I am going from a 58 cc combustion chamber with the l-98 heads to a 76 cc with the AFR heads .How much  hp will I lose . This is my combo.

zz4 short block.210 AFR heads XR280 cam with 570/578 lift 248/250 @50 % solid roller, 3500 stall 750 carb in an elcamino with 3.73 to 1 rear end .

Thanks

David

The old rule-of-thumb is about 4% per point of compression, but that doesn't tell the whole story.

You're swapping heads- better flow, but much lower compression.  I would calculate you are going to drop your compression from 9.5:1 (stock ZZ-4 compression ratio) down to a rather dismal 8.0:1.  That's a big drop.  Combined with the big cam, it's gonna be a pig.  It might make some decent power when you get way up into the rev range, but idling and part throttle drivability are going to be horrible.  You'll kill what little cylinder pressure you've got right now.  The massive compression drop may almost completely offset the better flow of those heads.

A cam that big is going to run MUCH better with compression up around 10:1.  If you can possibly swing it, you should get the 64cc version of those heads.  With 64cc chambers and some REALLY thin head gaskets (steel shim type) you'll be able to hold compression at your current 9.5:1

With 76cc heads you'll never accomplish that without changing pistons.

This engine will fall on it's face with that much cam and that little compression.

Cam timing and compression need to be thought of as companions. More timing, that is longer events, with increased overlap and late intake closing need more compression ratio to overcome the lost cylinder trap pressure that occurs with these long cams. This effect is seen from idle to well into the rpm range before the engine hits its torque peek.

The real compression pressure drops off with a long cam, as previously described, for a couple principle reasons. First, during overlap a certain amount of incoming mixture either passes out through the open exhaust valve or the residual exhaust pressure pushes the intake charge backward toward the carb, either results in a weak burnable mixture. Second,  at the other end of the intake cycle; the late closing intake valve allows the rising piston to pump the incoming mixture back into the intake. These two events conspire to reduce the amount of mixture trapped in the cylinder and-or dilute the mixture with exhaust gases. This condition continues untill engine speed produces enough mixture velocity within the intake system to supply inertial ram charging into the engine with sufficient force to overcome these lower speed flow reversals and exhaust pollution of the fresh charge. This is why big cammed engines are so "peaky" in their power output. A solution to this problem is to add compression, this forces more energy from the "thinner" mixtures going into the engine at RPMs below the torque peak. This also often requires a lot of early timing advance, but not more total advance. You need to be careful here to set the timing up to extract maximum low speed power without driving the engine into high speed detonation. 

An old general rule of thumb is that the power to be lost or gained relative to changes in compression ratio is that the percentage change in compression ratio is approximatly equal to the percentage change in power output. So lets say that your engine currently has a 9.5:1 compression ratio. Assume that the new heads will reduce this to a 8.5:1 ratio; that's a change of 11.5 percent in the negative direction. You can, therefore, expect to see a reduction in power at least equal to this. However, given your big cam, the effect in lost lower end torque will be even more pronounced. These effeccts will also be compounded in the negative direction by the larger ports of the AFR head, as these will further reduce gas velocities below torque peak RPMs and will further reduce off idle torque and will push both the torque and horsepower peaks toward higher RPMs. But getting to those RPMs is a different issue.

This doesn't look like a good move from a power stand point, and may also make the engine more detonation prone. It's certainly going to slow the car down and increase fuel consumption. When combined with a 3000 rpm stall converter this will behave like a mush ball off the line and may in fact surrender substantial top end rpms and power. More rear gears could help, like 4.11s or 4.56s. These would get the engine out of the doldrums quicker and allow sufficient "lever arm" to get the engine into the rev range where it's making power.

Otherwise, you've got to recover the compression by some means. Milling .060 to .080 should get it back in the game. The best solution from a compression, detonation, efficiency standpoint would be flat top or even  slightly domed pistons. But what you're currently doing will result in a very dissappointing  engine.

180 or 190/195 heads might be a better solution as these are available with a smaller chamber and don't surrender all that much flow. One of the things seldom if ever discussed in head selection is the ratio of flow in CFMs to the volume of the port. What you want is the most flow using the smallest port.

Bogie 

Im not sure how much I can take off these heads as far a milling ,but if thats what it takes I will do so ,Two corrections I have 10 to 1 com pression and I have a 3500 stall and a 150 shot of nos .

Thank For your input tell me more.

P.S in the fiture Im going to build a 406.

