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Old 09-14-2012, 12:05 AM   #1
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Default Compression + boost. Learn me

So as the title implies I'm interested in hearing opinions on compression ratio's and boost.

Everything I see on honda forum's is people want to go really really low,Like 8-8.5:1. Or the exact opposite people boosting stock honda k20's and f20's and making good numbers. This seems a bit ridiculous to me as I daily my car and I'd like to not have to be in boost to feel like I have power.

So here's my understanding (please correct if wrong, I don't want to spread misinformation). High static compression and lots of boost don't mix for a few reasons. The higher boost pressure makes a higher dynamic compression increasing the chances of pre-ignition, or igniting because of the compression and not spark. Also running turbo's into the higher boost area produces more heat. So more heat + lots of cylinder pressure = more chance for pre-ignition.

So from what I've seen there are trade offs to each. Low compression = the ability to run more boost and more timing. Also it's easier to tune on pump fuel.

With high(er) compression you get better out of boost response. Though you're limited to timing you can run in boost and not being able to run as much boost. May have to use E85 or higher octane fuel to have a reliable tune.

What I can't figure out is this. From a few examples I've seen low comp motors and high(er) comp motors can make almost the same power only on different boost levels. So in my mind, other then ease of tuning, why would you go with low comp rather then high(er) comp. I mean I know people get hard on's saying they run ALLOFIT. But If I make 300WHP on 10psi and you make 300WHP on 20psi what's the benefit? Other then saying "yeah I run 20psi"

So theoretical engine build time. Goal is 350whp. Maybe more maybe less
I want atleast 10:1 static compression but no more then 10.5:1 (note, this is not exactly high, but higher then the standard 8 and some change that's often seen). Let's assume all forged internals. No headwork. Tuning will be done with Megasquirt and possibly utilize vvt if I upgrade.
All exhaust piece's will be custom made. (Long tube header, 3" exhaust)
Ok so turbo selection time.

I daily drive the car (50ish miles a day highway). I don't care about super fast response, etc. If I'm at full boost by 4k that's fine by me, as long as the power is worth the wait.
So I have this Holset H1C in my garage. I like this idea because the bigger turbo should theoretically produce less heat and flow more air at less boost (I'm not very good at reading compressor maps but that's my deduction)
Or I could use a smaller turbo something like a gt2871 or 76 and run higher boost but then we get into the heat, high cylinder pressure issue.

Intercooling is a given. I would also run water/meth injection to help combat knock. E85 would be nice but there don't seem to be any stations around me.

So could this be a feasible plan? Would the water injection be enough to suppress knock at high boost levels? Would the high cylinder pressure necessitate lower compression or use of race fuel?

Thanks for reading through this ridiculously long post.
-Patrick
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Old 09-14-2012, 12:31 AM   #2
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You're comparing a budget, 20 year old Miata engine to a cutting edge, state of the art Honda engine with VVT on two cams. Every single thing on the Honda engine is superior to the Miata engine, that's why. Piston shape, combustion chamber shape, quench pad design, cam program, head ports...
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Old 09-14-2012, 12:40 AM   #3
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Fair enough. I don't mean to directly compare between our old crap engine(I know its crap I complain a lot about it to thirdgen) and fancy new honda stuff. But the trend with modern cars is boost and compression. I saw on ls1tech the stock zr1 has like 11:1 comp and runs 7lbs of boost and makes quite a bit of horsepowers. I guess that changes, or rather adds on another dimension to my question. What is it about new technology that allows more static compression and boost to be a good combination? Is it head design? New materials? Direct injection? Super fancy variable timing?
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Old 09-14-2012, 01:01 AM   #4
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Explain this to me: why do boosted k series or f series Honda engines make such good horsepower, but such crappy torque?
Or better yet, all v-tec Honda engines for that matter.
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Old 09-14-2012, 01:24 AM   #5
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Relation between HP and TQ mothafuka,

Do you know it?
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Old 09-14-2012, 01:26 AM   #6
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Answer: ALL OF THE ABOVE

Direct injection, heads that outflow ours without even trying, engine design, piston design, better rod ratio, etc etc etc. All this fancy new **** allows newer cars to do what we thought was UMPOSSIBLE just 5-10 years ago.

With miata's high compression is not that great cause we don't have the flow to reduce the heat with high compression and boost. You could use e85 or tons of w/i to "help" the situation, but again you can only get so far before an F20C or K20 or many other engines just blows you out of the water.


Regarding your last question: Because they have very efficient high flowing heads, rev for days, and have piston/rod relationship better suited for high revs/power and not as much lowend torque. Compare bore/stroke/rod ratio/cam profiles/etc etc etc of a subaru ej255 to a honda k20a2 and see for yourself why one is a torque monster and the other is a power monster.
Then compare them both to a BP and realize why there isn't millions of miata's out there running around with 600+ HP
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Old 09-14-2012, 01:30 AM   #7
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our cars are great because they're cheap and simple.

our motors are great cause they're cheap and simple and pretty strong for what they are. (and up to a certain power level)
Nowhere in there is any "advanced" technology mentioned though.

Its a motor that has a decent balance of power and torque, but is not a "master" of either. jack of all trades if you will.
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Old 09-14-2012, 02:15 AM   #8
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I only read about a third of your initial post here so far, but your persistence in using an apostrophe to express plurality takes away from the actual content of your post.

1 lemon 2 lemons

NOT 2 lemon's
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Old 09-14-2012, 04:40 AM   #9
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So consensus is that our engines are not high-flowing enough for high compression + boost to be feasible? I was just about to post this same question.

