Short induction - How does it work?
Discussion
Hi all - was wondering if someone with tuning knowledge could explain why a "short" induction kit (e.g. whirlwind pipes) gives a bhp gain on a 4.5 beyond the standard "long" purple pipes. As I understand it, the explanation goes beyond the fact that the purple pipes can contract at high revs and reduce air flow.
Also, given that this is the case, why didn't TVR fit a shorter system to the 4.5 as standard? I'm guessing shorter pipes have to be better-made than the longer ones, and therefore cost more.
Thanks
Neal
Also, given that this is the case, why didn't TVR fit a shorter system to the 4.5 as standard? I'm guessing shorter pipes have to be better-made than the longer ones, and therefore cost more.
Thanks
Neal
Hi Neal .. i can probably answer this one away for as it was me who first did a short induction 4.5 (outside of any factory stuff we don't know about).
I first noticed a trend in the way the hp production characteristic of the 4.2 cerbie differed from the 4.5 cerbie, and after doing a few basic calculations it became apparent that TVR had biased the 4.5 engine to emphasise the midrange, the induction length for the 4.5 calculates out to give it's primary pulse at around 4500rpm .. and if you look at any healthy std 4.5 you'll find peak torque is about 4500/4600 rpm.
However .. the longer length hinders it's HP ability at higher revs, so what was needed for those owners wanting to make the best use of the top end of the rev range on their 4.5s was a shorter induction length to shift the primary tuning pulse up the rev range.
Ceejay's car was the first cerb to wear a short induction and at the surrey rolling road shootout was about 15-20bhp up on the best of the rest remapped std 4.5 cars (ceejay's made 392hp that day, just 8hp shy of the magic 400 we'd seen a few days previously at noble motorsport), .. that car stil makes close to 400hp even now 3 years on from the original work.
There is a downside in that the shorter length does slightly compromise the cylinder filling at very low rpm, but at low rpm you're not interested in performance anyway . if you are you're either a caravan owner used to slogging the lower revs, or you're in the worng gear or in the wrong car
A remapped cerb 4.5 with short induction outperforms the std cerb 4.5 everywhere in the rev range though, this is beacuse the 4.5 mapping is nowhere near optimum as std for good power, so you can gain more from getting the mapping right than you lose by using a short inlet length.
Quite why TVR went this long induction and then compromised on the maping is anyone's guess .. but it does mean that owners can get their cars nearly up to what TVR claimed for them for not a lot of money in tuning terms
I first noticed a trend in the way the hp production characteristic of the 4.2 cerbie differed from the 4.5 cerbie, and after doing a few basic calculations it became apparent that TVR had biased the 4.5 engine to emphasise the midrange, the induction length for the 4.5 calculates out to give it's primary pulse at around 4500rpm .. and if you look at any healthy std 4.5 you'll find peak torque is about 4500/4600 rpm.
However .. the longer length hinders it's HP ability at higher revs, so what was needed for those owners wanting to make the best use of the top end of the rev range on their 4.5s was a shorter induction length to shift the primary tuning pulse up the rev range.
Ceejay's car was the first cerb to wear a short induction and at the surrey rolling road shootout was about 15-20bhp up on the best of the rest remapped std 4.5 cars (ceejay's made 392hp that day, just 8hp shy of the magic 400 we'd seen a few days previously at noble motorsport), .. that car stil makes close to 400hp even now 3 years on from the original work.
There is a downside in that the shorter length does slightly compromise the cylinder filling at very low rpm, but at low rpm you're not interested in performance anyway . if you are you're either a caravan owner used to slogging the lower revs, or you're in the worng gear or in the wrong car
A remapped cerb 4.5 with short induction outperforms the std cerb 4.5 everywhere in the rev range though, this is beacuse the 4.5 mapping is nowhere near optimum as std for good power, so you can gain more from getting the mapping right than you lose by using a short inlet length.
Quite why TVR went this long induction and then compromised on the maping is anyone's guess .. but it does mean that owners can get their cars nearly up to what TVR claimed for them for not a lot of money in tuning terms
Theoretically the shorter pipes would be better if they were made out of something that didn't flex.
Have you tried manufacturing metal equivalents of your shortened induction pipes? Or does the cost prohibit the investigation?
The 4.2 looked like the ideal inlet, but has anyone managed to produce a 4.2 inlet but with the right diameter bore?
Better still has anyone rotated and shrunk the airbox to fit them over small straight inlets.
One last question. Has anyone ever quantified the effect of the airbox on airflow. In other words the pressure in the airbox. Say I poked a hole in my airbox. Would I expect the engine to speed up or slow down.
Have you tried manufacturing metal equivalents of your shortened induction pipes? Or does the cost prohibit the investigation?
The 4.2 looked like the ideal inlet, but has anyone managed to produce a 4.2 inlet but with the right diameter bore?
Better still has anyone rotated and shrunk the airbox to fit them over small straight inlets.
One last question. Has anyone ever quantified the effect of the airbox on airflow. In other words the pressure in the airbox. Say I poked a hole in my airbox. Would I expect the engine to speed up or slow down.
trackcar said:
A remapped cerb 4.5 with short induction outperforms the std cerb 4.5 everywhere in the rev range though, this is beacuse the 4.5 mapping is nowhere near optimum as std for good power, so you can gain more from getting the mapping right than you lose by using a short inlet length.
I don't quite understand the last bit?Are you saying that you must remap in order to get some benefit from the short Induction kit?
