Ignition timing & compression theory

**NeedOne** · 02-13-2019, 08:22 AM

Hoping some of our gurus can chime in on the relationship between timing & compression

Background is I got a set of 34cc heads for my 200efi, before I go just slapping them on I'd like to know what I'm doing.

As compression is raised, is there a relation to what should be done with timing?
I've read on here it's ok to go to 23 or 25 degrees but is there some logic driving that?

**XstreamVking** · 02-13-2019, 09:40 AM

Not a guru, but I do know it ALL depends on the fuel you are running.

**NeedOne** · 02-13-2019, 10:03 AM

I consider you a guru man. Here's where I'm confused - from what I've read elsewhere, in general, increased compression requires retarring timing. But on S&F I'm reading cut heads and advance timing is common. Are people just saying that's the limit you can push on pump gas? If stock timing and cut heads is safer for the motor I'd rather do that

**XstreamVking** · 02-13-2019, 10:35 AM

Here's some info from Jay Smith, who is a guru....Read through this at least twice. He gives the exact way! You should have the "lost foam" cast heads....http://www.biggerhammer.net/mercury/

I followed his mantra and got 135#'s comp, timing at 25 deg on hottest cyl. and run 93 octane. Also changed the efi box to a 200 promax box.
1998 200 Merc EFI. It runs good and has for years.
Better to learn and know, than to have someone tell you this and that. You have to monitor your own mods with plug checking etc. Ask questions...
there are lots of guys running the same motor that have tons of expertise. I'm a glass guy so not a motor guru.

**FORBESAUTO** · 02-13-2019, 10:46 AM

Added compression will need less lead time from the ignition to prevent detonation. Higher octane fuel burns slower and has a higher flash point than lower octane which allows you to lead it more than lower octane fuel. If you don’t run enough octane for the amount of compression you have it may ignite from the heat generated from the compression without a spark and really cause detonation problems. That in a nutshell is the relationship of fuel/compression and timing (lead time). The 25 degrees you read about is on average the most you want want to run with 140-145 compression and 93 octane fuel without problems. Hope that makes sense and helps tie it all together. There is a million and one things that effect what a motor requires

**NeedOne** · 02-13-2019, 11:01 AM

Awesome fellas thanks a lot. That clears it up. I may follow up with a dumb question on indexing, I'm gonna try to figure it out solo first.

**Merc 2.5** · 02-13-2019, 06:13 PM

Originally Posted by NeedOne

Awesome fellas thanks a lot. That clears it up. I may follow up with a dumb question on indexing, I'm gonna try to figure it out solo first.

Forbes put out a great thread bout indexing. Just fyi.
I also run 140 -145 lbs but only run 21 22* because I haven't indexed , I'll get round to it tho

**NeedOne** · 02-14-2019, 01:02 PM

Yeah I read Forbes' write-up, very good stuff. I'm falling short on the calculation to determine thousands btdc for 23 degrees. Thinking zero cyl 1 and use the scale on the flywheel compared to indicator and just measure it out.

**FORBESAUTO** · 02-14-2019, 01:12 PM

Originally Posted by NeedOne

Yeah I read Forbes' write-up, very good stuff. I'm falling short on the calculation to determine thousands btdc for 23 degrees. Thinking zero cyl 1 and use the scale on the flywheel compared to indicator and just measure it out.

Thats correct, once you’ve determined true TDC, using your TDC gauge (dial indicator), measure the distance the piston travels from TDC to 23 BTDC and that reading will be your reference for each cylinder at 23 degrees. Or you can use the neat software to find out that Chaz shared with me.
http://torqsoft.net/piston-position.html

**Dave Strong** · 02-14-2019, 01:22 PM

34CC heads with 93 run 23deg and should be fine.

Dave

**mitch184** · 02-21-2019, 10:02 PM

As I newbie to 2 strokes in general.... can I throw squish height into this thread and how much it affects the amount of timing you can run or need to run?

and where’s the Forbes writeup

**Motv18** · 02-22-2019, 12:28 AM

Squish height won’t change the cc of the head. Just how the force is applied same with angle. Some can alter hot spot and force retarding due to heat.

Been thinking for a while how to add some engineering knowledge without just adding confusion.

