hot air engine temps

[kiwi ka]

This is probaly a set of questions that Jac Mac could answer off his head.
I am mid way between houses waiting to build my workshop so everything is packed away.
I have a 3.5ltr Rover running 4 x 40mm twin choke Del Orto`s with a medium cam. Now that I have started to run it full throttle using 6000rpm, its running hotter than I would like. I have noticed that all my hot air is not leaving the car so can provide more pressure / flow from the front which may assist that. What I can not check is what the distributor is set up to ( info in storage), Its the standard set up for the Buick MSD as supplied.
Question..
Will the amount of advance at higher rpm effect the top end heat production.
What sort of numbers should it be for that max rpm.
The motor runs and pulls well with no obvious objections and I feel is on the rich side.

Getting the air out of the back of the car will hopefully be sorted with more input.
Any comments please.
Russell

 

[Kevin Box]

Russell


Will the amount of advance at higher rpm effect the top end heat production. - Yes if it is over advanced you will get high speed detonation
What sort of numbers should it be for that max rpm. - 32- 34 see below

The motor runs and pulls well with no obvious objections and I feel is on the rich side. - May well be, but make sure with some sort of mixture analyser under full load

Advance probabaly needs to be mapped to suit the cam and compression and dependent on how efficiently your combustion chamber is working you may require more or less advance.
Probabaly a trip to a dyno shop that knows Rover engines would be the best place to start.

Typically an MSD would have about 24 crank degrees mechanical advance (12 cam degrees) and somewhere around 32-34 degrees is where your max advance will end up.
With the Rover if you have flat tops in it and zero deck as per the Stock car guys you may find it only needs 32 max advance.

Try on the 30 side of 34 and move up as over advance will cause high speed detonation which will melt things !!! On the dyno it is obvous what the engine needs as all you do is slowly increse the advance until max power and then bring it back slightly on the lower side. You can play slightly from there but increase in advance pushes you toward a melt down (High speed detonation.) Sometimes you may opt for this with a fatter mixture for better acceleration but set your base point properly first !!!!

Also depending on your cam you may find an advantage to cut back the mechanical advance to 20 crank degrees so your static figure would then come up to 14 degrees for 34 top end. This increased base advance often helps to soften a soggy bottom end from a cam and helps smooth out the idle. (often a neglected detail).

The next part of setting up the dizzy is part science from the dyno and part seat of the pants as you are trying to get the advance curve (how the spring allows the advance to progress) to give you the best torque to pull out of corners and accelerate the best. Dyno will get it pretty good but seat of the pants will tell you. Basically your are trying to get as much advance in early without detriment to torque. Usually excess advance will make the engine rattle (ping) under max acceleration but this is more obvious on a dyno sheet as you loose torque.

Dont get hung up by the saying "People don't race dynos". Ironically most successful race teams use dyno testing ????

A dyno test will tell you stright up where your advance needs to be and also let you get a better idea of what the mixture is up to. Often you you have to move both to get it right.

It would also give you a good idea of what is happening when you are ending up with higher intake temps and whether or not you need to fatten the mixture. (lean mixture may be a possible cause of your risng temp under load)

A good rule of thumb I have worked with is go fat on the mixture and short on advance until you know where you are going - play safe. Fuel is cheaper than engines !!!!

I dont know of any dyno shops up your way but I suggest you ring Jon Butler at Tuning & Performance in Hamilton (07 8466546). He has done a lot of stuff with Rovers and is "No Bullshit guy". He may also be able to advise of a good tuner closer to home. He worked with Tom Walkinshaw when they were playing with Rovers and also is a renown tuner in classic circles.


best of luck

Kevin B

 

[kiwi ka]

Kevin
Many thanks for that info, I am waiting to make an adaptor so that we can fit the wheel mounted units on at the dyno shop that knows carbs and who I would trust so your info will be a great guide. I will also give Jon a ring and see if he can fit my car.
Cheers
Russell

 

[Andrew Robertson]

Hi Russell, I agree with KB and emphasize the point that you're really shooting in the dark if you don't have accurate measurement of mixture under load.

Cheers, Andrew