Lotus Elan and Lotus Elan Plus 2 Forums

I have a +2 running on Strombergs with a low compression head and sprint (CPL2) cams. The engine runs well.

The current manifold (header) is the standard bore type. If I changed this to a large bore version and did nothing else, can anyone say what increase in air flow I might expect?

There are some old posts on this sort of thing, but I wondered what the current thinking was.

If there is an increase in air flow I imagine that the Strombergs will auto-compensate to give me more fuel and therefore more power, but at high throttle openings the needles will run out of range and the mixture will go weak. A change in needle profile should fix that.

Any thoughts?

Dave Chapman.

Exhaust system design does not directly influence intake air and fuel mixture flow

What it does influence is cylinder blowdown on exhaust valve opening, pumping losses during the exhaust stroke and scavenging effect on exhaust valve closing.

The design will have a secondary effect on how the inlet side flows particularly the scavenging flow during the overlap period but that influence is small compared to the first 3 effects.

In general the exhaust side of the Sprint level tune engine with sprint cam and the standard "small" diameter headers is reasonably well balanced with the cams and inlet ( especially Stromberg inlets) and not costing much in performance. So larger "fast road" headers will not help much without other matching modifications to cams and inlet system

cheers
Rohan

Thanks Rohan.

I suppose that if I had a higher compression ratio with intruder pistons, there might be a significant advantage in going to a bigger manifold. But my 170psi compression is just not enough to make the manifold flow the limiting factor.

So I might be making 110-120 bhp. If I wanted to go to 130bhp then intruder pistons and large bore manifold + new needles?

If I wanted to go to 150bhp, then it's Stromberg to Weber head, cam change, intruder pistons and larger manifold?

Maybe I should stay as I am - it's so easy to get to the distributor!

Cheers,

Dave Chapman.

Hi Dave

If wanting to get more from a Stromberg head engine then.

Higher compression ratio up to around 11:1 on 98 premium unleaded
high lift short duration cams something like QED 420 or similar with suitable valves and springs
big bore exhaust - TTR fast road exhaust system
bigger inlet valves - 1.625
some mild clean up of the ports and opening up of the inlet seats so the bigger inlet valve seats on its outer edge.
retune of carbs and advance curve on a dyno by someone who knows about Strombergs.

All of the above if done well should get you into the 140+ hp range with a Stromberg head at 6500 rpm with a 83.5 mm bore, standard stroke, 1600 cc twink. To go beyond that I believe you need to go to a well ported Weber head and / or bigger capacity and / or steel bottom end for more revs

A few other details can get you to the top end of whats achievable e.g.
careful managements of piston ad bearing clearances for minimum drag
match porting of carbs and exhaust to eliminate any steps at joins
short jack shaft and electric fuel pump
lightweight synthetic oil
careful control of rings gaps or gapless rings to minimise blow by
plus many other little details

Potentially more is available in a Stromberg head with a scatter cam ( scatter cam = timing of MOP for the siamesed cylinders is slightly different to minimise cylinder robbing of intake charge through the siamesed ports) This is used in the siamesed port MGs in racing but i know of no one who has tried that on a Stromberg Elan

cheers
Rohan

Thank you very much Rohan.

I will store your response for future reference if I can get a money pass from SHMBO!

Dave.