Downsides of intake port and duct mod?
 

It is time for a rebuild of my 1987 930 and I'm looking at increasing the intake ports and duct to 36mm. Are there any downsides to doing this? My reading of this subject on this site gives no negatives, but I want to make sure. I have found many comments about the cost of this modification. If you have had this done, I'm curious what you paid.

If you don't have an adjustable WUR, you will need one to fuel the increased mass flow that bigger intake ports will yield.
The only negative will be sluggish low speed performance. If I was going to larger intake ports I would increase the static compression ratio to 8:1 to gain some low speed performance back, then add twin plugs with higher compression ratio to forestall detonation and so the simple engine rebuild snowballs (as does the cost).
Another option would be to have whoever overhauls the heads do a port clean-up, which will improve flow (although not like 36 mm ports will) and re-grind the cam to a 964 profile.

Paul, thanks for the reply.

I don't want to change the compression, but I'm willing to add an adjustable WUR. If a compression change is necessary then I'm not going to increase the port and duct size.

Steph,

Food for though from the Old One.

Opinions are like ass holes everyone has one, but from experience, I would tell you to first and foremost define what your end objective is. How are you going to use the car, how important is retaining originality, do you want street car, a track car, or both. Are you going to maintain it or do you have to pay to have everything done. How much are you willing to invest.

That old slippery slope we talk about here is like the snow ball rolling down hill, it starts out a snowball and ends up being an avalanche. For every modification you make there will be 3-4 more you need to optimize the first one and each of those will require more to maintain safety, longevity, and reliability. From one evening in 1976, when I saw my first 930 and I knew what car I wanted. The next 22 years I spent figuring out what I wanted it to do.

Sit down and make a list from the most important to the least important
traits you want, and then post it here. We can then give you options, estimated costs, pros, and cons.

My Grandchildren think I'm the smartest man alive and I always tell them it's just that I have made every mistake there is but I learned from each one of them.
This forum is a gold mine of experience and had learned lessons and we are all here to help.

Cole