Intake/Head Port question (930 engine)

[cowtown]

Hi all,
I'm planning a conversion to EFI and distributorless ignition for my '77 930 engine (3.0 liter, 6.5:1 compression). Car is 95% street driven, with a few DE events. Nothing competitive. I'm planning to use (and have bought) a 3.2 Carrera intake manifold and throttle body, with full-bay intercooler.

For now, I don't want to pull the heads off to port them, since I rebuilt about 2 years ago. After I get the retrofit done and running for a while, I will probably pull the engine apart to port, re-cam, and possibly twin plug. But for now, I plan on using my shop mill to fab up six adapters that will mount between the 3.2 intake and 930 heads, taking the ports from 41mm measured on the intake side to 32mm on the heads. I believe this to be the right head port size - I haven't confirmed with measurements yet, but I will over the next few days.

Here's my question - with these adapters, is there a rule of thumb for what angle I should use for the hole that tapers down from 41mm to 32mm? For example, I can do a hole with walls that slope inward gradually at 10 degrees, resulting in a 25.4mm high adapter. Or, I could mill the hole at a 24.3 degree angle, giving a 10mm high adapter.

I realize this is not a great solution for horsepower, but I need to break this project into do-able parts or I will get overwhelmed and the car will be off the road for months....

Any advice?

Thanks!
Colin

[dean]

Colin,

I had the same problem. The reason I wouldn't go with the adaptors is that there is little clearance between the TB elbow and the firewall.

I seem to remember that the proper angle for the taper is 7degs.

What I ended up doing was running without porting of any kind (large step in intake) for a year. I didn't notice anything bad except a hesitation. But I think that was a tuning problem cause I port matched my heads and I still have it.

[WinRice]

Colin,

I'm working on the same thing. I wanted to get the EFI and new intercooler in place first to work out the major kinks, before I go into the engine. Just in case I really screw up somewhere and hurt the engine.

The factory Carrera phenolic spacers are 8mm thick, which is roughly 5/16". I planned to get a sheet of 3/8" phenolic and machine new spacers with a 41mm to 32 mm taper.

Shouldn't make a big difference with a 1/16" difference in thickness, once I go back to stock spacers with ported heads.

Win

[David]

Rather than a taper, you could use a radius in the spacer to change diameters.

Like this:

[Rob 930]

I considered mounting a Carrera intake (40.5 mm bore) to mildly ported 930 heads (36 mm bore). I, too, considered tapering the 5/16 spacer blocks. I eventually pulled the heads and had them match ported (and then some). Before doing that, I discussed the effect of leaving a discontinuity in the intake stream with a number of experts. The general consensus was that it would probably work okay, but with some flow penalty, which couldn't be quantified. The optimum, as Dean said, is to have a maximum angle of deviation of 7 degrees on each side. Without match porting, your only reasonable option is to make a taller spacer with a taper to bring the angle to something more acceptable. I found that such spacers would force me to elevate the manifold, which wasn't going to work -- the intercooler would no longer have fit under the stock wing in the engine compartment.

Here's another thing to consider. A 930 with EFI will not run properly (read: safely under high load) without significant dyno tuning. There's no out-of-the-box map that will work for any ECU or engine. I decided that if I was going to invest in proper tuning, I wanted the engine to be very close to "final form" so that I didn't have nagging issues that I wanted to improve upon later that may require re-tuning. There's so much time and cost in the tuning process (and so much potential cost in *not* tuning it properly), that you may want to consider building it once, exactly the way you want, and then having it tuned by a pro.