Hello all,

I am putting together a 393 build together for my 65 Galaxie, yes I know this is a truck forum, BUT, I have read a number of threads from members on here and well you guys know your stuff!

The plan is to run it on pump gas so I would like to keep the compression down, I am doing a 393 because I got a crank and pistons for FREE. New in box stuff so why not. The Galaxie was a 240 car originally and for an almost 4000 pound car it was underpowered to say the least. I do not plan on racing this car other than to see what it can really do, its mostly just going to be a cruiser and to keep up in traffic. It will eventually get a 4R70W trans to modernize it but I am sticking with a C4 for now.

So far I have selected
93 F4TE block stock
AFR 1351 185cc 64cc (I know these are small for my application but I don't want to be running 11.5:1 and STILL need to buy new pistons)
TFS-51403002 Cam ( I am open to suggestions on this changing this cam my other alternative is a comp 282 series cam but the TFS cam is 250 bucks cheaper with similar specs so)
Pistons I have are speedpro hyper 302 flat tops with 8cc reliefs on them I was looking at some DSS with an 18cc relief forged so I can run a bigger cylinder head but the Promaxx 210s are 56cc chambers which would still put me at a higher compression ratio than I feel comfortable with on the street

So I guess my question is cam related more than anything else. I have read that guys use cams to impact what kind of CR they run but I don't fully understand that concept.

Suggestions and guidance, clarifying questions, and general ridicule are welcome!

thanks!

 

A 393 is a nice easy stroker to build. Depending on things most of the time there's very little grinding that needs to be done on the block. The last one that I did took just a little bit at the oil pump boss. Depending on the pistons I've had to grind a little notch in the skirt to clear the counterweights at BDC before but not a whole lot. That was when I used a stock rebuilder 302 piston that had a pretty long skirt.

The way the camshaft effects the dynamic compression of the engines it easy to understand. Most people think that a cam with more overlap causes the engine to have a lower dynamic compression but this is not true. When the engine reaches BDC on the intake stroke the valve is still open for a while as the piston starts upward on the compression stroke. The later the valve closes the shorter(fewer number of degrees) the compression stroke becomes. A later intake valve closing happens due to a later intake centerline and additional duration.

A cam like you're thinking about using has a 112 lobe separation and it is probably in the engine 4 degrees advanced. This gives you a intake centerline of 108 ATDC. Using the seat timing figures for the cam(usually but not always at .006 tappet lift) you have 286 degrees of total duration. Assuming that the cam is a symmetrical design that means that there are 1/2 of 286 degrees that happen after the ICL so 286/2=143 so 143+108=251. This gives 251 degrees after TDC when the intake valve closes. 251-180=71 degrees ABDC on the compression stroke the intake valve closes. As you can see as the intake centerline increases(gets later after TDC) the intake valve closing point gets later which reduces the compression. Going the other way say down to a 104 intake centerline does the opposite thing with the same duration. This is why cams that are intended to produce low speed torque and work well in low compression applications have durations that are sort and ICL numbers that are low.

 

Almost 20 years ago, I came up with a 393W-stroker build that is now a cataloged item at cnc-motorsports.com.

I used Keith Black 364 22cc dish pistons, 4.030 bore, stock replacement 5.956" 351W rods, Eagle cast 3.85" stroke crank., and 64cc cylinder heads. This gave a 9.3:1 compression ratio.

The camshaft I initially used was a Comp X4262H 'Extreme Energy 4x4 grind combined with stock Cobra 1.7 roller rockers for low end torque in my '3800# '89 Crown Vic with a custom wide-ratio AOD trans and 3.55 rear gears. This actually passed Ohio's emission testing with a 660 vac sec Holley Street Avenger carb! I used that car as a daily driver for several years - even in Cleveland winters!

When this car became emissions-exempt after 25 years old, I upgraded to a Comp XR-276 Hydraulic roller cam, with Harland Sharp 1.7 roller rockers on the ported cast iron 64cc World Windsor Senior heads for the .545/.545 lift. This motor/trans combo propelled that 4000-pounds (with driver) to a 13.05 @ 106.55 at the track, using BBK shorty headers, a Mustnag H-pipe, and 2-1/2" duals!

