benefits of short trumpets ?
Discussion
The standard trumpet length is tuned for 5280rpm with the standard SD1 cam, most cams in TVR's are set to give peak power at around 5500-6000rpm so by matching the trumpet length to cam you get better power and less holes in the torque delivery.
Not only that the std 38mm trumpets are very close to the plenum roof on the middle 4 cylinders so you unmask these cylinders by lowering the trumpet.
25mm is a good start as the metal 'on' machine is harder to work
Not only that the std 38mm trumpets are very close to the plenum roof on the middle 4 cylinders so you unmask these cylinders by lowering the trumpet.
25mm is a good start as the metal 'on' machine is harder to work
THe intake tract length is not the same for all cylinders with TVR V8 engines, the outer cylinder tracts being considerably longer. This may have been a design cock up or design intent after much engine development testing.
Long trumpets induce torque, short trumpets induce power. Driveabilty is a balance between the two. With regard to the tall trumpets of the 4.0 litre engines and the minimal clearance to the plenum roof the simplestt solution is to fit the SC Power Plenum spacer, this raises the plenum increasing clearance and improving air flow, apart from increasing the overall volume of air in the plenum to the benefit of engine breathing and performance. In my opinion 25mm is far too large an adjustment and if applied across all cylinders will probrably be detrimental.
Long trumpets induce torque, short trumpets induce power. Driveabilty is a balance between the two. With regard to the tall trumpets of the 4.0 litre engines and the minimal clearance to the plenum roof the simplestt solution is to fit the SC Power Plenum spacer, this raises the plenum increasing clearance and improving air flow, apart from increasing the overall volume of air in the plenum to the benefit of engine breathing and performance. In my opinion 25mm is far too large an adjustment and if applied across all cylinders will probrably be detrimental.
The inlet tracts are all the same length, the 'longer' trumpets in the centre 4 are to compensate for the shorter run in the lower manifold.
Conventional tuning will reduce all the trumpets by the same amount
to roughly calculate the required inlet length
ECD = Effective Cam Duration- period for which valve is off its seat by an effective amount
RV = Reflective Value = 2 for 2nd reflection 3 for 3rd etc – the earlier the reflection = the stronger the tuning
ECD = 720 - (Adv. duration - 30)
Speed of sound = 396m/sec
intake runner length = ((ECD/360) × (1/(tuned rpm /60))* speed of sound)/(2*RV)
( ((470/360) x (1/6000 /60))*396) /(2 *6) = 0.43mtrs
Most of the 45mm trumpet bases I've seen feature a trumpet which is 30-50mm shorter than standard in addition to the larger diameter port, admittedly as the pipe size increases so the gas speed reduces so you need to compensate for the lower speed by shortening it still further.
I've run 25mm shorter and 40mm shorter on a 38mm trumpet set-up and it tends to shift peak power to 6K and 6250rpm respectively, this matches the cam better and suits a sports car better - well to my way of thinkng anyway.
If you prefer a lower peak power speed (and corresponding lower figures) then by all means leave them at standard length or fit the spacer.
Conventional tuning will reduce all the trumpets by the same amount
to roughly calculate the required inlet length
ECD = Effective Cam Duration- period for which valve is off its seat by an effective amount
RV = Reflective Value = 2 for 2nd reflection 3 for 3rd etc – the earlier the reflection = the stronger the tuning
ECD = 720 - (Adv. duration - 30)
Speed of sound = 396m/sec
intake runner length = ((ECD/360) × (1/(tuned rpm /60))* speed of sound)/(2*RV)
( ((470/360) x (1/6000 /60))*396) /(2 *6) = 0.43mtrs
Most of the 45mm trumpet bases I've seen feature a trumpet which is 30-50mm shorter than standard in addition to the larger diameter port, admittedly as the pipe size increases so the gas speed reduces so you need to compensate for the lower speed by shortening it still further.
I've run 25mm shorter and 40mm shorter on a 38mm trumpet set-up and it tends to shift peak power to 6K and 6250rpm respectively, this matches the cam better and suits a sports car better - well to my way of thinkng anyway.
