This was posted over on the C1 & C2 forum. I'm sure it would have interest here too!
https://www.corvetteforum.com/forums...on-1968-a.html

 

What did Motion do to the engine on the Phase 3 427 C3's? Being able to "guarantee" an 11.5 quarter at 120 mph is pretty brave for a street vehicle of that era. What engine mods were done to be able to guarantee that hind of performance?

I have a 1968 C3 that is currently a L88 clone and it is fun to enjoy that power the 427 can make when it is running the high compression. It is a shame that good high octane fuel isn't readily available on the East Coast. In the mid west I saw 100 and 105 Octane fuel available at the pump a couple years back and my engine would be in heaven.

That is a beautiful example of an Motion Corvette!

 

joel would usually swap in an L-88 crate engine for starters but this varied. they added headers, a c427x intake, a 3916 850cfm 3 barrel carb, a mallory double life distributor with a phase III ignition box (jacobs box) , rear gear change if needed, and a complete chassis dyno tune. sometimes a hone-overdrive was added.
again..this can vary upon customer requests and joels reccomendations..

 

Rosen did NOT put the L88 in for a standard Phase III Corvette conversion. Camaro and Chevelle is another story, they could get a engine swap. Why do that for the Corvette when GM would put in the L88 right out of the factory? He would have just ordered a factory L88 if you wanted one. He even advertised the ZL1. There are many articles over the years describing exactly the upgrades but in short he started with the L71 or L89 and switched out the manifold and put on a Holley 3bbl carb as mentioned. He then messed with the ignition and did the usual hot rod tricks to get more power out of the L71. And he would set up suspension mods, and body mods if wanted. The mods were really nothing extraordinary that anyone could have done themselves. What made it extraordinary was that the cars were invoiced through Baldwin Chevrolet and came with the factory warranty. If you did these mods in your garage on your own you would void the GM warranty. That was the selling point.

A good example of this is, in fact, that yellow 68 in the original post. It has a L89 motor in it. It’s not the original motor, but it is a GM factory L89 block installed early on. As far as I know, nobody knows what happened to the original L89 (this is a factory ordered L89 corvette) but he did start with that engine option and that yellow Corvette is the first Phase III built. Maybe they blew it up testing, who knows.

Interesting cars for sure.