 

[QUOTE=A cam like you're thinking about using has a 112 lobe separation and it is probably in the engine 4 degrees advanced. This gives you a intake centerline of 108 ATDC. Using the seat timing figures for the cam(usually but not always at .006 tappet lift) you have 286 degrees of total duration. Assuming that the cam is a symmetrical design that means that there are 1/2 of 286 degrees that happen after the ICL so 286/2=143 so 143+108=251. This gives 251 degrees after TDC when the intake valve closes. 251-180=71 degrees ABDC on the compression stroke the intake valve closes. As you can see as the intake centerline increases(gets later after TDC) the intake valve closing point gets later which reduces the compression. Going the other way say down to a 104 intake centerline does the opposite thing with the same duration. This is why cams that are intended to produce low speed torque and work well in low compression applications have durations that are sort and ICL numbers that are low.[/QUOTE]

Thanks for the reply and technical info Dave!

If I understand you correctly the longer the intake valve hangs open the lower the compression since air is given a chance to escape from the chamber prior to full valve closure. How does this relate to the exhaust valve? Both valves are open during this time correct?

 

The intake closing event is very important because once the valve is closed no more air and fuel can enter the chamber. The later it closes the shorter the "compression" stroke becomes. How the engine responds to this all depends on the velocity of the gasses in the intake tract. The higher the velocity the more it has a tendency to continue flowing into the cylinder even though the piston has stopped travelling downward and is now travelling upward in the cylinder. This is one of the ways an engine can achieve a volumetric efficiency higher than 100% without supercharging.

At lower speeds the velocity is not high enough to produce much inertial ramming and it does actually reverse direction and some gets pumped back out into the intake tract. What is cool is that it reduces the cylinder pressure at speeds(lower) where the engine is most sensitive to detonation due to high cylinder pressures. After the speed gets high enough for inertial ramming to start happening in most engines the speed is high enough that detonation isn't much of a problem.

The overlap portion of the camshaft happens at the other end of the intake cycle. The best way to think about it is that the exhaust cycle happens first, then the intake, then compression and then finally the power stroke. Overlap tends to reduce idle quality but it does not tend to reduce low speed cylinder pressures. I know that seems counter intuitive but that's how it works. At low speeds with the throttle closed(manifold vacuum) the flow can actually reverse during overlap and exhaust is pulled into the intake tract which contaminates the charge. As soon as the throttle is opened a little bit and the speed goes up the velocity gets high enough in the exhaust and intake tracts that this effect is mostly eliminated.

My 460 is a good example of how overlap tends to affect the way the engine runs. With the cam on a 107 lobe sep even though it only has 200 at .050 duration the cam has enough overlap that the idle is very slightly rougher than a stock 460. It'll idle hot in gear at about 16 inches at around 800 or so rpm. Just touching the gas pedal and increasing the rpm maybe 50 or 100 and the engine is totally smooth. It is just enough to get the velocity high enough that the lope completely disappears.

 

Thought I would revive this thread and give an update on my progress, which admittedly has been minimal.

I currently have purchased some knock off AFR 205 as cast heads which have upgraded springs rated for .600 lift, 7/16 studs, 58cc chambers.

I am looking at my previously mentioned TFS2 cam(TFS-51403002) to work with these heads. My goal for compression ratio is 10.5:1 to keep it on pump gas.

I am leaning towards a set of DSS forged pistons(DMS-8723-4030W), as I can use my rings that I already have, they have a 13cc dish in them and a compression height of 1.600.

So here is where I am at, IF I understand my engine building math correctly my stock block has a deck clearance height of .022 with the factory 1.774 pistons in it. I am ASSUMING this is true for all 351w blocks.

So if I use the formula stroke/2 + rod length- deck height I should get 3.85/2 + 5.956 - 9.503= 1.622 compression height. This tells me that if I use a piston with a compression height of 1.6 that puts the piston in the hole .022 inches. Is that correct? Or if I used a piston with a 1.608ch that would give me a .014 deck clearance in the hole?

If so that would give me a "larger" chamber and lower my compression or did I mess up the numbers and the two aren't related. Would I need a smaller or thicker gasket to achieve the correct quench? Would I need to 0 deck the block?

Anyone with DD willing to plug in some specs for me and see what I can expect output wise?

Thanks!

 

The compression ratio is the ratio between total volume and unswept volume. So if you add some volume by having more deck clearance you add it in both places and it does reduce your compression ratio. CC's gained in unswept volume reduce compression ratio the same no matter where you get them. In the combustion chamber, deck clearance, gasket volume, valve reliefs etc.