Milling down SM 68cc CNC heads?

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roadrunninMark

roadrunninMark

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I got a set of SM 68cc CNC heads on the way from the black friday sale. I really wanted the 65cc heads but they were out of stock and you couldn't order them for the sale. Using a compression calculator from Summit, I am showing a CR of about 9.16 to 1 with the 68cc head. I really wanted to be closer to 10 to 1. If I can get the head to 61cc, it would put me just about where I want to be. Using the "junkyard jewel" build up as rough reference, when they milled the heads .050, they picked up about 6-7cc.

1. Anyone with these heads mill them down some to pick up some more compression and what amount would you recommend?
2. Am I correct that the intake manifold will now be needed to be milled down as well to fit correctly?
3. Any issues that may arise with this idea?

Thanks.
 
Where does the piston sit in all this? CC wise and its location to the deck? What did you figure for a head gasket thickness and does it all jive with the dynamic compression of you cam?
 
MLS head gaskets, .040 compressed. KB167 pistons that have a +5 cc volume with a compression height of 1.810". Block has been bored 30 over otherwise, it is stock decking etc. Can't find any details online about the compression ratio recommendation for cam but here are specs: Advertised Duration (Int/Exh): 272/272; Duration @ .050 (Int/Exh): 206/206; Gross Valve Lift (Int/Exh): .447/.447; LSA/ICL: 114/108; Valve Lash (Int/Exh): Hyd/Hyd; RPM Range: idle-4800
 
With that little cam I wouldn’t be trying for 10:1 myself(although the 114lsa will help with that to some degree).

An unsolicited critique of the combo.......
A 318 with cnc ported heads and a pretty small cam seems like a combo that’s pointed in two different directions.
 
I didn't want to go too big of a cam and those heads were the only ones they had left in stock.
 
Someone on here posted up the flow bench results of the CNC head (may have been PRH or PBR) and the low lift numbers were less than impressive.
 
roadrunninMark said:
I got a set of SM 68cc CNC heads on the way from the black friday sale. I really wanted the 65cc heads but they were out of stock and you couldn't order them for the sale. Using a compression calculator from Summit, I am showing a CR of about 9.16 to 1 with the 68cc head. I really wanted to be closer to 10 to 1. If I can get the head to 61cc, it would put me just about where I want to be. Using the "junkyard jewel" build up as rough reference, when they milled the heads .050, they picked up about 6-7cc.

1. Anyone with these heads mill them down some to pick up some more compression and what amount would you recommend?
2. Am I correct that the intake manifold will now be needed to be milled down as well to fit correctly?
3. Any issues that may arise with this idea?

Thanks.
Click to expand...
I think the difference in compression from the recipe you're following will have almost zero effect on power.
Mill them .020 if you want ..down to 64cc
After that.. you're going to effect the pushrods,lifter preload, intake alignment milling any more..and its costly..once you have to correct the intake face too.

I wouldnt worry about it.
 
Those are the same 65cc heads from the start. They just take a pallet load of those heads to a machine shop close by and have them CNC'ed... The extra 3 CC's could be from the cleanup... In my opinion this is splitting hairs for that compression... I feel your real power loss is going to be in your cam...
 
roadrunninMark said:
I got a set of SM 68cc CNC heads on the way from the black friday sale. I really wanted the 65cc heads but they were out of stock and you couldn't order them for the sale. Using a compression calculator from Summit, I am showing a CR of about 9.16 to 1 with the 68cc head. I really wanted to be closer to 10 to 1. If I can get the head to 61cc, it would put me just about where I want to be. Using the "junkyard jewel" build up as rough reference, when they milled the heads .050, they picked up about 6-7cc.

1. Anyone with these heads mill them down some to pick up some more compression and what amount would you recommend?
2. Am I correct that the intake manifold will now be needed to be milled down as well to fit correctly?
3. Any issues that may arise with this idea?

Thanks.
Click to expand...

You might want to CC those chambers. The versions with the CNC runners and as cast chamber are 65cc, not the 68 the advertise.

If you got the version with the CNC chamber work, those might be 68cc, but I'd still want to check.
 
Car will mainly be a street cruiser.

They are the cnc heads, the description says they are 68cc. I can check them when they arrive.

While the engine is apart, I want to get the compression a little better before it all goes together.

What size cam should I use then, if you believe the 272 is too small?
 
roadrunninMark said:
Car will mainly be a street cruiser.

They are the cnc heads, the description says they are 68cc. I can check them when they arrive.

While the engine is apart, I want to get the compression a little better before it all goes together.

What size cam should I use then, if you believe the 272 is too small?
Click to expand...

