339x heads

I have been trying to get some information on these heads. I know they are 1.72/1.5 valves. I read that they are larger 170cc intake runners because they have the X. I am taking a class at the community college to build heads and bore and hone blocks. I am using my 65 283 as a project. The 283 has power pack heads. I also picked up some other practice heads I could use on my 350 to build some more power and torque. These are 993 and 339x heads. I will probably have time to build all 3 sets. The power packs are going back on the 283. The others may or may not go on the 350. The current heads are 882 and the engine does not perform well. I am going to a different cam, looks like I might go with the summit 1102. The heads will all need hardened seats. I am thinking about going with 1.94 intakes on the 339x heads since they already have oversized runners. Would it be better to stay with the small valves for velocity? Does velocity increase torque? Or do I go with the 1.94 valves to help with the flow?

I would also like to boost compression slightly and the 339x heads are 72 to 74cc from what I have been able to find out. The 882 heads are 76cc.

I know there is a lot of talk about the expense of porting old heads vs vortecs but I will be doing this in class anyway so the cost or time is not an issue.


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Junk, use them to practice on but don't put them an anything. The 993's a large chamber, lazy burning head, the 339X takes the same idea and shrinks the intake valve from 1.94 to 1.72 (go figure). The 882 heads you have are the same thing as the 993 but with hardened exhaust seats that's needed with unleaded fuel. The 339X and 993 do not include that luxury so you'll find yourself getting plenty of valve and seat replacement practice.

The difference between a 74 and 76 CC chamber is nothing in terms of compression. Besides compression isn't the only player, chamber shape is much more important and you don't see any improvement in that till the L31 Vortec for the Gen I block. Those babies will by themselves punch up the power from 20 to 40 depending upon the cam you use, more cam begets better output from them. Even port size isn't that strong of an indicator any more, shape and placement have much more to do than simply size. These old heads have neither.

The things you're looking for in the head is the spark plug moved to between the valves (as close to the middle of the chamber as possible) and canted toward the exhaust valve; a relief pocket on the spark plug side of the intake valve that guides the incoming flow into a swirl pattern and washes the plug's electrodes while doing it. On the opposite side you want a squish/quench step that crowds the chamber and projects a beak between the valves. For a piston you want either a flat top or a D dish if you need a dish to adjust the chamber volume to something pump fuel can live with in terms of compression ratio. This is the definition of the Ricardo combustion chamber pioneered by Sir Harry Ricardo in the last centry and mostly ignored by the automotive industry till it had to come up with low emission, low fuel consumption, and high performance engines demanded by government edict and customer desire over the past 10-15 years.

The other important thing is matching compression ratio to the camshaft. Back in the 1960's this wasn't so important if the ratio was high against the cam timing you just bought 100 plus leaded and purchased your way around the fundamental engineering issues. Today you can't do that, so you've got to work harder and smarter. The classic Ricardo chamber is a huge step in that direction. The other is appreciation the difference between Static Compression Ratio (SCR) and Dynamic Compression Ratio (DCR). The SCR is simply the cylinder and chamber dimensions added and divided to drive out a ratio of volumes. The DCR takes into consideration the effect of when the intake valve closes in crankshaft degrees and how that virtually reduces the engine's stroke. This is to say that at low to moderate RPMS if the valve closes late much of the mixture that was drawn in is pushed out again. To recover this loss the compression needs to be raised to force more work from less mixture. This is the DCR which needs to be from 8 to 9 and will be the result of going backward into the elements of the SCR to push them high enough to get the desired DCR with the cam being used. The math to compute the piston movement is the same trigonometry used for computing the effects of changing the rod length ratio against stroke. There are plenty of DCR calculators on line you can go explore. If you're desperate to fandango your math teacher just come on back we can help with the detail arithmetic. The basics are the piston does not move in a linear fashion with the stroke. The crank as it moves opens and closes right to oblique triangles, this makes the stroke appear to change as the the crank rotates which causes the piston to vary its speed and position in the cylinder in a non-linear fashion that is very slow going over the Dead Centers in-fact stopping and reversing. Then having variable rates of acceleration and deceleration as it travels between the centers.

If you really want to build fast motors you need to understand this stuff.

Anyway this is hardly kissing the surface. Forget the ancient heads except to practice on, start thinking of the many Vortec/Fastburn (Ricardo) designs out there at GMPP and the aftermarket.

Bogie

Wow! Thanks that is the information I needed. I'd much rather work on the theory on why things work than try a shot in the dark approach at picking systems. From what you said here about timing it could be that my lack of power with the 882 heads is based on the valve timing. I suspect that the cam was installed wrong and your thoughts seem to confirm that. I can advance my timing to a very high initial and still not get detonation. I was going to change the cam anyway and I will be able to check if it is off when I take the cover off. If I do change heads I will go to the L31.

Thanks a lot for the science lesson.