If you prefer a lower peak power speed (and corresponding lower figures) then by all means leave them at standard length or fit the spacer.
CHGRIFF said:
With regard to the tall trumpets of the 4.0 litre engines and the minimal clearance to the plenum roof the simplestt solution is to fit the SC Power Plenum spacer, this raises the plenum increasing clearance and improving air flow, apart from increasing the overall volume of air in the plenum to the benefit of engine breathing and performance.
The SC Power plenum spacer is very pretty but for some bizarre reason they decided to make it out of highly heat conducting aluminium rather than an insulator. A phenolic spacer would be cheaper and do a better job.CHGRIFF said:
In my opinion 25mm is far too large an adjustment and if applied across all cylinders will probrably be detrimental.
What's the basis for that opinion? Matt350 has already explained the relation between length and tuned rpm, and why the standard ones are too long. i have a trumpet base and a set of 38mm A.c.T carbon trumpets for sale, work well on the 4l 4.5 not sure about the 5l .these trumpets give the best extra bhp and torque on the dyno run .any one interested ?at the same time you could fit a insulator gasket between the trumpet base and inlet manifold . this chap sells a very good gasket for a good price matthew.feasey@sky.com.
Edited by daz the plumber on Sunday 19th July 20:49
I agree with Matts calculations, my point was concerning the 4.0 litre engine whereby TVR have seem to have set long trumpets to induce torque rather than power, probably to optimize drive-ability and acceleration at the bottom end of the rev range, maybe to optimise 0-60 times. The RPM at which peak power and torque occurs is moved by changing the tract lengths, if you want this at any specific rpm make the adjustment. The calculations be it 3rd or 4th iteration determines the changes to induction length. The point I made is that by reducing the trumpet lengths it will change the relationship of power and torque with a reduction of the latter. Reducing torque of the 4.0 litre may reduce low speed acceleration to the detriment of 0-60 times. However I agree, having more power feels quicker, but possibly not against the stop watch. Regarding the cylinder tract length from the valve face, the theoretical centre line length of the individual tracts of the valve differs. I believe this can be demonstrated by filling the tracts with equal amounts of oil and measuring the differential. Out of curiosity what base is used to determine current cylinder tract lengths, physical measurement or TVR documentation?
The plenum spacer is the simplest and cheapest modification to raise the plenum to improve engine breathing without changing existing power and torque characteristics, in particular the 4.0 litre, unless you can carry out the work yourself to modify the trumpet lengths. The 4.5 and 5.0 litre engines use shorter trumpets to influence more power as they already have substantial amounts of torque. TVR Power Taraka has a very short induction tract i.e. no trumpets. Does anyone have before and after power and torque figures to see how the data changed after their conversion.
Regarding materials, the plenum top is aluminium, this being quite an expensive material with associated tooling. This is a low stressed item and could have been produced from injection moulded plastic with considerable cost and weight saving. Why did Rover choose the aluminium over plastic? Be interested in any opinion.
Too often the trumpets are cut down by estimated amounts taken out of thin air. Any reduction will increase power but the problem is unless the dimension correlates to wave bounce you could end up with peak power and torque way out on the rev band. I MO anyone wanting to make this change should consult someone like Matt or others with the knowledge. It’s better to get it right first time. At the end of the day the results are subjective, road cars need torque, race cars need power, or anywhere in between.
The plenum spacer is the simplest and cheapest modification to raise the plenum to improve engine breathing without changing existing power and torque characteristics, in particular the 4.0 litre, unless you can carry out the work yourself to modify the trumpet lengths. The 4.5 and 5.0 litre engines use shorter trumpets to influence more power as they already have substantial amounts of torque. TVR Power Taraka has a very short induction tract i.e. no trumpets. Does anyone have before and after power and torque figures to see how the data changed after their conversion.
Regarding materials, the plenum top is aluminium, this being quite an expensive material with associated tooling. This is a low stressed item and could have been produced from injection moulded plastic with considerable cost and weight saving. Why did Rover choose the aluminium over plastic? Be interested in any opinion.
