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Discussion Starter #1
Now...when i rebuild my engine, I am gonna open it up 2mm and go to a 85mm bore on my b21. I am gonna get pistons to match it all. Now I have a couple of questions:

1. Will it hurt if I drive stock like this for 6 months? Like will there be enough fuel in the cylinder during operation after the enlargment?

2. I plan on going turbo, so how much will it affect performance if I go to a 2.5 exaust instead of the preffered 2.25 n/a? I want to know this for when i do my before rebuild baselines, and after....

Thanks for any info...
 

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Actually it makes more sense to get the 2.5 inch over the 2.25 with the turbo.

Back pressure is good, but too much of it causes you to lose power.
 

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^Incorrect. Backpressure is never good, ever. What a car needs is exhaust velocity not backpressure. You want to get the highest velocity possible at he lowest back pressure possible. A pipe that is too large is going to have very low back pressure but stagnant flow which is detrimental to low end especially.
Butters i was wondering the same thing about the exhaust. I have someone who will build me a 2.25 and a 2.5 system for the same price so i just have to decide. I plan on turboing but that'll be far in the futur but i don't want to pay for an exhaust system twice. Are you resleeving the motor when you over bore it or are you just boring it out? just wondering because I'm thinking on goin on pretty much the same path as you only with a b20. sorry, i kinda just asked questions but wasn't of much help lol.
 

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Discussion Starter #4
Boring and resleeling to use forged pistons...it's the only right way to do it. ;) without instant oil burn that is...
 

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sounds good. is that neccesary with the b20 block. not sure if i'm rebuilding the b20a5 or dropping in a b20. do you think that i could take out the pistons and rebuild it all with the block still in the car? I think that would be the deciding factor.
 

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Discussion Starter #6
I only have to do it with the b21 cause of the frm sleeves. I want to run 15lbs of boost.

You can rebuild the engine with it in the car...but it is MUCH easier with it out. But you can't get to everything with it in the car either...
 

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like what couldn't you get too? I feel bad lol. this thread was supposed to help you out but instead so far its the other way around. Tho I think you'd be aight driving with the computer stock. i don't think that 2 mm is enought to lean out the motor enough to be dangerous. From the a/f charts i've seen our motors get pretty damn rich up to especially.
 

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the only problem you are gonna run into is running a little lean if you overbore and change the exhaust, might be a little enough difference that an adjustable fuel pressure regulator would do the trick for a little time. does the B21 have the impregnated composite sleeves? just wondering i have no idea. thought they were only on the 5gens and NSX's.
 

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Discussion Starter #9
bluedlude said:
the only problem you are gonna run into is running a little lean if you overbore and change the exhaust, might be a little enough difference that an adjustable fuel pressure regulator would do the trick for a little time. does the B21 have the impregnated composite sleeves? just wondering i have no idea. thought they were only on the 5gens and NSX's.
The b21 have the first generation of FRM sleeves. They refined the design in the h22 motors...not too sure about the nsx motor.
 

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What kind of sleeves and pistons are you going to be using, butter? And isn't 85 mm a little large for the B21, even with sleeves? I heard that 85 was the absolute max.

--J
 

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Discussion Starter #11
Import_Tuna said:
What kind of sleeves and pistons are you going to be using, butter? And isn't 85 mm a little large for the B21, even with sleeves? I heard that 85 was the absolute max.

--J
I heard 86 was the absolut max? I want it to be a true 2100cc.

Here's the details on the sleeving I am going with:
http://laskeyracing.com/shop/sleeving.htm
 

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Hmmm... 86 sounds huge and ridiculous.

Have you talked to these pricks about your B21 yet? Hopefully there will be enough room for new sleeves with 85 mm pistons. I'm sure they can verify that. I think 85 mm would be insane, in a good way of course.

It's really too bad we have to do this with our B21s. I was planning also on getting sleeved for big pistons. I think FRM was a great idea, and that's why I think it's too bad that we have to sleeve our blocks.

--J
 

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Discussion Starter #13
Import_Tuna said:
Hmmm... 86 sounds huge and ridiculous.

Have you talked to these pricks about your B21 yet? Hopefully there will be enough room for new sleeves with 85 mm pistons. I'm sure they can verify that. I think 85 mm would be insane, in a good way of course.

It's really too bad we have to do this with our B21s. I was planning also on getting sleeved for big pistons. I think FRM was a great idea, and that's why I think it's too bad that we have to sleeve our blocks.

--J
Atleast i know my new custom b21 block will be able to handle 50 lbs of boost. :)
 

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Ha.

--J

EDIT: Holy fist, man. At first I thought you were joking. Then I read that Laskey shit you posted. 50 lbs!!? THAT is outrageous.

