1 Turbo or 2?

Eliot · Post by **Eliot** » Thu Aug 18, 2011 9:23 am

But with a single turbo you have the force of all 8 cylinders to drive it, whereas on a twin turbo setup (such as mine) you only have 4 cylinders to drive each one. So there's probably not much in it.
As stevie says (having also done a twin turbo setup as well) - its double the fab work etc. I enjoyed doing mine, but I would do a single setup next time if I found a suitable turbo. I don't think lag would even be a factor.

DEVONMAN · Post by **DEVONMAN** » Thu Aug 18, 2011 1:34 pm

I also would recommend the book on Forced Induction by A Graham Bell.
It covers many aspects of FI that other books miss.

As an example, it explains why different cams should be used for superchargers and turbos. There are cams on the market described as "turbo/Blowers cam". They can't be ideal for both. Using a blower cam on a turbo engine will result in dilution of the incoming mixture because of back pressure in the turbo exhaust housing.

Cheers Denis

kiwicar · Post by **kiwicar** » Thu Aug 18, 2011 1:50 pm

and it is in a good reading style.
Best regards
Mike

unstable load · Post by **unstable load** » Thu Aug 18, 2011 11:53 pm

2 small turbos running one per bank will spool up faster than one heavy one off both, so if you size the scrolls correctly you could come out on top of the equation with 2 over one, it also depends on how much boost you want.

JJMclure · Post by **JJMclure** » Sat Aug 20, 2011 10:50 am

if you're worried about spooling a big turbo you could always try one of these
http://www.spracingonline.com/projects/ ... Valve/3643
then get that 88mm turbo stuck in there!!!!!

stevieturbo · Post by **stevieturbo** » Sat Aug 20, 2011 10:07 pm

DEVONMAN wrote:I also would recommend the book on Forced Induction by A Graham Bell.
It covers many aspects of FI that other books miss.

As an example, it explains why different cams should be used for superchargers and turbos. There are cams on the market described as "turbo/Blowers cam". They can't be ideal for both. Using a blower cam on a turbo engine will result in dilution of the incoming mixture because of back pressure in the turbo exhaust housing.

Cheers Denis

Blower cam and turbo cam are a load of bollox.

Cams are cams, people just like to give them silly names. And the only way to determine which cam is optimal for any setup, is to test it. But obviously that isnt practical.
And yes, cam's can be ideal for both. Simple fact of the matter is a mild cam with little overlap will work superbly in pretty much any forced induction build.

And saying that bigger cams with overlap are better suited to SC than turbos....again wrong. Take a look at some of the fastest cars, and they too will be running big cams with lots of lift, and overlap. But of course they will also be running very big turbos and hopefully EGBP in the manifold will be pretty close to boost pressure.

Same could apply to SC where people say big cams dont work, and all the intake charge gets blown out the exhaust. Again, it's tried and tested that big cams do work.

So it's just a matter of selecting the cam that will give the best idle and drivability characteristics for you. There are loads of options.
But with FI, mild cams will always perform well.

I recently changed from a moderate cam, to one with no overlap at all. Power delivery is smoother, idle is nice and smooth at 850rpm, hydrocarbons at idle went from around 1200ppm, to 150ppm. And performance overall hasnt really changed a bit.
That works for me

And proper twin scroll on a V8 would never work the same way it will on a 4cyl. On a 4cyl it is possible to isolate firing cylinders from each other, so there will be less EGBP, less risk of contamination between cylinders, and better pulsing to the turbos. So a V8 would never see the same benefits.

Either way, given the multitude of turbo options out there, there is a big single to do whatever you need. Hell, I'm even tempted again myself !!

DEVONMAN · Post by **DEVONMAN** » Sun Aug 21, 2011 10:06 pm

I think you have reinforced my point about blower/turbo cams. The point I was making was that before I read the A Graham Bell Book, I rushed out and bought a cam labelled "blower/turbo" from a well known source, and was dissapointed with most aspects of the performance. Like you I have acheived much better drivability and performance with a milder cam in my case using twin turbos.

Just trying to help others who may make the same mistake.

Cheers Denis

stevieturbo · Post by **stevieturbo** » Mon Aug 22, 2011 11:28 am

In many ways this rule applies to all cams. It was in Vizards yellow Mini book.

"Choose the cam you want, then buy one stage below".

You have more to risk or lose by going too big with a cam, than you will ever gain. At least certainly for any engine that sees road use.