Tumpets

[MARSHO]

I have a 5 litre TVR v8 running quad downdraughts, question is what is the maximum length of the ram pipes /trumpets that I could fit without being detromental to the engine performance, as the ones that are fitted are only very short, and having another set which are far to long I was thinking of modifying these to suit and maybe get a little added performance ,cheers David.

[Slow M]

MARSHO said:

I have a 5 litre TVR v8 running quad downdraughts, question is what is the maximum length of the ram pipes /trumpets that I could fit without being detromental to the engine performance, as the ones that are fitted are only very short, and having another set which are far to long I was thinking of modifying these to suit and maybe get a little added performance ,cheers David.

David,

ram pipe length is a component of engine tuning; short ram pipes usually being fitted when you want to move your power band higher in the RPM range and long longer ram pipes or velocity stacks as they are called on this side, being utilised when you want to have more torque lower in the rev range. The ram pipe length should be matched to your camshaft profile.

Bernard.

[MARSHO]

Thanks for the quick reply Bernard, how do I work out the length from my cam profile ,the car is mainly used on the drag strip and has a crane cam, thanks David.

[Slow M]

MARSHO said:

Thanks for the quick reply Bernard, how do I work out the length from my cam profile, the car is mainly used on the drag strip and has a crane cam, thanks David.

David, first check with Crane where in the rev range the maximum power of your cam is expected to be. IIRC they post a fair bit of info on a .pdf somewhere on their site.

Once you've figured out what properties you have in your cam, look for what is available in the way of ram pipes. One thing to look for is that they have a decent radius. A few years some people were advocating a full radius as being even more important than length. See image below-

...and appreciate the SU mounting flange on this one (made me chuckle)...


You may even be able to get one of the Crane tech guys to assist you in determining optimal intake tract length.
B.

[MARSHO]

Thanks for the replys it looks like a bit more reserch is required , David.

[rev-erend]

If you have a standard manifold - measure with a piece of string from the mouth of the manifold nearest valves to the tip of the trumpets.. and that will be the max length needed and what the manufacturer tested as best for drivability / torque with the standard cam fitted.

Going shorter will probably suit more radical cams and higher rpm power.

[Redmist336]

David, you want to research Helmholtz. It's the resonance theroy behind naturally aspirated "supercharging" or getting over 100% VE. It's very hard to calculate, however some time spent on Helmholtz is extremely beneficial. Ignore any online calculator that doesn't take into effect, trumpet diameter, length to valve, cam duration, cam timing, swept volume. It is considerably more complex than that as the resonance can be upset by simple bends in the trumpet, the surface of the trumpet (ie boundary layer) the density of the air, where the fuel is injected, cam overlap and a tonne of additional factors. This is why many race teams run a sliding trumpet on the dyno to prove where in the RPM to get the Helmholtz "boost". (F1 teams use a continiously sliding computer controlled trumpet) You may also wish to tune the frequency for the lower end of your working RPM band. I've had great sucess tuning it to give an immediate boost when changing up. A hole in an existing dyno plot can point to a poorly tuned Helmholtz frequency or possibly a bad relective resonance in the exhaust. Don't try and fill a hole with your Helmholtz "boost", correct the issue and use the "boost" to spread your torque in the useable RPM band.
Don't alwasys think longer is better. It fully depends on how many bounces of the reflective pressure wave you're using. Again use an online calculator to point you in the right basic direction for length.
Other than Helmholtz, you can also reflect the reversion pulse off the back of an airbox which can also give you above 100% ve.

[MARSHO]

Thanks for the info, it loks like its getting quite involved, David.

[GreenV8S]

350Matt is heavily into this side of things and did explain the theory in one of his posts a year or so back, so worth a search. From memory, the conclusion was that the standard lengths were optimised for stump pullers and a higher revving engine might go better with about 20mm off.

I also remember Joolz reporting that replacing the standard TVR tapered tubes with nicely flared bell mouths made no significant difference.