bosscat
Mechanical
- Jan 29, 2005
- 13
My long-term project car is a '67 Triumph Vitesse.
This has a 1998 cc straight six ohv with 2 valves per cylinder.
The bottom end is still as built by Triumph in the 60s including the camshaft which is a 25/65 with 270º duration and 0.329" (8.36mm) lift at the valves. In spite of a long and occasionally hard life, engine condition is fair with decent oil pressure and compressions. The head has had a little work including 3 angle seats and removal of the sharp edges. Valves sizes remain as standard and the CR remains as standard at 9.5:1
Externally, things are less standard with multipoint fuel injection on a homemade plenum type manifold, 3D distributorless ignition and a 6-3-1 tubular manifold. This is all controlled by a megasquirt ECU, which although pretty crude compared to modern OEM stuff, compares more favourably with the original twin 1.5" Strombergs and Lucas clockwork dizzy.
I've run this for several years now and the seat of the pants dyno (not always the most accurate or objective) says I have gained power and torque. I have certainly gained fuel efficiency and it drives very well.
Recently I finally took the thing to a rolling road (chassis dyno), mainly to check that I had safe high rpm/highload settings as this is hard to establish by road mapping. The good/bad news was I was already pretty close and we could only find another 3 bhp.
It did show up an oddity though. As standard the manufacturers claimed 104 bhp @ 5300 rpm and 117lb/ft torque at 3300 rpm. These are probably optimistic.
We achieved 122bhp @ 5600 rpm (probably optimistic as the RR was using a whopping 34bhp trans losses for correction) and 117 lb/ft of torque @ 5100 rpm.
Now I know that comparison of the absolute figures is worthless but what has moved peak torque nearly 2000 rpm up the rev range? I don't have the graph available yet (hope to be able to add this later) but in actual fact the torque curve is quite good from 2000 rpm but with a marked peak between about 4900 and 5300.
As the mechanical parts are near enough stock, this is presumably a "plumbing" effect. I'd like to understand this better as I'd prefer it a bit further down the rev range. The exhaust manifold is a fairly well known aftermarket item and although it has plenty of issues, odd, high rpm torque peaks is not one of its known features.
This leaves my inlet manifold which was designed largely on a "what fits in the gap" principle.
It has 30mm dia runners (matching the head port diameter) 190mm long with another 80 odd mm induction length in the head itself. Runner volume including head port is 190cc.
The plenum volume is 1900cc (90% of displacement). The single throttle body has an ID of 52mm.
It's possible that my runners are a bit large, the head was designed for a 2500cc version and some say the ports are too large for the 2000cc. I think the runners should be longer too but that presents packaging issues. Also the plenum may be a bit large although I have read so much conflicting information on plenum size that I don't know what to think anymore.
I would welcome any thoughts or suggestions on this and may even build another manifold to test any emerging theories.
Thanks for reading
Regards
Nick
This has a 1998 cc straight six ohv with 2 valves per cylinder.
The bottom end is still as built by Triumph in the 60s including the camshaft which is a 25/65 with 270º duration and 0.329" (8.36mm) lift at the valves. In spite of a long and occasionally hard life, engine condition is fair with decent oil pressure and compressions. The head has had a little work including 3 angle seats and removal of the sharp edges. Valves sizes remain as standard and the CR remains as standard at 9.5:1
Externally, things are less standard with multipoint fuel injection on a homemade plenum type manifold, 3D distributorless ignition and a 6-3-1 tubular manifold. This is all controlled by a megasquirt ECU, which although pretty crude compared to modern OEM stuff, compares more favourably with the original twin 1.5" Strombergs and Lucas clockwork dizzy.
I've run this for several years now and the seat of the pants dyno (not always the most accurate or objective) says I have gained power and torque. I have certainly gained fuel efficiency and it drives very well.
Recently I finally took the thing to a rolling road (chassis dyno), mainly to check that I had safe high rpm/highload settings as this is hard to establish by road mapping. The good/bad news was I was already pretty close and we could only find another 3 bhp.
It did show up an oddity though. As standard the manufacturers claimed 104 bhp @ 5300 rpm and 117lb/ft torque at 3300 rpm. These are probably optimistic.
We achieved 122bhp @ 5600 rpm (probably optimistic as the RR was using a whopping 34bhp trans losses for correction) and 117 lb/ft of torque @ 5100 rpm.
Now I know that comparison of the absolute figures is worthless but what has moved peak torque nearly 2000 rpm up the rev range? I don't have the graph available yet (hope to be able to add this later) but in actual fact the torque curve is quite good from 2000 rpm but with a marked peak between about 4900 and 5300.
As the mechanical parts are near enough stock, this is presumably a "plumbing" effect. I'd like to understand this better as I'd prefer it a bit further down the rev range. The exhaust manifold is a fairly well known aftermarket item and although it has plenty of issues, odd, high rpm torque peaks is not one of its known features.
This leaves my inlet manifold which was designed largely on a "what fits in the gap" principle.
It has 30mm dia runners (matching the head port diameter) 190mm long with another 80 odd mm induction length in the head itself. Runner volume including head port is 190cc.
The plenum volume is 1900cc (90% of displacement). The single throttle body has an ID of 52mm.
It's possible that my runners are a bit large, the head was designed for a 2500cc version and some say the ports are too large for the 2000cc. I think the runners should be longer too but that presents packaging issues. Also the plenum may be a bit large although I have read so much conflicting information on plenum size that I don't know what to think anymore.
I would welcome any thoughts or suggestions on this and may even build another manifold to test any emerging theories.
Thanks for reading
Regards
Nick