The burn rate rate of the fuel isn’t dependent on aki, it depends on the additives in addition to the main component ( the chemical octane). An example would be iso-octane vs. 100ll both have 100aki but not the same burn rate due to 100ll being a mix of octane, tel, and cheeper versions of 100ll other things like detergent and stuff.

Higher compression benifits in creating a higher consumption rate easily confused with faster burn. It is a faster burn as burn rate is simply a defined rate of burn for chemical description. Confused yet?

faster consumption requires less advance for peak pressure to accour at the right point of piston travel creating maximum down piston acceleration. Without impending the up stroke, or generating knock ( the forcing backward of piston travel).

Lean also consumes faster

when setting final advance ( not max) you will typically be leaner then at other points prior to full throttle. 25* full throttle timing has a max typically close to 30* in the motors richest state. I.e sometimes people get a knock at 3/4 throttle need to retard max timing. Or richen up.

Each fuel and motor combo has a different max advance and any change in fuel would cause knock, as will air temp and current motor conditions.

So yes higher compression requires less advance. Hope this wasn’t to much don’t ask for math formulas I done forgot them just imagine pascals laws, and geometry.

Edit : another discussion is rpm and timing.

**powerabout** · 02-22-2019, 02:04 AM

Originally Posted by FORBESAUTO

Added compression will need less lead time from the ignition to prevent detonation. Higher octane fuel burns slower and has a higher flash point than lower octane which allows you to lead it more than lower octane fuel. If you don’t run enough octane for the amount of compression you have it may ignite from the heat generated from the compression without a spark and really cause detonation problems. That in a nutshell is the relationship of fuel/compression and timing (lead time). The 25 degrees you read about is on average the most you want want to run with 140-145 compression and 93 octane fuel without problems. Hope that makes sense and helps tie it all together. There is a million and one things that effect what a motor requires

Some higher octane burn slow like Avgas, I would be asking my fuel supplier.

**Chaz** · 02-22-2019, 07:53 PM

Originally Posted by powerabout

Some higher octane burn slow like Avgas, I would be asking my fuel supplier.

You got it right when you highlighted ...

Higher octane fuel burns slower
and has a higher flash point than lower octane

This is always the case . And unless you buy in bulk from a known supply or own a test kit. It's better to buy-mix on the side of caution than burn it up.

A little overkill

But you can throw a small cup of this on the concrete , a few seconds later you cant tell where it was. 93 still looks like an oil slick minutes later.
Stoichiometric @ close to 15:1 .. you have to really work at making it hammer.

https://vpracingfuels.com/product/c23/

That in a nutshell is the relationship of fuel/compression and timing (lead time). The 25 degrees you read about is on average the most you want want to run with 140-145 compression and 93 octane fuel without problems. Hope that makes sense and helps tie it all together. There is a million and one things that effect what a motor requires

To the OP .. as stated above, this is an average based on experience.
A ride to the ballpark with no guarantee that you will be drafted to play on a pro team. That will come from your own experience.
The million and one things .. only your motor can tell you. Go to a different gas station that has low volume sales, so the gas might have been sitting for a few weeks. Cool dry air, leave the "fast prop" {tight} on and take 3 of you football player buds fishing .. all make for a perfect storm ..

I also understand the need to understand ..

Put a fresh battery in the calculator and see what Dr. Blair has to say ...

http://dragonfly75.com/motorbike/2StrokeDesign.pdf

**Jay Smith** · 02-23-2019, 08:05 AM

I find that as the outboard folks report I ALWAYS inquire about customer availability of FRESH higher octane super unleaded fuel. If the perspective engine build owner or maching of heads live in a rural area I back a bit off the compression , if the customer lives where there’s lots of higher end autos that cycle lots of super unleaded through the stations it’s fresher and potential top of the octane rating . Working in the fuels division at EXXON for 35 years the fuel experts tell me fuel is at its rated octane only 2000 HOURS from time of catalytic cracking tru tankage , and storage SOO that the reason I strive for a station that’s top brands ( better QC) and over all movements are brisk ..

Timing : The nature of a 2 cycle front half is by nature sloppy and fat with fuel I find a lot more lead timing wakes my engines up BIG time.

Give me me a call and I’ll explane how you can turn your lazy dog into a tiger out of the hole , it’s an easy fix ..

Jay