And so I pressume that a bespoke remap will yield more hp than the standard map. But by how much? Also, how much more hp would a short induction (plus remap) give over just the bespoke remap?
thegreengoblet said:
trackcar said:
A remapped cerb 4.5 with short induction outperforms the std cerb 4.5 everywhere in the rev range though, this is beacuse the 4.5 mapping is nowhere near optimum as std for good power, so you can gain more from getting the mapping right than you lose by using a short inlet length.
I don't quite understand the last bit?Are you saying that you must remap in order to get some benefit from the short Induction kit?
And so I pressume that a bespoke remap will yield more hp than the standard map. But by how much? Also, how much more hp would a short induction (plus remap) give over just the bespoke remap?
thegreengoblet said:
trackcar said:
A remapped cerb 4.5 with short induction outperforms the std cerb 4.5 everywhere in the rev range though, this is beacuse the 4.5 mapping is nowhere near optimum as std for good power, so you can gain more from getting the mapping right than you lose by using a short inlet length.
I don't quite understand the last bit?Are you saying that you must remap in order to get some benefit from the short Induction kit?
And so I pressume that a bespoke remap will yield more hp than the standard map. But by how much? Also, how much more hp would a short induction (plus remap) give over just the bespoke remap?
julian64 said:
Theoretically the shorter pipes would be better if they were made out of something that didn't flex.
Have you tried manufacturing metal equivalents of your shortened induction pipes? Or does the cost prohibit the investigation?
The 4.2 looked like the ideal inlet, but has anyone managed to produce a 4.2 inlet but with the right diameter bore?
Better still has anyone rotated and shrunk the airbox to fit them over small straight inlets.
One last question. Has anyone ever quantified the effect of the airbox on airflow. In other words the pressure in the airbox. Say I poked a hole in my airbox. Would I expect the engine to speed up or slow down.
Have you tried manufacturing metal equivalents of your shortened induction pipes? Or does the cost prohibit the investigation?
The 4.2 looked like the ideal inlet, but has anyone managed to produce a 4.2 inlet but with the right diameter bore?
Better still has anyone rotated and shrunk the airbox to fit them over small straight inlets.
One last question. Has anyone ever quantified the effect of the airbox on airflow. In other words the pressure in the airbox. Say I poked a hole in my airbox. Would I expect the engine to speed up or slow down.
My short pipes are 4 ply silicone so don't flex much, plus silicone is an excellent heat isolator .. you could have them made out of lovely shiny stainless steel or polished alloy and they would look great, but bet they lose hp from heat soak raising the inlet charge temp, though I've never tried it back to back. The very slight flexibility in the silicone allows for the production tolerances of the TVR airbox to throtle distance
The airboxes are quite important .. I did an experiment once where I used just the backplate trumpets and blew cold air over the engine and isolated the exhaust heat as far as possible and the hp drop was massive, something like 40hp through most of the rev range.
http://www.pistonheads.com/xforums/topic.asp?h=0&a...
You'll only regret not doing it sooner
Edited by PhilCerbera on Friday 5th October 17:12
trackcar said:
thegreengoblet said:
trackcar said:
A remapped cerb 4.5 with short induction outperforms the std cerb 4.5 everywhere in the rev range though, this is beacuse the 4.5 mapping is nowhere near optimum as std for good power, so you can gain more from getting the mapping right than you lose by using a short inlet length.
I don't quite understand the last bit?Are you saying that you must remap in order to get some benefit from the short Induction kit?
And so I pressume that a bespoke remap will yield more hp than the standard map. But by how much? Also, how much more hp would a short induction (plus remap) give over just the bespoke remap?
Oh, and with a view of getting the short induction and bespoke MBE remap after.
FUBAR said:
trackcar said:
FUBAR said:
trackcar said:
Ceejay's car was the first cerb to wear a short induction
When did you fit them Joolz? 
you were 5 months later that's all thegreengoblet said:
So I'm off to get my Ohlins done this month. Next on my list is to replace my sports cat box with an SP performance system. Would you recommend this over simply replacing my cats with a linked decat pipe? (My cats are knocking and rattling away so I need to do it soon to preserve them for MOTs).
Oh, and with a view of getting the short induction and bespoke MBE remap after.
The SP exhaust is the best on the market so far Oh, and with a view of getting the short induction and bespoke MBE remap after.
Linking the decats will be of benefit even if you have the JP sports tails (the best flowing of the aftermarket loud exhausts) .. The SP system also uses linked decats and a link inside the back box .. ive got a system up and running at the moment which i hope will be the highest HP system yet but it's only been on one car so far so results aren't conclusive. If youve got the money for it though, there's no reason not to go for the SP now.
trackcar said:
thegreengoblet said:
So I'm off to get my Ohlins done this month. Next on my list is to replace my sports cat box with an SP performance system. Would you recommend this over simply replacing my cats with a linked decat pipe? (My cats are knocking and rattling away so I need to do it soon to preserve them for MOTs).
Oh, and with a view of getting the short induction and bespoke MBE remap after.
The SP exhaust is the best on the market so far Oh, and with a view of getting the short induction and bespoke MBE remap after.
Linking the decats will be of benefit even if you have the JP sports tails (the best flowing of the aftermarket loud exhausts) .. The SP system also uses linked decats and a link inside the back box .. ive got a system up and running at the moment which i hope will be the highest HP system yet but it's only been on one car so far so results aren't conclusive. If youve got the money for it though, there's no reason not to go for the SP now.
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