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Old 11-07-2017, 01:39 AM   #1
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Default M20B25 compression without opening the engine?

Hi guys, is there any way to know the compression ratio of the engine, without opening it?

My car was manufactured on nov 1988, its a euro 525i, no cat. Thanks for any help.


VIN info:

Vehicle Identification Number WBAHC210904237760
Type HC21
Model 525i - EUR
Development Code E34
Chassis LIM
Steering LL
Doors 4
Engine M20
Displacement 2.50
Power 125
Drivetrain HECK
Transmission AUT
Color Royalblau Metallic - 198
Upholstery Indigo Leder - 0293
Production Plant
Production Date 1988-11-14

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Old 11-07-2017, 02:32 PM   #2
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8.8:1 ish
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Old 11-07-2017, 05:42 PM   #3
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is there any way to be sure? im in doubt since this is an euro non cat car.

i got a chart for the different pistons for the B25, and it says that for 87-90 the CR was 8.8:1. but all of those had a cat.

then it says that for 87-88 there were some 9.4:1 cars, but it doesnt specify anything else.

and finally for 85-89 (i guess those are E30 years aswell) for the Euro i cars, they had 9.7:1

So almost a whole point difference between 8.8 and 9.7. i really hope its the 8.8 since i want to boost it lightly, and i would like to avoid having to open it and put a thicker gasket in there. Since i recently refurbished the head and put a new gasket...
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Old 11-08-2017, 12:03 AM   #4
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if you're only boosting it lightly on a street car I wouldn't worry too much.
put some ARP studs in if you can. Years ago we used to boost the
20v silvertops to 10psi and they run a 10.5 comp on a standard gasket and bolts.

at least being cat free it can shed the heat a bit easier. Not like it will be on full boost constantly.
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Old 11-08-2017, 02:18 PM   #5
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Though I highly doubt you have a 9.7:1 b25, even if you do it will be fine for boost. It's all in the tune. As far as I'm aware all car fitted with the 885 head were 8.8:1.
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Old 11-09-2017, 09:32 AM   #6
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Quote:
Originally Posted by Coyote View Post
is there any way to be sure? im in doubt since this is an euro non cat car.

i got a chart for the different pistons for the B25, and it says that for 87-90 the CR was 8.8:1. but all of those had a cat.

then it says that for 87-88 there were some 9.4:1 cars, but it doesnt specify anything else.

and finally for 85-89 (i guess those are E30 years aswell) for the Euro i cars, they had 9.7:1

So almost a whole point difference between 8.8 and 9.7. i really hope its the 8.8 since i want to boost it lightly, and i would like to avoid having to open it and put a thicker gasket in there. Since i recently refurbished the head and put a new gasket...
Even at 9.7:1 you are still in good shape for boost. That is still rather low compression.
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Old 11-09-2017, 03:46 PM   #7
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Can you figure out where the intake valve closes in relation to crank shaft angle?

This DCR calculator will give you a rough cranking compression number. This number changes almost 20psi changing from 8.8:1 to 9.7:1 all other variables the same.
http://www.wallaceracing.com/dynamic-cr.php

What does an 8.8:1 m20 make during a comp test, roughly 150psi? Could be a good ballpark indicator...
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Old 11-13-2017, 02:24 AM   #8
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Quote:
Originally Posted by 325ftmfw View Post
if you're only boosting it lightly on a street car I wouldn't worry too much.
put some ARP studs in if you can. Years ago we used to boost the
20v silvertops to 10psi and they run a 10.5 comp on a standard gasket and bolts.

at least being cat free it can shed the heat a bit easier. Not like it will be on full boost constantly.
Yeah already got a set of ARP studs. Still not sure if i will go with a MLS gasket, or just keep the stock one as a fuse. Im not aiming for high boost anyway, likely something under 10psi.

Quote:
Originally Posted by M20_Fever View Post
Though I highly doubt you have a 9.7:1 b25, even if you do it will be fine for boost. It's all in the tune. As far as I'm aware all car fitted with the 885 head were 8.8:1.
As per the guys in the E34 forums, it should be a 9.7:1 b25 since its a 88 euro non cat car.

Quote:
Originally Posted by travish325 View Post
Even at 9.7:1 you are still in good shape for boost. That is still rather low compression.
I hope so, im not aiming for high numbers anyway. Just get into V8 hp numbers. 280-300hp. truth be told ... this is just my weekend car (sort of a daily, but i work from home so i dont use a car during the week). i shouldnt be boosting it ... but i just cant keep it stock :P

Quote:
Originally Posted by So Live View Post
Can you figure out where the intake valve closes in relation to crank shaft angle?

This DCR calculator will give you a rough cranking compression number. This number changes almost 20psi changing from 8.8:1 to 9.7:1 all other variables the same.
http://www.wallaceracing.com/dynamic-cr.php

What does an 8.8:1 m20 make during a comp test, roughly 150psi? Could be a good ballpark indicator...
I think we measured about 160-165psi during cranking. i will check that calculator, thx.
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Last edited by Coyote : 11-13-2017 at 02:44 AM.
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Old 11-13-2017, 03:18 AM   #9
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def a great idea on using the stock gasket on these engines. Better a gasket going than pistons.
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Old 11-15-2017, 02:48 AM   #10
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yeah, i always like to leave some cheap fuse instead of the expensive parts. shit happens.
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