I have a 1970 mercury cougar with rear mount twins 67/62 1.01 AR, while testing my home built kit with my 351c I had no boost, I could hear the turbos spooling and had 5 lbs of pressure at the turbo but nothing at the manifold, my set up is as follows (untested with the 408)
408 scat forged stroker Cleveland, 2v open chamber ported heads. 750 quick fuel blow through carb, long tube headers 1 7/8 to 2.5” collector, left 4 cylinders feeding one turbo right 4 feeding the other turbo, 2.5” wrapped exhaust pipe to turbo, 3” exhaust outlet into mufflers, cold side of turbos are 2.5” into a 3” Y-pipe running forward to a 18x12x3 Intercooler, HKS sq4 blow off valve, then into the carb.
Im wondering if my AR is too large for a rear mount set up or my piping is too large.
With the stock 351c I had no boost at the manifold, I pressure checked the piping and ran her to 4500rpm without boost 20hg vacuum at idle and decreased to 0psi at 4500-5000, wastegates are connected to manifold. Blow off valve not sticking, 10psi spring on 50mm dual wastegates.

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Can you clarify how you have your wastegates hooked up? I see plugs and no reference lines hooked up. Do you have the fire rings installed?

Were you just free revving to 5k rpm or going down the road?

 

fire ring is installed, 50mm wastegates are both inline with the exhaust prior to turbo, hose is connected to manifold, the piston style Wastegate has 4 ports, 2 upper 2 lower, 1 upper and 1 lower are blocked off the lower port is connected and the upper is vented open

no free revving it was in 3rd gear as I was driving with motor under load

Richard

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Get a measurement before the carb and see if there's boost there. Carbs have alot of pressure drop.

 

I assume since it has 4 ports that they are water cooled. Can you confirm you are using the correct ports for vac reference? (id think it would be over boosting if wrong but worth the check). Is the vacuum reference right off the turbo? I suppose its possible you have a 5 psi pressure drop between the turbos and the engine. Are the gates opening up at 5 psi or just makes the 5psi and doesn't do anything else? if you ran a vacuum source from the manifold to the gates im curious if youd see a change

 

I had confirmation from the manufacture they are correct. I had the refrence from the turbo outlet initially but had concerns the wastegates were cracking open prematurely, I then connected them to manifold. I used the reference from the turbos to run another boost gauge so I could see if they were producing boost and where I had the drop. The only thing I can think of if the piping is too long and too large. It’s approximately 244” long to the carb. Not accounting for the column inside the Intercooler just length

Richard

 

You should pressure test your cold side, more than likely theres a leak somewhere, I'm running a single remote setup and when I first got it running I had a few boost leaks that made me have about 1 psi while driving, as soon as I fixed the leaks it was right at 5 psi, how much research did you do into your pipe sizes feeding the turbos, 2.5 sounds a little large to me, you may benefit from reducing the pipe size down a litttle about half way to the turbo to keep exhaust velocity up.