Irizarr

Question in the title. I know my car needs a rebuild and I'm trying to decide what I want to do in terms of compression. e85 is very tempting, and I'd like to build for it, but I'd also like to be able to take the car on trips and not have to worry about having access to fuel. What is the best option for compression with this in mind, and if I were to run both fuels, what would the best setup for that include? i.e. a flex fuel sensor and related parts.

shuiend

iTrader: (24)

Supertech 8.6:1 pistons if you are ever running pump gas. The safety margin you get from them far outweighs the tiny amount of benefit that comes from higher compression.

Irizarr

I know from reading a lot of threads on here that you guys (specifically you shuiend, and a few other regulars) talk about the torque benefits of lower compression/more timing specifically on pump gas, but what about turbo spool? How much benefit do you get from higher compression in terms of spool up? Also how do these engines handle forced induction while increasing rev limit, and how does that interact with compression? Also in regards to e85, how much loss is there when running e85 on 8.6:1 vs say 11:1? EDIT: Oh, and also why the supertechs? I like the wisecos.

concealer404

iTrader: (3)

This is the same thing.

Some. Until you can't hit MBT anymore, then negative benefit.

None of these things really "interact."

Loss? Depends on MBT like above. If you plan on running E85 the vast majority of the time i wouldn't do 8.6:1 pistons.

Because 8.6:1.

Irizarr

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What is? The relationship between torque and compression/timing and the relationship between spool and compression/timing are the same? That can't be what you mean since then you'd have immediately contradicted yourself.. I'm not clear on what you mean here, sorry. The rest makes sense, but what would you consider to be the vast majority of the time for e85 use? I mainly want to be able to safely run the car in the event I'm on the road and don't have access to e85, which would not be very often since I only travel maybe once every 2 years on average.

You seem to be implying that 8.6:1 is the optimal choice. Why is that/how do we know this?
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Lastly, Is there a benefit to doing head work for increased rpm? Maybe I'm starting to sound like a honda guy (I'm not, I mean I like the S2K, but who doesn't?), but I'd like to get to 8k rev limit if possible/if that even makes sense to do for any reason.

concealer404

iTrader: (3)

I do believe we've reached a point where you're not really sure why or what you're asking. Which is fine. It happens. I'd suggest you read most of the stickies in whatever forums we have if we still have them i guess? Then try again. We're kinda stuck in Fisher Price My First Turbo but asking deep dive questions without context, which are all things that have a few hundred threads each about them.


I will leave you with this in response to your first questioning thinking i'm contradicting myself: What creates torque in a turbo car?

Irizarr

I don't really agree with your assessment. My questions are specifically motivated, and I know exactly why I'm asking them. The what is what I was hoping to gain a better understanding of by asking, but since as of now I can't get the answers I'm looking for here (in this thread specifically, not the greater forum), I'll do some more research elsewhere. I also didn't come here to be quizzed, I deal with enough of that in my college classes. I understand that from your perspective as someone who's been around for years you can probably easily recall a lot of threads and knowledge that may or may not specifically answer my questions. Your responses to me here are that of a veteran forum user who has become jaded to new people. Which is fine. It happens. I'm not an expert, and I do need to do more research, if nothing else I now know this is not an approach I can use to do that research. Thanks for taking the time to respond at all, I'll keep an eye on the thread in case some more helpful responses come in either answering my questions directly or pointing me in the right direction.

patsmx5

iTrader: (16)

Lots of questions. I recommend you do some reading here and elsewhere, as there's a lot of info (facts and opinions) about these topics.

The summary for your first question about compression, it depends on a lot of things, there isn't a general answer to a question like that. Depends on how you use the car and your goals/priorities with the setup.

patsmx5

iTrader: (16)

In my experience, with 9:1 and excellent tuning, I can run low 20's psi on 93 octane, 40 on e85. With less than excellent tuning, can break things quickly.

The Australian

Three variables that affect ultimate torque.

One (increased compression) eventually limits the other sources of torque (boost and timing).

