Does anyone have any experience with larger valves on a 305 such as the L31 Vortec heads have? I'm thinking of installing them on a stock LO3 shortblock. I'm questioning valve to piston clearance with the larger 1.94 intake valve with a .480 lift cam.

Thanks!

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91 RS 5.0 TBI....LT4 cam....Edelbrock headers....3"Dynomax exhaust....5spd.... 3.08.....Ultimate tbi....afpr...ZR 255-50's...Koni's

 

The problenm on a stock bore 305 with stock 1.94+ valved heads is valve to cylinder wall
clearance. If the motor is bored oversize
its ok. I don't know off hand what minimum
overbore allows this.
World product makes a S/R head with a modified valve placement just for this application. When ported they will equal or surpass the performance of a stock vortec
head and don't need a special manifold. This cylinder bore clearance problem may not be an issue with a stock lowlift cam but....

 

I see on another post where you ported a set of 305 416 heads with good results. I was thinking of that too as I have a set of those. I know that RB83L69 has installed a set with larger valves on his but I don't remember his saying of any problems or what heads they were. It had good dyno results from what I can remember.

 

I've never tried this but if you must stay with a stock bore 305 you could cut down a set of 1.94's to 1.90". then regrind the valve face. That should clear. I never got to try my ported "416's". I finished them, sold 'em and bought vortecs.

 

There is no particular piston-to-valve clearance problem with the L31 heads, not to any flat-top pistons. The retainers will hit the valve guides at least .150" before the pistons will hit the valves. Valve diameter has nothing to do with any of that. As long as the valves fit in the bore, and don't overhang the block, they'll work. Obviously it would be no fun if the valve smacked into the deck when it tried to open; that's what happens with some heads and really large valves in a small-bore engine.

My motor has 186 heads, which are one of the old "double-hump" castings with the accessory bolt holes; it also has a Comp XM264HR-12 cam and 1.6 rockers, which net about .530" lift. No problems.

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"So many Mustangs, so little time..."
ICON Motorsports

 

It's been done by the magazines, with good results. But, consider what you have to do to the Vortecs to get them to work properly on a flat-top piston 305 (valve to cylinder wall clearance isn't one of the problems):
Shave to reduce chambers from 64cc to 58cc (~.007"/cc, or .042"). Shop here charges $40/.020 shave per pair, $80.
Shave intake to match shaved heads. Another $50.
The aforementioned special intake, and self-aligning rockers. IIRC, dealers are including those in their ~$300 "Vortec installation kit".
If you want to go over .450" lift on the cam, special springs, machining of spring pockets. ~$100.
With more lift, you don't want press-in rocker studs. ~$75 to get the heads machined for screw-in and the studs themselves.

Hummm, $605 plus the $425 Vortecs cost now, you're at $1030.

Oh, forgot - Vortecs don't have exhaust cross-over passages, so if you need EGR, you have to plumb exhaust from the manifold/header up to the cross-over passage in the intake. Not quite sure how to estimate that cost.

Or, go out and get a set of World 305's, $600 with upgraded springs and seals, spend another $200 to get them pocket ported and gasket matched (or do it yourself if you have a compressor and diegrinder for about $50), another $145 for an Action+ EGR intake, you've got something that does all that and was actually intended for your engine.

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Thank's RB. I thought that you had some larger valve heads on your eng. Are they 1.94's? I'm undeceided on porting the 305's or running the Vortec's.

five7 I already have the Vortec's setup to run .480 lifts and 1.6 rockers. The milling is what I don't know if I am going to fool with or not no matter if it does drop the C/R around .5 or not

 

That is an excellent point. What are you going to lose with 0.5 compression loss? Only about 4 or 5 hp--utterly neglibible! In fact, I heard it said many times that GM themselves say the Vortec combustion chambers are actually only about 61cc, not the 64cc generally quoted, so you are losing even fewer hp. In my book that is nothing to concern yourself over for a street engine

 

Right, as far as the CR. I basically just scraped together a bunch of stuff laying around on my garage floor (well, not quite... the cam was actually sitting on a shelf ) and I opted to just eat the loss of compression due to the larger chambers. In spite of that the motor seems to run pretty hard for a 305 at least, it has already surprised a few L98 owners. The guy that owns the chassis dyno that I use out here says he hasn't seen a single N/A 305 do any more than this one; in fact he races (oval track) in a class where you get like 300 lbs for a 305, everybody gives up the 300 and runs a 350, he was re-thinking that philosophy.

This pair of heads just has the stock 1.94" valves in them.

I prefer not to mill heads myself any more than necessary for gasket sealing (especially not hard-to-replace ones like double-hump heads with bolt holes!!), not so much because it causes problems or even because of the expense, but simply because it's irreversible. I hate altering hard parts in ways I can't undo.

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"So many Mustangs, so little time..."
ICON Motorsports