Too often the trumpets are cut down by estimated amounts taken out of thin air. Any reduction will increase power but the problem is unless the dimension correlates to wave bounce you could end up with peak power and torque way out on the rev band. I MO anyone wanting to make this change should consult someone like Matt or others with the knowledge. It’s better to get it right first time. At the end of the day the results are subjective, road cars need torque, race cars need power, or anywhere in between.
CHGRIFF said:
I agree with Matts calculations, my point was concerning the 4.0 litre engine whereby TVR have seem to have set long trumpets to induce torque rather than power, probably to optimize drive-ability and acceleration at the bottom end of the rev range, maybe to optimise 0-60 times.
I don't think they were that scientific about it, they just kept the standard RR 3.9 EFI setup which was designed for a big heavy vehicle with a stumpy cam. Matt's calculations show that this produces a tuning peak at quite lows rpms and it is beneficial to shorten it to give the extra torque closer to peak power. It's not a controversial suggestion - plenty of people have already tried this and found it works.
350Matt said:
The inlet tracts are all the same length, the 'longer' trumpets in the centre 4 are to compensate for the shorter run in the lower manifold.
Conventional tuning will reduce all the trumpets by the same amount
to roughly calculate the required inlet length:
ECD = Effective Cam Duration- period for which valve is off its seat by an effective amount
RV = Reflective Value = 2 for 2nd reflection 3 for 3rd etc – the earlier the reflection = the stronger the tuning
ECD = 720 - (Adv. duration - 30)
Speed of sound = 396m/sec
intake runner length = ((ECD/360) × (1/(tuned rpm /60))* speed of sound)/(2*RV)
( ((470/360) x (1/6000 /60))*396) /(2 *6) = 0.43mtrs
.......
I've run 25mm shorter and 40mm shorter on a 38mm trumpet set-up and it tends to shift peak power to 6K and 6250rpm respectively, this matches the cam better and suits a sports car better - well to my way of thinking anyway.
I am looking to shorten the trumpets on my Chimaera 450 4.6l RV8 to modify the rpm that peak torque occurs, in order to bring that into line with the TVR "S-Pack" cam with it's more aggressive cam profile (310degrees according to Kent who made it).Conventional tuning will reduce all the trumpets by the same amount
to roughly calculate the required inlet length:
ECD = Effective Cam Duration- period for which valve is off its seat by an effective amount
RV = Reflective Value = 2 for 2nd reflection 3 for 3rd etc – the earlier the reflection = the stronger the tuning
ECD = 720 - (Adv. duration - 30)
Speed of sound = 396m/sec
intake runner length = ((ECD/360) × (1/(tuned rpm /60))* speed of sound)/(2*RV)
( ((470/360) x (1/6000 /60))*396) /(2 *6) = 0.43mtrs
.......
I've run 25mm shorter and 40mm shorter on a 38mm trumpet set-up and it tends to shift peak power to 6K and 6250rpm respectively, this matches the cam better and suits a sports car better - well to my way of thinking anyway.
It looks like the 450 has shorter trumpets than the 400 anyway, but I can find no figures to identify exactly where peak torque occurs in the 450 with these trumpets.
Can anyone give me any information on the best amount to shorten these trumpets by in order to move peak torque to around 5800-6000rpm..?
My V8 tuner is thinking that 10mm would be a reasonable amount to take off the trumpets - but looking at the above posts, it looks like lengths like 25mm might be more appropriate to shift power up..? Only problem is that we don't know what rpm the 450 trumpets are optimised for in the 1st place...
Have PM'ed 350 Matt, but I don't know how often he is on this forum, so any info much appreciated!
Dom
Hello Dom
Best thing to do is measure the length of your exisiting port from the back of the valve to the top of the trumpet, if you pick one of the middle cylinders which has a reasonable run then a steel tape measure should give you a rough figure.
You can then punch this number into the previous equation and come up with a length for yourself.
Best thing to do is measure the length of your exisiting port from the back of the valve to the top of the trumpet, if you pick one of the middle cylinders which has a reasonable run then a steel tape measure should give you a rough figure.
You can then punch this number into the previous equation and come up with a length for yourself.
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