|01-22-2013 12:22 PM|
Compression remains the same. A better (but not perfect) definition is that there is an ideal combustion pressure to force the piston down. Iron heads retain the heat, so say this ideal is reached at 9.3:1 compression on my theoretical example. Any less and you loose power, any more and you detonate. The aluminum heads bleed much more heat out of the chamber, so another full point of compression is required to maintain the same amount of heat in the combustion process. So the process sees the same amount of heat with 9.3 iron heads and 10.3 in aluminum heads. Over simplified, but it gets us close to what is going on.
There are also limitations in stock cast iron heads that limit the amount of porting that can be accomplished. Since the aluminum heads are designed from the beginning as a performance head, there is more meat designed in around the port area so a better port can be designed, and better flow is the result. Aluminum heads save weight, and they weigh half of an iron head. Aluminum heads are cheaper and easier to produce than iron, and are also much easier to machine and port. Because of this more companies make aluminum heads - and that competition brings a better product to the market, and keeps the prices down for us.
|01-22-2013 05:27 AM|
yea i get what you are saying, ive changed most of the things i said in that first post already. But yea, i will probably end up with a 750 holley. But you are saying that i should be okay because i have a aluminum heads? ive heard that but never knew why, so since its taking heat away its essentially lowering the compression in the cylinder?
|01-21-2013 11:51 PM|
|lust4speed||You are very close with your static compression ratio estimate. I came out with 10.35:1 using your supplied info. Aluminum heads require an additional point of compression over iron heads because of their ability to suck heat out of the combustion chamber so you are completely safe. An 800 DP is way too much carburetor, and no reason for a double pumper on a street vehicle. Holley backs this up on their website recommendations stating that a double pumper needs a light vehicle, loose torque converter, and lower gears. It's not sexy, but a 750cfm carb with vacuum secondaries will be much more efficient on a 406. That translates to more power and much better gas mileage. I would also come down a notch on the cam choice. The 230/236 duration @ .050" is a little rowdy for a 406 in a heavy vehicle. You should also have a higher stall converter over stock, and vacuum for your power brakes will be marginal. While the cam would work, I'd just go somewhat smaller for a heavy vehicle.|
|01-21-2013 09:03 PM|
|01-21-2013 07:43 PM|
|01-21-2013 07:40 PM|
|BuzzLOL||.. Round off any razor sharp edges in the combustion space to prevent detonation such as on pistons, valve seats, etc...|
|01-21-2013 07:17 PM|
bringing this thread back because i have another question. I have settled on the skip white rotating assembly, i wanted internal balance but for the build this is going to be its not worth it to shell out the extra money. So this is the kit SBC Chevy Scat 406 Forged Rotating Assembly ft 30 4 155 Bore | eBay
As i was thinking about i think i will use a .039" head gasket and the piston will be about .010" in the hole, and typing that all in to a static compression calc, i got about 10.37:1 which as far as i know should run on 93 octane....
Lastly, for my dynamic balance i am trying to find a cam that will be mild and not cause any issues..
This is the one i have selected thus far: 12-246-3 - XTREME Energy
That cam gives me 8.34:1 dynamic compression...
I have aluminum heads, ive heard that plays into the picture...
Any suggestions? I dont wanna have any detonation problems, am i too close to the line? And will i be able to advance it or will i have to play with the timing to get rid of spark ping?
Edit: also if someone doesnt mind could they run my numbers through a compression calc and verify im doing it right? 4.155" bore, 3.75" stroke, 64cc heads, 14cc dished pistons, .010" deck clearance, .039" gasket (unsure of bore yet so estimate), 6" rods
|01-06-2013 02:09 AM|
.. A 650 CFM - 750 CFM carb. or even stock Q-Jet 4 bbl. carb. is plenty...
.. Stock oil pan and oil pump is fine at this performance level... maybe add a high pressure bypass spring to stock oil pump or use a high pressure oil pump... I would avoid the power-consuming high volume pumps as unneeded ... add a windage tray if it doesn't already have some kind of one inside the oil pan...
.. What's your tranny? If it's an automatic shifting at only 3800-4000 RPMs at WOT, then it'll need governor springs to bring that up to 6000 or so for the engine to make its new power... or manually shift it when wanting that power... If a manual tranny, of course it can still simply be shifted as desired...
.. Whether you need/desire a different stall RPM torque converter will depend a lot on your current rear end ratio... and driving habits... and final cam choice... and final compression ratio...
|01-06-2013 12:50 AM|
.. OK, at the performance level you're talking: "400HP" from a SBC 400 for a fun road truck, everybody is OVERTHINKING this!
.. The stock SBC 400 with stock heads and crank/rods with merely flat top or domed pistons and a $50 Summit #1105 224/234 cam/lifters/Z28 valve springs added (and 4bbl. carb./intake if not so equipped already) will make 400 HP or close to it...!! And do it by 5500 - 5750 RPMs... shift at 6,000 RPMs or so... with torque peak around 4000 RPMs... pulling really hard from 2000 - 2500 RPMs or so...
.. or another combo:
.. Stock block, crank, rods, your first pistons choice but for stock 5.565" rods, your first cam choice (biggish for a fun road truck, but usable), and your first heads choice -> 375 - 500 HP depending on the exact heads used (165cc - 220cc range intake ports)... still by 6000 RPMs...
.. The 2nd cam you picked was just too big for your goals... wouldn't pull hard till 4000+ RPMs... wants to make 500+ HP... not fun truck compatible... requires other engine parts made of expensive unobtanium...
.. 6" rods make more power above 6250+ RPMs... basically, out of your needed operating range...
.. I'd suggest doing something like one of the two budget combos above... just add an ounce or two of kerosene or other octane booster to a tank of gas if you just can't tune any/all spark knock out with ignition timing/advance curve and premium gas... or can only get 91 octane premium in your area... no 93 or 94...
.. KB, TRW, or Summit hypereutectic pistons would be plenty for this build... and KB especially usually come with good compression height... and all have good price... adjust them to the gram weight of your stock pistons/pins and avoid paying for an engine re-balance job at this performance level...
.. Here's an article about adding a cam of duration similar to your first choice 230/236 duration (only expensive roller lifter type) to a mostly stock block/rods/pistons/heads SBC 383" engine and what it did:
.. Edelbrock E-Street heads would be a good pair of American-made somewhat budget Aluminum heads... although on the smallish side for a SBC 400"...
|01-05-2013 09:29 PM|
|Wage92||what is a better piston brand Mahle or SRP? Or are they fairly close. This would be for forged dished pistons with 6" rods|
|01-05-2013 09:20 PM|
.. LOL! Nobody was considering building an engine with the piston 0.275" above deck... that was just somebody's math error...
.. With a longer rod, you use a piston with a shorter compression height... or, if budget allows, a custom piston with the perfect compression height if nothing proper close enough is available off the shelf...
|12-31-2012 03:35 PM|
The other way around the problem is to open up the quench area large enough that it also won't cause detonation problems. I can't remember whether it was pretty low around .080" or higher in the range of .120"? The tighter quench is definitely the way to go, but sometimes you don't get to have what you want. Basically, the big number was why we got away with milling out the old TRW forged pistons (years ago before the beautiful D-shaped dished pistons became available) to lower compression without going into ping city.
|12-30-2012 07:13 PM|
406 small block review
What I have read about DCR is you want to be in a range from 7.5 to 8.5. The engine makes the most power with a DCR of 8.2.
|12-30-2012 02:56 PM|
Edit: also would a dcr of 8.0 help at all, by using this cam: http://www.compcams.com/Company/CC/c...x?csid=88&sb=2
|12-30-2012 02:50 PM|
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