motronic 1.3 for 325i which is good with high and low comp

[markus_74]

hello,

need info again, as I build the engine with mixed parts and I don't know it will be high of low comp.

crankshaft from 325i 1989
piston and conrod from 525i 1990
engine block from 520i with bore up.

any info it will be high or low comp?

and I use motronic 1.3 engine loom and management, does it is good or not?

I mean that high and low comp engine which will give good combination and negative combination with motronic 1.3?

regards
markus

[pacerpete]

IIRC, 1990 525i pistons = 8.8 CR.

[Speedtouch]

Yes, Motronic 1.3 is the later and best version to use. It has more resolution (map sites) than the previous versions, and gives an overall more refined control of the engine.

It will probably be the 'medium' compression; early high compressions were 9.7:1, then they went down to 9.4:1 then 8.8:1 for the later low compression engines in around 1992.

[maggspower]

As above motronic 1.3 is the best.

But I agree with Pete, most likely the pistons will be 8.8:1 comp ratio. The 9.4:1 pistons are relitivly rare, they were only around a short time '88-'89 approximately.

[markus_74]

thank you for all that give me info so it is low comp engine.

but which is best to get benefit to combine with motronic 1.3? low comp or high comp engine?

[Speedtouch]

It doesn't really matter; Motronic 1.3 was used with both versions. High compression engines usually give better throttle response, power and economy though.

However, being in Indonesia, it may be better to have a low compression engine if your fuel grade isn't so good...

[markus_74]

It doesn't really matter; Motronic 1.3 was used with both versions. High compression engines usually give better throttle response, power and economy though.

However, being in Indonesia, it may be better to have a low compression engine if your fuel grade isn't so good...

thank you for the info and how much the different in the power? does low comp can produce 171 hp? or far less?

[Speedtouch]

Some low compression engine M20B25s, such as bss325i's, produced the factory 170bhp, but most were around the 160bhp mark.

[markus_74]

Some low compression engine M20B25s, such as bss325i's, produced the factory 170bhp, but most were around the 160bhp mark.

11 bhp is a big different between low and high comp.

there are 3 fuel octan here 88, 92 and 97, and 88 is cheapest .

if I use high comp piston and use 88 fuel it will be detonation and not good for the engine.

but how to increase the power and fuel economy with low comp piston?

[Speedtouch]

AFAIK, the M20 engine needs at least 91 RON fuel to avoid pinking.

A chip will improve the low compression M20B25 considerably.

[markus_74]

AFAIK, the M20 engine needs at least 91 RON fuel to avoid pinking.

A chip will improve the low compression M20B25 considerably.

so if I use high comp what ron should I use? 95? or 91 is OK?

160 bhp with low comp + mild chip 10 bhp?

[Speedtouch]

Yes, 91 RON should be fine. The chip gives typical gains of 8-12bhp, but more importantly and noticeably will be the extra low-down torque, making for easier driveability and typical fuel consumption gains of 2-3mpg when driven sensibly.

[Brianmoooore]

A low comp. M20B25 in good condition should and will produce 171 bhp. The drop in compression ratio was offset by the improvements in the management system that were introduced at the same time.

[markus_74]

thank you for all the info

[reggid]

likely the lower comp engines lose more over time as the CR is lower but a fresh engine produces what BMW claimed which is 168BHP (SAE), 171bhp is the metric hp number (PS), also 125kW but it is the same amount of power just different units.

i would use the 9.7:1 piston if available and premium performance is wanted. they also respond better to a mild cam upgrade due to the better compression.

[Acox99]

On a low comp engine, what is the best way of changing the compression without changing pistons? Decking the block or skimming the head to effectively lower the combustion chamber area.

[Brianmoooore]

Pistons hit the head if you take off anything more than the skimming limit (dimples in the head face).
Replacement piston are the only way practical way to significantly increase CR.