When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
i am happy w the final machine work. they head to Carter's tomorrow in order to be kitted to their respective turbos.
here's another pic of the approx downpipe.
simple and low drag. the additional V band was for testing numerous turbos. since my turbo (G40-1150) has a 4 inch outlet i now have a 4 inch 45 out of the turbo, a bit of 4 inch and a 4 to 3 reducer before the bottom turn. i like the noise level of my 3 inch exhaust. it makes wonderful (674) power w modest backpressure,,,
for those who have bought the manifold and are now awaiting the balance of the kit, or for those considering the system i thought it would be worthwhile to better introduce my partner, Carter Thompson. many in our community already know him both as a member of the RX7club or as HPPFAB. (High Pressure Performance Fabrication.com). Carter is Mazda first and is an accomplished road racer as well as bike and kart guy. he is an accredited Mechanical Engineer, has a Serious day job and somehow finds time for interesting personal projects... like this:
a couple of years ago this was a dark green Cayman w the usual broken motor. today it sports a single turbo intercooled Audi V6. (i did seriously lobby him for one of my motors but the wheels were already in motion). it has aways been my contention that all German cars are designed so their owners cannot work on them. taking a German car and swapping in a German engine from another brand, turbocharging it would appear in one of my nightmares. just another day for Carter.
Carter sells a very nice diff brace that will be onboard my uprated 4.30 Giken LSD rear under current construction:
apparently there are around 400 in current service.
i also am upgrading my lower trailing arms w his chrome moly alternative that adds a bit of additional clearance and features high end spherical bearings.
after a brief tour of Carterland i think you will agree that i have a strong partner as to the "kitting" of my low drag manifold. the kit will include both Mica and DEI heat shielding panels.
watch it appear in this thread and keep in mind there is an 8 week lead time on the casting (in Alabama BTW) til it arrives at HPPFAB/Tn.
$850 gets you in line.
Last edited by Howard Coleman CPR; Mar 15, 2026 at 10:29 AM.
Carter's picture is perhaps the best as to showing the design aspects of my manifold.
one of the key challenges in designing a proper manifold is positioning the wastegate(s). there's loads of diagrams on the web and the CPR manifold design successfully challenges the conventional wisdom. it may be up for debate as to whether a disadvantaged wastegate position is up to the job but it is not up for debate if positioning the wastegate directly into the exhaust flow or on the outside of a directing turn is not subtractive.
here's proof that my "disadvantaged" wastegate position works:
more boost than backpressure until 6796! this is w the boost control off, running off the wastegate spring. this was my first run w the cast CPR low drag manifold. it was cut short by an oncoming police car. peak torque on our motors ranges from around 5000 to 6300 for a very well ported race port motor. peak torque is max cylinder fill. this is the point of max misfire and detonation risk. it is really nice to know that during this period the exhaust is NOT backing into the intake charge air.
note also there is absolutely no boost creep even w the boost control off... this is w an 860 rotary rwhp turbo! all w a single "disadvantaged" wastegate.
as you may recall the cast manifold is a tuned up (thanks to SolidWorks and Carter) version of my welded steel manifold that i fabbed in 2013. i am delighted by the fact that the cast manifold bested the welded manifold in all aspects... crossover point is approx 300 RPM higher.
i do have additional data to share but it is from my welded manifold. i expect the cast manifold to best all metrics.
8032 RPM, 21.7 boost and 674 rwhp. thank you Garrett and their marvelous G40-1150. built for low backpressure and top end, not spool. Trim is at the end of the low spool range at 84. the welded manifold produced 32.2 psi backpressure V boost or 48%. keep in mind this is w a 3 inch exhaust, the wastegate plumbed into the downpipe, at a high power output and the welded manifold. crossover was at 6334.
notice the position of the wastegate diaphragm... wastegate diaphragms are simply fabric infused w silicone and as such are heat sensitive. i note many manifolds locate them near the downpipe. not ideal. note the diaphragm end of the CPR wastegate is located away from the heat and in a welcoming space in the subframe that Mazda thoughtfully provided. i am running the same Tial V60 wastegate that i purchased in 2013. (Tial makes real race stuff... like Tilton.)
i am excited to see the supporting kit soon to evolve and am working w Carter on the design aspects. i expect the kit to be as good as the manifold.
casting lead time on the manifold is 8 weeks for those considering it.
Last edited by Howard Coleman CPR; Apr 28, 2026 at 04:58 PM.
Is there not enough room in the chassis to make the neck-down farther down the downpipe? Would've been ideal to have the reduction after the wg merge.
I'll defer to Howard or Carter for an answer on that, but eyeballing the photo here, and the space available in my own FD, I think the problem you'll have putting the WG merge further down the DP is that you would need to run a tighter radius bend than what's shown here right off of the turbo to clear the available space at the firewall/tunnel area to achieve what you suggest.
"Is there not enough room in the chassis to make the neck-down farther down the downpipe? Would've been ideal to have the reduction after the wg merge."
yes, that would have been an add but...
"you would need to run a tighter radius bend than what's shown here right off of the turbo to clear the available space"
in addition, the DP WG stub would make it impossible to mount the DP without pulling the motor. probably a no on that.
once the WG flow enters the WG there is only one turn (which is in the WG) before the relief flow enters the DP at a nice blend angle. the location of the WG tube is determined by the pass side motor mount and the desire to keep the tube straight.
I'll defer to Howard or Carter for an answer on that, but eyeballing the photo here, and the space available in my own FD, I think the problem you'll have putting the WG merge further down the DP is that you would need to run a tighter radius bend than what's shown here right off of the turbo to clear the available space at the firewall/tunnel area to achieve what you suggest.
I wasn't referring to merging the WG farther down the DP. I was wondering if we could carry out the larger diameter of the DP farther along then neck down after the WG merge (same intended merge location). I was thinking it'd be nice to maximize the CSA of the DP for as long as possible and dump the WG exhaust gasses into a larger CSA tube, then neck down afterward. But alas, space and bend radius constraints seemingly wouldn't allow that.
Is there not enough room in the chassis to make the neck-down farther down the downpipe? Would've been ideal to have the reduction after the wg merge.
Yeah, there is, but it will become tight at some pipe diameter as room between transmission, unibody "frame rail" and unibody floor gets tight and the pipe will become the lowest part of car.
My downpipe is 6" inlet gradually reducing to 3" 2 bolt flange so it can hook up to standard FD exhausts. I am running the 3.75" Trust (Greddy) Power Extreme BL.
Last edited by BLUE TII; Apr 29, 2026 at 06:26 PM.
EFR 8374 10psi at 2,000rpm and full 12psi boost before 2,500rpm. Even in lower gears.
It's wild that you had to have something like a hybrid s4/5 turbo with open exhaust to get that kind of boost threshold 20 years ago. Now you can do it with a turbo that will make near 3 times the power.
