421 TriPower on run stand! - Pontiac GTO Forum
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post #1 of 3 (permalink) Old 10-01-2016, 07:45 AM Thread Starter
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421 TriPower on run stand!

Today is the first day of the week-long Crusin the Coast, the ONE vacation my wife and I take in the year. The 421 TriPower in her 65 Royal Bobcat GTO had to be rebuilt due to a failed big end bearing in the #3 connecting rod. Fortunately I caught it before it spun or caused other damage. This motor had been built out by a previous owner years ago. It is now .035 over, has RaceTech dished forged aluminum pistons (10:1 CR...from a previous 10.9:1), replacement ArmaSteel cast crank, 65 8777 ported iron heads, RollMaster billet steel double roller timing set, Comp Cams Xtreme Energy hydraulic roller cam and lifters (278/284 .505 lift), double springs w/damper spring, 10 locks, retainers, steel roller tip rocker arms, valve lash caps, Eagle H-beam rods, and ARP everything everywhere including ARP 4 bolt main cap studs with a Milodon oil pump and Pertronix Ignitor III billet distributor. I am going to set the rev limiter at 5,900 RPM based on cam profile (5,600 top range) and the formula for Feet Per Minute based on stroke x rpm 6. For a "street" motor it is suggested that 4,000 FPM be the limit, and given the quality of parts in her 421 I am confident that I am on the conservative side. 3,933 FPM is the number at 5,900 for the stock 4" stroke that it has. Opinions on the RPM limit I am suggesting?

Last edited by avman; 10-01-2016 at 08:00 AM.
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post #2 of 3 (permalink) Old 10-01-2016, 08:34 AM
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I think it would be hard for any of us to recommend what RPM level your engine will run at. That is something your builder or someone who runs a 421 can tell you. You have all the good parts, so you may be on the conservative/safe side. I am building my 455CI looking to spin 5,800 RPM, but it will be the cam/heads that dictate where the power drops off because the shortblock will certainly spin more.

I think your limits look OK to me as long as all clearances are right and your oil pump is up to the job. I would not be pushing the limits quite yet until you have gotten some miles on the engine and dialed in all your components, especially your distributor. If not done right, you can get detonation that you cannot hear and you may find yourself wiping out another set of bearings. I think I would go conservative and set my RPM limit at 5,200 RPM's until I got some mileage on the engine to seat everything and got my ignition dialed in.

10 to 1 compression is still a bit high for the "77" closed chamber iron heads for a street cruiser. You will most likely need a good 100 octane racing gas or true octane booster. The pump 93 octane may not cut it. Many topics found here on the forums under "pinging", "compression", "timing" etc. that should help you out.

Its your call, but personally, I would not be taking a car on a long trip (?) just out of the engine builder without sorting the engine out on the road locally first. But...... I have done it myself.
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post #3 of 3 (permalink) Old 10-10-2016, 04:10 PM
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The big journal engines don't like high rpms....they can have oiling issues and spin bearings. One reason why the small journal strokers are so popular these days. They are stronger and hold up better. That said, there should be no need to take a big journal engine up over 5200 rpm....it should be making maximum torque/power under that. That's the charm of a big journal engine: tons and tons of torque without having to wind it to the moon. I personally would not wind one up to 5900 rpm if I owned it. BTDT with a 428, (actually closer to 6500 rpm) and that was that. No more 428. YMMV......
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