How to write a MAP

Ryanesque · Post by **Ryanesque** » Tue Jun 16, 2009 11:14 pm

So, okay. I got a MAP that i created from Brent's "baseline" MAP and "miniman82's" map (I have a Corvair but it isn't a turbo) and the test run I had lots of power...so far so good.

but I got to thinking...In reality I have NO idea how to "write" a map. i understand that basically you have more advance as the engine goes faster...okay fine...but what does LOAD have to do with advance? I guess i really don't understand how load effects the needs of the engine as far as timing goes. i know what advance IS...i understand compression and strokes and all that pretty well...but I can't understand why the advance would need to be changed based on the load.

Anyone got a simple explanation? (actually, even a technical one wouldn't be beyond me to comprehend, so bring it on.)

Ryan

dave9469 · Post by **dave9469** » Wed Jun 17, 2009 9:21 pm

Hi Ryan

Here is an extract from Dave Andrews web site which I have found very useful. It is based on a distributer setup but translates well to the megajolt setup.

In brief the document describes how fast the fuel burns and this burn time alters with RPM and load so the point at which the spark is needs to fire is different.

Hope the document helps, maybe Brent can extract the useful bits and put it in the wiki?

Dave

Gassed'58 · Post by **Gassed'58** » Wed Jun 17, 2009 10:43 pm

For an interactive pictorial representation of what happens to ignition timing you may want to have a look at the link below, click on the graph to make it work, also a link at bottom of page to represent speed mapping.

http://www.gill.co.uk/products/digital_ ... ad_map.asp

Hope this is of some use.
Geoff

Ryanesque · Post by **Ryanesque** » Thu Jun 18, 2009 6:28 pm

So would I be correct in stating that in basic terms more load equals less advance?

dave9469 · Post by **dave9469** » Thu Jun 18, 2009 8:31 pm

Ryanesque wrote:So would I be correct in stating that in basic terms more load equals less advance?

Correct.

Just remember when you have heard pinking, it's always been at high loads, usually low revs. Basically you want complete burn and maximum cylinder pressure as the crank has just gone through TDC and the pressure is able to push down on the piston. When there is too much advance this pressure is at maximum before the crank has gone through TDC and the piston is still moving up when the cylinder pressure is trying to push it down.