Mark Stielow’s LT4-pow­ered Gun­ner Camaro is the pro­to­type for all-new mid-length and long-tube headers

Chevy High Performance - - Contents - TEXT & PHO­TOS: Barry Kluczyk

Mark Stielow’s LT4-pow­ered Gun­ner Camaro is the pro­to­type for all-new mid-length and long-tube headers

It has been a while since we’ve vis­ited the con­struc­tion of Mark Stielow’s lat­est Camaro pro­ject. We’ve cov­ered the build over the past year and you un­doubt­edly saw im­ages of its de­but at the 2017 SEMA Show in Hol­ley’s booth, and on the cover of Chevy High Per­for­mance’s April 2018 is­sue.

Hol­ley was a big con­trib­u­tor to the car, from the su­per­charged LT4 crate en­gine’s mounts to its ex­haust sys­tem. In fact, it’s the ex­haust sys­tem we’re ad­dress­ing with this in­stall­ment. Hol­ley’s Hooker Headers di­vi­sion used the first­gen Camaro to de­velop LT swap headers and we were there to doc­u­ment the process, fol­low­ing how a col­lec­tion of 304 stain­less steel tub­ing was care­fully cut, shaped, and welded by the com­pany’s ex­haust en­gi­neer, Doug Marino, to cre­ate a cus­tom set of headers that ul­ti­mately served as the mod­els for Hooker’s pro­duc­tion headers.

Hooker pro­to­typed mid-length and long-tube headers for the car, de­signed specif­i­cally for its Detroit Speed front sub­frame. The com­pany has also de­signed LT swap headers for first-gen Ca­maros re­tain­ing the fac­tory-orig­i­nal sub­frame.

Ad­di­tion­ally, Marino fab­ri­cated the rest of the car’s ex­haust sys­tem while on lo­ca­tion at Detroit-area Sled Al­ley, where the rest of the car’s fab­ri­ca­tion was han­dled.

“The LT en­gine swap is get­ting more pop­u­lar ev­ery day and it was im­por­tant to de­velop headers for not only the stock chas­sis but the Detroit Speed sub­frame that so many builders are us­ing for track-ca­pa­ble Pro Tour­ing ma­chines such as Mark’s Camaro,” says Marino. “In fact, eval­u­at­ing the in­tended use for the ve­hi­cle, from ba­sic street per­for­mance

to dual-pur­pose street and rac­ing, is the first step in our devel­op­ment process for ev­ery new header sys­tem.”

While we all know the pur­pose and func­tion of ex­haust headers, it’s worth re­vis­it­ing the im­por­tance of their de­sign for op­ti­mal en­gine per­for­mance. Ex­haust flow in­creases with higher rpm and higher horse­power lev­els, in­creas­ing the im­por­tance of quickly and ef­fi­ciently draw­ing ex­haust gases out of the cylin­ders. And be­cause com­bus­tion byprod­ucts don’t burn a sec­ond time, any resid­ual ex­haust gases left in the cylin­der dur­ing the next com­bus­tion cy­cle re­duces cylin­der fill­ing and can con­tam­i­nate the in­com­ing air/fuel charge. The bot­tom line is it re­duces per­for­mance and ef­fi­ciency.

In the old days, it was a given that swap­ping the fac­tory’s heavy, re­stric­tive, cast-iron ex­haust man­i­folds for a pair of high-flow headers was a sure way to un­cork a few horse­power—and more when ad­di­tional en­gine mod­i­fi­ca­tions were made. Th­ese days, the OEMs have got­ten much bet­ter at de­sign­ing

high-flow, low-re­stric­tion ex­haust man­i­folds.

Even with a high-flow fac­tory ex­haust man­i­fold, there are com­pro­mises, mostly to ac­com­mo­date un­der­hood pack­ag­ing and noise level tar­gets.

In a Pro Tour­ing pro­ject such as Stielow’s Camaro, all those fac­tory com­pro­mises go right out the win­dow, al­low­ing the ex­haust sys­tem to be op­ti­mized for ul­ti­mate per­for­mance. That gen­er­ally boils down to two fac­tors: the di­am­e­ter and length of the pri­mary tubes.

