Worth milling my heads?
 

So some of the info I'll give is known but alot is assumed.
The block is a .060 over 455 with forged rods and -15cc dished icon pistons.
The cam is a melling spc-8 cam (041) and I have Rhoades lifters.
The heads are 6x-4 heads with 1.5 roller tip rockers.
I'm fairly certain the deck height is stock and I don't believe the heads have been milled. If I pull the heads I will check these things.
But assuming a .016 piston to deck height and 94cc for the heads with a .039 gasket. The basic calculators put me right around 8.9:1 compression.
If I were to pull the heads and mill them to say 84cc which would put me around 9.6 unless the block happens to be zero decked then I would be closer to 9.9 but with good quench. And I'd clearance them for 1.65 full roller rockers.
What could I expect for a power gain? Obviously that's alot of assumed specs but If those were the numbers would the gain be worth the time and money? And how much of the gain be from the rockers and how much from the compression? Is it worth just doing one?

This chart may help you and if it where not for the fact that your running Rhoads lifters I would say go for, but I will leave it up to you and others who will chime in.

This cart is percent of power increase.

I'm guessing 20-30 horsepower gain with the head milling and 1.65 rockers. You will feel an improvement, get better mileage and see a difference on the time slip if you race your car. I'd also adjust the deck height with a thinner gasket if the piston isn't even with the deck.

Is it worth the work, depends how much the performance difference means to you. You'll need to mill the manifold or manifold side of the heads. To do the job right you should leave a step at the bottom of the intake side of the head (assuming you choose to mill the manifold side if the head which is usually the better way) for the valley pan to seal and you might need shorter pushrods.

Heads are angle milled. You then need to mill the intake side to get the angle back so it seals properly. IF you have manifolds, expect them to be touching the motor mounts.
Why even mention a cam when you mention 1.65 rockers? They change the cam specs. Got the springs and guides for that lift?
Your fuel requirements will sure change as you chase a "static" compression number.

If the modifications require a "de-tune" for pump gas....

Could end up with a TQ/HP loss.

Been there done that and don't like it
Clay

There is alot I don't know, but I'm always learning. Which is why I came to a forum to ask these questions. And I appreciate any input. I would be taking them to a very reputable machine shop in my area to be milled. I assume he would make sure they are done correctly to seal properly, but maybe I'm incorrect. The car has headers and plenty of clearance. As for mentioning what the cam is I was figuring if I asked if milling for a certain compression range was worth it people would want to know what cam I'm running. But maybe I'm figuring wrong. I do know that if I went to 1.65 rockers I (probably the machinist) would have to check spring pressures and installed height and spring bind. And I'd have to measure pushrod length. I don't know what I don't know so I appreciate being told those things because I've made alot of mistakes with my projects. At this point I'm really just asking what the power gain would be with those only two theoretical changes to see if it's even worth considering.

Paul, I've tried to do some research on thinner gaskets and just haven't found enough to feel comfortable using them. Do you have any you recommend? Thanks

Most of what I read has said the 041 cam can be run on 91 octane up to 10:1 compression with good tuning. I'm close to sea level and I'm running a cliff ruggles qjet and have an AFR gauge installed. I'm under the assumption I wouldn't have to de-tune. But I'd take advice on the matter from anyone if my assumption is wrong. I'm definitely not looking to have detonation issues.

Cometic will make them down to .023 thin.

You shouldn't be angle milling so you don't need to worry about alignment issues and correcting bolt hole angles.... No need to over complicate things.

You shouldn't have any detonation issues with 9.9 and an 041.

Many years ago, the 428 had 8.9 compression, magnum 292H hyd-flat cam and 225 cfm ported heads. So, like you, a cam too big for the compression, yet it ran a best of 12.31 in the GTO.

With your Rhodes lifters you will be fine for street driving with the lower compression. Like what was asked before, do you' need' the extra power? I think if you do the work of milling the heads and 1.65 rockers, then check for rocker stud length and pushrod length, you may have to purchase both, plus new springs. I feel it is not worth doing all that with stock heads. If you need the extra power, also port the heads.

As for the clearance in the heads for 1.65 rockers, you may need to remove material the whole length of the guide hole, not just the top portion.

Actually, when I sell the GTO I will put those lower comp heads back on with the RA4 cam, Rhodes lifters and 1.5 rockers. I think it run good enough for the next person.

SPC-8 has less advertised duration and less over-lap than a 041.

I suspect a .006" (instead of .004") measurement would put The SPC-8, advertised and overlap, closer to a 744. Faster ramps than a 041.

If you look at ICL @ ,004" adverised and ICL @ .050".. May even be some voodoo crap going on.

With HFT or SFT cams (make that any cam).. The same @ .050" numbers isn't enuff for me.

SPC-8 isn't a copy it's a replacement. Like a toned down more street friendly version of an 041.

Clay

Do cometics require any particular finish on deck and head surfaces for proper seal?

The last time we checked an SPC-8 it had identical numbers at .004. .006 .020, .050. .100, .200 as an NOS RAIV cam. Maybe they changed something since then as they did with the 744 copy. I'd think almost any 230 @ .050 with the intake centerline installed around 111 would be okay with 9.9 compression ratio, premium fuel, good tune up and a decent quench. I know we have a lot of 235ish HR engines using 10.5 compression with zero issues with detonation and those usually have an ICL around 109. Aluminum heads we will run at 11:1 with zero issues and higher than that with bigger cams.

They say they do. We have had zero issues and have run them on pretty coarse finishes including factory machined surfaces when using iron on iron.

The reason why the SPC-8 looks smaller on paper is Pontiac engineering originally rated cams different than Melling. Melling uses a SAE rating which is .006” lift out at the valve, rocker arm ratio effects those. From what I remember HO racing published that the Pontiac ratings were originally .002” open to .008-.009” in the closing. Oldsmobile supposedly rated their cams a 0” lift, OEMs ratings from the day were all over the place. The lasted SPC-8 that I checked was 3 years ago measured 291 intake and 302 on the exhaust, both measured @.006” tappet, one supplier rates them at 292 and 303 at .006” tappet, pretty close to what I measured. It has gentle ramps like you would expect to see from the 60s and 70s cams.

I am on the same page as Paul for compression using a spec-8. We have tried running it with just under 9 compression and were pretty disappointed. With low compression it’s street manors are like a 12 second cam and your doing good to get it into the 13s. But, it will run on any dishwater gas you put in it, and if you like the fenders to shake a little at stop lights, it does that well. Rhoads help street manors, we didn’t have Rhoads on the low compression 455 we did. I didn’t feel like it ever lived to it’s potential compared to when it has another point of compression.

Why did you start out with dished pistons? Flat tops would be my choice.

GT

Well like I said before, I've made alot of mistakes learning as I go. When I had the engine built I was scarred of compression and understood almost nothing about what all goes into detention sensitivity. If I had it to do again I would have done that and alot of other things different. But here I am.

if the 6x heads have oem valves I would go aluminum 72 cc heads with the dish pistons for the best bang for the buck. You will have a chunk in the 6xs if starting over with all hardware

The 6x-4 heads I have already had the 1.77 valves installed and then when I had the engine rebuilt it got new guides , springs and valve job. I'm hoping the springs I have, have the correct installed height to handle the extra lift of 1.65 rockers.
I'd love to have aluminum heads. But the quote I got for milling and clearancing the pushrod holes was $300. Out of the box kre 74cc heads are $2500. Not saying it's not worth the money, if I was tripping over bags of it that's what I would do.