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DIY Turbo Discussion greddy on a 1.8? homebrew kit?

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Old 04-08-2007, 07:05 AM   #1
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Default stroker crank?

hey does anyone know a good place to get a stroker crank, to make the 1.8 into a 2.0? is there a crank from another "B" engine that will fit but has a longer throw? the only place i've seen one is on flying miata, and they want like 3.5k for the rods, pistons, and crank. just wondering if there were any other options.

on another note, how come people lower the compression of their engines when building a turbo motor? i would think that you would want high compression to make more power, and be able to use a smaller turbo so it spools up sooner. but it seems that everyone in the industry builds a low compression engine for turboing. Theres probably something really simple that i'm missing here but i can't really figure it out.
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Old 04-08-2007, 07:53 AM   #2
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Lower compression prevents detonation...
@ 15psi you add 2x as much air and fuel into the chamber... Don't you guess that it should be lower??
As for stroker cranks... Racing mazda has some too... not as nice as the FM's...
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Old 04-08-2007, 11:11 AM   #3
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as for compression, the trade off for dropping compression is generally always favorable because the amount of boost you can add increases power way more than the loss of compression.
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Old 04-08-2007, 05:14 PM   #4
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that just doesn't make sense to me. greddymx5 i'm talking like say i wanna make 350hp. if i had a high compression engine i wouldn't need to have 15psi to reach my hp goal.

Y8s- isn't adding a turbo effectively increasing compression anyways? if detionation is your limit how can you run more boost by lowering the compression and end up with more hp?

thanks for the links the the other sites. still wondering though if you could swap in like a "B20" crank into the 1.8 miata motor.
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Old 04-08-2007, 05:28 PM   #5
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No matter how hard you compress 6psi from a turbo, you still only have 6psi worth of oxygen to burn. Drop the compression down a little bit and get to 12 or 15 or 18psi and you have 2-3x the oxygen to burn even if you aren't squeezing it quite as hard. Less compression and more boost is the way to get power.

If you could swap a B-series crank into the Miata, FM wouldn't have gone through the trouble to have their cranks custom-made.
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Old 04-08-2007, 05:31 PM   #6
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Quote:
Originally Posted by Savington View Post
No matter how hard you compress 6psi from a turbo, you still only have 6psi worth of oxygen to burn. Drop the compression down a little bit and get to 12 or 15 or 18psi and you have 2-3x the oxygen to burn even if you aren't squeezing it quite as hard. Less compression and more boost is the way to get power.
What?

Higher compression is always better for power and response.
Lower compression is easier to tune and can run on lower octane fuel.
Trade offs.

A well tuned high compression motor will pull away from a well tuned low compression motor. Assuming both are properly fueled.
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Old 04-08-2007, 05:36 PM   #7
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What?

Higher compression is always better for power and response.
Lower compression is easier to tune and can run on lower octane fuel.
Trade offs.

A well tuned high compression motor will pull away from a well tuned low compression motor. Assuming both are properly fueled.
I was talking straight out of my ***, but I still don't believe that higher compression is always better. Take superchargers for E46 M3s, for instance. AA and Dinan both keep stock compression, but EVOsport drops it down and makes like 100 more hp.
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Old 04-08-2007, 05:41 PM   #8
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Quote:
Originally Posted by Savington View Post
I was talking straight out of my ***,
No arguement


Quote:
but I still don't believe that higher compression is always better. Take superchargers for E46 M3s, for instance. AA and Dinan both keep stock compression, but EVOsport drops it down and makes like 100 more hp.
I don't know the first thing about blowing a BMW, but there is more going on than EVO lowering compression to make more power. All other things equal, lower compression means LESS power.
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Old 04-08-2007, 05:54 PM   #9
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want lower compression ... cheapest way possible. Get a thicker head gasket.
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Old 04-08-2007, 06:31 PM   #10
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Ben- you said that you can use lower octane fuel. Thats why probably most people run a lower compression engine. how low do you think you can go? how much power can someone running 8:1 CR or 9:1 make? wouldn't lower compression me you could run less timing advance also? if that is the case how come people don't make 7:1 or 6:1 compression ratios? logically if you did that (destroked the engine) wouldn't you be able to run like 12psi without too much timing advance on pump gas? talkin theortically here now...

sucks about the crank, could you imagine how fun it would be to have a cheap 2.0 miata engine?
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Old 04-08-2007, 06:35 PM   #11
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Quote:
Originally Posted by guitarrc6 View Post
Ben- you said that you can use lower octane fuel. Thats why probably most people run a lower compression engine. how low do you think you can go? how much power can someone running 8:1 CR or 9:1 make? wouldn't lower compression me you could run less timing advance also? if that is the case how come people don't make 7:1 or 6:1 compression ratios? logically if you did that (destroked the engine) wouldn't you be able to run like 12psi without too much timing advance on pump gas? talkin theortically here now...

sucks about the crank, could you imagine how fun it would be to have a cheap 2.0 miata engine?
The toyota supra runs a 8:5:1 CR and they make 1000whp on stock block.
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Old 04-08-2007, 06:50 PM   #12
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Quote:
Originally Posted by Ben View Post
I don't know the first thing about blowing a BMW, but there is more going on than EVO lowering compression to make more power. All other things equal, lower compression means LESS power.
No argument that less compression = less power, but EVOsport is able to CRANK the boost when they dump the compression. Can you explain theoretically why it is better to compress less air harder vs. compressing more air more softly?
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Old 04-08-2007, 07:23 PM   #13
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Originally Posted by PaKMaN View Post
want lower compression ... cheapest way possible. Get a thicker head gasket.
cheapest way is to get two head gaskets and stack em.
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Old 04-08-2007, 07:31 PM   #14
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cheapest way is to get two head gaskets and stack em.
doesn't sound as reliable as one thick gasket... but whatever floats your boat.
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Old 04-08-2007, 08:50 PM   #15
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For some reason I remember that the flame propogation gets affected with higher compression because you are affecting the quench point for the cylinder. This has many advantages, but can be really bad when you start to add boost into the mix. Don't quote me, but I think that it has something to do with this.
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Old 04-08-2007, 08:54 PM   #16
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lower compression + boost = safe power

Higher compression + boost = More power but my homie dangerous gets involved
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Old 04-09-2007, 01:23 AM   #17
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there is a book called Maximum Boost, read it!!
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Old 04-09-2007, 01:38 AM   #18
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put 12-1 pistons in youre car then run 15lbs from a t78- for the win.
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Old 04-09-2007, 01:57 AM   #19
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^ Turbonetics T-106 thumper.
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Old 04-09-2007, 04:57 AM   #20
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Quote:
Originally Posted by turbopezz View Post
put 12-1 pistons in youre car then run 15lbs from a t78- for the win.
Why i'm sure he can go with a GT55-90
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