My 85 LG4 currently has 143,000 miles on it. I want to do a few mods like new intake, headers or cat-back. Somebody said that I could screw the engine up if I modded it with that many miles. It doesn't smoke or anything, actually it runs great. I'm just looking for more power.

 

Nah, bolt-ons like what you described aren't going to make your LG4 into a piston-eating, high-revving, tranny-killing beast. I've done similar mods with more miles on my L03. If you started to mess with big cams, or a power adder, I'd question the intelligence of that, but...
Just keep things relative to the rest of the setup.

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No guts, no glory.

 

The best thing to do is have a leak down or compression test done. Any good shop can do them. Also have them listen for any loose main or rod bearings. As long as the block, bearings, rings, and valve seats are in good shape, it should hold up pretty well.

I got 170K Miles on my 305. I haven't done headers yet, but ya never know. I've done other simple stuff that has worked OK so far. The car still don't leak or burn a drop of oil. I was thinkin' of laughing gas, but I wanna have a leak down test before I bother with that.

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92 RS w/t-tops 305 TBI Auto.
170K miles and don't burn a drop o'oil
-K&N Truck filter #1500 w, open ele.
air cleaner
-Dynomax 2 1/2" cat-back
-B&M TransPak
-Jet Stage2 Chip
-180* T-Stat w/ 185* Fan Switch
-JVC CD--Alpine speakers & 4ch. amp
Rockford 2ch. to 2 MTX 12" subs

 

I currently have a 1989 305 TPI Iroc, it currently has 238028 miles on it. I beat the living sh-t out of it and it still takes the punishment. I took it to the track and I got 9.73 in the 1/8 at 75 mph. I was highly impressed. As far as I can tell it is the orginal motor in the car. I bought it with
113995 miles on it and the farthest I have been in to the motor was replace the valve cover gaskets. THAT'S IT!! As long as you change the oil every 2-3000 miles it should last a long time and alot of punishment. The 350 will be in about 1 month, can't wait!

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89 Iroc 350 TPI,
Forged pistions
Comp roller Cam .510 lift 230dur
Flowmaster exhaust
Dart Iron Eagle heads 200cc runners 64cc chambers
MINI RAM SOON!!!!!
stock TPI for now 24# inj