Jump to content

More Power


Guest Seanus01

Recommended Posts

Guest lotusPaul

So, simple maths.

 

Lets say you currently have 140bhp and you want 210bhp. Thats an increase of 70bhp or 50%. So at atmospheric presure, 1bar/14.7psi, you get 140bhp.

Knowing that you need an extra 'boost' of 50% means we know you need half 1bar, 7psi.

 

Point to note, i already said 3psi is equal to 1point compression. So a 9.2 c/r at 6psi is equal to 11.2c/r. A little high for my liking but can be worked with.

 

Next thing to understand is turbos dont work in psi, they work on flow and mostly cfm. Cubic foot per minute.

 

We then need to work out your engines cfm requirement and thus requirements.

 

Many different ways that all get to the same end result, but easy to do is work capacity and revs, converted to cfm. That is now multiplied by your efficiency factor.

This gives you your engines cfm of air.

 

So say 2000cc/16(cid)=125

125x6000rpm÷3456=217cfm of air assuming 100% efficiency.

Clearly you are not 100% efficient at this revs as this would be where peak torque would ocour. Most carbed cars are 0.7 to 0.8 seldom higher. Ill assume your currently at 0.8.

So,217×0.8=173 actuall cfm going into YOUR engine at 6000rpm.

 

This means that your chosen turbo must develop 173cfm of flow plus the over boost required. Which we shall say from above is 50%.

173+50%=260cfm of flow required from turbo.

 

So now you can see current air usage and new required to give 0.5bar boost or 7psi.

 

Again we dont want the turbo to be working at max effort all the time as this will cause failure. We idealy want it to work at 75-80% of its max. So in an ideal world you want one that will flow around 350cfm.

 

Knowing this info you can look at turbo graphs and make your choice, or take it from me that an S2 Escort RS turbo, t3 with an AR of .48 will do the job.

 

Yes thats a long winded, possibly over thinking way of looking at it, but this is why i stated its not as simple or refined if you just bolt stuff on. By following the maths you can have a turbo on a 9.2cr pinto, limited to 6psi that will make the power you want but with a torque curve to die for. You may have to retard the ignition at the top end a little, but not as much as you think. You will have minimal lag, but this will have little effect because your keeping the initial cr.

 

All done, you will have a drivable car with off boost performance and then a ton of torque when you want it.

Link to comment
Share on other sites

Excellent info Paul

Yes turbo head mods are quite different to N/A mods as you don't generally have problems getting air in, it's getting out where the restrictions appear.

What has not yet been said is that cam choice is very different for a turbo setup, you want less duration and as little overlap as you can with high lift as I understand it.

The little things help, 3 angle valve seats and penny on a stick valves.

Looking forward to seeing how this develops

Link to comment
Share on other sites

Hi lotusPaul,

good info above. To add a slightly different angle onto what you've said - your maths for the increase in air are based on CFM and you've used a linear scale to work out how much extra CFM you need to generate boost - I don't think this is quite right.

 

CFM is a measurement of volume, and as the air compresses within the intake system, it's density goes up and as such, I think your workings out should be looking at Mass Flow rates.

 

Not claiming to be any kind of expert, just regurgitating the info picked over time.

Edited by steamer
Link to comment
Share on other sites

Guest lotusPaul

Richard,

 

Indeed you are correct, the maths are to show what you need as a volume consuption.

To put into a better context the next step is to calculate pressure density to give the lbs of air.

So cfm x 0.076 to give it.

Further complications are discovered when we understand that 1lb of air in a compressed induction is whats needed for 10 bhp.

I say complication because clearly for 200 bhp, 20lbs is needed. 350cfm x 0.076=26.6 thus the spec of turbo i suggested will mathematically develop 266bhp.

 

If you look at the 260cfm figure in the requirements and multiply by 0.076 you get just off 20lbs. Theres your 200bhp.

Link to comment
Share on other sites

Guest lotusPaul

Richard,

 

Just to confirm thats,

 

1lb per min for 10bhp (average between 9.6 and 10.4bhp)

 

All depends on a few other factors but its about as accurate as we need.

Link to comment
Share on other sites

Guest kevin booth

Spent a small fortune on mine to get it to push 320bhp. New gearbox.diff.halfshafts. plus all forged and steel. Only to turn it down to 220bhp so I can drive in on the road without it trying to switch ends on me. Also think of when you get caught in the rain. I have to tip toe around to stay alive. 200 bap is plenty in a Seven

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...