Jason Moyle Racing

I was working through some options earlier to ensure a customer had the right trumpet length for his 16v Corrado and figured an article based on the popular variations for 8 & 16v carb & ITB conversions would be a help...and I found some common ground whilst armed with a tape measure!...read on.

To start with, some basics ...

8v...

Head port length...valve seat to manifold face...100mm
DCOE type inlet manifold...83mm
Misab plate (carb to manifold gasket/seal)...5mm
DCOE / DHLA sidedraught carb...118mm

For those without a calculator to hand the above adds up to 306mm. A good starting point for "total" induction length for open type engines...i.e carbs or T/B's rather than std type plenums is 320mm from the trumpet tip to the valve seat. Now it's not quite an exact measurement as the distance through the head port and the manifold will vary going through the turns & around both the long roof side & short floor side of the head...so I've come to the above figures by measuring the long & short turns and averaging them out, then comparing them with a measurement through the middle of the ports & manifold.

So based on the above, we need a trumpet for 8v engines of around 14mm to start with. Well it's quite handy then that the shortest current trumpet that Weber can supply is 16mm!

I've had good result's in the past with 40mm trumpets on 45 DCOE's, giving a total of 346mm on 1800 hillclimb engine's....making 172hp @ 7,200, and when changed to 60mm trumpets the owner reported back a noticable performance improvement on track as expected, though we did'nt get to re-check it on the dyno.

On my own hillclimb engine I've always used 60mm trumpets on my 48's, giving a total length of 366mm. The old longer for low end and shorter for top end doesn't always ring true as my engine, and others I've done since with similar lengths make peak power between 7,500 & 8000...so dont lack top end with longer intake lengths in 8v form. When I'm next on the dyno and time permits, I'll try a few different lengths and see how it rocks the power curve, but for now the above guide will get you in the ballpark.

And for the 16v brigade...

Head port, valve seat to manifold face...96mm
JMR DCOE inlet manifold...75mm
Misab plate/gasket...5mm
DCOE/DHLA carb...118mm

So we have a starting point not a million miles off the 8v in terms of the head port and overall length. I factored the head and carb length into the equation when I speced our manifold a couple of years ago, along with installation room & common available trumpets.

With the above, our starting point is 294mm, so to get to our 320mm overall length we need a 26mm trumpet. Weber have been good to us again with the availability of...a 26mm trumpet!

A popular trumpet that many used sidedraught's come with is 40mm, so giving us a total of 334mm. I've seen 175hp on this length on 1800 16v's with a fast road ported head & 276 cams. To get to the total length we've found to work on the 8v in high spec form we need a trumpet of around 72mm. Weber are good to us again with a 75mm trumpet, though the common size in the older steel trumpet is 60mm...giving 354mm total length.

And finally...for now...DTH ITB's.

Head...96mm
DTH Jenvey...112mm
Common trumpet...120mm

So a total starting point of 328mm. I've recently seen just shy of 250hp @8300  with 167ib/ft of torque from this length on a rally spec ABF, so it obviously works ok to a point, but maybe an improvement can be had with more length...time & the dyno will tell. Jenvey do a 150mm trumpet which would bring us up to 358mm total length. I would suggest at that length tho that DTH bodies wont enable sufficient under bonnet clearance, so DCOE style bodies on a more conventional manifold would allow for the extra length extended into the front panel.

Well that should get you all thinking, and give anyone new to carbs or T/B's some starting choice....dont be affraid to ask tho for specific applications...we're here to help.