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Discussion Starter · #101 ·
Can you elaborate on this?

Also, you didn't answer my question on the timing chain tensioner.


The 2.5 and 2.3 blocks are identical other than 1 oil passageway that connect to the head and the bore is larger on the 2.5 block.

The crankshafts have the same diameter main journal bores but the 2.5 uses a larger connecting rod journal.

The 2.5 crank shaft has a 100mm stroke vs the 2.3 94mm to the 2.5 uses shorter rods to compensate.

If you had a custom set of rods you could use the 2.5 crank in a 2.3 block with 87.5mm pistons
 

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So, what is the advantage to using the 2.5 block if you are still running a 2.3 crank, and have a minimal increase in displacement?

I am not trying to be critical, I just want to understand the advantages as from what you've said so far it seems like you are reinventing the wheel somewhat.
 

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The 2.5 block has a slightly larger bore but the motor was originally intended to be boosted and make 800 or so hp. The 2.5 sleeves are made of a different steel which is supposed to be stronger.
 

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We are going to build a bigger motor soon though.
I liked this and I have to say that I like this. A lot. For those of us looking for cheap, reliable, easy to access horsepower, displacement is, has been and always will be the answer. I would really like to see a solid build and product offering for a 2.5 Focus.
 

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sounds like you need a 2.5 for your track car:thumbup:
I am enjoying my little blower right now, but at heart I know I will be better off if I can get naturally aspirated what I want powerwise. Problem is, I have an 01 so the Duratec swap isn't so easy.
 

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So, playing devil's advocate:

Going N/A you're going to have to rev the hell outof it anyway. Why not go with a 2.5 block and a forged 2.0 crank (fairly cheap from a miata engine)?

I believe you would need custom rods, but this is one of the lightest cranks available, it would decrease piston velocity, the larger bore would optimize the dispersal of the intake charge, shorter stroke would improve vibration of rotating assembly and promote longer life at high rpms, and you would still end up somewhere between a 2.1 and 2.2 (I didn't do the math, correct me if I am wrong).
 

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So, playing devil's advocate:

Going N/A you're going to have to rev the hell outof it anyway. Why not go with a 2.5 block and a forged 2.0 crank (fairly cheap from a miata engine)?

I believe you would need custom rods, but this is one of the lightest cranks available, it would decrease piston velocity, the larger bore would optimize the dispersal of the intake charge, shorter stroke would improve vibration of rotating assembly and promote longer life at high rpms, and you would still end up somewhere between a 2.1 and 2.2 (I didn't do the math, correct me if I am wrong).
If the engine is larger, you aren't going to have to rev the hell out of it. Even though the specific output of the smaller engine can be better, it won't be that much better. All else being equal, if I were shooting for 220 fwhp, I'd rather have it from a 2.5 than a 2.0. The piston speed is probably going to be the same maybe a slight advantage to a 2.0 particularly if you optimize the bore/stroke like you are talking about, but the bearing speeds will be slower, the valve spring loads lower, so the oil and coolant heating will be much less, which to me is the key. Nearly everything sees less stress because the engine is turning less RPM and making less heat.

This is, in part, why 8 liter Vipers and 7 liter Corvettes did so well in endurance racing. The air inlet restrictor meant everyone would make the same HP, so why not get a big honking engine that can make that same horsepower with lower stress? Since I am looking for a track day car engine (24 minute sessions not 24 hours but way lower budget) I am apt to apply the same thinking. Go big for lower stress at the same power level.

Now, I could be wrong, but that is where I stand right now. I certainly enjoy hearing other opinions and thought processes.
 

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I am not trying to be difficult, I just want to understand the thought process involved in the part selection for this engine.

Though, 9000 rpms is (in my world) reving the hell out of an engine. I believe that is what this one spun to on the dyno sheet.
 

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I am not trying to be difficult, I just want to understand the thought process involved in the part selection for this engine.

Though, 9000 rpms is (in my world) reving the hell out of an engine. I believe that is what this one spun to on the dyno sheet.
You aren't being difficult. I think a discussion of thought process is good.

As for 9000 RPM, yes it is spinning the hell out of an engine. If you want 300 hp in a Focus, IMO, you are looking at forced induction. However, what I am hoping is the Top Speed's research into the Duratec will yield us mere mortals an easy, cheap and reliable 220ish fwhp setup that you can do whatever you want with.
 

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You are being difficult. I think a discussion of thought process is good.

Agreed. Your points about bearing speeds, valve spring stress, and heat are well-taken...and things I didn't ponder.

That being said, I enjoy high RPM n/a engines much more than I enjoy turbo engines. I definitely see the advantages to a setup like this. I do think the build requires more in-depth discussion as to the 'why' it was done the way it was...so people can make informed decisions.
 

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I would love to have a cheaper alternative to FI. It would be great to make 220whp in my DD without having to spend through the roof to get it, not to mention extra crap to break and go wrong...
 
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