I am building a 400 motor for real good power but to still maintain streetability. I am using aussie heads (ported and polished out to about 62cc chamber volums) boring it .030 over with KB Claimer hypereutectics (I am bushing the rods for 351C pistons). These pistons have a -4.5cc piston (dome) volume. I am also decking the block and heads only enough to clean them up to straight. My problem is, even without any machining, I am looking at 10.66:1 compression. That seems awfully high to me. And it will only get worse as I machine the heads and block. So what can I do? Should I take more out of the combustion chambers, like 2 more cc's? Or what can I do. The main thing is that I don't want to be running so much compression that I'd have to be easy on my motor running premium. My brother used to have a super-hot 400 running 10.5:1 c/r and he could run midgrade fairly easily, unless he was really into his motor, like off-road. I could live with that but I think that the closer I get to 11:1, the more I am gonna have to run premium. Assuming I only have to machine .005 off the block AND Heads, I am still running close to 10.8:1. How can I reduce compression? What are my options, who has to do them, and how much will they cost? Thanx

 

Well the easiest way to lower your compression on that motor would be to mill down the -4.5cc piston (dome) to -2.0cc or even make them flat tops.....

 

Sorry ShaZam, I said that wrong. They are flat tops with 4.5cc valve reliefs...

 

OK what's the compression height of those pistons .

 

The Compression Height is 1.670 and the pin dia is .9122 FYI

 

OK check out the 351C pistons with a better compression height that would lower your end compression...Or you could look at cams that have a better overlap to lower you compression too.

 

Sorry for busting in with a newbie question, but I'm still trying to learn half as much as everyone else here..

> The Compression Height is 1.670 and the pin dia is .9122 FYI

What do these numbers mean? I understand combustion chamber volume, dished pistons, etc, but I got lost here.

Also, can someone explain or point me to a site that explains how to decihper camshaft numbers? Duration, lift, etc are still a mystery.

Thanks!

 

andym,

Compression height is the distance from the center of the piston pin to the top of the piston (not counting any "dome" that might rise above the piston top). It is important when calculating deck clearance, which is a factor in calculating the compression ratio.

To determine the affect that a specific piston has on compression ratio, you need to know the piston's compression height and head volume. Piston head volume is the volume of any piston top features, such as dish, valve reliefs, or dome. A true flat-top piston has 0cc piston head volume.

For the 351C engine, there are many pistons available with different compression heights to "adjust" the compression ratio. Also, most "stock replacement" pistons have reduced compression height (from the OEM spec) to allow milling the block and cylinder heads when rebuilding an engine without changing the compression ratio.

 

Andym,

Camshaft numbers aren't really that hard to understand. Duration is just that. It is the length of time (or duration), in degrees, that a valve is open. If it says intake duration, then it's an intake valve. There are two kinds of duration - advertised and .050. I am still struggling with this but I believe that advertised is the actual duration of the lobe and .050 is the duration that the valve is lifted .050 or more. Anyways, .050 duration is the important figure. Now lift is simple also. Most camshafts have lifts between .450 and .550 inches. This is the actual distance, in inches that the VALVE is lifted off it's seat. This is called VALVE LIFT or GROSS LIFT. You have to keep in mind however that this figure is not how much the lobe lifts the lifter. The amount the lobe lifts the lifter is known as LOBE LIFT or NET LIFT. The lobe lifts the lifter, pushrod, and rocker arm. The rocker arm pushes the valve down and open. However, most rocker arms are not 1:1 ratio. Most are in the 1.6:1 to 1.8:1 range. So it is possible for you to change your valve lift by changing rocker arms. For example, if you have a camshaft that has .500 GROSS LIFT and you have stock 1.73:1 rocker arms then your NET LIFT would be .289 (.500/1.73=.289). Now if you changed to lower ratio rocker arms, such as 1.7:1, GROSS LIFT would now be .491 (.289*1.7+.491). In this manner, if you have changed your rocker arms from stock ratio, it is possible to have more/less valve lift than advertised by the camshaft manufacturer.

 

The best way would to be to find a good shop and have them turn or mill in a dish, that would lower compresion and improve flame front travel. Problem with this is with flat tops you need to be carful not to make the dish to big, you dont want to significantly weeken your pistons. If you want to go the cheep rout gust use two head gaskets but watch out for pushrod length, If you had a valve job it should have put your valves farther in the hole so you could get away with it but i would look at the tolerance.

-jim