It hurts scavenging, but the overlap is not that severe in stock form, and the aftermarket cams don’t do much to change it since there’s hardly any room for valvetrain geometry to change with this much compression, were not trying to make 1000hp here, but 400-500hp, so we’re not counting every nickel of airflow so to speak. In ideal form you want drive ratio to be 1:1, but it’s basically unachievable and you will never see 1:1 in the real world, 1:1.5 is a more realistic goal for an efficient setup. 20 psi in the intake and 40 in the exhaust is pretty null when you consider that the exhaust coming off the power stroke is in the 100k psi range, 40 or even 100 psi of drive pressure doesn’t matter as much since it’s got all that pressure coming off the piston driving the spent exhaust out the exhaust port, the overlap actually helps get the exhaust out of cylinder and pull more air from the intake, not the other way around, what you loose with the overlap is actually turbo efficiency, you get boost going out the exhaust but that does also help push the hot air out. What high drive pressure does is create higher egts since the exhaust gas is being compressed more, and gas velocity slows down if it’s getting backed up in the turbine, which would mean more exhaust left in the cylinder. That’s not a problem because of pressure, but because of flow. No matter what exhaust pressure is going to increase along with boost, so as long as you try and keep below 2:1 you’re really not doing too bad for airflow efficiency.
Back pressure dropped towards the end of the engine’s life for a few reasons. One, going to the Walbro GSL392 and bypass regulator meant that I wasn’t dropping fuel pressure at wot, which retards the timing and creates more drive pressure since more combustion is occurring out the exhaust (makes for good flames and high egts) instead of in the cylinder pushing the piston down making more power, so having the extra tune-ability helped me drop it and pick up some efficiency. The turbine housing never changed, it still the stock factory turbo housing. Having the wastegate delete streamlined the exhaust but meant more boost and more drive pressure. The other reason why I think I saw the drive pressure lower during the last couple months of the engines life was the na rods. Pretty sure it was making enough power ever since pulling off that 14.8 quarter mile to bend the rods, which pretty much is like shaving the pistons and dropping the compression ratio. Less compression means less cylinder pressure, which is probably why it got to a point and stayed together until I turned it up for a Hail Mary on the dyno, where it stressed them enough to fail.