Thank you so much everyone for all the input! I think I’ll go for Diamond performance Thunder & Lightning cast headers.

Excellent move.

Ok, so, is that 646 ft.lbs of torque and 690hp with tubular dyno headers???----- or with the Diamond Thunder & Lightning cast aluminum manifolds???

Are those numbers with the Sonic or the Excalibur ?

If you are serious about making power, buy the Thunder and Lighting headers (KE headers). I've used Gil's, Stainless Marine, and Eickerts all on the same engine. Stainless and Gils will give you exactly the same results. I switched to Eickerts and instantly gained 2-3 mph. Nothing else was changed and that was on a single engine boat, with a 439 BBC with peak hp at 6100 rpms. midrange pull was noticeably stronger and top speed improved and stated. I would be putting them on my new boat, but I don't see how they will work in a staggered set up. Go with the Eickerts and don't look back!

I just ran across this thread from about 5 years ago.... and as best as I know the Diamond (KE design) full length cast aluminum exhaust manifold/header system is no longer being made. I don't think it's much of a big announcement anymore though.
It's actually too bad as it's a very good exhaust cast aluminum system that possess true "header" performance quality.
It would be nice if someone with a few bucks in their pocket could buy the manufacturing rights (whatever) from Diamond and get the show back on the road again.

Many US foundaries have closed or gone offshore. Largely because of the EPA. Showed it to a friend who has a foundry and a machine shop. Said it was a difficult and large casting and was not something he could do.

Padraig

Good thread on Speed-talk about log manifolds vs tuned tubular headers.

https://www.speed-talk.com/forum/viewtopic.php?t=8305

IOT have unrestricted exhaust flow, you need 2.2 cfm/hp.

The BBC Mercruiser cast iron exh manifolds have a large enough collector size for 420hp before restriction.

Dry vs wet flow in the risers riserbe a factor as well.

Tuned headers draw exh flow energy and use it to 'supercharge' the intake. You won't get that benefit to the same degree with log manifolds, but correctly designed, the logs will flow very well.

Once in the riser, the exh doesn't have far to get out of the pipes, and it's like a large collector, compared to a vehicle with mufflers and curved tailpipes etc.
3" exhaust (dry) will support 740hp with no restriction.

It would be interesting to see what the Stainless Marine manifolds flowed on a flowbench.

It would be nice if the GIL or Stainless Marine cast aluminum exhaust manifold systems could support more than just a mere and meesly 500hp to more like at least 600hp.
I say this in reference to some previous posts within this thread.
I can say that I absolutely agree with removing the way the water is introduced into a tailpipe as it can have an adverse effect on the performance of an engine by choking the exhaust gases. It also creates a lesser chance for reversion ---- besides all of that a DRY exhaust system sounds much healthier ---- just my .02

I would think that now that Trump is in charge that perhaps many of the EPA restrictions and crazy regulations are in the process of being removed for these kinds of productions.
A lot of this kind BS regulatory infringement has killed a lot of business.... and this is especially since all the Covid-19 crap.
Hopefully companies (even new companies) are able to start up again, but many have sadly disappeared.

Does anyone have actual sheets to prove GIL and the like don't handle more than 500? I'm pushing 600+ through an NA bbc with gil offshore manifolds

Wondering if there is any merit to the claims or if it's all marketing to sell headers