 | Home > V8 > 3.9 V8 power increase limitations |
 
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| Devon-Rover Member Since: 22 Jan 2015 Location: South Devon Posts: 917   
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Hi.
Before reeling off a list of good performance mods can you ascertain that it is a A) healthy and B) all basic settings and tuning is done. V8's are known for eating camshafts so I trust yours is a good reasonable mileage and been well serviced? It is good to know that the valves are moving to the right extent, before even considering helping improve the airflow. Is the Kickdown cable adjusted correctly? Having it too slack means the box won't change down when you accelerate hard giving the appearance of slow performance. Same for the accelerator cable isn't working to full travel. It is worth checking the MAF to make sure it is working correctly. I used a dual channel scope to pin in on the MAF plug and see the voltage changes when the engine is running. IACV or the stepper motor located on the back off the plenum chamber should be working fine else you would be suffering from Idle issues too. I also put her on a gas analyser to make sure the mixture was right too. Ignition timing should be spot on or at least a tiny weeny bit advanced. Basic ignition system should be of quality items, Whilst as Mallory distributor & Coil and Magnecor KV85 leads are out of most peoples budgets they are the best and guarantee the right spark at the right time. My old disco had that above setup and ran a dream. My old 90 ran the standard lucas dizzy magnecor electrosports 60 leads, Bosch coil, and common on both NGK PFR6j-11 platinum spark plugs. Now with a correctly tuned motor there is a few things that can be done. I would start and say get the book Tuning Rover V8 engines by David Hardcastle as there is a lot of core knowledge in there to aid the understanding and give good hints. You can out a performance air filter on it to free up the intake breathing. Modifications to the intake manifold and the plenum trumpets are covered in the above book^^ but really the standard injection setup quite free flowing enough and free from major obstructions. -The heads can be skimmed a little to improve compression ratio too. -Portmatch the exhaust manifold to the heads and also the gaskets to minimise disruptions. This can be expanded onto actually opening up the ports a little (same for the exhaust manifold) to aid the breathing. The twin down pipe manifold is alright in flow rates for standard 3.9's and depending to noise preference then the 50th anniversary exhaust style with either including or not the centerbox gives good breathing. Hope this helps |
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25th Jan 2017 7:41pm |
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| calumj Member Since: 25 Jan 2017 Location: Northampton Posts: 3 
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Wow, what a great response, an exhaustive list of things i will start to look at. I had feedback that there was little i could do as the gearbox was at its limits from a torque perspective but just getting up to where it should be would be a big step forward.
Thanks |
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25th Jan 2017 9:38pm |
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| Devon-Rover Member Since: 22 Jan 2015 Location: South Devon Posts: 917
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I'll guess you have the standard 4 speed from the disco doner. It is to be exact a ZF 4HP22 and whilst not the strongest model fitted to Land rover models it can hold it's own. Don't abuse it, keep the fluid level correct and hopefully you have a cooler of some sort fitted to it.
TBH it is a nice box and in terms of life they outlive the V8's. It'll be fine behind your 3.9 For the time I ran one the gear box was the last of my worries 
Infact it was when my 3.9 died I then lumped my special 4.2 (ported, skimmed heads, mild road cam) into the void and the gearbox never quibbled. For the odd moments of haste when planting the rightfoot into the floor only resulted in either wheelspin or when In the dry minor tyre squeal and plenty of squirming shortly followed by starship acceleration down the road. 
I liked the Auto as It took the shock loading out of the system and made the 90 a nice drive without spending ages stirring the manual gearbox. To the best of my knowledge I sold her to a chap in west sussex and is still going strong. Have an underbonnet picture of the old 3.9 lump  Click image to enlarge |
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25th Jan 2017 10:29pm |
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| 100inch Member Since: 15 May 2012 Location: Brunswick Posts: 411  
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Maybe worth considering your tyre size and transfer box ratio. A tad over geared and the 3.9 doesn't 'pack' it anymore. m
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28th Jan 2017 9:08am |
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| Lodelaner Member Since: 04 Feb 2010 Location: Lambourn Posts: 632 
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Agree re gearing - the first performance mod on a 50th was to change from a 1.2 to a 1.4 ratio transfer box. With 1.2 and auto you'll be doing close to 30mph per 1000 in top. JB
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30th Jan 2017 9:03pm |
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