Don Terrill’s Speed-Talk

Someone brought this up over at the corvette place. What are you opinions on the longevity of a flat plane crank engine as a daily driver and if tracked?

Thanks -Jim

Assuming you’re talking about a ‘flat plane’ V8 crank …...

If the harmonics are kept in check (dampening) and the fundamental design is good I don’t see any issues over the average expected lifespan of a particular application

Why should there be ? ...... there’s more to it, but a FP crank is essentially an I4 crank and they’re hardly snapping all over the place

Ok, there’s the out of imbalance secondary, twice crank speed rolling couple that needs to be accounted for, but that’s mostly been known since the days of Lanchester, probably beyond

There’s maybe an lesser argument to be made that in light of this, in a performance application relative to a correctly engineered ‘cross plane’ crank, it could be potentially harder on bearings, and will certainly vibrate more without balancing shafts …… going up in a non-linear way relative to reciprocating weight which is why most FP engines have a modest displacement ...... but I doubt it's a deal breaker

What was/is the reasoning behind the C8 Vettes going with a flat plane crank?

fabr wrote: ↑Wed Aug 03, 2022 6:57 pm What was/is the reasoning behind the C8 Vettes going with a flat plane crank?

Knowing car guys....because they sound neat?

In the seventies flat plane cranks were starting to be run in F5000 here, the most common issue was the integrity of the tubs.
I don't recall what rectification was employed but as BCjohnny pointed out, if harmonics were addressed issues could be controlled.
Exhaust tuning was the aim then.

Cheers.

fabr wrote: ↑Wed Aug 03, 2022 6:57 pm What was/is the reasoning behind the C8 Vettes going with a flat plane crank?

Better intake manifold tuning, better exhaust tuning, lighter crank.
Lot's of negatives though.

And to better mimic a ferrari like they were going for

fabr wrote:What was/is the reasoning behind the C8 Vettes going with a flat plane crank?

I'm with the other posters ........ I'd say it likely has more to do with marketing than engineering

The trade offs for a 'street' engine are hardly insignificant ....... JMO

A successful 180°, V8 engine, was designed over 80 years ago, the Novi. It ran at Indy, for 23 years, with only three engine related failures. Magneto, one broken crank and one piston. Excellent record, I would say.

It was a 181 CID engine, three main bearings, with a large centrifugal, crank driven supercharger. It started out at 450 HP and ended, with 742 HP.

Maybe 3 mains are a advantage for 180° V8 cranks?

frnkeore wrote: ↑Thu Aug 04, 2022 2:31 pm A successful 180°, V8 engine, was designed over 80 years ago, the Novi. It ran at Indy, for 23 years, with only three engine related failures. Magneto, one broken crank and one piston. Excellent record, I would say.

It was a 181 CID engine, three main bearings, with a large centrifugal, crank driven supercharger. It started out at 450 HP and ended, with 742 HP.

Maybe 3 mains are a advantage for 180° V8 cranks?

The Cosworth, and Ilmor engines were also 180 cranks.
The Turbo AMC V8, and Turbo SBC my dad developed for Indy were also 180 cranks.
A certain IRL team made a 180 crank(and cams) for one of their N.A. GM engines. They ran it during practice, and the sound was so different, everybody at the track knew something wasn't right, so they didn't try to race it.

fabr wrote: ↑Wed Aug 03, 2022 6:57 pm What was/is the reasoning behind the C8 Vettes going with a flat plane crank?

Because Ford did.

There was a time when GM Engineering was in a leadership position.

I agree with most that it is basically marketing. No real/substantial performance benefit.

The flat plane crank is an advantage on the IMSA engines and how it draws on restrictors.

There are performance benefits for sure, especially when you’re chasing those last few percentage points in gains, and not just peak.

If the engineering controls are maintained and used within their design criteria there shouldn’t be an issue, just like anything else.

hoffman900 wrote: ↑Thu Aug 04, 2022 11:22 pm The flat plane crank is an advantage on the IMSA engines and how it draws on restrictors.

There are performance benefits for sure, especially when you’re chasing those last few percentage points in gains, and not just peak.

If the engineering controls are maintained and used within their design criteria there shouldn’t be an issue, just like anything else.

If the engineering limitations are maintained, i.e stroke length.

Tom68 wrote: ↑Fri Aug 05, 2022 4:22 am

hoffman900 wrote: ↑Thu Aug 04, 2022 11:22 pm The flat plane crank is an advantage on the IMSA engines and how it draws on restrictors.

There are performance benefits for sure, especially when you’re chasing those last few percentage points in gains, and not just peak.

If the engineering controls are maintained and used within their design criteria there shouldn’t be an issue, just like anything else.

If the engineering limitations are maintained, i.e stroke length.

Right, but the aftermarket is notorious for this stuff. The OEMs spend millions of dollars to make something work and some JoeBlowHotRodder modifies it in a way that takes it outside what it was designed to do with zero consideration for the entire package and wonders why it blows up.