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aventari

Detonation w10:1 Compression and QSC pistons

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(edited)

https://www.qscusa.com/collections/bmw/products/bmw-pistons-set-90mm

 

I'm rebuilding the stock M10 in my 71 2002. It has a 121 head and flat top pistons right now and it needs an overbore. I guess these would yield a 10:1 compression, but wouldn't I have detonation issues with our crappy 91 octane California gas?

 

 

I just have a stock distributor and crappy 1bbl Solex carb. I'm not looking for performance, just good drivability. I'm not familiar with this motor's detonation tolerance, but on my 8v VWs 10:1 is too high on a 2 valve/cyl motor without a knock sensor ignition and good fuel control.

 

Edited by aventari

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QSC currently only sell pistons for the e12 head so they're not an option for you.

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(edited)

10:1 on CA 91 gas shouldn't have detonation issues

Edited by rcf925

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9 hours ago, aventari said:

I'm rebuilding the stock M10 in my 71 2002. It has a 121 head and flat top pistons right now and it needs an overbore. I guess these would yield a 10:1 compression, but wouldn't I have detonation issues with our crappy 91 octane California gas?

 

And, keep in mind - the "10:1 compression piston" is an 'estimate'. They don't know the volume of your head chamber (typically smaller than stock if it's been surfaced), how much the block has been surfaced, etc. That said, choice of camshaft overlap can make 10:1 build more friendly. -KB

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Cool thanks for the advice, where can I find 9.5:1 pistons? (affordably)

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Ireland Engineering used to have 9.5:1 forged pistons  They were Mahle knockoffs  but I think they had problems with the supplier and no longer offer them. They offer custom made forged pistons for $650 a set  Do a little research about pistons and you will find that for most street applications, cast pistons offer  greater affordability - since you mentioned it - and practicality in use.

As kbmb02 has eluded there many factors to consider in an engine build. including  head height , block height, camshaft overlap etc.

It doesn't sound like you have the motor apart yet and don't know exactly how much oversize you need.  It may be safe to assume you need the second overbore, but you wont know until you have the motor apart and the blocked checked

Don't rush into pistons without having a clear receipt for  your motor build.

9.5:1 pistons have a very good chance of working well with California 91 octane gas regardless of the other details They just give a better margin to work with all the other components of the motor I've built two 9.5:1 motors in San Diego with no detonation and very pleasing results. The difference in power  between a 9.5:1  compression motor and a 10:1 motor will not be significant, that's why I favor 9.5s

I notice you are local  so PM me to exchange contacts and we can talk about this if you like

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I was told 9.5 would be easiest for Cali gas

 

my 10.0 pistons were cut down to 9.5 and I've had no issues in 3 years

 

1024475990_RussatStevesMachine(18).jpg.7066dd6a265e3d42e975851e55e20bf2.jpg

 

800785159_RussatStevesMachine(16).thumb.jpg.50cbc2ab01ee40e150ed65688df504e1.jpg

 

 

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I'm having Racetek make me some custom forged pistons right now, That's pretty much what I'm waiting on for my build. They should be  between 9.5 and 10.0 with slight more valve relief so I shouldn't have to worry about valve and piston clearance although I'll still use some clay for clearance

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1 hour ago, chargin said:

800785159_RussatStevesMachine(16).thumb.jpg.50cbc2ab01ee40e150ed65688df504e1.jpg

 

 

 

1 hour ago, rcf925 said:

I'm having Racetek make me some custom forged pistons right now, That's pretty much what I'm waiting on for my build. They should be  between 9.5 and 10.0 with slight more valve relief so I shouldn't have to worry about valve and piston clearance although I'll still use some clay for clearance

 

I hear you, I liked the relief idea 

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I mean i know it has a knock sensor and variable valve timing to prevent detonation but my daily is a 14:1 turbocharged 2 liter... it runs on pump 91. i really cant see 10:1 causing detonation with 91 octane

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It'd come back to this:

 

 

Quote

I just have a stock distributor and crappy 1bbl Solex carb.

 

Add in a cylinder head design from the 50's, ecofuel designed to burn in

a modern car, not an antique, and yeah, it might detonate

as low as 9:1.  Modern pent- roof 4 valve heads and good electronics are 

not to be underestimated.

 

That said, a bit of careful tuning with a wideband on a good carburetor

and decent ignition can let you do things that the stock parts simply can't manage.

 

Especially, as Ken says explicitly, if you cc your heads, break sharp edges, etc...

 

t

ported his tub faucet.

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Makes sense. That would be a fun thing to test ... stick a knock sensor on it and see how high compression you can go before it starts detonating at all

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I agree.  I think, though, if we could invent the 'compression ratio adjuster'

we'd make billions.  Sell an app- "Adjust your compression ratio on the fly

using your phone via blootooth!"

 

It'd spawn 3 new Fats and Furryless movies!

 

heh

 

t

 

Smokey Yunik did things like that.  He was nuts.  And had no social life.

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2 minutes ago, TobyB said:

I agree.  I think, though, if we could invent the 'compression ratio adjuster'

we'd make billions.  Sell an app- "Adjust your compression ratio on the fly

using your phone via blootooth!"

 

It'd spawn 3 new Fats and Furryless movies!

 

heh

 

t

 

Smokey Yunik did things like that.  He was nuts.  And had no social life.

 

Variable compression ratio is already a thing.

 

https://www.roadandtrack.com/car-shows/paris-auto-show/a30996/infinitis/

 

I enjoy the Smokey legends.

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