Don Terrill’s Speed-Talk

On several boards there are discussions about setting cam timing to maximize cranking compression pressure. Supposedly it may have originated in Hot Rod or Circle Track magazine.

I was thinking if the cam had was ground with any high rpm capabilities at all, using such a test might probably lead to installing it several degrees advanced to make the intake close earlier than intended by the cam designer.

Am I missing somethig?

thanks

Dan T

Are the pressure traces from the RPM of concern or just cranking?

I believe Just cranking, as described in the posts.
" (with a good battery) spin the crank and check the cylinder pressure with a good compression gauge. "
Some say go for max, some say 200-220 is OK.

I have not found the magazine article yet

Maximum CCP will always occur with the cam advanced until the intake valve hits the piston.
Also known as "a really stupid thing to do".

A great example of "if it's on the internet it must be true"...

Exactly! If what you want is maximum cylinder filling at 150 RPM, then just keep advancing the cam until intake P/V = zero, or IVC is at BDC, which ever comes first.

If on the other hand, you'd like to pump up the cylinder pressures at idle speed or higher.... Fuggediboutit

Dan Timberlake wrote:I believe Just cranking, as described in the posts.
" (with a good battery) spin the crank and check the cylinder pressure with a good compression gauge. "
Some say go for max, some say 200-220 is OK.

I have not found the magazine article yet

You might have to look back pretty far. I first read that when I was in jr high school and they still sold leaded pump gas.

panic wrote:Maximum CCP will always occur with the cam advanced until the intake valve hits the piston.
Also known as "a really stupid thing to do".

A great example of "if it's on the internet it must be true"...

Have you tried this? I did this years ago trying to maximize a cam I had and had no money to replace. I tried different cam positions and valve lash until I got the maximim cranking pressure. Advancing the cam did not develop more cranking pressure. I ended up retarded a few degrees and with a little additional valve lash. I tried running it that way, and the car actually picked up a bit. By no means am I saying this method works.....it just worked once for me. That's hardly proving it out. The cam I have in the engine right now was installed incorrectly (not by me) and several degrees retarded. When I advanced it to where the cam card called for it to be, it showed a lot of increase in cranking compression. I'm just not sure we can conclude that cranking compression always increases with advancing the cam indefinately.

if you visualize what's actually happening in this scenario, it's not possible to conclude that the proposed process can be beneficial:
1. At cranking speeds, there can be no pulse/ram/inertial tuning effects at work.
2. Therefore the compression process cannot begin until the intake valve closes.
3. A well set up typical production engine's intake valve might close at say 30°* after bottom dead center, thus generating compression over almost the full piston stroke, so an engine with a 10:1 CR might crank at 170 psi.* (*Hypothetical examples; I'm not doing the math for actual values.)
4.The cranking compression would increase as the cam was advanced, thus closing the valve sooner after BDC and so giving a longer compression stroke, until the effective valve closing was at BDC (i.e., 30°advanced) and the CC reached perhaps 190 psi, at which point further advance would slowly start to reduce CC. At this point, the carefully selected position of every valve event has been subverted to the single irrelevant goal of maximum cranking compression.
5. For best dynamic cylinder filling at high RPM, a radical race engine might have the valve closing at 100° or more after BDC, so that even with 16:1 CR, the compression stroke is so short that CC is still only ~ 170 psi. Ignoring piston to valve interference issues, if you started to advance this cam, cranking compression would rise to probably 450 psi or more as the cam was retarded 100° What are the odds the engine could even be cranked, much less perform better?

Ive cranked many n/a tractor pulling engines that do not exceed 3000 rpms thats cylinder pressure at cranking speed would peg a 350 psi automotive compre ssion tester.ps this is the reason I posted thread about dcr. With a dcr that high benefit from more cam with such a low rpm . Not changing subject but feel post is along with dcr post.

A give cranking compression is a dance between IVC and CR.

Stan

Stan, can you provide a similar graph that shows the cranking psi on the right & each trace would be a different compression ratio?

panic wrote:Maximum CCP will always occur with the cam advanced until the intake valve hits the piston.
Also known as "a really stupid thing to do".

A great example of "if it's on the internet it must be true"...

panic wrote:Maximum CCP will always occur with the cam advanced until the intake valve hits the piston.
Also known as "a really stupid thing to do".

A great example of "if it's on the internet it must be true"...

This fairy tale pre-dates the internet by a couple decades. I was hearing this one in the seventies. Rates right up there with the "carb formula"...

I think it appeared (again) within the last few months as an editorial ""'tip" in the print version of HotRod or Car Craft

I think its in isky tech papers maybe