Thread: need of feedback on compresion

I need to know if any one has tested compresion on a motor with n1 or sr16 cams. Vs kerlford will those kelford lower it by how much it does not have to be so specific

Probably not much help. My old SR16 pistons in a 20ve block pinned the compression tester with N1 cams and my Kelfords.

Does this mean with n1 or kerlford compression. Should stay the same or not

Yeah, you should peg any normal compression tester with or without n1's

I recently compression tested my N1 piston'd motor and I came up with 280ish. This was on a cold motor though..

Your camshaft will not change your compression test readings compared to stock (Edit: much at all). If it did I would not buy the cam.

Ok second edit since I'm anticipating an argument.

The compression result will be impacted by the intake valve closing time between each cam. Nothing else will impact your compression results. The intake cam closing time is a critical feature of a cam with regards to timing since the nominal value will vary with RPM.

That said, it is possible that the aftermarket cam will change this, therefore changing your dynamic compression slightly. Since there is "dwell" time at BDC when the exhaust valve is closing, it will be a very small alteration to your dynamic compression (1%-5%). Looking at Kelford's site the exhaust valve closes around 50deg after bottom dead center (but does not specify if this is high or low profile, I assume high). Not sure if anyone has the specs for stock or N1 cams. If you do, we could find the exact amount of compression ratio you sacrifice or loose between cams, then find the expected pressure reading you should see from the tester. It should be noted that the low lobes will have a much earlier close time because of the low air velocity through the engine, again another reason I say the cam will have nearly no effect.

This is why most say to look for consistency between the cylinders when doing compression tests, as the exact compression ratio is usually not known.

Walker Clark

n1 cams
Intake Low: Opens at 0deg before TDC, Closes at 40deg After BDC
Intake High: Opens at 40deg before TDC, Closes at 68deg After BDC
Exhaust Low: Opens at 64deg before BDC, Closes at 0deg After TDC
Exhaust High: Opens at 74deg before BDC, Closes at 34deg After TDC

Dudeman, your psuedo-engineering explaination is highly entertaining, lol.

Since when does the exhaust valves "dwell" around BDC (bottom dead center) or close 50* after BDC? No piston engine in the world does this. Learn how to read a cam card. Here's a hint, the exhaust valves close around tdc. Ever heard of the term overlap?

You can't figure out the cranking compression # by looking at the big lobe #. Cranking rpm is 300 rpm. You have to look at the small lobe. The big lobe kicks in above 5,000 rpm. If you can crank the engine above 5,000 then the big lobe # becomes relevant.

You wrote:

"The compression result will be impacted by the intake valve closing time between each cam. Nothing else will impact your compression results."

"It should be noted that the low lobes will have a much earlier close time because of the low air velocity through the engine, again another reason I say the cam will have nearly no effect."

Contradicting yourself.

Guys i just want to know if with kerlford vs n1 the compression with a tester stays the same or lowers

Mr. "5speed".
I did have a gross technical error in my post and I am thankful you called me out on it since it needs to be explained for everyone to understand my answer.
That said, glad I was able to entertain you, your not the first to find my writing so incorrect it must be a joke to you. Take it or leave it.



I had mistakenly placed exhaust valve twice where I meant to write intake, that was my only error; my apologies. For the record the exhaust valve will never impact the compression in any way, nor will overlap. I am not referring to dwell of the valve (which does exist FYI), but dwell of a piston. A brief explanation: As the piston reaches TDC and BDC the piston will dwell due to the obliquity of the rod and crankshaft. This is to say, the speed of the piston's linear travel will greatly reduce near TDC and BDC. This directly impacts why you would leave your valve open through this time and nearly hundreds of other things I could probably "make up" for you but I won't.

To make my error completely clear here, check out happyharrysco1's post above yours:



See the bolded text above. The N1 cam closes the intake valve 40deg past BDC as the piston is travelling up. This is what we are concerned with.

Since the intake valve is open during the upward motion of the piston, you get dynamic loss of compression because the valve is open and the air inside the cylinder is able to escape (no pressure is built). This is dynamic compression and is defined when the valve actually fully closes. From that point to TDC, you measure your compression. The later the intake valve closes, the more static compression you sacrifice.



Correct. Thus my comment: "It should be noted that the low lobes will have a much earlier close time because of the low air velocity through the engine, again another reason I say the cam will have nearly no effect."

In my quote above I was referring to the fact that under cranking the low lobe is active (I assumed everyone knew this). I went on to say, that by looking at the Kelford cam, (which does not specify low vs. high lobe intake valve close time that I could find) which reads 50deg ABDC (that's AFTER BDC) it is safe to assume the low lobe closes sooner (aka 40deg ABDC like the...N1 cam does). My air velocity comment is prudent because this is the primary reason to keep the inlet valve open past BDC (most don't know why you would do this). To take advantage of intake air inertia and pack the cylinder, as well as the increased density of the harmonic effect if the inlet was tuned (which next to no SR's are).



To further explain the two quotes you have.

The first quote is saying that since the intake closing time determines our dynamic compression; if you are to compare the two cams (N1 vs. Kelford) that the change in your pressure reading on the compression tester will be different between the two due to this difference (which I doubt there is one).

Funny how one word can make such a difference. Hopefully this is understandable for the OP now.

And to happyharrysco1: Thanks for the data man.

Walker Clark