Flow

Hi folks,

Can someone please help me get my head around this.

As I understand it so far:
- The better your boost to back pressure ratio is at ~max torque, the more efficient your intercooler is and the less head room you have on your turbo compressor efficiency map the higher the CR you can run for a given boost and octane rating?

So if you are currently not knock limited and able to run ~min timing best torque between max torque and max power and your inlet temps are say less than ambient +10 and your turbo is starting to fall out of the sweet spot at max power you can look at running a higher compression ratio?

Have I got the relationships correct? Are there any other factors I am missing?

Many thanks in advance!

Savington

iTrader: (31)

You're overcomplicating it. The only thing that matters is detonation. If you can get to MBT (timing for best torque) without detonation, you can add base compression and/or boost. Lots of things affect detonation resistance, including most* of the things you listed.

Practically, you need to focus on everything else first before adding base compression. It requires a TON of detonation resistance, typically through extremely high octane fuels (think 105+ US octane rating) before you can start to add base compression on top of even a mild amount of boost. As you are in NZ, I assume that E85 is unavailable there. If you are limited to standard 93 octane (98 RON) pump gas, you need to keep the base compression low to make the best power/torque in a BP.

* - I have a huge problem with the idea of a "boost to backpressure" ratio. If you have a 1:2 boost to backpressure ratio, that could describe an engine running 1psi of boost with 2psi of backpressure, or an engine running 15psi of boost with 30psi of back pressure. The former engine can run near-NA levels of timing advance. The latter engine, um, can't. Manifold pressure and backpressure independently affect detonation resistance, and they should be evaluated independently.

Flow

Cheers Andrew. We have E85 here in NZ. I guess my question is what factors do I look at in theory to make the add compression or add boost decision. Would that be where you look to efficiency factors of the install eg. how far out the other side of the compressor map you are at max power? Also is manifold to manifold boost to back pressure ratio a good efficiency indicator for guidance on dialing in the compression to pressure ratio sweet spot (assuming 1:1 is about as good as you can realistically expect)?

So on 98 RON if you can't get to MBT and you have head room with the turbo, lower the compression and if you are on E85 and running out of turbo, raise the compression. How am I doing?

codrus

My impression is that if you're on E85 and you're running out of turbo, you're probably better off adding more turbo than more compression.

--Ian

Flow

Thanks guys, so do you think I can run a custom divided manifold with a twin scroll EFR 6258 and pretty much straight exhaust on a OEM 1.6L with rods, the NB head, manifold/throttle body and maybe a light port and polish or am I going to have surge issues?
Have I got the got the volumetric efficiency numbers right here for the 1.6L NB Mazda? Matchbot

Savington

iTrader: (31)

You add boost until you stop seeing system flow (horsepower) increase. If you know how to read a compressor map, and you have an idea of how much power your engine should make at a given boost level, you can very quickly plot the points and tell whether or not you're running out of turbo. At that point, you can either go larger on the turbo (if you want to add lots of power and are OK with a loss in response), or you go higher on compression (if you want to maintain response and are OK with a much smaller gain in power). In general, adding compression will increase the peak cylinder pressure, but not dramatically increase the average cylinder pressure (simple physics of how an engine works). Adding boost will increase both peak and average cylinder pressure (and thus torque) dramatically.

Unless your application's specific racing class demands it, I am really not a fan of high compression engines. They combine most of the abuse of an extremely high-boost application with a fraction of the benefit.

No. Give up on that ratio. It's a terrible ratio and you should never use it to do anything. Don't even bother calculating it, in fact. As stated above, it's a useless metric which obfuscates the operating conditions of the engine. The ONLY reason you would look at such a ratio is if you were matching compressors to turbines. That is a black art that turbo companies perform for you (and not always very well).

Flow

Understood, good explanation, thanks!

DeltaP / (boost to back pressure)
Also interesting, would you be able to add some color to which operating conditions it obscures/mixes?

How are my Matchbot VE numbers looking for the Mazda NB 1.6L BTW? That was a rather rough guess.

borka

iTrader: (1)

What are your goals? Power? Usage of car?

Flow

This is the car https://www.miataturbo.net/meet-gree...-under-100862/ it is currently ~220whp with a K26 on a 1.6L. No more than 250whp is in the cards. The power delivery is somewhat 1980s Audi and I would really like to produce a tight, responsive N/A feel in the max torque to max power range that these cars (7s) really thrive on. The usage is essentially to the track and back. I am looking at moving the BOV from atmosphere to intake and dialing in some boost by throttle position, but I am not sure the old K26 is up to it, hence my Matchboting

Do you think I should start another thread on this? It seems the boost / CR considerations have been pretty well covered haven't they.

patsmx5

iTrader: (16)

There are a lot of variables, so you can't answer what's best without a lot of information and prioritizing what's more important. Cost, reliability, safety margin in the tuning, and a lot more. Driving for fun vs racing for the win. I tune my stuff soft because I don't want it to break. This is bad for power and response but helps reliability.

I run 9.5:1 comp on my setup, E85, all the boost. I keep the AITs low, the turbos big, and it doesn't detonate. But the tuning is conservative, and response suffers from soft tuning and oversize turbos. I've run smaller turbos that were more fun, but I killed 3 of them from overspinning them so now I run a bigger turbo that doesn't break under the same loading.

Savington

iTrader: (31)

Get a 1.8L

Flow

I have had a good long look at that however the amount of fabrication and fitment issues I would have to work through in this chassis leave me really with two options. Keep working within a budget around the 1.6L Mazda or make the jump to F20C / Sadev. We have a good community here in NZ around Mazda so I'll be sticking with 1.6L power train... for now.