Inlet Manifold length vs torque and power

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Inlet Manifold length vs torque and power

Post by Charlie54043 »

Inlet Manifold length and its influence on torque and power of a 1040cc Hillman Imp engine

GT Power Engine Modelling Software was used to model various inlet manifold lengths on a Modified Hillman Imp engine to assess the effect of manifold length on Power and Torque.
The Imp engine is the 1040cc unit in YKE512J and has the follow spec:
R20 Camshaft
1.4” inlet valve and 1.25” exhaust valve.
Ports and Combustion chamber modified to full race spec by Reg Pattern.
Exhaust manifold is a stainless version of a small bore Janspeed with a stock Hyabusa silencer.

To confirm the accuracy of GTPower, a Model was created using the actual engine dimensions including
2x1.5” strombergs/standard Sport inlet manifold, that is in the Davrian.

The real-life engine was run on a rolling road and torque/power recorded.

The Model plots were compared to the Rolling Road plots, these are shown below.

YKE 512J.
Pwr curve for YKE512J.jpg
Column B = Actual power, Column C = Actual Torque, Column G = Model Power, Column I = Model Torque

As can be seen, the Model and the Rolling Road plots are very close, confirming the measurements for the Inlet/Exhaust ports, Combustion chamber, Cam timing, Inlet manifold/Carbs and Exhaust manifold/silencer were accurate.

Models were then created for different lengths of inlet manifolds ( Head/manifold face to Bell mouth ) ranging from 50mm to 650mm, in 50mm steps and included the dimensions of a set of 4 x 36mm dia throttle bodies from a Triumph 600TT.

The model results are shown below:

50mm manifold.
Curve for 50mm.jpg
100mm Manifold.
Pwr curve for 100mm manifold.jpg
150mm manifold
Pwr curve for 150mm manifold.jpg
200mm manifold
Pwr curve for 200mm manifold.jpg
250mm Manifold
Pwr curve for 250mm manifold.jpg
300mm Manifold
Pwr curve for 300mm manifold.jpg
350mm manifold.
Pwr curve for 350mm manifold.jpg
400mm manifold.
Pwr curve for 400mm manifold.jpg
450mm manifold.
Pwr curve for 450mm manifold.jpg
500mm Manifold.
Pwr curve for 500mm manifold.jpg
550mm manifold.
Pwr curve for 550mm manifold.jpg
600mm Manifold.
Pwr curve for 600mm manifold.jpg
650mm Manifold.
Pwr curve for 650mm manifold.jpg
Studying the power plots of the various lengths, it can be seen that as the manifold length increases, the revs at which the torque and power peaks occur earlier in the rev range, however, the longer the manifold the lower the peak power at high revs.

The short manifold plots demonstrate that if ultimate power is required, such as in a race or rally car, then throttle bodies directly to the head face should be used and although the torque is tailing off above 7500 revs, 100 bhp should be attainable ( if the engine can withstand revs above 8000 rpm ). Note these models are using the dimensions of an R20 cam, which is not a race cam, but a cam that can be used on the road.

YKE512J is a road going Mk5 Davrian and if a short manifold was selected the engine would be a real handfull on the road and ultimately be unreliable.

So looking back at YKE512J Rolling road power plot, the torque peaks at 58 ft/lbs at 3250 rpm and stays approximately the same to 6000 rpm, then falls away to 50 ft/lbs at 7500 rpm by which time the power is also falling. Now if there was a manifold that produced more torque than the present 58 ft/lbs at usable revs and it's peak power was below 8000 revs, then a few Horsepower at the top end might be worth sacrificing.

The 400mm version fits this requirement, it shows a steady rise in torque, peaking at 72 ft/lbs at 5500 rpm ( 25% increase in torque, good for overtaking ) and does not drop back to 58 ft/lbs (previous peak) until 8000 rpm. The power peaks at 90 bhp (25% increase) at 7600 rpm, which might mean the engine stays in one piece a bit longer!

However there's a snag! a straight 400mm long inlet manifold would be impossible to fit into an Imp or a Davrian, but it was found that if it were configured as a Down Draft, it fits in the Davrian with room to spare.

Below is the plan of the Down Draft 400mm manifold and throttle bodies.
inlet manifold Mk 2.jpg
The 400mm manifold dimensions are: 27mm ID at the head face, 27mm ID to length 250mm, then flares out to 36mm ID by length 325mm where it meets the Triumph 36mm Dia throttle bodies, the overall length inc the throttle bodies is 400mm from the head face to bell mouth.

A model of the proposed Down Draft manifold was run and the results were almost identical to that of the theoretical manifold, so it was decided to go ahead and build the 400mm version.

The injectors are placed as close to the head/manifold face as possible to prevent fuel drop out at idle and low revs.

And finally! Here is the real manifold on the real 1040 engine!
My thanks to Robert, who did all the theoretical work.
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Re: Inlet Manifold length vs torque and power

Post by intrepid »

Wow, fantastic read - thanks for taking the time to share
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Posts: 147
Joined: Wed Aug 26, 2009 4:01 pm
Location: Hornchurch Essex

Re: Inlet Manifold length vs torque and power

Post by Dave6091 »

Hi Charlie,

Just reread your posting and results, great stuff, will be interesting to see how the car performs!. Dave Cooper tells me his injection kit is nearly finished so we may all be putting the webers on ebay soon!.

Looking at the last pic, is that another Davrian in the background??.

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