David 

AFR advertises their thick .750 decks so some substantial angle milling could be done, but plan on re-locating bolt holes in some way after this, either by offset doweling, head studs, or modifying the bolt holes on the cylinder heads, also milling of the intake manifold side of the head and header flange side of the head will be required, it pushes the machining costs up but if you already have the large chamber heads then its your only option because with that big of a cam and that little compression and your stall and gears i cant find it possible to find a place where this would help your performance, also a 1/8th mile track would pronounce these drastic losses in low speed torque even more in comparison with ET

AFR gives a spec of .008 angle mill per cc of chamber volume drop, so 10-15 cc could be removed pretty easily

Those AFR's will work well on a 406  ...  even if you mill them down to 68cc's now to make them work with the 350.  (a 68cc head on a flat top 406 is about 10.75 to 1, give or take deck height and head gasket thickness)

Instead of milling the heads (which will cost money), I would recommend finding small domed pistons for your 350 that weigh close to what your pistons now weigh. Swapping out pistons will give you the compression to make that combo really work in the mid to upper rpm range.  As you can tell from the different responses you have gotten, there are different ways to make it better... but you need to do something to make it better.  Perhaps the most logical thing to do is to sell those larger AFR's and buy the 195's. They come with 68cc chambers which will work on your 350 (even though you will lose some compression, you will still probably improve performance), but have been proven to work great on 406's (assuming you're not going solid roller with it and seeking 7000rpm ... in that  case I'd say keep the bigger afr's ). 

So many decisions to make...  it's all about the right combination. You can "make due" with those bigger AFR's temporarily, if you are going to make a long rodded, high rpm, race type 406.  But, if it's going to be a street car motor .... I'd say get rid of those bigger AFR's and go with the 195's. 

To touch on your initial question.... I was running a 355 flat top motor with angle milled steel 472 angle plug ported gm heads.  They picked my car up a full second over the regular "smog" heads the motor had before.  They were approximately 60cc chambers. I bought the AFR 195's and lost compression due to their 68cc chambers, but the car ran the exact same ET due to the better flow characteristics. (if it matters... 12.70's in a 3500lb car with a TH400, holeshot 3600, 410 gears)

To touch on your initial question.... I was running a 355 flat top motor with angle milled steel 472 angle plug ported gm heads.  They picked my car up a full second over the regular "smog" heads the motor had before.  They were approximately 60cc chambers. I bought the AFR 195's and lost compression due to their 68cc chambers, but the car ran the exact same ET due to the better flow characteristics. (if it matters... 12.70's in a 3500lb car with a TH400, holeshot 3600, 410 gears)

So you spent a extra several thousand dollars on AFR heads and ran the same ET with the GM castings, boy would i be disapointed. I think it goes to show you that ported factory castings are better than people think

How do you caculate how much you can mill off these head and what cc you will end up with ,and what compression you will have ? flat mill.

Dave

AFR advertises .006 per cc of chamber volume drop when flat milling, .008 when per cc when angle milling

im sure with their very thick decks you will have no probably cutting the chamber down to the size you need, to help you find the size you need you can look here

So you spent a extra several thousand dollars on AFR heads and ran the same ET with the GM castings, boy would i be disapointed. I think it goes to show you that ported factory castings are better than people think

$1225 actually... and I was dissappointed ... but the old steel heads,  worked by an old speed shop ( now know for their fuel systems, Bo Laws Performance, or BLP) needed valves, valve guides, valve job... maybe seats... that can run close to, or even over a thousand dollars at the machine shop .... assuming they don't hit a water jacket when they machine in the new valve seats. This is why I went with new heads. (I traded those old steel heads for a standard bore 400 short block) I went with the AFR's because I knew I'd be upping the cubes in the near future.... I just didn't know I'd be going to a long rodded, roller 406; if I did, I would have gone bigger with the heads than 195cc's. 

Thats why I went with the 210 s.I hope they will work out,the peanut heads i have now are slowing me down.Did any body take a look at the total set up on the car ,,,,stall ,,,gear,,, n.o.s,,,,and knowing that im going to build a bigger moteer later.

dave

How much can I get rthe compression up to by millig with a flat top,with the heads I have now they are listed as 10 to 1 with 58cc chambers.

Dave

Having read both of your threads, the question you have to ask yourself honestly is: What is your final combo going to be? Are you going to buy racing components for your 406 like an aftermarket block, forged crank, 6" forged rods, full floating pistons, splayed 4 bolt main caps, and the machine shop bill that accompanies this? Are you looking to build a 7000+ rpm motor? If not, then those heads are indeed overkill even on a 406. The 195's will give you more low and midrange umph. You will be sacrificing some top end, but considering how much time you spend above 6000rpm, it's a good sacrifice. 

We arent trying to rain on your parade, just give you some knowledgeable advice. 

If you insist on using those big heads on your ZZ4, your machine shop had better be able to tell you how much they will need to cut to give you the compression ratio you're looking for.

Good luck

Even if your compression is actually 10:1 right now (don't trust piston manufacturer's compression specs as absolute truth- it depends on more than just the pistons) you would still be down around 8.4:1 with the head swap (no milling of the heads, everything else held constant).

That's still going to be low.  Is there NO WAY you can trade them in on a set of otherwise identical 64cc heads?  It's going to be a lot easier to put some more heavily dished pistons in your future 400 than it is to mill the heck out of your brand new AFRs (not to mention the annoying gasket sealing issues that can arise) to try and work up to some barely livable amount of compression on this 350.

Barring that' I'd have to go with other's recommendation for some domed pistons in the current 350.  I rarely use domed pistons unless I have no other option but this is a situation that might call for them.