改裝排氣之馬力與扭力的取捨

上一篇 / 下一篇  2009-05-21 05:40:08

很多車主都覺得改裝排氣系統是最基本的,最沒有什麼太多的技術含量可言。甚至有些剛接觸改裝不深的車主認識排氣管越粗、越響、直排就是最好,往往都是在改裝的時候注重馬力的提升,對扭力卻沒有關注過,其實這是個誤區。那麼排氣管該怎麼改?在細說選擇排氣管粗細、型式、材質等等的疑慮之前,先來瞭解排氣管的本質吧.
  在我們所接觸到的量產車裏,原廠的設計往往屈就製造成本和環保法規使得發動機的真正性能都有所限制,就好像是一個封印封住了發動機真正的實力。現在的量產車都採用四行程發動機的設計,原本就把進排氣各個行程獨立分開,造就成一個可完全燃燒的設計,也因為獨立進排氣行程的設計,讓燃燒過的廢氣,有充分的時間排出,空出氣缸的空間,以便吸入更多更密集的新鮮空氣,來使發動機有更良好的輸出功率。但由於發動機的氣缸數多,各個氣缸之間無法有獨立的排氣管,同時需考慮噪音、空間整體佈局與量產的成本因素和法規規定的環保要求,排氣管只淪為消音及排除冷卻廢氣和淨化尾氣的作用。量產之下的排氣管,於是就有排氣不夠順暢的問題產生,進而降低了發動機的性能,在討論改裝排氣管是否增加了多少馬力時候,也可以理解為是挽回了多少發動機原本就有而要遷就量產所損失的馬力。
  排氣管的排氣順暢程度,可以理解為回壓取捨的多寡,也就是排氣管內部的排氣阻力。排氣阻力的大小和頭段的設計角度、中尾段的管徑粗細以及三元催化的大小、總體長度和彎曲角度、尾部消音器的回路也就是回壓都有相當的關係。改裝排氣管主要的用途,就是在於減低回壓讓進排氣的運動迴圈交替更加順暢,也可以說增加氣門重迭時間的延伸;因此可改變發動機的特性、轉變扭力輸出的時機、提升高轉速的反應和流暢程度。
  排氣管回壓的尺度是非常有講究的,過多和過少都屬於缺陷,回壓得多少也都有利有弊。減低回壓有助於排氣的順暢,對於高轉速是好的,但過低的回壓使得排氣毫無阻力,中低轉混合氣在還沒有完全燃燒完的時候便被排出,將造成扭力的損失。而且當發動機回油時,排氣管內的回壓過低,廢氣極有可能回流至燃燒室。所以在改裝之前車主最好應該考慮清楚,對自己想要改裝出來的實際效果有所認識。中低轉速的使用,相當程度的回壓仍然是必需的,也就是低轉速的扭力,降低回壓就會降低中低轉速的扭力,但是可以使排氣通暢並提高高轉速的發動機性能。很多車主只知道改直排會增加馬力,但改裝完了以後反而覺得車子沒有以前有力就是這個道理。
  原廠發動機管徑加大比率,應該以10%至15%為上限,一般而言,改裝排氣管大多先從中尾段下手,常見的手法不外乎增加管徑的尺寸、縮短消音器回繞的行程,努力使其消音路線呈直線化。提到直線化設計原本是很單純的原理和方法,卻因為市場上出售的量產車有底盤及油箱的干擾,想做到筆直路徑的排氣管,是有相當的困難,除非像參加職業大賽的各廠家車,拿掉原廠油箱、改裝FIA合格的防爆油箱,以空出底盤中央的空間,好讓排氣管在車底盤中央再從車尾中央直接通到車外。
  當排氣路徑縮短,各彎角也逐漸平滑、減低排氣阻力之後,順暢程度即有相對的提高。但就之前所說的原則,粗短型式的排氣管,是適合於高轉速時的馬力輸出,細長型的排氣管則擅長於低轉速的扭力輸出;造成兩種現象差別的原理,是因為細長型排氣管管內的壓力高,中低轉速時廢氣能夠迅速從排氣口排出,但高轉時大量的廢氣將面臨阻塞排氣管及排氣溫度過高的缺點。而相比之下,粗短型排氣管便擁有中低轉排氣慢的問題,可是到了高轉速區域便能發揮的淋漓盡致使排氣通暢無阻。
  一般量產車的排氣管,應先考慮選擇排氣行程比較直的排氣管,然後才是在管徑上變化,這樣才能兼顧到全轉速區域的表現。根據現在改裝界比較流行的選擇,中段排氣管管徑的增加到原廠的10%至15%最好,也就是自然進氣發動機在52mm至60mm左右,渦輪發動機則在65mm至75mm左右。當然發動機的排氣量越大,改裝目標動力越大的話也要相應的增加,也有可能達到80mm。談到管徑的配置比例,由粗變細或從頭至尾一樣口徑,對重視扭力需要的車主會比較有用。如果只追求性能提升,那選擇逐漸放大的排氣管比較好,這種放大式的排氣管設計,是採用慢慢擴大管徑的方式,使越往後越膨脹的廢氣加速排出,特別是在持續高轉速的情形下效果會更加明顯,這種方式也是日系改裝車選擇比較多的排氣管。
