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Tech / General EngineIs your car making a strange sound or won't start? Thinking of adding power with a new combination? Need other technical information or engine specific advice? Don't see another board for your problem? Post it here!
Let me explain : I've been told a stock LU5 305ci bottom-end cannot handle more than 5500RPMs. Or you will severely alterate reliability.
I mean, a LU5 bottom-end with Trick Flow 30310001 heads, Summit 1103 cam, Weiand Street warrior intake under Holley Sniper EFI. The engine already breathes through headers and true duals.
Goal is not drag-racing, but long trips at high speeds.
So, is 5500 true ? If so, I guess it doen't worth the cost to reinforce the bottom-end. Better switch to a different motor.
Last edited by 70's iron; 12-26-2019 at 06:31 PM.
Reason: more precisions
Keep in mind, this isn't a case where at 5501.3745 RPM the matter will meet the anti-matter and annihilate the entire known universe into a microscopic black hole even smaller than my brain. It's more a matter of probabilities. Kinda something more like, multiply the 100s of RPMs above 5500 times the total cumulative # of minutes they occur, and divide your 150,000 normal lifetime by that number. I.e. if the RPMs go very far over 5500 for very long, the lifetime of the engine is drastically shortened. The crank will break, probably at the front of the #1 rod journal.
Don't get me wrong, I'm convinced my old 305 TBI would still run after using it as a boat anchor...That said the crossfire one is a different story
Did the old 305 TBI engines have a forged crank VS a cast one on the Crossfire ?
I ask because if the 305 TBI and the 305 Crossfire both used cast cranks , and the cast crank is the limiting factor in how high of RPM will be tolerated , then it pretty much shouldn't matter which intake is bolted to it whether it grenades or not at 5501.3745 RPM when Sofa's matter and antimatter collide ?
The bigger issue is putting an aftermarket top end on an LU5 bottom end. If the goal was sustained high RPM, might it have made more sense to address the bottom end first? Like building a house over a sinkhole.
305 TBI engines have a forged crank VS a cast one on the Crossfire ?
AFAIK there are NO 305s with cast cranks. (that said, someone will probably come along and tell us that the 305s in his vans had them) ALL 305s had cast AFAIAA..
There is NO DIFFERENCE WHATSOEVER between the Cease-Fire and TBI motors, as far as bottom-end durability. About the only significant difference is 1-pc vs 2-pc rear main seals.
Should have clarified I wasn't referring to bottom end durability but I know of a couple crossfire's that actually caught fire, and dad always used to say they were fire hazards so I just put 2 and 2 together. In reality I'm sure there were way more TBI 305s that overheated and caught fire.
[QUOTE=70's iron;6345849]Isn't that a good question ?
"Goal is not drag-racing, but long trips at high speeds."
This alone should be a concern. You're dealing with an engine that was intended for light duty use in an application producing about .57 HP per cu in. Even at that level, long term high speed operation would be questionable. Now you're talking about closer to 1 hp per cu in, using the same rotating assembly and just as important, the same oiling and likely the same cooling system. My concern would be insufficient lubrication and cooling to support that kind of use. This is in addition to the obvious concerns of running a cast crank and pistons along with stock rods in a high output application. You can build a pretty solid steel crank 350cid bottom end for around $2,500, including machine shop labor for balancing and clearancing. The best money you can spend.
The L69 305HO motors in the Trans ams had a 6K redline tach, same cast bottom end as the LU5. Probably was fine at 190hp, but add 50% more power though and stress goes way up. Do the top end upgrades, keep the rev's below 6K and only quick bursts up there running through the gears. It may live a long time, or it may start knocking soon. Hard to tell. If it does start knocking, shut it down before it grenades and transfer the top end parts over to a 350 short block.
Cast crank, cast rod, cast or hyper piston, 2 bolt main 305 marine engines run 4,800-5,200 rpm at WOT pushing a ski boat down the lake. 5,000 rpm continuous or 6,000 rpm intermittently even making 400+ HP usually is not an issue with a fresh engine. If it has some miles on it, who knows. Marine 305 was 230-260 hp and had the same bottem end. I have run the stock bottom ends 6,500 and they lived for a long time. I had a TBI 350 that never saw 5,000 rpm that snapped the crank in half.
Smart-asses will (for sure) have noticed that I live in Europe. Where Audi 4.2 V8s are cheaper and easier to get than 350s. Or maybe they can't consider a given reality.
So, thank you for the serious answers. My engine has 67XXX miles, and is not aimed at more than a couple thousand miles a year. I notice the advices of not going over 5500RPMs often. Restarting from toll boothes should be enough.
The original heads have been modified (yes, smartass, I know, they are the worsts heads ever). So instead of buying TFS 30310001s ($1100+shipping and taxes = 2000€) I should save money to get a 350 or 383 long-block...
If you are using ported stock 305 heads and a reasonable cam, your power peak will be below 6000rpm anyway. Also assuming you are getting rid of the crossfire injection and using a 4bbl? With ported stock heads, mild cam, 4bbl and headers, that will be a nice, sub 6,000rpm street motor for not a lot of money.
12-26-2019, 06:07 PM