Realistically, though, what sort of boost are you thinking of? More specifically, what kind of WHP are you shooting for? I totally understand if it's top secret.
 

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Discussion Starter #15
Import_Tuna said:
Ha.

--J

EDIT: Holy fist, man. At first I thought you were joking. Then I read that Laskey shit you posted. 50 lbs!!? THAT is outrageous.

Realistically, though, what sort of boost are you thinking of? More specifically, what kind of WHP are you shooting for? I totally understand if it's top secret.
Nah....I plan on going to ~15 lbs of boost. My TRUE goal is to equal out tq and hp to be ATLEAST 300. I am shooting for 300whp with 315 tq. Since my b21 produces 3 more torque than Hp now....I think I might get my goal....15lbs should get me right about there with proper tuning.
 

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Amaymi said:
^Incorrect. Backpressure is never good, ever. What a car needs is exhaust velocity not backpressure. You want to get the highest velocity possible at he lowest back pressure possible. A pipe that is too large is going to have very low back pressure but stagnant flow which is detrimental to low end especially.
How can you say back pressure is never good, and then say very low back pressure is detrimental to low end (power)?
 

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Attaus said:
How can you say back pressure is never good, and then say very low back pressure is detrimental to low end (power)?
I didn't mean that low back pressure was detrimental to low end i ment that an effect of having a pipe that is too large is having little to no back pressure (which is good) but you also have very poor velocity. Its like this, try blowing threw a coffe stirring straw its gonna be really hard but the air exiting the straw is going to move fast; high velocity but also high back pressure. Now try to blow through a 2 inch pipe, its going to be really easy but not very controlable and the air will exit the end very slowly; low back pressure low velocity as well. Now take a good sized drinking straw and blow throught it, its not goin to be very hard at all and the flow of air is going to be at a good speed.
Back pressure = pumping loss which = less power
Velocity is needed for scavenging, it causes a vacume to develope behind closed exhaust valves which in turn helps to "suck out" the spent exhaust.

I know my stuff, taking a college fluid dynamics course cuz i'm a mechanical engineer major specilizing in engine design.:)
 

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Amaymi said:
I didn't mean that low back pressure was detrimental to low end i ment that an effect of having a pipe that is too large is having little to no back pressure (which is good) but you also have very poor velocity. Its like this, try blowing threw a coffe stirring straw its gonna be really hard but the air exiting the straw is going to move fast; high velocity but also high back pressure. Now try to blow through a 2 inch pipe, its going to be really easy but not very controlable and the air will exit the end very slowly; low back pressure low velocity as well. Now take a good sized drinking straw and blow throught it, its not goin to be very hard at all and the flow of air is going to be at a good speed.
Back pressure = pumping loss which = less power
Velocity is needed for scavenging, it causes a vacume to develope behind closed exhaust valves which in turn helps to "suck out" the spent exhaust.

I know my stuff, taking a college fluid dynamics course cuz i'm a mechanical engineer major specilizing in engine design.:)

He's right, backpressure is NEVER good, it ALWAYS lowers power output.
Yes, that was always AND never in one sentance, futhermore they are both true, despite what people might say about never saying never.


The trick to velocity is that the higher the velocity the larger the pressure pulses, so with a narrower pipe (that has more backpressure AND more velocity), you can arrange the pulses so that just as the exhaust valve opens there is a vacuum on the exhaust side of it, despite the exhaust as a whole having pressure in it. That gets the exhaust gasses in the cylinder moving more quickly to start off with, so that when the vacuum stops (an instant after the exhaust flow starts), the gasses, though having backpressure, are already moving, and can bash their way through that pressure.

With too large of pipe, you lose the pressure pulses, so the exhaust gasses in the cylinder have to open their own path through the air/exhaust outside the valve, rather then being SUCKED out.

Thats why huge exhaust hurt power, despite having no backpressure.



TL-DR version:
Velocity = always good
Backpressure = always bad
 

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Discussion Starter #19
So.....in my temporary situation?
 

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I would go ahead and run the 2.5 inch pipe. Usually you hear people say that 2.25 is the max for honda engines but when most think of honda engines they think the 1.6 and 1.8 liter engines. True 2.1 is pretty big for 2.25 i think. I wish i had the equipment to experiment cuz i think that would be interesting.
So yeah go ahead with the 2.5 if it hurts low end it shouldn't be by much and you should gain alittle up top not enough to be noticable tho. If anything else just get the 2.5 so that you only have to get an exhaust once. i really don't think that it will be too detrimental right now and if anything it'll just make that much more of a difference when you go turbo.
 
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