McFeng

From the preceding three articles on evaluating an engine's power potential it should be clear that you can too much inlet valve area. It directly determines power potential - every 10% increase in inlet valve area gives a 10% increase in power potential. However, in many engines there is a trade-off between inlet and exhaust valve size. By making one smaller you can increase the size of the other. Equally this is not always the case. Sometimes a larger inlet valve will hit the bore wall long before it interferes with the exhaust valve or vice versa. On engines with angled valves, i.e angled towards each other, the point of interference depends on the cam duration as well as the absolute valve size. Long duration cams with a lot of overlap will have both valves open a long way at TDC on the exhaust stroke which is where they can hit each other. With shorter duration cams you can make either valve bigger and still not have clearance problems. Clearly all engines need exhaust valves and if these are too small power will suffer. So when the choice is between increasing inlet valve size and exhaust valve size what do you do? 
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從先前三篇評估引擎潛在動力文章中明白的告訴你不可能有太多的進氣閥(俗稱:把嚕卜)空間(註:指面積)。很直接的評估潛在動力-每增加10趴進氣閥門區就增加10趴的潛在動力。但是，對很多引擎來說卻是進與排氣之間大小的抉擇。廠商製造一具小的機件另一方面又讓你可以去改造他，這並非每一家廠商都會這麼做。
有時候較大進氣閥門在造成排氣干涉前會引起氣體敲擊管壁(應該是指活塞)反之亦然。在有角度閥門的引擎上，例如，面對面角度相同的閥門，造成干涉的點是不只依據凸輪軸轉動週期而且和閥門大小有關。Long duration cams (同high cam指高角度凸輪軸lobe lift)伴隨較多重疊角而進氣閥與排氣閥同時打開在TDC(top dead centre下一篇再跟你解釋= =) 在排氣動作時，這也顯示了閥門在哪裡互相撞擊。較短週期凸輪軸可以讓每一個閥門都加大同時沒有空隙的問題。很清楚的所有引擎需要排氣閥，假如太小動力將會下降。所以要增加進氣閥大小還是增加排氣閥大小兩者之間你會怎麼選擇又該怎麼做？(待續)

The simple answer is there are no simple answers because optimum exhaust valve size, or more correctly optimum ratio of inlet to exhaust valve size, depends on a number of factors including the state of tune of the engine, cylinder head design, port efficiency, cam profile and especially compression ratio. However there are some very good rules of thumb which will put you very close to optimum for most normal tuned engines.
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單純的回答其實不簡單，因為理想的排氣閥尺寸或更精準的進氣側或排氣側閥門尺寸的數值，全仰賴許多因素的數目值其中包含引擎改裝的協調性、汽缸頭設計、容積效率、凸輪軸數據和最重要的壓縮比。然而這邊有一些非常好的準則可以非常的接近正確數值給大部分改裝過的引擎。

The first thing to appreciate is that exhaust valve area does not directly relate to power potential. As an approximation for an average engine if you increase exhaust valve area by 10% you only add about 1% to 2% to the engine's power potential. This is a very much less direct relationship than that between inlet valve area and power. What happens when the exhaust valve is too small is you generate a pumping loss where the piston has to do the work of forcing out the last of the exhaust gasses but this is a small penalty compared with the direct loss of losing inlet valve flow because the inlet valves are smaller than they otherwise could be. Only when the exhaust valves are very much too small do the power losses start to become significant. What also follows from this is that you have to be talking about very highly developed engines where the last couple of bhp is important before you have to worry too much about whether it's better to make the inlet valve 0.5mm bigger at the expense of a 0.5mm smaller exhaust valve or vice-versa. For most engines fitting bigger exhaust valves will make an absolutely negligible difference to the engine's power output. 
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第一件事是感謝排氣閥區並不直接的與潛在動力有關。有數據顯示平均每顆引擎增加排氣閥面積10趴只能增加1到2趴引擎的潛在動力，這是非常不相關的對於進氣閥與動力來說。那排氣閥太小會發生什麼事？答案是會產生漏氣，在活塞運作擠壓動力最後排出氣體至排氣管但是這裡有一些小缺點在同時直接流失掉進氣閥的流動，因為進氣閥門太小，不然不會發生。只有當排氣閥門非常的小，動力的流失就會非常明顯。這也是本篇也就是你必須討論關於非常高完整度的引擎榨出最後一匹馬力是非常重要的，在於你必須非常的在意關於是不是增加個進氣閥0.5mm或是同時損失縮小排氣側0.5mm或相反來說。大部分引擎改裝較大的排氣閥對於動力的輸出絕對是沒必要而且沒感覺的。

