XU5 8V engine mid range power / torque.

[mollisk 2 1 Cars]

I recently converted a rally spec engine with twin 45 Weber carbs back to a standard inlet manifold with DTA management for fueling and ignition. ( To allow the car to be entered in historic class  )

When the engine ran on carburetors there was  a loss of torque / power around between 3500 to 4500 rpm.

With the carburetors replaced the loss of torque / power is  now between 4000 to 5000 rpm and is greater than the power loss with carburetors. ( The engine was set up on a rolling road with carbs and with the standard inlet manifold fitted. ) This make the engine difficult to drive competitively on gravel.

Does anyone have a definitive answer as to the cause of the dip in mid range torque / power that seems to be fairly common to XU competition  engines ?

 

Regards,

 

Mollisk

 

 

 

 

[welshpug 1,621]

never heard of this 'common power dip'  !

 

your mapper surely should be able to tell you

[mollisk 2 1 Cars]

The 8V XU engines that I have built have all shown a dip in the torque curve at approx 4500 rpm, I have seen this with dyno print outs from other builders 8V XU engines also.

I have experimented with different length ram pipes, 4 into 1 exhaust manifolds, but I have yet to determine the cause of the torque dip.

 

Graph 2 is the dyno print out of a 1.6 8V engine, 11.2 :1 CR, Cat Cams 4900368 cam, standard exhaust manifold and twin tube down pipe, twin weber DCOE carburettors.

DTA used for ignition control.

 

The first dyno print out is the same engine with standard inlet manifold fitted. DTA ECU for fuel and ignition control.

 

Edited January 28, 2022 by mollisk
More information added

[welshpug 1,621]

[welshpug 1,621]

Standard 1.9, no dip.

 

something very wrong with your setup somewhere, mapper should be able to spot something to cause that big of an issue, be it going lean or rich, or pinging

[petert 549]

I thought we were only talking about 1.6L here? They are very different. Rod/stroke ratio of 2 vrs 1.6.

 

The XU9J4 suffers badly from the dip, due to the combination of rod/stroke ratio and exhaust manifold. I didn’t think the XU5 did.

[Thijs_Rallye 86 4 Cars]

Do you have the specification sheet of this camshaft? Do you have a plot where the AFR is logged? What strategy are you running with the DTA, alpha n or map? I would suspect some harmonics are in play.

 

 

 

[mollisk 2 1 Cars]

The first graph has a plot of the air / fuel ratio ( AFR )  on it.

When the engine was run on carburettors, the AFR was kept at 13 : 1 as best as possible.

 

My guess was that it related to the exhaust manifold and that a maniflow 4:2:1 designed manifold might help. Additionally with this type of manifold it would be possible to increase the length of the secondary pipes easily.  I had seen exworks 306 maxi cars where the secondary pipes did not join together until they reached the rear beam and was under the impression that this was done to increase mid range torque.

 

I was hoping that someone may have come across this problem and found the cause.

 

 

 

 

[petert 549]

The exhaust would be my first pick. Replacing the standard Mi16 manifold with a long secondary design removes that 4000-4500 dip. I guess it would in the XU5 as well.

[mollisk 2 1 Cars]

Further to the above.

The original standard manifold, down pipe and 2.5 " exhaust were removed and a 4/2/1 manifold  and 2.25 " exhaust fitted.

This did help with the mid range dip, but it was still very  noticeable.

The secondary pipes were then extended by approx 500 m/m. 

With this completed  the mid range dip is not now noticeable and the engine pulls cleanly from 2500 rpm.

However, the engine has not been dynoed ( the fuelling was adjusted to suit the exhaust system using an Inovate wide band meter )

[Thijs_Rallye 86 4 Cars]

That lambda sensor is not going to last long in this position. I know the space is tight, but you should try to have position the sensor around the 11 o'clock position to keep moisture damage at bay.

 

Also Innovate is known for frying wideband sensors, personally I wouldn't touch those controllers with a barge pole.