high compression vs low compression

[munky30]

I hear this mentioned a lot...

Early high compression engines, better than late low compression ones...

Could someone please explain why/how?

And tell me if my pre facelift engine is low or high compression (thus giving/denying me some bragging rights and superiority)

[oakey]

pre-facelift m20b25's are high compression so get bragging!
the higher comp engines apparently produce closer to the BMW quoted power figure and are a bit more rev happy. that is all that i know

[Ziggy]

Pre facelift is high comp. In theory, higher CR gives benefits to power & economy. The facelift CR was reduced because of emissions / cat convertor blah...
Most people say that pre-facelifts feel a bit faster because of this, but I've not really done a back-to-back comparison myself.

[harry_p]

higher compression is better for power, but you run closer to knock limits.

[munky30]

pre-facelift m20b25's are high compression so get bragging!

Pre facelift is high comp. In theory, higher CR gives benefits to power & economy. The facelift CR was reduced because of emissions / cat convertor blah...
Most people say that pre-facelifts feel a bit faster because of this, but I've not really done a back-to-back comparison myself.

Ideal, well I have had a post and pre facelift m20b25 in this car, but the post only lasted a day so I dont really have much to go on. This wont stop me claiming my engine is better than others though, it is my god given right.

[pnd]

MORE TORQUE MORE POWER and better fuel economy in my experience. The difference really is obvious ,I think Ant suggested on here that it amounts to about 10 bhp and he should know. The switch was in early 88 I think e plates have both high compression and low compression engines.Chrome bumper saloons have the early engines. Also they seem to respond better to chipping than the later units. I would love a good early car but I foolishly let mine go some years ago and they are so rare in decent shape now.

[munky30]

well my car is post lift, but the engine in it now is pre lift, as I found out to my cost when i tried to attach it to a post lift gearbox.

Best of both worlds... no nasty chrome but the better engine.

I take it bmw quoted the same power and torque figures for both engines even though there is a difference?

Which figures are accurate then? iirc they quote 170bhp ish for the m20b25... is that a figure taken from the early engines and carried over onto the new ones for marketing reasons or are the figures for the early ones closer to 180bph?

[e30_Turbo]

There is talk that a early high comp engine being run on a facelift motronic is even more powerfull, although you need to swap over some hardware to get it working right.

I'm sure there are a few zoners who have done this, maybe they could share there findings??

Mark.

[Templ8e30]

There is talk that a early high comp engine being run on a facelift motronic is even more powerfull, although you need to swap over some hardware to get it working right.

I'm sure there are a few zoners who have done this, maybe they could share there findings??

Mark.

I did such a transplant to Danstable's 325i Touring a couple of years ago.

We fitted a healthy 1986 high compression engine in place of his late low comp engine, (suspected cracked head but it was a split hose ).

We fitted all the moronic bits to the early engine including the standard chip, results were impressive, nice and responsive, good strong power band above 4000rpm and loved to be thrashed

Cheers,

Iain T

[fowler]

well the earlier engines make the 170 bhp qouted by BMW.withe a comp ration of 10.1.1
Post Facelift engines have a comp ratio of about 8.8-1
i have had both engines and out of the 2 i prefer the earlier engine as the give a better response.
best way to get the most out of an early high comp engine is to run it i on post 88 Motronic as the software level is a bit better. but rember to change the crank positioning wheel and sensor as this is diffrerent between the two engines.

[oakey]

I take it bmw quoted the same power and torque figures for both engines even though there is a difference?

yes. i think so

Which figures are accurate then? iirc they quote 170bhp ish for the m20b25... is that a figure taken from the early engines and carried over onto the new ones for marketing reasons or are the figures for the early ones closer to 180bph?

I have an '87 tech1 sport and in the manual it claims 171 bhp. when the engine was standard i had it dynoed at about 165hp after 130 odd thousand miles.
so no. deffinately not closer to 180hp im afraid, it would be nice though

[fowler]

azs fozzy said read my previous post

[Ziggy]

This seems like as good a place to ask as any - has anyone written out a definitive list of what you need to run the later Motronic (1.3?) in an early car with an early engine? If not, would anyone like to?!

[munky30]

well my ego and bragging rights are getting a nice build up from this topic.

Because of me using a late model gearbox on an early engine, I had problems with the loom from the early engine needing sensors in the bellhousing that werent there.

So I swapped the entire engine loom, sensors and front pulley system from the late model engine over onto the early one, ecu included...

List of parts.... some pulleys, an ecu, and lots of wires....

