Supercharging a 3.9

cammmy · Post by **cammmy** » Thu Jun 16, 2011 12:50 am

Hi Guys

I am getting a 3.9 from a Discovery and am planning to twin supercharge.

The chargers I have are TX15's (SC14's)

Does anyone have an idea of the gearing I should use on a 3.9? Would I be looking at an (engine) rev-limit of 5,500rpm? so would 2-1 be a good ratio? Would the boost level (dependant on cam profile and intercooler efficiency, I know) be acceptable? Also, I would be looking to change the camshaft. Does anyone have a good idea for a suitable profile for a setup like this? The chargers have electric clutches and a bypass system so will need to work with them turned off; but in saying that I imagine even a reasonably aggressive profile shouldn't have much trouble with moving a 1250kg (2750lb) car at low RPM when you have nearly 4 litres working for you.

I was thinking or a more aggressive profile to limit low end torque and push power up nearer the top (as a form of traction control). The Triumph is much lighter than the Discovery the motor has come from and I don't think the standard profile (for an auto box) will work too well when supercharged and put into a much lighter (and manual) car. I think I would have too much torque down low to be usable.

Cheers
Cam

kiwicar · Post by **kiwicar** » Thu Jun 16, 2011 1:12 pm

Hi
You want to run them in that 65% efficiency region as far as possable, so aim for peak power to coinside with 5000 rpm on the blower, but if you are running two your pressure ratio may be a bit high, the easiest thing to do is to plot your engine requirments onto the compressor map and see how it works out. What boost are you aiming at? do you have a Volumetric efficiency plot for the engine (or one very like it)?
I would look at a fairly mild cam for this, standard 3.5 maybe, or a stump puller from RS?
Best regards
Mike

cammmy · Post by **cammmy** » Thu Jun 16, 2011 9:14 pm

I don't have a VE plot sorry. That would change depending on the cam I chose anyway?

I've been told that the motors handle 8psi fine in road applications so I would be looking at <10. This means I can drive the blowers more slowly and push the highest efficiency island up closer to where peak power will be? I'm not after a target boost though to be honest. I understand that (especially with roots type chargers) making things breath more efficiently reduces boost pressure but increases the VE of the cylinders, so you end up with more power but 'lower' boost?

I've not attempted anything like this before though so am going solely from my understanding of research I have done. I am currently reading the forced induction performance tuning book by A.Graham Bell (have already read four stroke performance tuning by the same author)

Edit: Do you recommend a milder cam so that low end performance is less-affected and I can slow the blowers down to move the peak efficiency up the range without needing lot's of boost to get moving? That makes more sense

Although I have to admit I adore the sound of a lumpy RV8. Those 4.0 Chimaeras get me every time

I should also note that I'll be looking at proper tubular manifolds and a bespoke exhaust.

JP. · Post by **JP.** » Sat Jun 18, 2011 5:35 am

cammmy wrote:.... I understand that (especially with roots type chargers) making things breath more efficiently reduces boost pressure but increases the VE of the cylinders, so you end up with more power but 'lower' boost?
.......

Thats spot on.
Lost 4 psi with the Merlin head swap alone...........It gave me 20 bhp extra on the rollers.
You did your homework correctly

cammmy · Post by **cammmy** » Sun Jun 19, 2011 9:47 pm

JP. wrote:You did your homework correctly

That's always good to hear

I'll take a look at the stump puller cam. Does anyone have the specs on the standard disco cam for a 3.9?

I have a duplex chain kit with presets for changing timing. Would retarding the standard cam a few degrees in lieu of a new one be a decent idea? although as it's not twin cam I won't be able to adjust the LSA to see what works best.

Cheers
Cam

ged · Post by **ged** » Sun Jun 19, 2011 10:16 pm

Scroll down to page four mate.

http://www.realsteel.co.uk/section1.pdf
Regards Ged

cammmy · Post by **cammmy** » Sun Jun 19, 2011 10:36 pm

Cheers bud, did a google for Real steel camshaft's and found it.

cammmy · Post by **cammmy** » Mon Jun 20, 2011 3:14 am

Does anyone have the LSA for the stump puller?

cammmy · Post by **cammmy** » Thu Jun 23, 2011 9:17 pm

Just been re-reading my Rover V8 book. This motor has more km's on it than the amount that he says the rockers etc normally wear out at

Was thinking that I will probably want to take the heads off to see if any pistons look cleaner than the rest (could indicate cracked cylinder). Was thinking that I could use that as an opportunity to fit up-rated head gaskets and ARP studs. I was also thinking about steel rockers/shafts and isky anti-pump lifters with the new cam (depending on cost). According to Des Hammil's book this should mean the motor would be okay for 6,000rpm (although I think I may have sintered spring retainers so may need to replace those with something better too)?

Does anyone know how much the isky anti-pump lifters normally are for a set?

Is there anything else I would need to take into account and would these be cost effective modifications for reliability/longevity and performance? I don't want to get too far ahead of myself here.

bigaldart · Post by **bigaldart** » Fri Jun 24, 2011 6:57 am

Keep it simple, Real Steel blower cam is ideal, use hi-rev lifters or Rhodes lifters. Real steel dual springs and matching retainers, better still are the Rover steel retainers for a single spring, they need a little touch up on the lathe to stop the inner spring catching. The Rover dual spring retainers don't fit the Real Steel springs at all well. Is this for lots of road use or mainly track?

Alan

cammmy · Post by **cammmy** » Sun Jun 26, 2011 8:56 pm

The car will be used on the road almost exclusively.