I was wondering if the Corvette L98 aluminum heads (118s) would just bolt onto my 1990 Iroc with the LB9 305 engine. Decent upgrade? Same bolt patterns? No mods needed? -Collin

They will probably bolt right on. Not sure of the valve size for a stock L98 but I would suspect they are not the larger performance valves.

In reality, "corvette" heads mean nothing. Just another set of L98 heads similar to every other L98 engine out there. Out of the box aftermarket aluminum heads will probably outperform them. For the 305, installing a head designed for a 350 can also change a lot of things. A different sized combustion chamber will change the compression ratio. Port flow will be different and may not always be beneficial. Stock valve size will probably work however bigger 2.02/1.60 valves will be too big for the small 305 bore.

If you can get the L98 heads relatively cheap then it will probably be a worth while swap. Price out aftermarket aluminum heads first. If you have to do a lot of work to the L98 heads, valves, seats, guides, resurfacing etc then I doubt it would be worth it. "Corvette" engine parts are not a buzz word like they were in the 1960's and 70's when aftermarket parts were rare and expensive. The biggest advantage to the head swap will be lighter weight.

Yes they bolt on. The 1.94 x 1.50" stock valves do fit a 305. If replacing 58 cc heads the CR stays the same.. Use a 350 head gasket.
These heads are modest flow in stock as cast form.
Not a. big upgrade over 305 heads in stock form.
Tbe good news is if you do want increased power both the L98 heads and the 305 heads do both respond well to generous hand porting to increase port flow. They both need generous /SERIOUS work.
198 ish stock cfm.. 235-245 cfm when fully ported with stock 1.94 valves. Max valves on oem stock re machined seat inserts 2.00"x 1.56" (LT-4 size valves). the 1.94" valves are fine with porting for power. Can be milled a bit to increase CR too.
53-56 cc if needed. The D shaped Exhaust ports will need headers gasket selection attention.
Consider these heads a start point that needs full porting to realize your power increase dream in a head swap. Again stock as cast is modest flow and power.
Your stock 305 TPI "081" heads have THE SAME port flow and power potential with the same full hand porting EFFORT. You just need to get busy on what you got.

Center 4 intake bolts are at a different angle.
The bolt holes on the intake may need modification.
Angled bolt washer/ spacers needed to fit the 305 TPI manifold to The aluminum L98 heads.
The L-98 aluminum heads do not have the angled 4 center bolts.

So will they just bolt on or not? I'm going to have the engine out anyway, and I'm just looking for a small, easy performance increase, not a huge upgrade. Not really looking to modify the computer yet, nor am I looking to mess with bolt holes or anything like that. I'm just wondering if they would provide the small performance increase I'm looking for, and if they will just bolt on without having to modify the block or intake. They would be the 113's, from a late 80's early 90's C4 TPI, so I figured all of the bolt holes would match, I guess that's the biggest thing.

Maybe if you wait long enough someone will come along with some "alternative facts" for you..
You could look for a set of the "113" heads that have already been fully ported and re-worked/upgraded. Some aftermarket companies did/ may offer these in CNC ported form.
eg: Lingenfelter, Or contract with a professional cylinder head porting shop to get a pair built for you ready to bolt on.
Re: bolt angles difference. If you also had the TPI intake base from a L-98 aluminum head corvette motor that matches the heads You will not have a "bolt on" issue.
Nothing good comes easy, fast or cheap.

Do you currently own a pair of the 113 casting aluminum heads?

If you want a performance increase do the L98 cam and get your 305 heads ported and polished.

You have to realize that the TPI induction system itself is very restricted air flow wise.. So.......

But here is a thread with a ton of TPI head and manifold porting info
https://www.google.com/url?sa=t&sour...W_6AhTvz_DG2YS

There was a time when that was a very popular swap for 305 owners. They’re better than your run of the mill 416 or 601 head, and will support 99.9% of the needs of most 305s on this site. Finding a pair that aren’t destroyed would be the challenge. However, you lose your EGR function due to the lack of exhaust crossover, which would only be a minor issue if you still smog the car. As mentioned above, the aluminum L98 heads use the pre-86 90 degree intake bolt angle. Just ream the center 4 bolts on your base. Been done thousands of times over the years. You may as well slide a cam in it while you’re in there to take advantage of the heads.

Would probably make more power and be more cost effective to have your 305 heads ported a bit and add a lil more cam to the 305.

His 90 already has the L98 cam. From GM ALL speed density TPIs use the same cam

Personally if you have the 113s use them! I put a set of ZZ4 heads that were cleaned up a bit on a 312 TBI I built and ran in the 10s. The aluminum heads save you ~50 lbs on the front end. The ZZ4 heads made 423 hp on a 305 at the crank. I also like that they are screw in stud and can use a hardened guide plate eliminating the garbage self aligning rocker arma.

If EGR is needed, can get a Vette base and plumb in the external EGR. The intake stays cooler and cleaner without it though.

A good Street HR cam for your 305 with fully ported chev heads. 113 or 081 or 083... 10:1 cr.
Each needs equal amount of (generous) port work.
Very similar results in ported form.
4.10's high stall. higher the better 4000-4500 staal behind the 305 is non too much. or 5 spd manual.
Melling #22280 1.6 in 1.5 ex RR.
Install on 104 in C/L to max 305's perf with this one.
115-125 seat. 300-330 open springs

Nothing good is going to happen with stock as cast 113 heads and a "L-98 cam"

This cam will run with a stock converter but needs a high stall (4000 ish) for best drag ET