If the ultimate torque goal is lower, then higher compression may net you faster spool (and torque), but with a lower ceiling on boost and timing before knock at a given point in the rev range.

My engine is tuned for PULP and E85 and has 9:1 Supertechs. On PULP it’s happy at 330hp. On E85 it makes 460hp+. It would probably make a little more on PULP with lower compression pistons.

Irizarr

So let me clarify that I'm not that interested in maximum hp/torque. I'm interested in building an optimal engine for making reliable power that can be used anywhere, contributing to a car that is fun to drive. As an example, If my engine will be reliable for many years at 250hp (the numbers aren't important here and don't reflect my expectations) but will be significantly less reliable at 300hp, then I would go with 250. Similarly, I will always go with the option that correlates to quicker response/spool, which is why my turbo of choice at the moment is likely the Garrett GT2554/2560R.

My partner and I love our miata, and we want it to be fast, but we also want it to live. I'm not about chasing power records or having the fastest miata on track. I want to build something that's fun but will last, but I also want to do that in the most optimal way possible so that I only need to do it once, and so that I know for sure I got everything out of my setup that there was to get out of it.

The Australian

Decide how much power you want and optimise for that.

I have two cars, both on e85.

One has 2560 and makes 256hp on E85. The turbo is the limiting factor, nothing else is at its limit. The small turbo means it spools quickly and makes torque early. It has 8.6:1 pistons. Very fun to drive.

The other mentioned above has 9:1 pistons and a much larger turbo, and makes much more everywhere, well beyond the limits of the gearbox etc etc.

Compression ratio has not determined the ultimate result in either case (turbo and fuel system has). For you, it probably doesn’t matter which CR you pick so long as you accept that each could have some impact if you push the limits in the future.

Irizarr

To be honest I'm kind of torn about the 2554R, I know it has awesome spool up but I also know it's limited to around 250hp. I would consider a bigger turbo and therefore more power if I could do other things to get it to spool as fast or even almost as fast as a 2554R. There's also the fact that I kinda decided on it before I knew for sure the engine was going to need a rebuild. The reason it sounded like the best choice then apart from fast spool is that it would max out right around the stock internals/drivetrain limit of ~250hp. Since I am doing a rebuild now with forged parts I'm open to other options as long as they will result in a same or very similar driving experience.

rascal

This ^^^^^

If you only want 150 hp then compression ratio prob not that important.
If you want 500hp then no compression ratio will work for PULP..

Irizarr

I don't have a specific power goal though. And it's not because I don't understand how important that is in the decision making process, it's because that's not my focus. I can say I would like to have 250hp or more, but what I actually want is to max out a turbo that gives a responsive driving experience and justifies a built engine.

patsmx5

iTrader: (16)

maxing out a turbo is good for making reliable power

Irizarr

Is it actually? I'm slightly suspicious of sarcasm here lol. I think the phrase "max out" may mean different things depending on context. Because there are turbo graphs that determine optimal operating range for turbos based on.. cfm or something? I need to review that part of my turbo research. My line of thinking is that if I'm going to buy a turbo and build my engine for said turbo it doesn't make sense to not try to max it out. Otherwise aren't you just not getting your money's worth?

patsmx5

iTrader: (16)

It doesn't last as long when running at it's max. It can be about anything.

Irizarr

That makes sense. Max within reason then. My previous posts indicate I want to keep the engine healthy and tuned underneath its maximum capability for reliability, in the case of the turbo, as with all of my other part choices, I want to do what's optimal but also reliable, whatever that may be.

Gee Emm

A built engine is a means, not an end.

You decide the end state you want, and then build to that. It may or may not involve a 'built' engine.

As others have, I am also going to suggest that you have some (ie a LOT) of reading to do, starting with the stickies. That will probably answer most of your questions. Right now we are going around in circles.

If your goal is c250whp and max responsiveness, go look, I'd be surprised if you couldn't find an answer, or information to help you narrow your search.