Follow-Ups to Answer from Expert Josh Fields

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jong wrote at 2007-05-16 05:09:04
hi, j. pak here, i'm a bmw technician for a small bmw shop in the west loop of chicago.  i've been around modified m20b25 engines ever since i started working around bmws.  porting and polishing an m20 engine is a relative subject.  i was reading josh's comment and just got depressed.  match porting and polishing will increase cfm by 5-10%, the amount of volume you can flow air through the cylinder head at which given velocity created by a numerous amount of physics involved when an engine is running will definitely increase a substantial amount of significant power.  however, in ANY engine, porting and polishing NEEDS to compliment the target of what the engine is BEING BUILT FOR.  if you port and polish a cylinder head and flow test, it is inevitable you are going to see different cfm readings.  if you install that p and p'd head with stock configurations, you will not see much difference.  p and p'ing is a great idea if it is complimented by components that'll HELP create the cfm when the modified head is installed.  the engine is a huge vacuum air pump, right?  you flow test a head, what's creating the air going through it?  now, if you have "vacuum helpers"  such as, in all simplicity of course, a combination of, bigger pistons with a stroker crank, or, a hot cam for bigger lift with bigger injectors, or, different configuration of headers that create different exhaust backpressure, or, scavanging, etc.



the comment about m20 motors being tapped out?  creating 270+ horsepower, n/a, out of a 12v inline six doesn't render the m20 tapped out....you don't even need that much if you have a strong lsd and consider the vehicle weight to output power ratio.  and plus, if you are a true racer/tech, you don't just bolt things on and hope things work out, leave that to the little kids who's eyes twinkle when they get a gift card to autozone for their birthday, it's all strategic.  you can spend 6000 dollars on a stage 2 turbo kit for 75% power increase at the wheels, which is significant.  don't make the m20 sound weak josh, cuz in sweden, a refurbished m20 engine, completely stock, will run 11 seconds boosted at 17psi aaaallll day without breaking anything.  cylinder heads do not crack just because, if i went into that issue it'll be too long.  the temperature of the engine is regulated by the coolant system, i wouldn't worry about that.  if you run cool you run rich anyways, if you run hot you run lean, you want it cycling at all times, that's why there's o2s or wide bands.  as for rocker arms breaking,  majority of the m20 engines out there are more than 100,000 miles old, if you replace the rockers, even stock ones, it'll last you another 100,000, duh.  they're made out of cast, of course they're gonna break if it's old....



i hope this helped.  thanks for reading

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pld0vr wrote at 2008-08-06 15:41:10
I sort of agree, but not really.



I just had my 325i head flowbenched and larger valves put in. I had mine ceramic coated, and I'll mention why in a moment.



On my flow chart:



Stock Intake   165cfm

Stock Exhaust  108cfm



Ported Intake  195cfm

Ported Exhaust 136cfm



It picked up a small amount more flow at .600 but my cam doesn't have that much lift.



There is definitely room for improvement... mind you... I had a fellow who builds race engines do mine.



Now as for the other part of the argument... I am also running boost. Highly recommended. Use a Megasquirt with large injectors to ditch the stock ecu, and slap on a big fat turbo.



I'm running 24psi and sitting over 500hp with this engine. Stock bottom end, but 2.7l stroker.

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