As an exercise in aging-movie-geek nostalgia, “Grindhouse” is a bloody good time. It’s an homage to the theaters of the 1960s and ’70s that would show double features or marathons of schlocky, exploitative films — kung fu flicks, blaxploitation, zombie movies, spaghetti Westerns — the kinds of films whose titles and posters were always more creative than the movies themselves. Gratuitous violence and nudity were the rule. The prints were usually scratchy and damaged, having been run through a thousand projectors a thousand times already.

My description comes from a purely historical, not personal, standpoint. For the most part, the grindhouse theaters died with the ’70s, and it’s a good bet that the vast majority of “Grindhouse’s” target audience has never actually been to one. We’ve seen some of the films, sure, but mostly on video or at midnight screenings at the local college theater. The whole grindhouse experience — the battered prints, the noisy projector, the salacious coming attractions — that’s all academic. We’re taking Robert Rodriguez and Quentin Tarantino’s word for it that this is how it was.

“Grindhouse,” with its faux trailers during the intermission and its intentionally rough edits, comes about as close to approximating the grindhouse double-feature experience as it can without actually handing out popcorn and marijuana to the audience. Rodriguez and Tarantino each wrote and directed a full-length film, got some horror buddies to make phony previews for others, scratched everything up, and packaged it into one three-hour hootenanny of blood ‘n’ mayhem.

Though the two directors — old friends and kindred spirits in the realm of raucous underground cinema — intended to complement one another, the finished product actually serves to highlight their differences: Robert Rodriguez can immerse himself in a concept and have fun, while Quentin Tarantino can’t lose his ego long enough to let go.

“Planet Terror” comes first, with direction (and screenplay and cinematography and editing and musical score!) by Rodriguez, he of the “Spy Kids” movies and (more germane to the topic at hand) “From Dusk Till Dawn” and “Sin City.” It belongs to the genre of Zombie B-Movies, being the story of a Texas town that is infected with a flesh-eating virus that turns its victims into flesh-eaters themselves. A rowdy band of locals — a mysterious gunman (Freddy Rodriguez), his stripper ex-girlfriend (Rose McGowan), a sadistic anesthesiologist (Marley Shelton), and so forth — fight off the ghouls while looking for an escape.

Rodriguez enthusiastically and skillfully re-creates a zombie flick circa 1970. It’s set in the present, but the muddled plot, the often incoherent editing, and the outrageous gore make it look authentically like something a no-name director would have churned out 35 years ago. The colors are washed out, the print has been made to look scratched — there’s even a reel missing, causing the story to suddenly lurch forward 15 minutes.

By making a movie that’s intentionally bad, Rodriguez has made a movie that’s very good. It’s genuinely entertaining and audacious even without the grindhouse gimmicks, with memorable characters and eye-popping morsels of action-hero coolness. (Who knew the junior mortician from “Six Feet Under” could kick so much butt?) The elements that are fun but that you’d be forced to consider “flaws,” technically, are there on purpose — which means you can enjoy, say, the ridiculous dialogue and the over-the-top violence without feeling guilty. Part of the pleasure in watching grindhouse-style films has always been laughing at the filmmakers’ ineptitude. This time, we can laugh WITH the movie, not AT it.

Tarantino’s contribution is significantly less enjoyable, though still proficient. It’s called “Death Proof,” and it’s a sort of slasher film in which the killer uses not a knife or a chainsaw to dispatch pretty young victims, but his car. His name is Stuntman Mike (Kurt Russell), and he’s a grizzled old veteran of the TV and movie stunt industry. Now retired, for fun uses his car as a weapon, killing his passengers or the people he runs into but leaving himself more or less unharmed, thanks to his expertise and to the modifications he’s made on the driver’s side of the vehicle.

Stuntman Mike meets his match, however, when he picks a fight with a car being driven by two women who are also stunt professionals. They’re Zoe (Zoe Bell, a real-life stuntwoman) and Kim (Tracie Thoms), out in the sticks for a film shoot and taking a super-powered old Dodge Charger out for a test drive on their day off. Their actress friend Abernathy (Rosario Dawson) is in the backseat, providing a non-stuntperson’s perspective on the day’s proceedings.

Because it’s a Tarantino film, there must be a lot of sitting around talking, and the dialogue must crackle with pop-cultural references and carefully worded snark. That’s all well and good (though it’s not exactly in keeping with the grindhouse formula), but there’s a snag: These characters aren’t nearly as engaging as the ones in Tarantino’s previous works, and the things they say aren’t nearly as clever as QT wants them to be. A lot of it is boring, frankly.

The film’s justification for its own existence is a marvelously terrifying car chase and a few other choice moments of action. Kurt Russell is hilarious and spooky at Stuntman Mike, too, particularly in the film’s final moments. As disappointed as I am in the overall product, I don’t want to tell you skip it, because then you’d be missing some sections that are pretty fantastic.

The old grindhouse films often had one or two very cool scenes that were obviously the whole reason the film got made at all, with everything else necessary just to pad it out to feature length. “Death Proof” follows that archetype to a T; the problem is, I’m not sure Tarantino was doing it on purpose. I get the feeling he thought everything in the film would be interesting and fun, when in fact much of it feels indeed like padding. What’s more, the old B movies may have been pretty thin, plot-wise, but at least they were short. Seventy or 80 minutes was a typical running time. “Death Proof”? Something like 95 minutes — and that’s with a jump forward accomplished through a “missing reel.”

Something else disappointing: Halfway through the film, Tarantino seems to forget about the grindhouse gimmicks. The picture stops being scratched up, the editing is no longer choppy, the colors look natural again. It becomes a fairly straightforward not-very-good movie, no longer associated with “Grindhouse” in any noticeable way.

If Tarantino wanted his film to evolve in such a way as to somehow comment on the grindhouse experience, or to subvert the slasher genre, that’s fine. But why do it in what was supposed to be a double-feature of straight-ahead homage? He comes off looking like a spoilsport, like he couldn’t forget his reputation long enough to go through with the stunt of making his product look delightfully bad. Rodriguez embraces the concept; Tarantino starts to and then chickens out.

As separate features, I’d give “Planet Terror” a B+ and “Death Proof” a B-. Together, accompanied by the fake trailers (directed by Rob Zombie, Eli Roth, and Edgar Wright, and each one astonishingly accurate and breathlessly funny in its own right), I give the whole experience a grade worthy of two B-movies.

B (3 hrs., 11 min.; R, abundant harsh profanity, abundant graphic violence and gore, brief nudity, some sexuality.)