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Old 06-24-2010, 01:21 PM
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Will my CR be too high?

Thinking of putting 461 heads on my SBC but don't want to go over 9.5 because of detonation with the hyper pistons. Engine specs are:
355 (4.030" bore) SBC
Stock crank (3.48" stroke)
Regular Fel-pro head gasket (thickness?)
block at stock deck height
H345NP+30 Fed Mog pistons
461 double hump heads

I read that the 461 heads have 60.5cc chambers?!? I currently have 336x heads on it which should have 72cc chambers from what I read. The specs (from here) on the pistons say they should have a cr of 9.35 with 64cc heads.

Using this I calculated that the cr with 60.5cc heads should be 9.58. Can someone check to make sure I did this correctly? I don't want the car to detonate if it gets hot since I only run pump gas. Will i still be ok with 92 octane with this cr?

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Old 06-24-2010, 02:41 PM
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With the piston 0.025" in the hole, a 0.041" head gasket, 4.1" gasket diameter, 5cc dish, and 64 cc chambers you will be at 9.75:1 CR.

60 cc chambers, all else the same is 10.2:1.

Zero deck, 64 cc= 10.34, zero and 60 cc= 10.85.

You need to find a calculator you like and get used to using it.
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Old 06-24-2010, 03:19 PM
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hypers or not, 9.5:1 cr with a stock cam and old iron heads will detonate and break pistons. This also depends on temperature, load, a/f ratio, and how much timing, but in general avoid 9.5:1 cr with old school iron heads and a stock cam.

if you have a cam with more then 265 degrees of more advertised duration then it should be OK.

461's should be 64cc. should measure them to make sure. 1cc of water weighs 1 gm. got a small scale and a water bottle with a squirter?

adding a little compression doesn't make that much difference in power. If you want to make power power then put on a good set of heads (vortec, dart, arf, edelbrock.......).
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Old 06-24-2010, 03:24 PM
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Quote:
Originally Posted by cobalt327
You need to find a calculator you like and get used to using it.

The problem is that i do have a calculator I like and it gives me similar numbers to what you're getting. I'm wondering why the piston spec page I quoted says 9.35 for 64cc heads with these pistons??
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Old 06-24-2010, 04:39 PM
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Isn't the H345NP piston a 1.540" compression height piston, instead of factory spec 1.560"?? This fact would make their published #'s close to correct.

Just asking, I don't ever use hypers.

I just looked and yes it is, means that piston is down the bore .045" on an uncut block. Lowers compression ratio and KILLS quench with an .041" gasket. Will be detonation prone and want better than 90 octane at 9.35-1 because of it.

I'd find a better piston or zero deck the block to reduce octane sensitivity.

Last edited by ericnova72; 06-24-2010 at 04:44 PM.
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Old 06-24-2010, 05:08 PM
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Wow, I didn't know they have a different compression height than stock... that sux. I wish I knew that before I put them in. Would some thinner head gaskets help?

The engine runs good and doesn't detonate as far as I know. I guess I better stick with the 336x heads.

The engine has very run little time on it so if I did end up changing pistons in this engine, should I reuse the bearings, rods, and rings? I don't have much of a budget to rebuild this engine again... The other thing is that the engine was balanced for these pistons and rods. What pistons can I use that are roughly the same weight so I don't have to rebalance the engine?

Last edited by enduro54; 06-24-2010 at 06:43 PM.
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Old 06-24-2010, 06:54 PM
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The compression height info is on the Flatlander page you linked, all you had to do is look, same as the TRW catalog.

Thinner head gaskets would help but it could have sealing problems if the block deck isn't flat(and they rarely are after 70.000 miles) and the head remachined flat.

The 461 heads do have a better, less detonation prone chamber than the 336, have them skimm cut to make sure they are flat and use the #1094 Felpro rubber coated shim gasket.

On an apples to apples comparison of unported heads, the 461 head(and any fuelie chamber/camel hump head) will make roughly 15-20 hp/20-25 ft lbs Tq more than a smog chamber head like the 336x, just from the better chamber shape. Gets better yet if they are ported.

I don't have a clue what other piston might weigh the same, you'll have to do your own homework there.
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Old 06-24-2010, 07:44 PM
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I'm surprised to hear the fuelies have a better combustion chamber shape than the 336x's. I'd probably stand the chance of detonating with the 461's because of the higher cr, right? I definitely don't want that with hyper pistons. I think I'll just stick with the 336x's for now.
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Old 06-24-2010, 09:08 PM
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The basic fuelie combustion chamber shape is quite close to the same as all modern aluminum and iron aftermarket SBC heads. There are slight differences and refinements with the newer heads but it is the same basic shape.
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Old 06-28-2010, 05:11 PM
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Knowing the lack of quench in my engine will raise my detonation risk, I don;t feel comfortable putting the fuellies on and raising my cr. I think I'll just leave it alone or this season. If I end up tearing it down this winter, can I use the same rings, bearings, etc and just change the pistons? The engine has very little run time on it. I think changing the pistons would be better than having it decked since I'd have to take it apart to deck it anyway.
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Old 06-28-2010, 06:09 PM
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Quote:
Originally Posted by ericnova72
The basic fuelie combustion chamber shape is quite close to the same as all modern aluminum and iron aftermarket SBC heads. There are slight differences and refinements with the newer heads but it is the same basic shape.
I'm thinking there's a lot more to the modern head's central plug location, unshrouded valves, larger quench surface, tumble (instead of swirl) inducing chambers and ports that favor them quite a bit over the fulie heads.

In any event, between the chambers and the ports, there's no doubt that the Vortec head outperforms the fulie head. Just as the fulie heads outperform the later large chamber SBC head.

Old:


Vortec:
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Old 06-28-2010, 07:40 PM
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Yea... we can't run vortecs
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Old 06-28-2010, 07:50 PM
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Quote:
Originally Posted by enduro54
Yea... we can't run vortecs
What- this is some kind of race car/class and no Vortec heads allowed?

I missed that part. Can you point me to where you listed the rules? I don't recall what vehicle this is, or the set-up either so if that is already here somewhere I'd appreciate a link to that, too.
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Old 06-28-2010, 07:57 PM
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Quote:
Originally Posted by enduro54
Yea... we can't run vortecs
Do the rules allow porting/chamber modifications?
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