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Old 01-10-2017, 03:31 PM   #1
Sykohtic
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Compression Ratios

Hey everyone,

I'm not really sure how I would search this without pouring through hundreds of threads, so I'll just ask here.

I'm building a stroker setup off of the M52 crank, eta rod, and early B25 piston combo (2.8L). I have read that this should put compression around 9.5:1, and with shaving the block, around 10:1. The car will serve as a daily when complete until my E36 is ready to take the job back, then the E30 will again become dual purpose weekend warrior/mountain carver and track car.

I plan on staying NA for now. I would probably choose a different piston and cam setup for if I ever supercharge it like I one day would like to. I know I can run meth injection to compensate for knock on a higher compression motor, but as it will be a daily I'm nervous about having to supply the car with meth to get from point A to B. If you have reassurances on running meth injection on a car thats going to operate between 3-5 days a week, please let me know.

So what is really the ceiling on compression ratio in this situation, like running on 93 octane? the 10:1? Or could I get custom pistons and shoot for a bit higher like 10.5-11:1 and be safe? I ask this obviously not knowing a massive amount about the subject, so the more you can do to help me learn (even turning me onto the appropriate literature) would be great.

Some more details: I plan to run a 284 regrind (Bimmerheads), Ebay headers, megasquirt, and either a custom intake or ITBs if I have the budget (ITBs being the best option obviously). Planning for +1 mm oversized intake valves, and full lightening and balancing of the internals.
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Old 01-10-2017, 09:55 PM   #2
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With 93 octane ...

10:1 is safe and gives you a decent tuning window. You can go higher, but your target window gets exponentially smaller.

Please read these two very recent threads involving very similar builds.
http://www.r3vlimited.com/board/showthread.php?t=400437
http://www.r3vlimited.com/board/showthread.php?t=400602
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Old 01-10-2017, 10:02 PM   #3
ThatOneEuroE30
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Personally 10:1 is max on pump. I will be running 12:1 on my race engine for my 2002 which is m10 and will be using 110 octane.
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Old 01-10-2017, 11:23 PM   #4
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My take on this is don’t sweat over compression too much if its street car. as others have said 10:1 is good choice for a mild cam

Note that removing material from block or using custom thickness head gasket to increase/maximize compression with whatever pistons you have also tightens up the quench/squish clearance with OEM style dome pistons which is a good thing for combustion quality and reducing knock regardless of the choice of CR.

http://www.hotrod.com/articles/0311em-power-squeeze/

http://www.hotrod.com/articles/0606e...ression-ratio/
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Old 01-11-2017, 07:58 AM   #5
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Quote:
Originally Posted by digger View Post
My take on this is donít sweat over compression too much if its street car. as others have said 10:1 is good choice for a mild cam

Note that removing material from block or using custom thickness head gasket to increase/maximize compression with whatever pistons you have also tightens up the quench/squish clearance with OEM style dome pistons which is a good thing for combustion quality and reducing knock regardless of the choice of CR.

http://www.hotrod.com/articles/0311em-power-squeeze/

http://www.hotrod.com/articles/0606e...ression-ratio/
great articles, thanks for the link. The first is a gold mine. I always was wondering about how much compression affected detonation. Here are a few quotes from the article. Its too bad some of the pictures don't work, it turns out the article is 14 years old.

Quote:
For every octane number less than 93, the compression pressure needs to be about 5 psi less to avoid detonation under normal circumstances.
Quote:
In practice we find that for an engine on the edge of detonation, every 8-10 degrees F reduction in intake temperature is equivalent to adding one more octane number to the fuel.
Quote:
Most stock engines of the post í70s emission era are on the edge of detonation and run with water temperatures in the 200-210 range. While this may be good for lower emissions, it is not good for power or staving off detonation. For a high-performance street machine running service station fuel, water temperatures of 170-180 deliver a couple percent more power and the ability to use about a quarter of a ratio higher for every 10 degrees reduction in water temperature. Your first move toward utilizing higher CRs then is to use a 170-180 degree thermostat.
Quote:
One of the greatest sources of intake charge heating is the intake valve. Because of its significantly greater area, this can absorb more heat during the combustion and power stroke than the exhaust valve. The reason it does not get so hot is that it is cooled by the intake charge, which is just what we donít want.
Quote:
15 COMPRESSION BOOSTING MOVES

