Legality of crank swaps

[Z3_GoCar]

So, here's a question that's not been covered. Rods and Crankshafts are legal for exchange. So, what if I were to swap my 84mm crank for a 75mm crank. Could I then claim it's a 2.5 and use the weight of a 2.5 liter motor?

[Greg Amy]

If all the rest of the parts of the engine are the same as the 2.5 (e.g., head, intake manifold, throttle body) then yes. But keep in mind IIDSYCTYC applies in ST too... -GA

[Andy Bettencourt]

This is a great question...

[Greg Amy]

It is a good question.

I guess the easiest way to explain my opinion would be that if you swapped in the shorter crank, and what you ended up with was exactly the same thing as what you'd have in the car if you swapped in a 2.5L engine complete, and all you're doing is looking for a cheaper way to do it, then I'd say it's compliant. But if doing so would give you a performance advantage over a similarly-prepped 2.5L engine (e.g., the 2.8L engine has a better head/manifold/t-body), then you're likely contrary to the regs.

Just to be clear, above you said "rods and crankshaft" are free. Did you mean "rods and pistons"? Crank can be an alternate part but must be identical dimensions. Honestly, I'd like to clarify the intent of that rule, as I think it's a World Challenge holdover...but it's safest to keep IIDSYCTYC in mind.

GA

[Chip42]

This is a great question...

I agree, and not because I don't think it's clear form the standpoint of the intent of the class (and tGA sumarized that well).

it's a great question because, going back to the first principle of the displacement to weight theory, why does it matter? if the ruesultant motor has 2500cc, then according to the concept, it should be wighted and run accordingly.

I support the "cost caontainment" or enforcement ability of running stock parts from the USDM etc... but the ruleset is, effectively, an excersize in identifying the best stock bottom end and head/intake/TB combination as offered in a car for sale in the US. displacement/weight only works if the other variables are removed. it can look close to right, but we all know better.

under the general pretenses of the class, I think that parts bin motors should be ok. I think that non USDM motors should be OK. and am apparently at odds with the CRB and many others.

I agree that it's easier and cheaper to have stock long blocks with the allowed modifications, as well as other implications of marketing and rule making import, but it simply doesn't add up from the engineering view.

[Z3_GoCar]

So, this is the applicable section:


2. The crankshaft shall be a stock OEM part or

an aftermarket part

as long as it is of identical dimensions and material as the OEM
part for the specific engine. The crankshaft may be

balanced.

The

maximum weight reduction allowance for balancing of the

crankshaft is 0.5 lbs. The

maximum weight reduction allowance

for the balancing of the reciprocating assembly is 15 grams.
Alternate connecting rods are permitted

, but must be ferrous
unless OEM supplied.

Part of the exercise is to get a better oe intake manifold, part is to keep the weight down. Dimensionally, my 84mm crank is the same as the 75mm crank except for the stoke. My rods are shorter than the 75mm crank, so I could also swap those out for a matching set. Also, head's the same casting.

Oh, one more twist. Does the intake manifold match the head or the crank? See I could use a newer generation 75mm crankshaft, but my head matches the older generation intake manifold.

[Greg Amy]

Does the intake manifold match the head or the crank? See I could use a newer generation 75mm crankshaft, but my head matches the older generation intake manifold.

James, you can't build a Frankenstein engine; you can only mimic the specs of an existing engine. If everything matches the specs of the BMW 2.5L engine (with STU allowances) then you're good. Otherwise, you're not. - GA

[Matt93SE]

Here's my take on it...

2. The crankshaft shall be a stock OEM part or
an aftermarket part
as long as it is of identical dimensions and material as the OEM
part for the specific engine.

In other words, you can not take a 2.5L crank and put it in a 3L block & head UNLESS BMW made a 2.5L engine using that specific combination of parts.


I'm glad this thread came up, since the non-USDM request has been denied, I'm trying to find another decent engine to put in the car. The only other "small" RWD engine Nissan has is the VQ25 from the Infiniti G25. 2.5L V6.
It's a baby-brother of the VQ35 in the G35 and 350Z.
The problem is Nissan only sells it as a long block (minus manifolds and lots of sensors), and it's around $8k. I haven't seen any on the used market yet for a junkyard donor.

Some of the Maxima guys have been Frankensteining these VQ engines for years using a 3.5L block and a 3.0L crank to come up with a short-stroke 2.3L engine that revs to 8,000,000,000rpm before throwing rods.

IF we can parts-bin an engine together using OEM parts, I could conceivably build a 'decent' engine doing that...

But the way I read the rules, that's not allowed.