I've been told the 18.5cm hot side is waaay too big. I can't really find any specs on it but would like to keep it and see what can be done with it. Any info on what can be done to improve it? I'm open to any suggestions

bullseye power sells different hot sides for holsets. they have a .42 AR dsm housing that ive herd good things about.

Hoping to be able to get 500hp out of it without having to rev to 10k or spend more than the cost of a more capable turbo. I'll check out bullseye

what motor are you running?

Built D16Z6.. Driveability isn't a concern as it will be a track only car

track 1/4 mile or real track ? If real track, just resell that turbo and get something much much smaller :)

I am assuming 1/4 mile, and you say a built D16 I am not that familiar with hondas, but I assume you are going to rev to ~8k as thats a pretty typical rev limit for old school toyotas as well (with normal main stream builds) So what is your gear spacing like? Your widest RPM drop will tell you the minimum rpm you need to spool... If you redline it at 8k and make peak power there ( which would be likely on a small motor with a huge turbine, nearly regardless of the cams and intake manifold unless they are terrible) Then 8k - gearchange RPM drop , say 3k as a guess would mean you need to be spooled by 5k to stay on boost hard for the whole run. Swapping to the bullseye .48 A/R housing is supposed to increase spool quite a bit, but it is a poor bandaid that really limits a turbo. It is better to run a smaller turbine in a larger housing than it is to run a large turbine in a small housing. Bullseye may have gotten better with time, but their initial batch of housings for like the S200 turbos - S256 S258 etc were pretty poorly designed and had mediocre results compared to the "too large" A/R housings that came with the real schwitzer S200. Hopefully they have gotten better with time because they do have an excellent product idea / niche market to provide much needed housings.

Anyways, Before you jump at anything else, you could keep an eye on ebay and pick up a junk turbo with a smaller turbine housing just for that housing. You will have to do some research before hand to make sure you get the right housing that will work. Another option is... this is home made turbo.. throw that beotch on the car and see what happens.. it might suprise you and spool closer to 4k than 6k who knows, be a trend setter. Good luck, and btw, a quick google told me the 16cm housing is a .68 A/R dunno if that is accurate, but it might be

would the .48 housing still limit the turbo if you were to run a big(60mm-70mm) dump waste gate? so it would still flow at higher RPM

it is less efficient. wastegate size does not have any relation to turbine efficiency. The sole purpose of a wastegate is to bypass enough exhaust around the turbine to control boost at a target pressure. Some engines /setups will require a larger wastegate than others, but a car that can maintain say 14 psi to redline with a 30mm wastegate will still maintain 14 psi to redline with a 60mm wastegate, the 30mm will just be open a lot more than the 60. The actual target pressure might not be very stable with the 60mm wastegate becasue a tiny movement will have big flow changes, so the target pressure will fuctuate around 14 psi with more fluctuation than the smaller wastegate.

Either setup would still require the same flow through the turbine.

Taking a large housing and replacing it with a smaller housing should increase backpressure. I have no idea what ratio will then go through the wastegate vs the turbine, but the point is the higher backpressure will reduce power. A smaller turbine in a larger housing will also increase backpressure, but not as drastically as choking a big wheel in a small housing. The reason you would want to increase backpresure ever is the tradeoff in quicker spool. If you can achieve 500 rpm quicker spool with a solution that adds 4 psi back pressure or a solution that adds 1 psi of backpressure, the 1 psi solution is obviously the choice to make!

Gonna rebuild it since its got a little play and just go with it. I talked to majestic turbo and they suggested just running it and seeing what it will do then go from there. I was told the 18.5cm hotside might be too big but o well