With that, what are you running for a transmission, gear ratio, tire size and if automatic are you willing to change converters or if you have one what is the stall? Also, what intake and carb (or EFI) do you plan to run? Basically the more we know the better the answer.

A lot will recommend a custom cam, and the grinder will ask you a lot of the same questions.
 
8hp70 w stock convertor, 2.76 rear gear, thermoquad from a 340, weiand 8007 intake, probably about a 28” tire diameter. I already have that 272 cam.
 
It's been said time and again the average difference in power from one full compression to another is about 3%. 3%. 3%. I would leave it be. It's totally not worth the trouble.
 
on a 318 with that cam, that gear and a stock converter. the cnc ported heads wouldn't be my first choice. but sometimes you gotta run what got.
i think some magnum heads would be a nice fit for your combo actually.

but then again, if you just want a good cruiser and nothing too impressive right off the line then it'll be fine and you have room to grow huh?
 
roadrunninMark said:
Car will mainly be a street cruiser.

They are the cnc heads, the description says they are 68cc. I can check them when they arrive.

While the engine is apart, I want to get the compression a little better before it all goes together.

What size cam should I use then, if you believe the 272 is too small?
Click to expand...

I understand what the listing says. I have a set of CNC ported that were sold as 68cc, but they have the same as cast chambers as the unported heads. I cc'd mine, and they were 65cc.

The only heads that might actually have 68cc Chambers are the version with cnc on both the runners AND the chamber, and I'd still check those to be sure.
 
Hey guys, I got my cam specs incorrect. Here are the correct specs:
  • Lift: .515''/.530''
  • Advertised Duration: 270°/278°
  • Lobe Separation Angle: 112°
  • RPM Range: 1800-5800
 
PRH said:
With that little cam I wouldn’t be trying for 10:1 myself(although the 114lsa will help with that to some degree).

An unsolicited critique of the combo.......
A 318 with cnc ported heads and a pretty small cam seems like a combo that’s pointed in two different directions.
Click to expand...

We see this more and more lately.
Everyone is throwing there stock heads away when all they needed was a valve job and a truing of the deck, for race heads and then trying to cruise with them.
 
roadrunninMark said:
I got a set of SM 68cc CNC heads on the way from the black friday sale. I really wanted the 65cc heads but they were out of stock and you couldn't order them for the sale. Using a compression calculator from Summit, I am showing a CR of about 9.16 to 1 with the 68cc head. I really wanted to be closer to 10 to 1. If I can get the head to 61cc, it would put me just about where I want to be. Using the "junkyard jewel" build up as rough reference, when they milled the heads .050, they picked up about 6-7cc.

1. Anyone with these heads mill them down some to pick up some more compression and what amount would you recommend?
2. Am I correct that the intake manifold will now be needed to be milled down as well to fit correctly?
3. Any issues that may arise with this idea?

Thanks.
Click to expand...

What part of GA u in ??
 
I would leave the heads alone, and true up and square the block. That might take ten or twenty thou off of it, and still be taller than blueprint spec.
And if the cam is really just over 200° at .050, I sure wouldn't run more than 9 to 1 as a cruiser.
 
rumblefish - the stock 318 heads aren't worth putting any money into. For the money, the aluminum heads are way better to use. If SM had the non cnc heads in stock, I would have bought those. The cnc heads were all they had and I am not waiting another year to get a set.

12many - I got a set of the 1.5 roller rockers from SM.

1badcolt - I am in Fayetteville, how about you?

33imp - thank you for the advice.
 
roadrunninMark said:
rumblefish - the stock 318 heads aren't worth putting any money into. For the money, the aluminum heads are way better to use. If SM had the non cnc heads in stock, I would have bought those. The cnc heads were all they had and I am not waiting another year to get a set.

12many - I got a set of the 1.5 roller rockers from SM.

1badcolt - I am in Fayetteville, how about you?

33imp - thank you for the advice.
Click to expand...
Just checking we have a Mopar specialist up in Ballground if u need assistance and or machine work performed, if interested pm me and I will give you his info .
 
roadrunninMark said:
rumblefish - the stock 318 heads aren't worth putting any money into. For the money, the aluminum heads are way better to use. If SM had the non cnc heads in stock, I would have bought those. The cnc heads were all they had and I am not waiting another year to get a set.

12many - I got a set of the 1.5 roller rockers from SM.

1badcolt - I am in Fayetteville, how about you?

33imp - thank you for the advice.
Click to expand...
Im in Hampton near Nascar track
 
1Badcolt said:
Im in Hampton near Nascar track
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Cool, not too far away. Do you go to any of the shows near there or know of mopar specific meet ups? I am always looking for more car shows, garage build expertise, etc.. I do have the 318 here apart, looking to get it back together soon.
 
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