IMSA thread has me wondering. Does anyone want to take a stab at the list and a brief explanation on what's legal (stock, mods, engines, tires, etc) for each class.
Whenever I've built a race engine I just asked what pertained to the engine I was building and honestly I've never looked any further. For anyone thinking of going down this rabbit hole it might be nice to know the differences.

I know some of them.. but depending on the level of detail. That’s a long document

I'll take a stab. No guarantees on accuracy though

SCCA classes:
Production classes - no full tube frames, more limitations in general on chassis mods:
C production for 914/6 in the 70s and until mid 80s or so. I think tire width and displacement varied over time, but got up to 2.5L by the early 80s. This class was ended in the mid 80's or so when the GT classes started.
E production for the 1.8L 4's from the 70s until 10-15 years ago when it got moved down to F prod. At some point they started allowing 914/6's in E prod with limited engine prep, not sure if they got moved to F prod.
I think the 914/6 GT was classified as B prod back when it first came out, which was a disadvantage. B prod also ended when GT classes started.

GT classes - full tube frames
GT3 for small bore 6's and 4's. Not sure of the rules differences, but I'd guess the 6's get more weight and/or more limitations on engine prep.
GT2 - I believe when the class started in the 1980s, the 914/6 was allowed to run a 2.8 or 2.9L engine at a little over 2000 lbs. 10 inch wide wheels I believe, first 15's, then 16's. Maybe 18's now? But that would be difficult for most 914's without a lot of work. I think displacement has probably grown over time too. Cup cars are allowed in GT2 these days!

In IMSA, the 914 ran mostly in GTU from the early 70s and did well. In the early/mid 70s it was basically the 914/6 GT. At some point 2.5L engines were allowed, then 2.8 by the 1980s. Around 1987, IMSA stopped allowing 914's to race because they were too old, even though there were a bunch of them still competing. There was one guy who built a 914 with turbos around 1980 and ran in GTX with the 935s. I don't think it raced much or did well.

PCA Club Racing classes:
Not sure which stock classes, and really no one races them anyway.
In the GT classes, which allow many mods but must keep the stock floorpan, I've seen 914s in GT3, GT4, and GT5. It's a sliding displacement to weight scale, but usually small bore cars run GT5, 2.7-3.0 in GT4, and big bore in GT3.
Now there are vintage classes in PCA CR, and I think most 914's will migrate to those. Rules are evolving, but for now I understand there are Vintage 4 cyl under 2.4L, Vintage 4 cyl over 2.4L, Vintage 6 cyl under/over 2.4L, and separate classes for cars with wings and aero mods under/over 3.0L.

That's what I know!

Scott

SCCA Improved Touring B (formerly A).

- Bolt on suspension mods (springs, shocks, swaybars, bushings), blueprinted engine built to service specs and you can 40 over and 1/2 pt compression, stock cams and intake but open ECU.

- Stock transaxle but you can change final drive and run an LSD

- Interior stripped but must retain stock dash, 6-pt cage, racing seat, 5/6-pt harnesses, window net. Fuel cell allowed but not required if stock tank is within wheelbase.

- Stock body panels all around, allowed to roll inner fenders.

The car is not competitive in ITB; wasn't in ITA either. Blake Meredith tried to build one a few years ago but it didn't work out. Mine is an ex-ITA car that sat after it popped the engine at Watkins Glen in 2003, I resurrected it a couple years ago for historics/vintage and have kept the general baseline of ITB but with reasonable mods (dual Dells, mild cam and compression, stuff like that). Chassis is still all IT-spec. No one looks very hard at it during historics events. Northeast Division also has a class called "ITEZ" that I run it in, it's a catch-all class for any cars that meet IT safety specs.

Duplicate

The problem with answering this question is that there were small changes over the years in the rules

Then in about the 70’s New sanctioning bodies (with new rules) started to form

By the time PCA became a racing sanctioning body in the late 80’s there were over 50 vintage clubs that were also sanctioning bodies

So a current explanation of where a 914 can run involves 50 different (although with lots of overlap) sets of rules

This is the reason vintage racing is frozen to a point in time
And the major philosophical difference between the FIA and say a vintage club

The fia changed rules every year and marketed towards manufactures. After a new car debuted it was uncompetitive after 3-4 years because the rules had progressed and the car hadn’t The FIA facilitated this and would drop cars from eligibility... effectively

Your question should be about an era or specific sanctioning body to get a specific answer.

For example I could say that a fast 914 race car ran with a large v8 in lemons or nasa
But not as any manufacturer ever intended. More as an individual dream or vision was developed by one guy. Nothing wrong with that but I think your asking about air cooled motors.

This the reason most vintage cars define rules in periods such as 1967, 1972, 1981. Or periods from different sanctioning bodies such as the scca. The FIA or IMSA

Cars (from a collectible appeal perspective) were made the most famous or gained notariety when they were winning a class. Often as built by a manufacturer or as modified by a particularly successfull race team

Think Joest Porsche’s (not 914)

So I would say the big easy classes and clubs would be:

-the scca with 2.0/6. And 4/cylinder’s
-the fia with 2.0/6
- IMSA with a 2.5, and later 2.8, or 3.0

After those periods then it became 50 different answers with 50 different clubs

Too many series with differing rule sets; most of them changed over time. Not gonna fit in one spot, unfortunately.

--DD

Thanks, I knew it was a quagmire, but didn't realize how all over the place it was.