  在現在量產車的排氣管當中有一個阻礙排氣的因素,就是我們常說的三元催化。三元催化的基本功能在於淨化尾氣,並且它還有消除共鳴的功能,減少共鳴噪音傳至駕駛艙中。三元催化是由許多重金屬密密麻麻組成的蜂巢結構,它的存在阻礙了排氣,而且又是排氣管中高溫的聚集點。所以拆除三元催化,就能使排氣順暢,提升馬力三到五匹左右。三元催化的外型,看起來設計成膨脹室的功能,因此能提供一個排氣的阻力點,也就是在排氣上提供了一定的回壓功能。
  改裝的排氣管一般都做的比原廠小,以增加排氣的速度。某些管徑偏粗的型式,是為了達到中低轉速扭力的平衡。當拿掉了三元催化時,排氣管的管徑又比較粗,那麼就需要最後段增加一個小的回壓槽。擁有消音功能的尾段排氣管,是最大的排氣阻力的來源,消音過程靠的是利用隔板讓排氣撞擊,由反射波的形成減低音量,原廠大都是採用如此的型式,而改裝的排氣管則都採用直線筒式,纏繞消音棉來吸收噪音,這樣無隔板直線式的設計,自然對馬力的提升有相當大的幫助。
  想要降低排氣尾段的排氣阻力、又想擁有相對的排氣阻力來保持相當的扭力,本來是相反的論點,但近年來設計師的鑽研,運用閥門裝置在進入尾段排氣管之前安裝活動閥門,便能由負荷的多少來控制回壓的改變,例如新的Integra-R、BMW 328等都有這樣的設計。改裝品牌上很多採用直排的尾段,在價格及耐久度上往往有著很大的差異,都是使用的材質及消音棉的原因;在直通式上噪音的吸收是靠消音棉來完成,一般都使用單純的玻璃纖維棉,但是長時間後玻璃棉會因高熱而劣化,燒蝕會使噪音變大。為了消除這個現象,使用高耐久度的不銹鋼絲消音棉,這種設計的不同點,是先用不銹鋼包覆內管的孔道,然後再將玻璃棉填入,目的是不銹鋼絲防止高熱傳導到玻璃棉上,使玻璃吸音棉延長壽命。
  由於考慮到驗車及噪音方面帶來的不必要的麻煩,許多廠家也推出了低噪音並且尾氣排放達標的排氣管;這類排氣管無論外型長相、聲音等都比誇張的純改裝排氣管更低調一些,而且表現也還可以,主要原因是利用原廠的大筒身,增加更多的消音棉來吸收噪音,但其內部依然為直排模式,所以才能在提升馬力之外,保持最大的靜音效果。
  現在我們來聊聊整個排氣系統的頭段,也就是我們俗稱的芭蕉。在排氣管的改裝中,最重要的部分就要算是頭段了,由於原廠排氣管頭段幾乎都是大量模具製造的鑄鐵材料,內管壁比較粗糙,各歧管長度也都不盡相同,加上接合的方式、距離、形狀等等不是純粹考慮動力性能,所以基本上都會干擾排氣降低發動機性能,使得各缸排出的廢氣相互衝突產生阻力。尤其頭段又是最為靠近發動機缸蓋的位置,對進氣燃燒會產生非常明顯的影響。現在改裝廠商推出的頭段絕大多數都採用等長的設計(當然也有不等長設計的頭段,比如在北京,翼豹車主使用的比較多的日本HKS所推出的頭段就不是等長的)。絕大多數都是使用管內壁平滑的不銹鋼材質,有的廠家則在歧管底座和介面部份使用模具鑄造的方式,實施無段差的介面,並儘量延伸彎角的角度,使得阻力減少、加速排出氣體的速度。
  更好的型式則是將支氣管的長度統一,最大程度的做到支氣管等長,讓排氣距離都一致,徹底消除各支管之間的壓力差。這樣不但利於後段排氣管的回壓,而且使整體的吸排氣效率也能大幅度的提升。
  以上所說的這些原理也許網友們一時間還不能全部消化,總結成一句話那就是在改裝排氣管之前,最好是先考慮自己車的特點是自然進氣的發動機還是渦輪發動機,是要求低轉高扭力的輸出還是要高轉速馬力的輸出,因為在這些方面必須有個取捨。舉個例子如果是自動檔的車,最好別換管徑太大的排氣管,會影響自動檔的換檔,嚴重的話會使轉速根本就達不到高轉速就被強制換檔了。而更換排氣管使發動機排氣順暢,勢必就需要更多的進氣量,當進氣和排氣都有所增加的時候,噴油量也需要有所調整。所以改裝是各個部位都有所關聯的。想要能夠真正的把所改裝的排氣管發揮到最大效果,就必須瞭解跟它有關的其他部分。這句話不光是用於排氣管的改裝也適用於所有改裝的原理。

 


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