1. Feed cold air to the induction
2. Keep water as cool as possible (170 F or less)
3. Keep the air cool in the intake ports
4. Put a heat-reflective shine on the outside of the intake manifold
5. Minimize heat transfer through the common exhaust/intake port wall
6. Keep fuel temperatures down (cool can)
7. Run with plugs a little colder than the minimum required
8. Use an ignition system that is gross overkill
9. Utilize as large a spark plug gap as possible
10. Use no more ignition advance than is necessary
11. Maximize quench action
12. Minimize head chamber volume
13. Use flat-top pistons if possible
14. Minimize under-hood exhaust heatĖuse coated headers
15. Do not ram in but vent out hot air through hood vents
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Old 01-11-2017, 10:28 AM   #6
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Originally Posted by ThatOneEuroE30 View Post
Personally 10:1 is max on pump. I will be running 12:1 on my race engine for my 2002 which is m10 and will be using 110 octane.
Are you saying this for an m20 (or other BMW), or just because of octane?
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Old 01-11-2017, 11:12 AM   #7
ThatOneEuroE30
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Compression Ratios

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Originally Posted by ForcedFirebird View Post
Are you saying this for an m20 (or other BMW), or just because of octane?


Was mainly talking about m20 but have noticed its a good rule of thumb to follow for the older BMW engines. Although i feel like I've read the s38/14 can be taken higher like 10:5:1 and 11:1 but I'm not 100% would have to research it again.

Edit and by older engine i mean 91 and before. I dont touch 24v unless s38. Have yet to tinker and find out what my m42 to handle
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Old 01-11-2017, 01:17 PM   #8
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Quote:
Originally Posted by ThatOneEuroE30 View Post
Was mainly talking about m20 but have noticed its a good rule of thumb to follow for the older BMW engines. Although i feel like I've read the s38/14 can be taken higher like 10:5:1 and 11:1 but I'm not 100% would have to research it again.

Edit and by older engine i mean 91 and before. I dont touch 24v unless s38. Have yet to tinker and find out what my m42 to handle
I'm not sure I agree, but also have not pushed the limit to have proof. Seeing how detonation-resistant they are when doing turbo builds, I have a feeling they would do fine with high compression. The Euro eta has decently compression, but it uses the smaller heart shaped chambers and flat pistons - we have pushed almost 14:1/93oct on a setup like that in a GM car with a large cam (that also has flat pistons and heart chambers).

Curious to see what the domed pistons and angled squish band could take. Morerevsm3 did a 14:1 m20 with side draft carbs, but don't remember what fuel they were running.

I read an article a long time ago where the "baby six" had a German nickname that loosely translated to "resistant to detonation". Tried looking for it recently online, but haven't had any luck. Seems as not everything that was on paper made it to the internet lol.
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Old 01-11-2017, 01:48 PM   #9
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Originally Posted by ForcedFirebird View Post
I'm not sure I agree, but also have not pushed the limit to have proof. Seeing how detonation-resistant they are when doing turbo builds, I have a feeling they would do fine with high compression. The Euro eta has decently compression, but it uses the smaller heart shaped chambers and flat pistons - we have pushed almost 14:1/93oct on a setup like that in a GM car with a large cam (that also has flat pistons and heart chambers).

Curious to see what the domed pistons and angled squish band could take. Morerevsm3 did a 14:1 m20 with side draft carbs, but don't remember what fuel they were running.

I read an article a long time ago where the "baby six" had a German nickname that loosely translated to "resistant to detonation". Tried looking for it recently online, but haven't had any luck. Seems as not everything that was on paper made it to the internet lol.