Gen­er­ally, a longer pri­mary tube will pro­mote in­creased torque lower in the rpm range by main­tain­ing ex­haust ve­loc­ity and pro­mot­ing low-speed scav­eng­ing. The shorter, larger pri­maries will en­hance high-rpm scav­eng­ing. Mid-range and topend power will ben­e­fit from greater vol­u­met­ric ef­fi­ciency and bet­ter in­er­tial scav­eng­ing at higher en­gine speeds, when the ex­haust vol­ume is in­creas­ing rapidly. That will shift the torque curve higher in the rpm range and low-speed torque will suf­fer due to poor in­er­tial scav­eng­ing at low en­gine speeds.

Mid-length headers are a pack­ag­ing so­lu­tion, par­tic­u­larly for ve­hi­cles em­ploy­ing a dry-sump oil­ing sys­tem, which re­quire more room for the oil lines. Com­pared to nat­u­rally as­pi­rated com­bi­na­tions, su­per­charged en­gines do not seem to suf­fer as much from the lack of pri­mary length, which makes for a good trade-off.

Scav­eng­ing—the prac­tice of us­ing the neg­a­tive pres­sure gen­er­ated near the ex­haust valve to draw ex­haust gas away from the cylin­der—is in­stru­men­tal in an ef­fi­cient header’s de­sign and has ev­ery­thing to do with

pri­mary tube length and size.

“Ideally, equal-length headers, where each pri­mary tube is the same length, of­fer the best scav­eng­ing,” says Marino. “When the tubes are the ‘cor­rect’ and equal length, or very close to, the tim­ing of the ex­haust pulses be­tween the tubes is op­ti­mized to cre­ate greater, more ef­fi­cient spillover sig­nal at the col­lec­tor. Wave tun­ing is more ac­cu­rate and the en­gine be­comes more bal­anced.”

The eco­nom­ics of mar­ket­ing af­ford­able headers, how­ever, typ­i­cally means com­pro­mise. In other words, most headers out there aren’t equal length, in­clud­ing the LT swap headers de­vel­oped on Stielow’s car.

“The re­al­i­ties of de­vel­op­ing and man­u­fac­tur­ing truly equal­length headers makes it a pricey propo­si­tion,” says Marino. “Our ex­pe­ri­ence and test­ing has shown that when the tubes are very close to be­ing equal in length, as they are with th­ese new LT swap headers, the per­for­mance dif­fer­ence be­tween them and true equal-length headers isn’t sig­nif­i­cant for most ap­pli­ca­tions.”

To put it an­other way, the per­for­mance delta be­tween them ain’t noth­ing com­pared to the cost delta, so al­most- equal length of­fers the best cost/per­for­mance bal­ance.

Marino spent the bet­ter part of a week at Sled Al­ley craft­ing the headers and ex­haust sys­tem on Stielow’s Camaro. Af­ter the pro­to­type headers were com­pleted, he re­turned with them to Hol­ley’s head­quar­ters in Bowl­ing Green, Ken­tucky, where they were laser-scanned in prepa­ra­tion for man­u­fac­tur­ing. They’ll be on the mar­ket when you read this.

The rest of the cus­tom ex­haust sys­tem’s con­struc­tion is de­tailed in the ac­com­pa­ny­ing pho­tos—and we’ll spare you the pun of call­ing it an ex­haus­tive look at header con­struc­tion. You’re wel­come. CHP

17 | Fi­nally, racks of new headers are prepped for ship­ping.

15 | Back at Hooker Headers’ pro­duc­tion fa­cil­ity, the dig­i­tally scanned di­men­sions of the pro­to­type headers are fed into a com­put­er­con­trolled tub­ing ben­der, which pro­duces per­fectly ac­cu­rate pri­mary tubes by the score.

13 | Here’s a look at the fin­ished pro­to­type headers, which served as the mod­els for Hooker Headers’ new LT swap headers. Along with ver­sions de­signed to fit the Detroit Speed sub­frame, they’re also avail­able for the stock first-gen Camaro sub­frame.

16 | Fix­tures hold the flange, pri­mary tubes, and col­lec­tor in place so they can be welded by Hooker’s crafts­men.

