[F-1Rodder]

I received a private message asking why I recently advised starting with good heads to build power. Since I wrote a response, I will copy it here for all to read:

Its very simple if you think about it. Anything you can do to increase the violence of combustion in the engine will create power. Higher compression, a larger combustion volume, more frequent combustion, a more complete combustion efficiency, a properly timed combustion, all these will create more power. Heads are important because this is where combustion occurs. Heads that are designed to move more air and fuel in more quickly and scavenge spent combustion out to the exhaust pipes quickly and completely are better than those who do it less efficiently.

Until very recently, auto manufacturers were willing to trade off combustion efficiency for larger motors because the cost of developing highly efficient engines was greater than building bigger motors and the consumer believed that bigger motors were better than highly stressed small motors. But with the shortage of fuel and higher prices in the past 20 years, the emphasis and regulatory climate is turning toward greater efficiency. An efficient head design will have valve openings that are less shrouded to allow combustion gasses to flow smoothly, valve pockets will have no sharp edges, exhaust and intake chambers will be very smooth and will exactly match the intake and exhaust flange openings so that there is less turbulence. Porting and polishing head ports helps, but the best porting job will not even come close to a well designed set of heads. Efficient head and piston matching also take advantage of a principle called "quench" at TDC. A very thin distance between the piston top and the combustion chamber surface (less than .035") will not support combustion and therefore no pre-detonation can occur. With this observed, the chamber is able to withstand higher combustion pressures without pre-detonating.

If you have a great cam duration, high lift and a big carburetor that is capable of moving a lot of gases and a big exhaust header and big cubic inches this is great potential, but totally useless if the heads are inefficient and cannot use the potential. Bad heads are like crimping the water hose….plenty of pressure, but there is not enough flow to create work.

I don't know what kind of motor you are interested in, but there are efficient after-market heads available and in some cases some good factory heads recently developed which observe these principles. It is pretty easy to produce a lot of power with the right combination of pieces. You have to know where you want your power at what rpm and what torque characteristics. From that you determine what size intake runners you want and what size exhaust ports and select high flowing heads that will give you what you want where you want it. The worst heads were those built in the smog era of the 70's to middle 80's where emissions regulation was the highest interest of manufacturers. These are also the most plentiful of hardware.

It is also a fact that a highly stressed motor will not last as long. You just won't get 200,000 miles out of a small block which is built to 500hp. It will wear faster and will break sooner so the auto industry wants cars to be very durable and not break.

This is why when I see young guys wanting to create power from different cams and manifolds, I will usually tell them that they are spending there money in the wrong place if they don't have good heads already. Bad heads just don't allow you to get a good return from your money spent. Start with the heads and you will get the greatest gains for your money thereafter.

Hope this helps you. There is a good book by John Lingenfelter on modifying small block chevy engines, the principles apply to all race motors. He discusses how these issues come together to make power. this and other books are available from Amazon

[ January 23, 2003: Message edited by: F-1Rodder ]</p>

[dk01_99]

[QUOTE]Originally posted by F-1Rodder:
[QB]A very thin distance between the piston top and the combustion chamber surface (less than .035") will not support combustion and therefore no pre-detonation can occur. With this observed, the chamber is able to withstand higher combustion pressures without pre-detonating.

hey thanks a lot for replying to my PM i sent you, especially since you did it so quickly. i understood all of it except for what i quoted up there, do i just didnt get what you meant by the combustion chamber surface. you mean the walls of the cylinder as well as the surface area of the heads? or what? but thanks a bunch. it helped. later

[F-1Rodder]

Lets assume a flat top piston for simplicity. Most modern cylinder heads have a flat portion with reliefs for the valves. With the piston at TDC, there has to be a distance between the head and the top of the piston to provide for expansion of parts. Hot parts expand and the rod and piston expand in length, so when hot there is less distance than when cold. Usually the gap is just the thickness of the head gasket. And the distance can also be affected by milling material off the top of the block. This is known as setting deck height or "decking". For a perfomance motor the piston is usually "0" decked meaning the top of the block and top of the flat top piston are at the same height. Getting the distance from the top of the piston to the flat surface of the head chamber down to less than .035" is when you get quenching. You can buy head gaskets as thin as .025" from GM or somewhat thinner on the aftermarket. An example of this is the GM fastburn or L31 vortec heads and the use of D" cup flat tops pistons.

I do not know why quenching works except that it does and its not my idea. See http://www.speedomotive.com/Building%20Tips.htm

I am successfully running 10.3:1 at sea level on pump gas because of the quench factor.

[dk01_99]

wow man that helped a lot. thanks. some of it is still being tossed around in my head but maybe someday it will straighten out. needless to say, thanks. this board rocks

[Making Waves]

Hey Rodders,
What right are the hot setup heads on the poor mans budget? It's to the point right now for me with the ole lady and all the rest to just stick to stock type engine set ups to swap for the old iron for a cool ride with gas mileage and a just a smidgen more power instead of dyno shoot out goals. (I fried another transmission today). I see the jap stuff getting 400 ft lbs at the wheels wiith only a small turbo on my local DYNOJET machine. They running stock gear boxes on FWD too. What's a fells to do?

[ap72]

well if you want cheap power I would suggest following the Jap guys and building a turbo system- if you have a small budget use junkyard parts and ebay'd generic parts- they usually workfairly well, though not quite as good as the name brand stuff.

As far as heads go- Vortecs still seem to have the cheap SBC market, though you'll need different pushrods, rockers, valve covers, and a manifold, so getting them used and complete is probably the best way for you to go- watch valve lift though as these are quite limited in stock form.

[ap72]

oh, and while heads are importatn I highly disagree that they are the first thing to get- the first thing would be a CAM and a good exhaust setup, most people don't realize how limiting stock cams are, the same goes for exhaust.

[machine shop tom]

Originally Posted by ap72

well if you want cheap power I would suggest following the Jap guys and building a turbo system- if you have a small budget use junkyard parts and ebay'd generic parts- they usually workfairly well, though not quite as good as the name brand stuff.

As far as heads go- Vortecs still seem to have the cheap SBC market, though you'll need different pushrods, rockers, valve covers, and a manifold, so getting them used and complete is probably the best way for you to go- watch valve lift though as these are quite limited in stock form.

Enginequest has Vor-tec style heads with the older intake manifold and valve cover bolt patterns. Might be a good choice if you already have an older style intake and valve covers and want to use them. They are also set up for screw-in studs.

tom

[techinspector1]

Originally Posted by ap72

oh, and while heads are importatn I highly disagree that they are the first thing to get- the first thing would be a CAM and a good exhaust setup, most people don't realize how limiting stock cams are, the same goes for exhaust.

Disagree. Everybody starts at the wrong end of the car. Gears first.