How much horsepower did a 1970 PLymouth Duster have with a 198 Slant Six engine
Executive summary
The 1970 Plymouth Duster fitted with the 198 cubic‑inch (3.2 L) Slant Six is most commonly documented as producing about 125 horsepower in period and retrospective sources, a figure cited repeatedly in enthusiast and reference writeups [1] [2] [3]. Reporting is not unanimous, however: some community threads and later compilations give lower ratings (as low as 95 hp), so the 125‑hp figure should be read as the mainstream, broadly accepted rating rather than an uncontested absolute [4].
1. What the 198 Slant‑Six was and how sources report its power
The 198‑cid Slant Six was introduced as a stroked, more powerful base engine for the Valiant/Duster line in 1970, and multiple published histories and fact sheets list the 198 as the entry six for the 1970 Duster and attribute about 125 horsepower to that 198 displacement in 1970 model specifications [3] [1] [2]. Auto outlets that synthesize factory specs and period brochures commonly repeat the 125‑hp number as the 1970 rating for the 198 Slant‑Six, reflecting the engine’s role as a modestly peppy base six in a compact car [1] [2].
2. Why some reports show different numbers
Not all references agree: at least one enthusiast forum post and a community writeup list the 198 Slant’s early‑year rating as lower — for example 95 hp in an Allpar forum summary — and such variance can come from transcription errors, later changes to emissions and compression, or confusion between SAE gross and SAE net measurement standards that shifted ratings in the early 1970s [4] [5]. Sources explicitly note that from 1972 onward manufacturers adopted SAE net standards and also reduced compression to meet emissions rules, producing lower numerical horsepower ratings compared with earlier gross figures, which complicates direct comparisons unless the source specifies the measurement standard [5].
3. How to reconcile the contradiction and state the practical answer
Given contemporary fact sheets and multiple independent retrospectives, the best-supported figure for a factory 1970 Plymouth Duster with the 198 Slant Six is 125 horsepower — that is the value most automotive historians and reference articles report for the 198 in that model year [1] [2] [3]. The lower numbers that surface in forums and some listings appear to reflect either later‑model detuned variants, misremembered data, or ratings under different measurement standards rather than the factory 1970 specification [4] [5].
4. What this meant for the car’s performance and buyer choice
The 125‑hp 198 made the Duster an economical, reasonably brisk compact in 1970 — the model lineup also included the larger 225 Slant Six and V‑8 options for buyers who wanted more torque or outright power, and the 198’s presence as a base engine helped keep price and economy attractive for many customers [3] [6]. Period coverage and later retrospectives emphasize that the Duster’s appeal was its combination of light weight, compact dimensions and a choice of engines from a thrifty six to more muscular V‑8s, so the 198’s 125 hp should be seen in that model‑mix context rather than as an isolated performance benchmark [7] [8].
5. Limitations in the available reporting
The assembled sources corroborate the 125‑hp claim for the 1970 198 Slant Six but do not produce a single factory brochure image or NRA‑style spec sheet in these excerpts that would be the definitive primary document; likewise, discrepancies in enthusiast databases and forums demonstrate how later reporting can diverge [1] [4] [5]. If absolute confirmation from a scanned 1970 Plymouth factory brochure or original dealer literature is required, that primary documentation is not present in the provided source set and would need to be checked separately.