The other commonly quoted situation is that of turbocharged and supercharged engines. Ask most people, professional engine builders included I'm afraid, and they'll tell you forced induction engines need bigger exhaust valves at the expense of smaller inlet ones. Well after all it stands to sense dunnit mate? The inlet side has a thumping great turbo pushing the air in and the exhaust side has nothing extra to help it. Utter bollocks I'm afraid and anyone who tells you this can be discounted as any sort of authority on how engines work and probably best avoided if you're looking for someone to spend your money with. When you turbocharge an engine you increase the cylinder pressure in direct proportion to the boost because you've crammed more air into it. This extra cylinder pressure directly increases the exhaust valve flow when the exhaust valve opens. In effect everything stays in the same proportion as when the engine was normally aspirated. The turbo basically boosts the exhaust valve flow in exactly the same proportion as it boosts the inlet valve flow although the relationship, being one step removed, is too remote for most people to appreciate it. So forced induction engines actually follow exactly the same rules about exhaust valve size as normally aspirated ones. 
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其他一般特殊情形是渦輪増壓和機械増壓引擎。問大部分的人，包括我擔心的專業引擎製造商，他們會告訴你這類大量進氣的引擎需要更大的排氣閥而犧牲掉使用較小的進氣閥，最後成為不成文法則？進氣側有個大家都贊同的說法大顆蝸牛推動氣體進入，排氣側完全沒幫到忙。聽的我想吐血(註:直接翻這句你也會吐血)，我很擔心假如有人這樣告訴你，最好離他們遠一點。當你的引擎在渦輪增壓時同時增加了缸壓直接傳遞給增壓機因為你已經把它塞的很滿了。當排氣閥處於開啟時過度的缸壓也直接增加了排氣閥流體，結果影響的是原廠引擎下所有機具負荷是相同的(重)。渦輪基本上推動排氣閥流更精確的說在相同情況下相對上它也推動了進氣閥流。其中有一個步驟停止，很少有人會感謝它的。所以動力傳導引擎精準的跟隨著些規則關於排氣閥尺寸相較於原廠引擎。

Can the exhaust valve be too big? Indeed it can unless certain steps are taken to design other aspects of the engine to suit the valve size. The main adverse effect of large exhaust valves is in the overlap period at the end of the exhaust stroke when the exhaust valve is closing and the inlet one opening. While both valves are open at the same time some rather nasty things can happen. Fresh inlet charge can skip straight across to the open exhaust valve and disappear down the exhaust port leading to high hydrocarbon emissions and poor cylinder filling. Also the exhaust gasses can reverse flow at low engine speed and come back up the exhaust pipe and into the cylinder leading to contamination of the fresh inlet charge. To combat these effects you can reduce cam duration but this then adversely affects high rpm power. Alternatively you can increase the cam lobe centreline angle (LCA) so the exhaust cam closes earlier and the inlet one opens later. This reduces the 'valve overlap triangle' but again might not be what you want at higher rpm. What we can deduce from this is that large exhaust valves might have little adverse effect on low tuned or standard road engines with short duration cams but progressively worse effects on higher tuned engines. In fact it leads us to the inevitable conclusion that the higher we tune an engine the smaller the exhaust valve might need to be. This flies in the face of everything most people believe about tuning where bigger is always better. 
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So let's get down to some rules. How do we decide if the exhaust valve is too small or too big? One way is to set a ratio between inlet valve size and exhaust valve size. Unfortunately this ratio changes as the state of tune of the engine changes so we need to specify the state of tune of the engine.

Another way is to gauge the exhaust valve size in relation to the cylinder size. If we tune the engine by fitting larger inlet valves, longer duration cams and raising the CR the valve size ratio changes but the optimum ratio stays fairly constant in proportion to the state of tune of the engine. What I set out below is a table which shows a sensible exhaust valve size for a given cylinder size. If the exhaust valves in your engine are at least that big then you'll see very little if any benefit from making them bigger even if you tune the engine for fast road, rally or race use. Given that it costs the same or more to fit bigger exhaust valves as inlet ones then it's clearly a sensible idea to spend the money on the inlet side of the engine. Of course if you're trying to build something world beating then no doubt you have the budget to dyno test all possible combinations of valve size and you might well deviate from the table.

Copyright David Baker and Puma Race Engines