I've been talking with some well known Honda tuners (not just the new stuff) about supercharging the miata engine and they recommend upping the compression with a supercharger. (lets put aside the super/turbo conversation for now) I was told with the tuning capabilities and possible e85 use that high compression is the best option for something like a supercharger. This may not apply to a high boost turbocharger option but thought I would throw in this little tid bit
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Old 09-14-2012, 08:43 AM   #10
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Blowers love compression. On the ecotec engine that GM supercharged the non-blower version has lower compression that the blower version. If you can get the heat out of the intake air and get the exhaust out of the head you can run the high compression. Its harder on a turbo car because you pretty much need to have low backpressure to get the exhaust and heat out of the combo chamber.

I wish there was a vvt DI engine that was available from mazda that easily swapped into these cars. I would love to be able to run 9.2:1 compression and cram 35psi from a ko4 into the thing and eek out 400 hp on e85.
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Old 09-14-2012, 09:18 AM   #11
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Quote:
Originally Posted by thirdgen View Post
Explain this to me: why do boosted k series or f series Honda engines make such good horsepower, but such crappy torque?
Or better yet, all v-tec Honda engines for that matter.
stock for stock they don't

<all images pulled from google results>

Stock B6 (1.6 litre Mazda engine fitted to MK1 MX5's/Miatas yadda yadda)


Stock B16 (1.6 litre Honda engine fitted to similar era Honda Civics etc etc)


The Honda makes the same torque that the B6 makes peak pretty much flat until the 2nd cam kicks in and then it makes MORE torque to a higher redline.
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Old 09-14-2012, 10:39 AM   #12
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Ok sorry my grammar isn't top notch. Though I said K20's rather then K20s so no on would think I was making up an engine.

Anyway. Thanks for the info 18psi. That's pretty much what I wanted to know. I know the BP is crap. I just didn't know how crap, or how far it could be pushed. I would rather spend money doing some ridiculous engine swap then trying to re-engineer a 20 year old engine...

Really this was inspired by a budget build idea of just throwing rods in and trying to run more then 250whp with some level of reliability. I just didn't know if compression would limit me to the same boost levels I run now, which would defeat the purpose..

I run 14psi at full tilt and my car is doing fine so far.
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Old 09-14-2012, 01:48 PM   #13
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Rods, DW 600's, and a Godspeed 3076 turbo... There's all your engine needs to make all your dreams come true.
I'm telling you man, if you pull that BP, that car will never be the same and you'll hate it.
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Old 09-14-2012, 01:50 PM   #14
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Oh yeah...you'll need a good clutch too.
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Old 09-14-2012, 02:15 PM   #15
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There is an in-house option from Mazda that would make the ridiculous power you speak of.

But for 250whp, you're re-inventing the wheel with no payoff. Your compression is not your limiting factor.
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Old 09-14-2012, 03:22 PM   #16
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Our motors are great to 250. Good to 300. Nice to 350. Ok to 400. Meh for 450. Fail for 500+

That's kinda how I look at it. Its whole lot more complicated than that, and some might argue they're worse/better, but that's the ball park. The level of parts and labor required past 300 starts to shoot up with the quickness.

I don't see any reason why you wouldn't enjoy the heck out of it at 300 though.....Very VERY fun amount of power
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Old 09-14-2012, 04:19 PM   #17
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The answer is quench. These Honda people you see running 8.0:1 car are probably just honda-tech retards, don't listen to them.
In the d series world the availability of he very cheap and very strong vitara pistons lead to a lot of people running cr in the 7s and 8s. It was less than ideally, but cheap, easy, and proven to over 600 HP. Now people are using the same pistons with custom length rods and heavily milled heads to create quench and bring the CR up into the 10s and 11s.

You have 3 basic factors to think about. Static compression ratio, dynamic compression created by valve overlap, and quench. The right balance will create the best results.

You brought up k20 f20/22 crowd who are running lots of boost on 11:1 cr. The reason they get away with this so easily is because they are high performance NA engines with wild cams. In other words they have lots of valve over lap which creates a lower dynamic compression ratio. In addition o his the heads have pretty good quench.

The BP can do the same ting but its going to require cams which man you're moving the power band up and you run into the infamous high rpm issues that we all love so much.
I wouldn't want to try to run much more than 11:1 car on a turbo BP and even then it would require careful tuning on pump gas. You can't be a typical Honda retard running 20 psi on a ******* base map.
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Old 09-14-2012, 04:25 PM   #18
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Quote:
Originally Posted by hustler View Post
You're comparing a budget, 20 year old Miata engine to a cutting edge, state of the art Honda engine with VVT on two cams. Every single thing on the Honda engine is superior to the Miata engine, that's why. Piston shape, combustion chamber shape, quench pad design, cam program, head ports...
They only have VVT on he intake cm, but a lot more range than the bpz.
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Old 09-14-2012, 05:41 PM   #19
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Thirdgen: That's kinda my plan. I talked to pdexta about his setup and was thinking about copying that. But he went with 94-97 bottom end for the lower compression. So I wasn't sure how having the 10:1 would affect it. I don't want to switch engines. Unless its an ecoboost 3.5L, lol.

Let me be clear I don't want just 250whp. I want to push past that I just didn't know how soon after that mark I'd be limited, if at all, by compression. 350whp is my "dream" number 300whp would be more then acceptable.

Also I want to at least try the Holset I have. If I hate it I'm sure I can find some other T3 twin scroll turbo to switch to.
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Old 09-14-2012, 06:36 PM   #20
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Y8S has the same motor as you...check with him about his setup. Something in my memory recalls 265whp on 9.5lbs boost with a gt2876?
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