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Why do people choose low compression pistons for turbo 4 cylinder builds

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Discussion and questions related to the course EFI Tuning Fundamentals


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Specifically Honda B/H/D series. Most forums I see someone ask what pistons to use for they're B18, D16, H22 and the typical responses essentially are "turbos need 8.5:1-9.5:1 for room for tuning errors" or "lower compression is safer for boost"

Why don't companys make more 10.5:1 or 11.0:1 compression pistons if the added compression means cooler exhaust gas temperatures, bigger expansion ratio and a more efficient engine? Let's say I have a 400whp goal with a stock b18c and silver surfer turbo( http://speedtrappconsulting.com/Silver-Surfer )

If that turbos efficiency range is 14-31psi and can produce respectively 400whp-540whp then wouldn't it be better to be at 11.0:1 and run less boost vs 9.0:1 and higher boost for the same 400whp goal? I can see how big HP builds get more risky but 300-450whp should be relatively easy right?

Andre, and others, can give more info', but it basically comes down to the fuel limitations.

Perhaps the easiest way to look at it is that compressing the mixture in the cylinder causes the gases to heat up and, in extreme cases, it can cause the mixture to spontaneously explode (especially if there are hot spots in the cylinder or localised pressure spikes). With forced induction there is a greater mass being compressed into the same volume and this will increase the temperature even more, with the attendant risks. However, if we increase the volume in the cylinder the mixture is compressed less and the temperature doesn't increase as much, which is 'safer' and less likely to ignite spontaneously.

I said fuel limitations, earlier, as some fuels are more tolerant of compressive heating, etc, than others, so the engine may be able to run a higher static (piston) ratio. Generally, the higher the OCTANE number the better the fuel is for boost and/or high compression ratios.

If you have, literally, tens or even hundreds of millions to spend on developing and building an engine and have fuel suppliers prepared to do the same, as in Formula 1, you can run very high compression (reportedly up to 16:1), high boost levels (reportedly up to 2.5bar - don't know if guage or absolute, though) and/or lean fuel mixtures (up to 16:1) reliably - all to get the most energy from a limited fuel flow.

The thing to keep in mind with those 90s Honda engines is that they were never designed for boost. They were designed for high revving n/a power. Therefore they have high flow heads and intake ports, large duration intake cams, and high compression pistons. You can make those work with boost if you have knock resistant fuel (E85, race fuel) or if you have a big turbo meant for high rpm, or preferably both.

Look at a modern Honda turbo 4 you might find in a Civic Si or an Accord or whatever. They are direct injected (helps reduce knock), they rev lower, they don't have high lift cam profiles anymore. Those are designs used for low end torque and running on lower octane fuel.

I've seen tons of stock high compression honda engines be turbocharged on pump gas and last a very long time, But when people build a motor they drop a couple points of compression? I've seen k20z3's last year's on pump gas making over 400hp revving to 8600rpm regularly.

If you have a good intercooler setup and proper pump gas tune at a higher compression ratio that's the better option right? I just think the whole "lower the compression for boost" is a old way of thinking from the early 2000's, and it can be done but your more knock limited. I'll be using e85 on my civic but was just curious and thought I'd ask.

I've haven't looked into the new civic's too much so I cant comment.

Here's an example...

***Modifications***

-Synapse Turbo Kit

-1000cc injectors

-No Nonsense FuelReturn

-GTX3071r .82 a/r turbo

-Upgraded fuel pump

-Hondata 4 bar map sensor

-Thermal 3″ Exhaust

6psi climbing to 11psi

91 octane

Attached Files

The laws of physics haven't changed.

If you keep the stock compression ratio, you WILL knock if you don't control the air temperature, boost, spark timing and AFR, or use more knock resistant fuel. Enough strong knocking events (or just one bad one) and something is going to blow. Retard the spark and turn down the boost and your risk goes down, but so does the output of the engine. The people who make it last a long time on pump gas are still working within the same constraints. At any given level of fuel knock resistance and charge air temperature you need to run Less boost, richer AFR, and more retarded spark as compression ratio increases. This is spark ignition combustion 101.

Making 400hp revving to 8600rpm tells you a lot right there. Power is a function of torque and speed. You can make a lot of power by revving high, and that's easy with a big turbo, because big turbo has an efficient compressor and a low backpressure exhaust housing (which reduces knock).

In your case of having E85, well that changes things a lot. E85 is far more forgiving with knock, but you need to make sure you use very cold plugs as it can tend to preignite more. You could definitely run stock compression ratio. However, you still have to be careful. Too much spark advance on E85 increases the cylinder pressure which accelerate wear and break pistons, rods, etc. The stock components are not meant for high cylinder pressures.

Now, if you tune spark and lambda like it's on pump gas, the E85 buys you a significant safety margin for normal spark knock, but if you get too aggressive you will break things. It's not a silver bullet. If you have the money for stronger pistons etc then go for it; even if you don't drop the compression ratio or change the heads and cams it will still be a benefit, because you can better take advantage of the E85's improved knock margin.

Also, it's easier to make high rpm power with the stock heads and high lift cam profile of oldschool VTEC. High flow heads and long duration cams = high volumetric efficiency at high rpm, but poor turbulence in the cylinder and high tendency to knock on pump gas. It's bad for fuel economy and low end torque, but it's good for high rpm power You don't need a ton of boost to make power, especially if you have big turbo.

Ok, that makes sense. Thanks for the explanation!

I bought a h22 Euro-R engine and want to test the limits with the turbo I have on pump gas and ethanol. I have a spare block that's getting sleeved and forged internals.

I'm working on making my way through the videos, but there's a lot of them. Still lots to learn!

cool, good luck on your project!

Yup, sounds like a fun toy :-D

I do make a point of recommending folks buy a core to build up as it means they can still use the vehicle in the mean time, just as you have done, it also means that any parts or budget holdups aren't critical, and it allows the whole lot to be completed without intermediate compromises - "do it right, do it once"