and the next three runs varied from 11.39 to 11.41 ET at 117.5 + mph
My pump gas Duster had 3.0 inch exhaust system from the same Hooker fenderwell headers with a set of Magnaflow 3.0 inch straight threw mufflers mounted at the rear bumper, that car didn't vary .02 ET and less than .02 MPH in the 1/8 mile by removing that exhaust system
Use 3.0 inch pipes from the headers back to the end of your system

Do you have any kind of a performance goal? Without one, its impossible to answer your question. Street car has such different meanings to different folks.

Of course you're giving up something, but If 575 hp is the goal (low to mid 11's in full street trim and crappy 60 ft), you can get there with the 1- 3/4" x 2.5".

The devil is in the details