The ideal rpm range for these is over 4000 or so but that doesn't mean the 'low end" is dead.
There are common misconceptions that an induction tract is suited for a given rpm range and to not be in that range is to barely run at all. It's not so extreme in reality. It's also better to err on being too short than long on a performance engine and the trumpet shape plays a role too.

 

Reduce, rather than kill...... but anyone wanting to go to ITBs for performance gain is almost certainly going to take advantage of more radical camshafts (not practical with a MAF system) . And they will certainly make up for the "losses".

As I said previously there is a 5 litre Alpha-N GT being prepared at present, I think the intention there is to extend the intake runners.

 

Quotes from above "I want to produce about 400+HP from the upcoming rebuild, a ground up re-build...."

"Engine work- Haven’t decided on that as of yet (who/what) so open to suggestions. I figure a budget of 8-10K should get it done…"

If you are not doing the labor you would be lucky to get a stock engine rebuild for that kind of money and as soon as it goes custom/ high performance it easily doubles for just the engine.
I think it is a great idea and could be lots of fun just add another $10,000 to the budget.

 

I have an ITB boner

 

20K? That is a lot of cheddar. What does that buy me?
Reminds me of Jay Leno, when he was house shopping in L.A. " For that kind of money, when I open the drapes in the morning, I want ***** pressed against the window".

 

Less, I think, for a stock 5.0L displacement. S4's are around 270 rwhp, and I've always considered them to be well-balanced: intake, cams, and exhaust are all moderately restrictive. You can only do so much by changing one thing.

GT cams are pretty good, and I think most stock GT's dyno in the 300-310 range. Add headers and a properly-sized exhaust, and careful tuning, and you are in the 340-350 range. You can get a bit more with different cams, but you are still looking at the same intake. Getting much further either requires more air pressure (i.e. boost), more displacement, or doing something different with the intake.

There is no reason that a new intake needs to be part of a rebuild. That's another whole layer of cost, but independent: If the engine needs rebuilding, then rebuild it. The intake bolts on top, I've heard that some folks take it off just to repaint it ...

 

Completely agree - once cams and improved exhaust are done, the intake may well be worthwhile. The OP has S4 cams - so he's a fair way off GT performance, and getting his 87 up to the level of your GT will take a fair wad of cash in itself.

It'll be interesting to see what your car does (I'm assuming the car John alluded to is yours ), especially as I have a set of S3 cams and also a custom set of cams to fit to my S4's, and I need to get my cars sorted for sharktuning now that I have a cable that fits my '87's too.

 

A $10,000 rebuild of a 5 liter...plus set of custom cams, decent headers, and individual throttle bodies, modified LH .....

Then you need hours and hours of reprograming of the computers to optimise the tuning @ $100+ per hour.

Just taking the engine out of your car and putting it back in is probably $2,000 at most shops if they did NOTHING at all to the engine.

When Porsche still had new GTS engines they cost over $30,000 each !