Stroke, Cams, and Timing

[Sonny S.]

Do you find that when using the same cam in different stroke motors (with all other things being equal) that the ignition timing requirements are totally different?

things that make ya go hmmmmm

[crazycalvin]

Does the stroke affect the compression ratio?  I was looking at the SERT files for 103 BB w/ 255 cams and they had -4 degrees timing in some cells!  -4 would end up being 4 degrees after TDC wouldn't it?  Like you said, things that make you go hmmm.  Later, Calvin.

[Scramjet]

Calvin,

The -4 degree spark timing on that table is in an almost unused area; 750-1000 rpm at full throttle.  It is probably so people do not grenade their engine by doing something stupid.  Kind of an error proofing things I suspect.

B

[crazycalvin]

Calvin,

The -4 degree spark timing on that table is in an almost unused area; 750-1000 rpm at full throttle.  It is probably so people do not grenade their engine by doing something stupid.  Kind of an error proofing things I suspect.

B

What do you think of the rest of full throttle timing cells?  Thanks, Calvin.

[ToBeFrank]

The -4 degree spark timing on that table is in an almost unused area; 750-1000 rpm at full throttle.

I used to think that, until I started noticing funny timing values in my logs when hitting the throttle from idle. Turns out the MAP spike is large enough when you first hit the throttle to put you in that part of the map.

[FLTRI]

The -4 degree spark timing on that table is in an almost unused area; 750-1000 rpm at full throttle.

I used to think that, until I started noticing funny timing values in my logs when hitting the throttle from idle. Turns out the MAP spike is large enough when you first hit the throttle to put you in that part of the map.

....Plus the ignition map is not based on throttle position but M.A.P., or load, even though there can be some times the same as TP.

[crazycalvin]

Does that mean that in first gear at 2000 rpm and you go from steady 2000 rpm to instant WOT that you might have a different M.A.P. than steady 2000 rpm in 5th to WOT?  Thanks, Calvin.

[FLTRI]

Does that mean that in first gear at 2000 rpm and you go from steady 2000 rpm to instant WOT that you might have a different M.A.P. than steady 2000 rpm in 5th to WOT?  Thanks, Calvin.

Yessir.

[crazycalvin]

Sweeeeeettttt!  Learn something new every day!  So I guess it would be difficult to relate M.A.P. to throttle positions?  I am trying to visualize what the previously mentioned M.A.P. might be.  I was originally thinking that instant WOT from 2000 RPM would give you maximum M.A.P. regardless of which gear.  Thanks for taking the time to answer my questions, Calvin.

[ToBeFrank]

The -4 degree spark timing on that table is in an almost unused area; 750-1000 rpm at full throttle.

I used to think that, until I started noticing funny timing values in my logs when hitting the throttle from idle. Turns out the MAP spike is large enough when you first hit the throttle to put you in that part of the map.

....Plus the ignition map is not based on throttle position but M.A.P., or load, even though there can be some times the same as TP.

Yep. Going from 0 to 15 TPS right off idle and my MAP jumps to 80 before coming back down.