That could be true. And like you said you've never pushed it to find out and not many people want to nor do they have the pockets too. I know domestics can be ran at crazy compressions on pump gas
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Old 01-11-2017, 03:15 PM   #10
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I know domestics can be ran at crazy compressions on pump gas
Not necessarily. Some of the later model stuff can. Once domestics figured out how to use fast burn chambers, they could with large cams.
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Old 01-11-2017, 03:50 PM   #11
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Quote:
Originally Posted by ForcedFirebird View Post

Curious to see what the domed pistons and angled squish band could take. Morerevsm3 did a 14:1 m20 with side draft carbs, but don't remember what fuel they were running.
They ran E85
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Old 01-12-2017, 07:59 AM   #12
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Thanks everyone, this is great information. Thanks digger for the links, I'll invest some time into reading them, just by glancing over the quotes they seem like they are indeed a gold mine of information. It's definitely already turned me onto the fact that I really need to explore certain options in the cooling system I hadn't considered before to get the most of any upgrades I do.

The M20s resistance to detonation I'm really interested in hearing more about. I probably will end up playing it safe at 10:1, but if there's a chance to go higher and stay reliable then I'm all for it. I want this thing to be able to drive around the country reliably, which is also why I'm hesitant to make it rely on E85 or Meth to do so. Those aren't super easy to get everywhere lol
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Old 01-12-2017, 08:25 AM   #13
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I always understood that because of the effects of dynamic compression ratio, the static compression ratio should go with your cam choice. With a milder cam, you want a lower static ratio (because the dynamic ratio will be higher), whereas with an aggressive cam, you want a higher static ratio. For example, you wouldn't want a 12:1 ratio with a regrind 272 cam, and 10:1 would probably be too low for a Schrick 304 (and even a 288). This would be due to the increased overlap between the intake/exhaust lobes, which means at low engine speeds a bigger portion of the VE charge simply leaves the exhaust, effectively lowering the dynamic ratio. Basically, it isn't really a static value.

As long as you are using premium, I don't see a point or two of octane making a huge difference on an M20 - in my experience, the M20 will make peak torque well before knock threshold. The other common assumption about tuning is you keep increasing ignition advance until it knocks and then back off, but that's not necessarily true either (unless the engine is knock limited). It's perfectly common that peak torque will be reached well before the knock threshold and that any ignition advance beyond that point will not produce more torque (but gets you closer to the knock threshold).

The one time I did have knock on my 2.8 is because I had the dwell set too high on my D585 coils and they were autofiring about 8 degrees earlier than they should have been, but that's more of a hardware/software issue and it was simple to address. Other than that, I found MBT on my M20 to be significantly before the knock threshold with 92 octane pump gas. I don't see it changing too much if I had a higher CR - because I would also match that with a cam with more overlap/duration.
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Old 01-12-2017, 11:47 AM   #14
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Nanado, you are correct about dynamic compression. When we design cams, we use Dynomation to calculate trapped mass. Keeping trapped mass up (ie dynamic compression), but not to the point of pre-ignition, the cam can be optimized for a particular engine. Then from there we can use the lift limitations (and/or racing rules) to choose a lift.

This is much easier on pushrod engines since they are much more forgiving of ramp rates. The inherent design of the m20 limits us, and you can see what happens when converting to roller cam and going with more aggressive grinds. It's nice to have the software and digital copies of over 5,000 cam lobes - several of them not open to the public. Too bad Comp Cams doesn't do m20's :/

Yes, the m20 doesn't need a great deal of timing for a low compression engine of the 80's. We found that 24-27* is ideal for most engines, a couple builds went closer to 30, but not common.
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Old 01-12-2017, 11:52 AM   #15
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It's the "fast burn" of the M20 combustion chamber that helps.

on the subject of the M20 head - I was looking at flow charts of the N54. Which in stock condition, flows about as well as a stock 885. Not so great for a modern 4 valve head. Even ported, it barely matches a worked 885.
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