I did the OE ones on my 68 RR, gasket matched to the RPM alum ported heads
a lil' material removed big casting marks & smoothed all the radius's a tad
as far in as I could see or reach with a carbide tip & then a paper roll...
4+ hrs worth at a min. maybe more...
Used TTI 3" mandrel head pipes too, to tie into my existing
American Muscle Flowmaster 3" mandrel exhaust system w/H-pipe
& Delta 50 series mufflers, all the way out the back of the car...
I know the motor made appr. 590hp-ish crank/appr. 540hp-ish to the rear wheels,
M/T DR street tires
don't remember corrected HP that day
with the Heddman Mid-length headers (not the best for flow or HP either),
going into same exact 3" Flowmasters mandrel full exhaust system
a repeatable consistent 10.50s thru the full exhaust @ 132-135 mph 3520#s,
80's/82's staggered jets in outboards & 65's on the inboard carb
(12.7-12.8 AFR readings WOT, fuel at 6 psi flowing, bypass reg.)
with a safe street tune, 36* advance
they were pissed I didn't have a rollbar, so I was limited some
I knew them all, & they still let me run
After I went back almost a month later like Aug. now,
to try it with the HP manifolds on the car,
I knew it was going to be slower, I could feel it in my butt
& the Unisteer rack & pinion steering
the Heddman mid-length 1-3/4" headers I had,
wouldn't fit with the rack & pinion I now have,
it (the HP's) killed a tone of power, even after the mild porting...
I'm 100% sure, it flowed better than the OE 68 HP manifolds ever did
the headers just flowed that much better...
No I don't have a side-by-side comparison of the flows...
But with the HP ported iron manifolds
best of 11.09s @ 125-128 mph thru the full exhaust 3520#s
on M/T DR street tires still
(the mph took a big hit, that's HP lost)
still same deal, walking it out of the hole, maybe 4200 rpm
the same exact procedures, same psi in the DRs, for burnouts etc.
60' foot was off, way slower, was never anything impressive anyway
(don't remember exact #s now, 1.6 (? maybe)
but 60' times were erractic, before I got a good (well decent jetting)
carbs tune in it, finally...
It was NOT that the porting job, that was wrong, in the HP manifolds
they were just that much more restrictive & are serious HP killers
(I have mad respect for the F.A.S.T. guys, that have to run them POSs)
I do remember, it lost about 0.5 tenths in the quarter on similar day,
(prior mid length headers aren't the best & 1-3/4" tubes too, not ideal either)
as track conditions & RAD
with the 68 HP manifolds installed 'after porting',
I did not run it with them un-ported, for a side by side comparison...
&
I had to lean it out a tone too,
it was almost the same exact conditions at the track 100+* day,
just a month later, same track (Sacramento) no prep, like normal T&T Sat.
almost same RAD, same sunny day,
but a slightly better & a slightly lower altitude & humidity RAD
(I had 80's/82's staggered) went down to 73's all 4 corners,
7-9 jets sizes down in the 'outboard carbs',
2300 Holley 500cfm VS w/Flowmaxx plates with Holley jets
it was still at 11.5-11.8 on the AFR (dead fat still, 1 full ratio off from prior)
& then I did the center carb
(I had 65's) went to 62's, 3 jets down in the inboard 350cfm
it's a trick flowed to 415cfm carb 2300 Holley inboard (6bbl)
it was still way too fat,
(full 0.5 AFR a 1/2 a full point off the previous combo, only change
was swap the mid-length 1-3/4" headers to the ported HPs manifold)
12.0-12.3 AFR, but I didn't have any smaller jets with me...
Car was 0.5 tenths slower anyway...
All that equates to 470 RW HP (vs 540 w/headers) at the track
& 522 at the crank (vs 590 crank HP w/headers) at the track, real HP
by Wallace racing calcs anyway, they don't take in, the RAD either
(air quality or temps)
w/HP manifold installed,
was a 70+HP loss at the RW & 68+ HP loss at the crank
I'd assume them #s are good, give or take 10-20hp up or down max,
for the conditions...
But the car still was working much like it did previously, just slower everywhere,
it was diminishing results anyway...
I don't know if that helps, but the headers are leaps & bounds better for free flow...
