New engine issues
Here’s a question regarding the on line cranking compression calculators......
Or something you can try when you’ve got the compression tester out.....
"Assuming you have an adjustable valvetrain........ how far can you crack the intake valve off the seat by removing any lash, and going a bit further...... before the compression tester shows “zero” compression?"
For example, if you tag a valve by over reving the engine and tweak it a little...... the compression reading is lower, but unless it’s severely bent, it’s not showing “zero”.
Yet, a bent valve never completely “closes”.
What’s the point of this?
My feeling is........ with cams that have extremely slow closing ramps, some compression starts to build before the valve is actually “seated”.
The valve is closing, the piston is ascending....... and if the piston is displacing more area than can squeeze through the small opening the almost seated valve has provided, some pressure will start to build.
So, if you tested two cams with the same exact actual closing points, but had radically different closing ramps(one really slow vs one that just slams the valve down), they would show different readings on the compression tester.
Although the calculation would say the cranking pressure would be the same.
It’s the same reason that a 292* cam could have 20* of vacuum.
The number of overlap degrees might be large, but the area under the overlap curve is still rather small because the valves aren’t open very far.
This 292/292-112 cam has the same overlap as the MP 484 cam, which is 284/284-108. Both have 68*.
Think that 484 cam would idle at 20” in this 318?
"Assuming you have an adjustable valvetrain........ how far can you crack the intake valve off the seat by removing any lash, and going a bit further...... before the compression tester shows “zero” compression?"
I have done this test and at cranking speed the valves have to be on the seat and closed to begin making pressure.
As you surmised, Mopar cams are still not closed at the advertised numbers, AFAIK the intake is still .008 off the seat.So the actual seat to seat, closed, duration numbers could be quite a bit more and the measured cylinder pressure even lower than the calculators indicate.But it's the best we got.
Furthermore, it may be that at some higher than cranking speed, after the air is steadily moving into the cylinders, the .008 numbers give new an more accurate meaning.
What I mean is this; if the true seat closed to seat closed duration number of say the 284 cam was 300 degrees, and the calculator showed that the Scr needed to be 11.5/1(just guessing) to make decent pressure, and if we then built the engine to that Scr, we might find that at 3000 rpm(again guessing) and under some load,that the engine would be in some incurable detonation with full timing, at anything over say 50% throttle.
So,since the Wallace calculator spits out numbers that many here on FABO have proven to work,it would seem that we can safely ignore those numbers lurking in the shadows. And in fact, it may be best to. Unless of course you are a racer looking for that last little edge,I guess.
the 292/114cam,in at 110,closes the intake to it's adv'd number at 76* ABDC
the 284/108cam,in at 104,closes the intake to it's adv'd number at 66* ABDC
Here is the 292/114 cam in at 110, at 700ft, Ica of 76*
Static compression ratio of 9.0:1.
Effective stroke is 2.27 inches.
Your dynamic compression ratio is 6.48:1 .
Your dynamic cranking pressure is
120.14 PSI.
V/P (Volume to Pressure Index) is
79 ........................... 79
And the 284 offers plus10* of compression; and the new numbers are;
Static compression ratio of 9.0:1.
Effective stroke is 2.52 inches.
Your dynamic compression ratio is 7.08:1 .
Your dynamic cranking pressure is
135.60 PSI.
V/P (Volume to Pressure Index) is
99..................................... 99
this is a very significant pressure rise, translating to a higher VP.
Here is the same 284cam but with compression boosted
Static compression ratio of 10.:1.
Effective stroke is 2.52 inches.
Your dynamic compression ratio is 7.84:1 .
Your dynamic cranking pressure is
155.56 PSI.
V/P (Volume to Pressure Index) is
114 ................................ 114
again more pressure and vp
This is the factory engine, the yardstick to compare to from idle to 3000 or a bit beyond. I used 48* as the ICA and still at 700ft elevation
Static compression ratio of 8:1.
Effective stroke is 2.89 inches.
Your dynamic compression ratio is 7.10:1 .
Your dynamic cranking pressure is
136.12 PSI.
V/P (Volume to Pressure Index) is
113 ............................ 113
as you can see even the factory engine was pretty dismal
here is factory engine with Scr increased, no other changes
Static compression ratio of 9.0:1.
Effective stroke is 2.89 inches.
Your dynamic compression ratio is 7.97:1 .
Your dynamic cranking pressure is
159.01 PSI.
V/P (Volume to Pressure Index) is
132 .................................132
I have operated a 318 like this, and it was very nice
To make that 292 fly,with iron heads
Static compression ratio of 11.2:1.
Effective stroke is 2.27 inches.
Your dynamic compression ratio is 7.98:1 .
Your dynamic cranking pressure is
159.28 PSI.
V/P (Volume to Pressure Index) is
105 ............................. 105
This is as good as it gets, check the VP. This indicates a super-soft bottom end,requiring a clutch or a fairly hi stall TC, PLUS gears, to overcome and make into a streeter.And this at 11.2Scr.
This cam will begin to ramp up after 4500, and in a 318 will probably make full power around 5800. At 11.3Scr it will carry the power(with good heads) well into the mid 6000s, or beyond. It's a great cam if you have the heads for it,and buzz the engine up. However, with 3.55s and a 904, 4500rpm/first gear doesn't come around until 39mph, and the 105VP is just too soft to have fun with...... unless you have a pretty hi stall TC and get around it; or really skinny tires,and get them wound up.
I tried this cam in a 367 cube streeter, at 11.3Scr, and after advancing the cam twice, I yanked it and sold it to a racer. I had a clutch and 3.55s, and they had to stay. The manual trans locks the rpm into the soft zone to about 30 mph, and that was just too soft for me.
Believe it or not, the next cam was a 270/110 (three sizes smaller) and that was heaven.