[Morat]

early is better until its time to turbo

[Jhonno]

early is still low enough compression to turbo with good results

[Ant]

I've seen a couple of touings of late, both 89 models with the high comp unit , engine checks out as O.E

Find the engine number and all will be revealed, 84B25E is the 9.75:1 CR variant 84B25K for 8.8:1

Later engines feel lazier imo, I still not 100% convinced on the cam either, early lump has a spikier profile when lined up eye to eye, part number changed too, not seen 100's to compzre to though , so no quoting me later on

make sense, drop the CR and go tamer on the cam, would make the engine feel torquer, and more refined maybe.

[munky30]

errrr.... mine is 256E125544023........

[Turbo-Brown]

I'd imagine that if the low comp cam is a little less agressive than the high comp cam it's to do with trying to increase the dynamic compression of the LC engine.....though that is just a guess

[jonb]

I've seen a couple of touings of late, both 89 models with the high comp unit , engine checks out as O.E

Find the engine number and all will be revealed, 84B25E is the 9.75:1 CR variant 84B25K for 8.8:1

Yes

Ive had loads of later cars with the high comp lump.

Its a myth that all the later ones came with low comp motors. They do seem to go better. Although there is nothing in it in the real world.

[reggid]

both are slow so who cares..............

[slippo]

how can you tell if you have a high comp lump? is there any numbers or anything? mines a chrome bumper cab 88 e reg

[Morat]

Mebbe there were later cars without the "emissions equipped" option and they came with the high comp engine?

Or something.

Damn, I want to check now!

[Ant]

Ant wrote:
I've seen a couple of touings of late, both 89 models with the high comp unit , engine checks out as O.E

Find the engine number and all will be revealed, 84B25E is the 9.75:1 CR variant 84B25K for 8.8:1

the E and K denote the CR, get in them engine bays gents

[munky30]

so does the E in this one denote its a high compression one?

-25----------6----------E---125544023
(2.5)(cylinder count)(?)(serial number)

Thats the only number I saw on the engine and it looks nothing like the ones quoted here

[Speedtouch]

Yes, the E block is high comp, K is the low compression block (suitable for catalytic converter).

[tim-ix]

My German's fairly ropey but wouldn't K stand for Katalysator? Presumably you could specify a cat on your UK supplied car as an option until they became compulsory. Seems logical, but it may be b0***cks.

[Jesus325iTouring]

I had a very early post facelift 325iSE,it was without doubt quicker and more aggresive than my Touring,even now with it's new motor.
I reckon,although it was a post facelift,being an early one,it had the high comp motor in it.

[e30bmlover]

pre-facelift m20b25's are high compression so get bragging!
the higher comp engines apparently produce closer to the BMW quoted power figure and are a bit more rev happy. that is all that i know

yer boi! ahhhhh the b25 pre facelift.... i can almost hear her purr... not long now.......

[e30bmlover]

Because of me using a late model gearbox on an early engine, I had problems with the loom from the early engine needing sensors in the bellhousing that werent there.

will i have this trouble when doing my swap? useing the facelift box and the prefacelift engine?

[gooner1]

Aaaaaah pre facelfts, faster and with lots of loveley chrome, perfect

[munky30]

Because of me using a late model gearbox on an early engine, I had problems with the loom from the early engine needing sensors in the bellhousing that werent there.

will i have this trouble when doing my swap? useing the facelift box and the prefacelift engine?

as far as i know, yes.

I put the engine in, went to start connecting up the loom and had two connectors left over...

Apparently one of them is a spare, the other is for the crank position sensor in the bellhousing on a pre facelift engine. (on the flywheel).

Post lift has the cps on the crank pulley at the front of the engine, so you need to use a post lift loom, crank pulley, cps and cps mount.

[Brianmoooore]

Because of me using a late model gearbox on an early engine, I had problems with the loom from the early engine needing sensors in the bellhousing that werent there.

will i have this trouble when doing my swap? useing the facelift box and the prefacelift engine?

You won't have anywhere for the crank position sensors, so the toothed wheel and management system from the later engine will have to be fitted to the older one. (Which would be a good idea, in any case.)

[munky30]

woo.. I got it right.

[tristan325]

Because of me using a late model gearbox on an early engine, I had problems with the loom from the early engine needing sensors in the bellhousing that werent there.

will i have this trouble when doing my swap? useing the facelift box and the prefacelift engine?

You won't have anywhere for the crank position sensors, so the toothed wheel and management system from the later engine will have to be fitted to the older one. (Which would be a good idea, in any case.)

OK the consensus appears to be that this is a good mod, but Brian can you explain to me why? What do I gain for the fruits of my labour?