REV limiter is 40 degree retard TOO much?!? Overboost Prote

PrOxLaMuS© · Post by **PrOxLaMuS©** » Mon May 29, 2006 11:21 pm

Hey guys... i have a quick question..

what is the proper spark cut for the soft rev limit retard?? I am using 40 degrees.. which I fear is WAY WAY WAY too retarded after some searching on the forums...
( I am running a inline 6, 2.8L SOHC, non cross flow head and 17psi on a 8.3:1 CR and MegaSquirt n Spark extra 021u and the stock Nissan dizzy)

is 10 degrees adequit?

I noticed when I go into the HARD CUT mode.. the RPM's go from like 6,200 RPM down to almost 4,200 RPM .. its a HUGE RPM loss.. is this correct?

another question is overboost protection..

is this damaging to the engine? during my datalogs when the hard cut kicks in when I overboost.. the MAP spikes up dramatically.. and looks like a MIS FIRE (which it is obviously) I fear this is damaging the engine rather than protecting it

Post by **jsmcortina** » Tue May 30, 2006 2:48 am

Setting 40deg for your soft rev limit is a great way of damaging the engine. Perfect in fact.

40deg is way ADVANCED for full throttle on boost. 10deg is a sensible value to pick. You will have to play around with the cut x sparks ouit of y setting on the hard cut to see what works best for you.

James

A4seen Racer · Post by **A4seen Racer** » Tue May 30, 2006 9:37 am

for the soft rev limit retard it is the actual timing you want for your soft limit right? so if i put 5 in there then my timimg will be at 5 degrees BTC when i hit the soft rev limit rpm?

also I'm a little confused on the hard limit settings. is the base number a timing value?

PrOxLaMuS© · Post by **PrOxLaMuS©** » Tue May 30, 2006 10:44 am

huh?? no the soft rev limit RETARDS timing.. it doesnt ADVANCE timing..

advancing timing under boost = catastrophic engine failure

Post by **jsmcortina** » Tue May 30, 2006 12:13 pm

If you tell it to use 40degrees timing then you have asked for advanced timing. That number is the absolute timing, not a retard from present setting.
Trust me, I helped write it and I use it on my car.

James

JesseT · Post by **JesseT** » Tue May 30, 2006 7:58 pm

This has always puzzled me too. The way it's put out in Megatune makes it hard to judge if it's the absolute value or the amount of retard.

Post by **jsmcortina** » Wed May 31, 2006 3:47 am

I agree that the wording can be confusing - I'd never looked at it that way before.
It should say "soft limit timing (absolute deg)" and will be changed at the next maintenance release of 029q

James

GarageWorks · Post by **GarageWorks** » Wed May 31, 2006 2:41 pm

So if you put 10 in the box for soft rev limit retard, it will make the timing 10 degrees any time the soft limit is activated? Not subtract 10 degrees from the value in your map?

Post by **muythaibxr** » Wed May 31, 2006 2:45 pm

GarageWorks wrote:So if you put 10 in the box for soft rev limit retard, it will make the timing 10 degrees any time the soft limit is activated? Not subtract 10 degrees from the value in your map?

yes.

Jon k · Post by **Jon k** » Wed May 31, 2006 10:16 pm

Yeah because you don't want to just start adding/subtracting timing from whatever the current value is... what if your current value is 5 and you subtract 10? Also, you said you were thinking 40 would subtract 40 degrees... if you are subtracting 40 degrees at your rev limit that means you're hitting 40+ degrees at that rev limit... that seems very bad.

PrOxLaMuS© · Post by **PrOxLaMuS©** » Thu Jun 01, 2006 4:30 pm

Ok so for an example..

the base timing on my motor is 24* BTDC ... general timing of 38* BTDC across before boost hits... and at 5,500 RPM and 17psi the timing drops down to around 21* BTDC...

soo would hitting 5* BTDC be way way too retarded?? or should I drop the timing to ohhh around the base 24* BTDC??

this is very interesting

Jon k · Post by **Jon k** » Thu Jun 01, 2006 10:18 pm

PrOxLaMuS© wrote:Ok so for an example..

the base timing on my motor is 24* BTDC ... general timing of 38* BTDC across before boost hits... and at 5,500 RPM and 17psi the timing drops down to around 21* BTDC...

soo would hitting 5* BTDC be way way too retarded?? or should I drop the timing to ohhh around the base 24* BTDC??

this is very interesting

I am not sure - I will let James answer, but i can tell you that 21 degrees @ 17 psi is a lot of timing even on a low compression motor

PrOxLaMuS© · Post by **PrOxLaMuS©** » Fri Jun 02, 2006 1:43 pm

I'm running on the general rule of thumb.. 1 degree of boost retard for every pound of boost... so I am pulling timing back 17 degrees at 17psi..

my motor is a 8.3:1 and with some wear .. im sure its around 8.1:1 now..

Anyway very very interesting thread!

PSIG · Post by **PSIG** » Fri Jun 02, 2006 3:01 pm

Lance has a useful set of guidelines for initial ignition map settings here http://www.msefi.com/viewtopic.php?t=9941 that may be helpful in sorting your sparks. Be careful using spark retard rev limiting on a turbo as extended periods of hefty retard can overheat the turbine. The shimmering orange glow is pretty, though...

Jon k · Post by **Jon k** » Fri Jun 02, 2006 3:58 pm

PrOxLaMuS© wrote:

my motor is a 8.3:1 and with some wear .. im sure its around 8.1:1 now..

Typically when an engine has wear the compression goes up, not down. as carbon in the cylinder chamber populates space otherwise free. So, I am not too sure how the CR would go down.

PrOxLaMuS© · Post by **PrOxLaMuS©** » Sat Jun 03, 2006 12:59 pm

hmm interesting.. I always assumed that as the rings wore down.. the BLOW -BY would increase over time.. so the pistons can no longer hold as much compression.. therefore reducing the overall compression ratio

i guess a logical assumption but I never really looked into what it does..

Jon k · Post by **Jon k** » Sat Jun 03, 2006 1:19 pm

PrOxLaMuS© wrote:hmm interesting.. I always assumed that as the rings wore down.. the BLOW -BY would increase over time.. so the pistons can no longer hold as much compression.. therefore reducing the overall compression ratio

i guess a logical assumption but I never really looked into what it does..

Well there is a difference between your static compression ratio changing and compression loss. I would really like to think your motor is strong enough that it does not have significant air blow by. Throwing oil into the chamber because of worn oil rings is common, but when motors start losing compression it's generally not a good sign.

Post by **jsmcortina** » Sat Jun 03, 2006 1:19 pm

I would expect the compression to go down too. My engine is nice and clean inside but the pistons slop around in the bores and it breathes heavily. All pots are down on compression.

James

Jon k · Post by **Jon k** » Sat Jun 03, 2006 1:33 pm

jsmcortina wrote:I would expect the compression to go down too. My engine is nice and clean inside but the pistons slop around in the bores and it breathes heavily. All pots are down on compression.

James

I guess it'd be a case of which is greater, ring blow by or carbon build up

PSIG · Post by **PSIG** » Sat Jun 03, 2006 3:53 pm

Jon k wrote:
PrOxLaMuS© wrote: my motor is a 8.3:1 and with some wear .. im sure its around 8.1:1 now..
Typically when an engine has wear the compression goes up, not down. as carbon in the cylinder chamber populates space otherwise free. So, I am not too sure how the CR would go down.

There is some crossover confusion here. The static (or calculated) compression ratio (8.3:1) can be increased by carbon buildup over time, while the dynamic compression can suffer from wear (e.g. - rings) simultaneously.