New downpipe design - Will I need a minor remap?

I'm running an RB26 in my Datsun260Z. Since i built it its been running a non equal length downpipe with a merge design that could probably be better.

I am looking to try and see if I can build an equal length downpipe with better designs, honestly I want equal length mainly for the sound but if I manage to improve airflow from a better design will I likely change it enough to need a remap in order to keep the fuelling safe?

I was thinking I would also improve the port matching on the exhaust manifolds and possibly try to somehow add a filter with a velocity stack instead of the hard cut pipe it has at the moment, although I haven't figured this one out yet.

If I am going to have to remap for just the downpipe I might as well make as many other improvements as I can at the same time.

I don't reckon a tubular set of manifolds for the RB26 will flow any better than a ported cast stock set but will be prone to cracking?

That's a very tidy installation! Good work!

The reality is that any changes to the exhaust or intake 'may' affect the VE of the engine and hence require a re-map. That's not to say what you're proposing will have an effect but it may. I'd always recommend at least performing a check up on a dyno to make sure everything is still ok after your changes. Adding a smooth bellmouth to your intake pipes instead of a straight cut pipe will definitely improve airflow into the turbos and again can affect your VE and even your boost control. If you can get a feed of fresh air to the filters instead of picking up air from behind the radiator then this would also be beneficial.

Thanks, I put a lot of thought into it to make the installation as tidy as possible.

It seems like it would be best for me to just carry out all the mods at the one time and just face that it will need a minor remap.

I think with a better designed merge from the downpipes, and with downpipes that are sized smaller than 3" going into 3" so theres a step up (at the moment the downpipes are 3 inch the whole way) that I can make better flow, I am not sure the equal length (assuming I can make it work in the limited space I have) downpipes will add any more flow but it will at least give me the sound I am looking for.

I really like the idea of a smooth bellmouth on the intake pipes, but from all I can find it looks like it will have to be fully custom (do you have any sources?) most bellmouths are for bigger piping and filter sizes, mine need to fit 2.5" stainless pipe.

Ideally just want to make the setup I have the most efficient it can be, I like how the turbos work for the road so only want to improve the parts around them.

I'd love to have the filters sucking from cold air, but I can't work out how to make it happen, if I were single turbo I could probably work it to have the filter in front of the intercooler but not with the twins. I could add an airbox behind the headlight and in front of the wheel with a feed from the airdam feeding it, but I am still trying to keep the feel and style of the original car. How much difference do you think it would make?

I think I'll go all out on this now and then remap.

Make a new equal length downpipe with a nice merge and step up in diameter where they merge.

Smooth off the piping more at the inlet to the turbos

Try and get custom bell mouths made for the filter side

Make sure the exhaust manifolds are properly port matched

Cutting a bit more out the front panel to hopefully allow some more fresh air near the filters

Will make up screen to divert radiator air away from the filters, will start will alloy sheet but may use this as a mould for carbon fibre.

I'm thinking all that lot should make a good difference and be worth the effort of a remap.