14 | For Stielow’s car, some care­ful fin­ish-weld­ing and fin­ish-work with some steel wool on the tubes pro­duces a gor­geous, pre­mium brushed ap­pear­ance.

09 | Here’s the pri­mary tube for cylin­der No. 4 mocked up. Note how it pushes far­ther away from the flange to po­si­tion it at the far side of the col­lec­tor. Rout­ing the tube from cylin­der No. 2 to the near-side po­si­tion on the col­lec­tor and the tube for...

07 | The first tack­welded pri­mary tube is shown here mocked up on the en­gine and routed to the col­lec­tor. Note how an ad­di­tional length was added to the tube for cylin­der No. 2 at the flange, push­ing the pri­mary tube out far enough to pro­vide...

10 | The steer­ing link­age and starter are the two big­gest ob­sta­cles when it comes to header de­sign. In some cases there’s no choice but to run the steer­ing gear through the tubes. When it comes to the starter, the chal­lenge is not sim­ply keep­ing a...

08 | The LT swap headers fea­ture a con­ven­tional four-into-one col­lec­tor with a 3-inch out­let. Some mis­tak­enly call this a merge col­lec­tor, but a merge col­lec­tor necks down af­ter the tubes come to­gether and widens at the rear to cre­ate a ven­turi ef­fect.

11 | When the header’s de­sign is mostly de­ter­mined, a po­si­tion for the oxy­gen sen­sor bung must also be ac­com­mo­dated.

01 | Mark Stielow’s Gun­ner 1969 Camaro was built with a Chevro­let Per­for­mance LT4 crate en­gine ob­tained from Scog­gin-Dickey Parts Cen­ter and in­stalled on a Detroit Speed front sub­frame—a great com­bi­na­tion that didn’t have an off-the-shelf ex­haust...

02 | Rather than start­ing at the ex­haust ports on the cylin­der heads, the headers’ de­sign ac­tu­ally be­gins at the end point: the col­lec­tors. Hooker Headers works from the rear of the ve­hi­cle for­ward to en­sure the rest of the ex­haust sys­tem fits as it...

05 | An­other length of tub­ing is cut to add length to the first pri­mary tube. It is an­gled slightly to en­sure clear­ance within the chas­sis.

04 | Hooker Headers’ Doug Marino holds up a curved length of 1 7/8-inch­di­am­e­ter 304 stain­less steel tub­ing to the ex­haust port out­let on the flange for cylin­der No. 2 as he be­gins to gauge the gen­eral rout­ing re­quire­ments for the LT en­gine in the...

03 | Build­ing the headers on the car starts with bolt­ing on the same 3/8-inch, wa­ter jet-cut, stain­less steel flange that will be used on the pro­duc­tion headers.

06 | The ad­di­tional length is tack-welded to the first length of tub­ing. At this point, it’s only nec­es­sary to hold the two pieces to­gether to check the over­all fit in the chas­sis, so no fin­ish-weld­ing here.

21 | Here’s a look at the ex­haust sys­tem flow­ing to the rear of the car. Ideally, it would have been great if the drive­shaft wasn’t trapped by the ex­haust sys­tem, but it was the only way to keep the sys­tem tucked up in the chas­sis to main­tain ad­e­quate...

18 | The rest of the 3-inch ex­haust sys­tem on Stielow’s car con­sists of a mix of pre­vi­ous first-gen LS swap ex­haust com­po­nents de­vel­oped on his Jack­ass Camaro, and the new X-pipe that will com­plete the LT swap ex­haust sys­tem. The ex­haust is a di­rect...

20 | A pair of Hooker’s VR304 stain­less steel mufflers com­ple­ments the headers. They fea­ture a straight­through flow de­sign for min­i­mal re­stric­tion. In fact, they can be mounted in ei­ther di­rec­tion, which makes cus­tom fit­ment eas­ier. They each mea­sure...

23 | Like the headers, the ex­haust out­lets are made of 304 stain­less steel and af­ter the tips were trimmed flush with the bot­tom of the body, a lit­tle more treat­ment with steel wool pro­duced the fin­ished, brushed ap­pear­ance. From stem to stern, the...

22 | The ex­haust out­lets are routed over the axle and as far away as pos­si­ble from the fuel sys­tem’s com­po­nents.

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