XO vs. XXO clarification

[Gary Perkinson]

Hey, folks...so the engine in question is a stock 1951 218 cid Mopar/Chrysler/Plymouth inline-6 flathead. Looking to run in /PRO. In the Vintage Engines section of the rulebook (2.A.1), the first sentence of the XO description reads, "XO class consists of in-line overhead valve (OHV), in-line flathead and flathead V-8 (except Ford & Mercury) and V-12 passenger car and pickup truck (or the same engine design family, Section 4.N) engines, 1959 model year design or earlier, up to 325 cid."

So that would seem to put my engine in XO. However, the second sentence of that section reads, "In the spirit of the class, XO engines shall typically be those run at the Southern California dry lakes in the 1940s and early 1950s. Examples include Chevrolet, GMC, Hudson, Packard, Buick, Lincoln and Cadillac." Okay, so does "typically" apply to Mopar flatheads, too? I'm guessing it does, since otherwise, why would "typically" be in there?

So then we move on to XXO, which is described as "an XO engine, as described above, with a specialty cylinder head as described below." The description of a specialty cylinder head (which my engine does not have) reads, "A specialty cylinder head is fabricated from billet stock, cast or a modified OEM head that has added ports. At least one valve per cylinder shall be in the head. All X class engines, as described above, which are over 325 cid, but less than 375 cid (a range that does not apply to my engine), shall be classified as either XXF or XXO."

Okay, so that too seems to point to XO for my engine. However, the XXO section then goes on to read, "In addition, all 1959 model year design or earlier non-Ford & Mercury flathead V-8s, flathead and OHV in-line engines, up to 375 original cid, shall be classified as XXO. Specialty cylinder heads are not allowed in this instance."

So that part of the section would seem to contradict the first part of the section, or at least be redundant. Is the rule thus saying that ALL non-Ford/Mercury flatheads under 375 cid (without a specialty head below 325, and with a specialty head between 325 to 375) are in XXO? I guess it is, and if that's the case, that's the rule, but just for the sake of transparency, I'm curious why sub-325 cid non-Ford/Mercury flathead engines without a specialty head were kicked out of XO and into XXO, when XXO is specifically defined as an XO engine with a specialty head...

Anyway, thanks in advance for any help on this. And no, I haven't contacted anyone on the rules committee, but I'll be happy to if it helps clarify the rule...thanks!

[71GSSDemon]

I will start with ask the committee.  Then I would say you will likely be in XO, unless you increase CID or modify the head.

[desotoman]

"so the engine in question is a stock 1951 218 cid Mopar/Chrysler/Plymouth inline-6 flathead."

You would run in XO/PRO with motor under 325 cubic inches.  Since you are a flathead forced into the overhead class, would you be allowed to run an aftermarket flathead head without jumping you into XXO/PRO? I would think so since no ports are in the head, but better to ask than be sorry. 

These rules have changed so much over they years it just amazes me. They used to let you run in the XXO class if your engine was just 326-375 with a stock head, but apparently that is no more unless you have added an extra port to a stock head.  JMO.

Tom G.

[Gary Perkinson]

Hey, guys, thanks for the replies! Yeah, you would think that I'd be in XO, since I have a non-Ford/Merc flathead under 325 CID, but that last paragraph (which I believe was added in the last couple of years, although I could be mistaken), seems to shut the door on the matter. To me, it's pretty clear: All non-Ford/Merc flatheads up to 375 CID, regardless of head configuration, are in XXO. Now, the only reason I really care is that the XO/PRO record is 10 mph slower than the XXO/PRO record (not that I would have much chance for either with this engine), but it just seems weird to me that a little 218 CID non-Ford/Merc flathead with a stock cylinder head suddenly has to compete head to head (excuse the pun) with a 375 CID non-Ford/Merc flathead with a specialty head. And hey, I would say that, okay, I guess I have to get myself a specialty head, but they're not allowed under 325!

Anyway, I'm not looking for any kind of change in the rules or the class breaks--I just want to confirm which class I would be in, but I'm also genuinely curious about the rationale behind the change. And I will double-check with the committee...thanks!

[71GSSDemon]

So, they mention what was Typically run, give examples, etc.  But they specifically say flathead V-8 and don't specifically exclude a mopar flathead.  To me, still XO

[desotoman]

So, they mention what was Typically run, give examples, etc.  But they specifically say flathead V-8 and don't specifically exclude a mopar flathead.  To me, still XO

Hey, folks...so the engine in question is a stock 1951 218 cid Mopar/Chrysler/Plymouth inline-6 flathead. Looking to run in /PRO. In the Vintage Engines section of the rulebook (2.A.1), the first sentence of the XO description reads, "XO class consists of in-line overhead valve (OHV), in-line flathead and flathead V-8 (except Ford & Mercury) and V-12 passenger car and pickup truck (or the same engine design family, Section 4.N) engines, 1959 model year design or earlier, up to 325 cid.

The XF class is only for Ford and Mercury passenger car motors. No other flathead motor is allowed to run in that class. I caused the wording of " Passenger car motors " to be added to the rule book.(long boring story) Since those are the only motors which can run in the XF class, all other flathead engines produced 1959 or earlier must run in the XO or XXO class. That designation comes down to the cubic inch of the motor. Example:
337" Lincoln Flathead V8 would run in the XXO class, but sleeve the block to 324" and it would run XO class because of its displacement. 308 Hudson 6 cyl. motor would be an XO with stock bore and stroke. Hope this clarifies your question.

Tom G.

[panic]

Way, way roo small.

[jimmy six]

You didn't state what your vehicle would be but if it were me I would find out what the biggest stock MOPAR engine flat head six came in. In the late 40's and early 50's they had a 265" Chrysler 6 for sure. (Chrysler Crown in Boats) Usually an XO is a Chevrolet 235/261 over head that can reach over 300". In the mid 70's when I started Hagertty and Dozier competed with a 30's Chrysler Air Flow and were very successful.  To reach the 325 -375" XXO class they went to the Chrysler straight 8 flathead even using a supercharger at the end. Don't try to over read the rulebook. You may want to start with the USFRA 130 class where there are not as many safety rules for street driven cars. Good luck and have fun with what ever you choose.

[Gary Perkinson]

You didn't state what your vehicle would be but if it were me I would find out what the biggest stock MOPAR engine flat head six came in. In the late 40's and early 50's they had a 265" Chrysler 6 for sure. (Chrysler Crown in Boats) Usually an XO is a Chevrolet 235/261 over head that can reach over 300". In the mid 70's when I started Hagertty and Dozier competed with a 30's Chrysler Air Flow and were very successful.  To reach the 325 -375" XXO class they went to the Chrysler straight 8 flathead even using a supercharger at the end. Don't try to over read the rulebook. You may want to start with the USFRA 130 class where there are not as many safety rules for street driven cars. Good luck and have fun with what ever you choose.

Hey, Jimmy...thanks! Yeah, that would definitely be the way to go...I happen to have a '51 Plymouth Concord with the 218 on hand, and I thought it might be fun to run it in XO/PRO, even if the 139 mph record was going to be a little ambitious...  8-)  The real limiter (among a lot of other things) would be the transmission, but I have a T5 that will probably go in this coming spring and will hopefully solve a lot of problems...