I just bought a 79 F250 2x4 with a 460 in it (going to swap the 400m engine in my other 79 F250 4x4 for it). No history on the truck, engine looks fairly new and it has a Edlebrock Performer Manifold & Carter 4V (not sure what size.), new Distributer. Compression test reveals 100, 110, 120. No increase after the third compression stroke and no difference when doing a wet test. This truck really hauls, feels like twice the torque and power my 400M ever had! Questions I have:
1) Any way to determine CR based on compression test?
2) Any way to determine if this engine has an aftermarket cam w/o tearing it down?

 

If the exhaust has a lope to it it may have a aftermarket cam.

 

Thanks for the reply, it doesn't have much of a lope so I think I'll order the cam (and headders & timing set) I want and change it while I have the engine out. Does anyone have any # on their compression test? This engine is really sweet but 120 seems a little low?

 

79's I think were 8.0:1.

 

The compression test done on the engine in my dads race truck broke a 300 psi gauge, but it figures out to over 14:1.

 

Yeah, now that's some serious CR!, but what I want to know is it seems like there should be some relationship between Compression Ratios and the compression #'s on the gauge. Is 120 lbs. normal for a 8.0:1? My stock worn out 400m has 145 lbs. but doesn't even begin to compare with this BADDOG in terms of power & torque.

 

Because of the small cam, the compression ratio, as tested, will be higher in the 400. And you can't really compare a 4 bbl. 460, to a worn out, 2 bbl. 400.

 

Your'e right, there is no comparison. I have a lot to learn I'll admit that but how does the size of a cam make any difference in the compression test #'s? If both valves are closed and piston comes up, you get your reading, right?

 

Go to www.iskycams.com, and read all of the tech articles section, and it will give you a lot of info. It will explain everything.

 

Well that does explain alot, thanks. So as I understand now, you can't really know your CR based on the test #'s, at least on a modified engine. If I could compare my 120lbs. to somebodys stock compression test lbs. I might be able to determine if this engine has an after market cam. It does have a slight lope to it but I thought it may be the idle setting. Does anyone know what is a typical compression reading for a stock late 70's 460? Also, if I measure the distance the intake & exhaust valve stems move would this be the same distance as the cam lift? If so, I could compare it to some stats from CompCams. I'm trying to figure out what I have without tearing it apart because it looks and feels like a new motor, I'm going to swap it into my other truck and want to upgrade it (if it's not allready) while I have it out.

 

you can measure the lift of the cam lobe two ways, if you use a dial and measure the va;ve stem movement you ned to know the rocker arm ratio as that multiplies the lobe lift. stock is 1.6:1 and most aftermarket are 1.7:1. simply divide the valve measurment by the appropriate ratio. or directly measure the lift of the cam with a dial gauge that can reach the cam, comp sells a tool just for that. however i do believe for any of these measurements to be accurate you'll need a solid lifter in place of a hydrolic one. so if there hydrolic then it's not accurate. and when measuring the valve lift, the dial needs to be inline with the valve or the geometry will effect the reading.

so i guess its not that simple, and there is a slight possibility of having to remove the intake manifold. most times people by the edelbrock package; intake, cam, lifters. as a apposed to seperate peices.

 

I disagree, the rocker arm ratio of a 351C/M/400/429/460 have a factory ratio of 1.73:1, not 1.6. 1.6 is a popular ratio for small block(289,302,351W) Fords and (350) Chevy's.

 

And please don't call it a 400M. There is no such motor. The M designation is for the 351M only.

Put a Crane 268H cam in that 400 and I bet it would come close to the 460.

 

Agreed. Put a decent cam and even an SP2P with a 500 Carter, and your well over the 300 hp mark. Increase the comp. by a point or 2, and your over 365 easy.

 

oops sorry, your right the stock ratio is 1.73:1. i confused it with the 351c i'm also dealing with.