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New exhaust....to go 2.5 or 3 in?? Thoughts?
- Thread starter jjsaul
- Start date
Michael Montgomery
Well-Known Member
Yes I agree looking for better boost rather than higher boost hence wanting to upgrade to a 100 series turbo.
The thing is, while it’s tempting to think that a bigger exhaust is better, there is a ‘sweet spot’, beyond which it will be too big.
The thing is, if you’ve got a free flowing exhaust at a certain speed, and you let off the power, the slug of exhaust is continuing rearward towards the exit at the same speed it was at full chat. Now, Air is elastic and so is exhaust gas, so while that which has just left the turbo is already slowing down, that towards the tail pipe is still going at full chat so a partial vacuum is created which increases along the exhaust pipe back to the turbo where it will be strongest. So when that wastegate opens due to overpressure in the intake, the vacuum extends through it into the manifold before the turbo. The intake overpressure then backs up on the turbo, slowing it from the intake side which is why the boy racers fit dump valves.
If the exhaust is too big, the slug of gas moves along it at a far lower velocity.
Think of a kinetic recovery rope. If you take it slow, the force exerted on the stuck vehicle will be less than going faster as the pull will be less. This principal is used to great effect to increase scavenging on two stroke engines.
Then there’s eddies and turbulence within the pipe which can impede free flow, as will any bends or constructions.
My pal has built and run road rally cars for some time. It’s thanks to him going down from 3” to 2 1/2” that’s given me some 3” parts and straight tube. Apparently his tests showed the 2 1/2” exhaust was giving him some extra horses over the 3”.
He’ll be impressed, I was listening to him when he explained it all.
The thing is, if you’ve got a free flowing exhaust at a certain speed, and you let off the power, the slug of exhaust is continuing rearward towards the exit at the same speed it was at full chat. Now, Air is elastic and so is exhaust gas, so while that which has just left the turbo is already slowing down, that towards the tail pipe is still going at full chat so a partial vacuum is created which increases along the exhaust pipe back to the turbo where it will be strongest. So when that wastegate opens due to overpressure in the intake, the vacuum extends through it into the manifold before the turbo. The intake overpressure then backs up on the turbo, slowing it from the intake side which is why the boy racers fit dump valves.
If the exhaust is too big, the slug of gas moves along it at a far lower velocity.
Think of a kinetic recovery rope. If you take it slow, the force exerted on the stuck vehicle will be less than going faster as the pull will be less. This principal is used to great effect to increase scavenging on two stroke engines.
Then there’s eddies and turbulence within the pipe which can impede free flow, as will any bends or constructions.
My pal has built and run road rally cars for some time. It’s thanks to him going down from 3” to 2 1/2” that’s given me some 3” parts and straight tube. Apparently his tests showed the 2 1/2” exhaust was giving him some extra horses over the 3”.
He’ll be impressed, I was listening to him when he explained it all.
Towpack
Well-Known Member
With a turbo, the freer flowing the exhaust the better, get the gases out as quickly as possible especially on a low revving Diesel engine. This isn’t some highly tuned 4T NA petrol engine where the entire exhaust is tuned and matched to the intake to aid gas scavenging and deliver a specific torque/power curve.
It’s cooling that really puts the limit on increasing boost and fuelling on the HDT/FT engines in the LC’s. The Yanmar marine versions deliver considerably more reliable power but obviously benefit from a level of cooling virtually impossible to replicate in a road vehicle. JMO
It’s cooling that really puts the limit on increasing boost and fuelling on the HDT/FT engines in the LC’s. The Yanmar marine versions deliver considerably more reliable power but obviously benefit from a level of cooling virtually impossible to replicate in a road vehicle. JMO
Well his car is naturally aspirated so that backs up what you’re saying TP.
It seems the throat of the turbo is around 60mm (thanks Mike for the measure) so the elbow off the turbo is larger at the 3 bolt flange at 65mm. It’s fair to say it’s a pretty tight bend that’s more of a plenum with a hole at the bottom than anything nicely swept so a nice easy bend would be better I’m sure.
I’ve decided there are far more restrictions within the factory system than at the dump pipe so at least for the time being I’m going to leave this part and concentrate on the downpipe followed by the rest of the system as and when it needs replacement.
I’ve decided there are far more restrictions within the factory system than at the dump pipe so at least for the time being I’m going to leave this part and concentrate on the downpipe followed by the rest of the system as and when it needs replacement.
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Tom Edwards
New Member
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- Jun 18, 2018
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I have no experience on Land Cruisers, but for my sons 1971 MGB, we bought from Bell a complete stainless steel system including 3 into 2 into 1 manifold for £120 3 years ago. They do fabricate, they were going to do a system for my 260Z, and did a complete system for a friends Cresta.
Maybe worth having a chat - based outside Swindon.
http://www.bell-steel.co.uk/index.htm
Maybe worth having a chat - based outside Swindon.
http://www.bell-steel.co.uk/index.htm
If you decide to leave the dump pipe and go for the three bolt flange thisTypically now i've booked my truck in to Prospeed on monday telling him i can bring the flange i can't find one
https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.co.uk/ulk/itm/132245719958
was the one I received today from Germany after ordering Saturday.
Nearly finished, just got to trim off the excess at the flanges, fit an internal reducing cone to get round the step from 76mm to 65mm into the factory centre section, add some gaskets and bolt on. Then fabricate a stainless clamp for the enginevend and that’s that section complete. If I need a resonator in the first section I can cut one in later.
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- Feb 2, 2013
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- 16,204
Nah i was just trying to save him some time Rich he can make it , he made my last one .
Fair to say the 3" exhaust on my 90 improved things a lot but went i went back and told him i want it to start at the turbo it improved things a lot more , hence i wouldn't even consider not starting at the turbo flange even if the 80 is different .
Fair to say the 3" exhaust on my 90 improved things a lot but went i went back and told him i want it to start at the turbo it improved things a lot more , hence i wouldn't even consider not starting at the turbo flange even if the 80 is different .
I can see why the 90 one is restrictive. It’s insane! The 80 is a whole different ball game but with Charlie’s experience, why not. At a guess I’d say the 80 one has 5% restriction, and the 90 one 15%. Looking at the Centre section the pipe goes down from 65mm to probably 50mm or so. That’s one of the 80’s mad bits, the other is the bit over the back axle that Chris pointed out. It gets flattened slightly. Bonkers!
nielsc
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I went from a stock exhaust to 2.5" side exit exhaust with resonator, some difference noticable in responsiveness.
I then went to a full length 3" with a resonator including the dump pipe on the turbo and this added again to the responsiveness of the engine, it's not that the turbo is giving full boost earlier in the rev range, it just picks up better and just goes better over the entire rev range.
I then went to a full length 3" with a resonator including the dump pipe on the turbo and this added again to the responsiveness of the engine, it's not that the turbo is giving full boost earlier in the rev range, it just picks up better and just goes better over the entire rev range.
