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LJDB

Best way to achieve correct compression ratio

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Hi guys, I came across a post on Facebook in a blow thru turbo page about the best way to achieve a desired compression ratio. It was pretty interesting to see different people's approach to the same goal. So it was basically which is preferred a larger cc chamber or dished piston. Now most of the guys on it are Yanks with big capacity v8 running high comp with boost making big power.

Now this had me thinking about our old trusty xflows which have a variety of head and piston types to choose from. So which would be better, zero deck flat tops with larger chamber or small chamber with large dished pistons down the bore? I know there would be more to the equation but ideally flat tops, zero decked would promote quench, whilst dished pistons zero decked would do the opposite so would there be a point? Or is 10.1 still 10.1 no matter which way you achieve it? The Yanks seem to get away with high comp, big turbo set ups with e85 ect but we don't have the luxury

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Look at the engine you are building and decide on the most cost effective way. There will be positives and negatives each way. For a given engine, example a 250 crossflow, a zero-decked flat top might give you too much compression with even a stock head, but a stock piston will give you only average compression with a stock head. If you want something in the middle a mild dish and machined head may be the way to go.

 

There is some evidence floating around that a mild dish helps centre the force of the combustion and helps with initial squish and also pre-ignition. This passes the common sense test in that you will get some good charge distribution from the narrow sides forcing charge into the centre as the piston rises, and have a friendlier shaped combustion chamber floor with regard to pre-ignition.

 

The shape of a chamber is more important than the volume, but there is a limit where a good shape and unshrouded valves of appropriate sizes can all co-exist. Generally this is a nice semi-hemispherical chamber with rounded, smoothed contours focussing on the valves edges.

 

Short answer? See what is reasonably available for your engine and go from there.

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I'm not in the process of building an engine just found it interesting. When I put together my turbo motor I just went largest chamber and dish, zero decked to keep comp down.

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It's really down to the individual builder.  As with most things in engine building everyone has there own theories of what works and what doesn't. 

The biggest deck height I'd ever have on an engine would be .010 with zero being the norm. I prefer a slight dished piston over a flat top, but sometimes you need a flat top. Chamber shape can be modified to assist in relieving comp. 

 

i guess it just comes down to the builders own theories and experience. 

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On 16/12/2018 at 8:19 AM, LJDB said:

I'm not in the process of building an engine just found it interesting. When I put together my turbo motor I just went largest chamber and dish, zero decked to keep comp down.

 

Compression won't kill a engine,tune will. Barras and 2js run over 10 comp. Need to talk to people who build other engines not just xflows to go forward. Or be stuck with a VHS player if you get my drift.

 

Aus fastest RBs run more comp than you think too. My xflow runs 11.2 comp and has seen 10psi @ 7500rpm with a big cam and big cyl head. On a small or stock headed combo with a small cam it would show ALOT more boost but wouldn't make the same hp.Boost is only a measurement of inlet restriction after all. So a bigger head with less restriction means it shows less boost but makes more hp and you use more comp which means little to no lag time if turbo is sized right. 

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Mines on 10psi but only revs to just over 5k. I start making boost at 2500 and 10psi not long after. I killed my first low comp engine because of the tune on less boost. This engine i'v ran more boost and timing without issues. Both motors are anything but build, just bits and pieces I have laying around. This motor is mid 8 comp, mild na cam with alot of overlap and shows 150psi on comp test. I was just amazed about the hp figures the Yanks have on low boost blow thru set ups. I suppose it comes down to experience and knowledge of product. Or comp v boost/timing and when too much of one starts to effect the other. Get them both right and it's happy days.

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Turbos are odd things and Im not on board my self but Dans LS Centura with Chinee turbo but sized right has ZERO lag. Like powerskids from idle to max rpm in top long as you can from a standing start like your doing a brake skid.

 

Get the exhuast on the smaller side and it will still make the hp and make it such a better deal...IMO based on stuff that actually works on real cars not internet talk. 

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Bingo. And there's no use having eleventy billion horsepower if it all comes in at once at 8,000rpm. You want area under the curve as slydog says above. I'd take a nicely set up mild 400-500hp street engine any day over a peaky 800-1000hp unit. If you can put your foot down and get a wall of instant torque shunting you forward you will shit all over the guy with a peaky engine who has to shift, spool up/rev up and finally start launching.

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My little corty would love 400hp on the street but who wouldnt want 800hp. I live fairly close to calder and heathcote and been to neither. Although one day i will, its fun telling the hsv boys its just a xflow at the next set of lights

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Drive a modern turbo engine for evidence of what Slydog is talking about.

Instant torque and a perfectly flat torque curve (plateau/table?) from about 1500 rpm (under stall speed) to 4 or 5 thousand rpm.

Perfect drivability with great fuel economy, win win.

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Absolutly Fingers. Can't tell you how many cars using more boost than the ute that are slower. Shows 2 things,wrong turbo choice and or engine combo needs work. 

 

You dont just put "a turbo" on a stock engine and expect it to work or get the most out of it.The main reason Barras and LS engines respond to turbos is the cyl heads flow so well and there engine control/efi works well.

 

Mark @ JPC helped in the design of my boosted cyl head which is part of the reason it went 1.50 in the 60ft untuned with belt slip in a real 1500kg with driver street driven car. Its quite literally hp been left on the table by people using xflows,sohc,holdens and ford v8's and for the hope of saving money or been uneducated. Spend $1000 on porting and seats and valve shaping and gain a potential 100plus hp over non ported. MTOD show the difference between a stock ls turbo and a built ls turbo and the outcome was even better with a extra 300plus hp. Who doesnt want that???

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