How do you decide on how much boost?

[Blasted]

Simple question I hope,

When building a turbo engine how do you determine how much boost you want? From reading various threads I get the impression that people aim for specific figures?

If that’s the case what's a good figure to aim for? (VW type 1)

Thanks

[yamaducci]

That depends on many variables.
1) Common detonation points known by trial and error.
2) Type of Fuel.
3) Static Compression
4) Dynamic Compression.
5) Shape of Piston Top / Combustion Chamber.
6) Timing Advance / Retard.
For the street the max is usually in the 10-12 PSI range. Some will stretch it and risk detonation.
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[kangaboy]

The only thing stopping me is Intake Air Temperatures. I only run 8lbs right now and can't safely run any higher without my IATs skyrocketing because I lack and intercooler or water/meth injection system.
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[Max Welton]

Just give it a good hard run then count the number of parts on the ground afterwards.

Max
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[gprudenciop]

As much as possible! I'm at 19 with intercooler in my daily.
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[c77owen]

All of it....

[Blasted]

Thanks for the responses.

I'm going to order the turbomania book and do a bit of reading but it would be nice to know I'm on the right path with my thinking.

As I understand it boost is the difference in flow between what the turbo pushes out and what the engine can consume i.e an increased pressure in the inlet side of the system.

Primary factors controlling boost are the rpm of the engine, the size of the impeller/compressors of the turbo, the manifold diameter and the flow rate of the engine.

Secondary factors are the fuel type, fuel injectors, ignition, cylinder head flow rate.

Too much boost is bad for the life of the engine. Too little boost defeats the purpose of turboing.


So hypothetically speaking, I thought about building the following, where does the boost factor into the equation?

2007cc engine – 78x90.5
T3/4 turbo
EFI
CB heads with match ported manifolds

[buguy]

Compression and octane of the fuel. The less the compression the more boost you can run. But not enough compression and its lazy off boost. It's all about compromise. The higher the octane the more boost (within reason) E85 would be your friend if you have access to it.

[modok]

Ultimately you turn it up until you hear a little knock or the clutch starts slipping, then back it down a few pounds.

knock is mainly a function of fuel VS heat. Colder air, colder engine you can get away with more. Lower compression allows more, but important to not go too far with that idea for reasons like you mentioned. What is practical?

Simple setup and premium pump gas, 5-7 pounds
with intercooler and fancier stuff, 10-12
higher octane fuel, or ethanol, 15psi+

[clonebug]

The turbomania book.... while not a bad read is really quite dated especially if you are planning on EFI.
There are setups running upwards of 30 lbs boost with well designed intake and fuel management systems.
You can run 20 plus lbs. boost on a almost stock engine if you pay attention to detonation and IAT's.
Remember.... the cooler the air the more dense it is and therefore the more fuel you can add without getting too rich.
The two easier ways to gain hp with a turbo is rpm and keeping the intake temps as close to ambient as possible.

Water/Alcohol Injection works pretty good for me and allows me to run 22 plus lbs. boost on an almost stock engine.
Without water injection I might be able to run 15 lbs but with severely retarded timing and only in the cooler times of year.

If I switch off my Electronic Boost Control and go with the stock 6 lb. Subaru waste gate I can go anywhere and everywhere without a worry in the world. I need very little timing retard and no water injection.
It gives me enough extra power to climb any mountain grade at 60 plus mph and no shifting down.
In fact I went 3500 miles to California and back that way.

Flip two switches and I am right back to 22 lbs. boost.

Oh.... and don't forget......once you taste the boost you will be addicted and will turn it up at any cost.......

Here is a log when I was testing boost and hit my overboost protection.

It's kind of hard to read ....

23.2 lbs boost
3608 rpm
129 degrees MAT

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[oprn]

Way too many variables to give a single answer!

One thing that I found was huge was the ambient air temperature. Summer temps would only allow 10 or 11 psi before detonation reared it's ugly head but at -30 she would run well over 18 psi with no sign of the problem at all! That was running 9.5/1 CR and 120 octane fuel.

Intake air cooling is so important to get consistency!

[Blasted]

Thanks for the great replies.

Two things I'm going to have little control over is the quality of the fuel and air temperature/humidity. The UK is variable at best but I would expect more of my driving to be in the summer than the winter.

So, going back to my original point. My confusion came from do you:

1. Build an engine aiming for a specific boost figure,careful components selection and a bit of maths?

or,

2. Build an engine around personal preferences and with turbo in mind, find out your boost and then just tweak the engine to make the most of what you get?


What I'm essentially doing is building a set of requirements, these requirements then influence the engine build plan and component selection.


Requirements to date:

Reliable,requires little ongoing work aside from general maintenance.
Value for money, I don't want to spend a lot of money for small gains when a different set up for a similar price produces a lot more power.
I don't want to have to chop into the body or drill holes in firewalls etc. Rear lids got to close.
Combination of steady town motorway driving, but picks up quick from low down and puts a smile on my face if I plant it.

Thanks

[oprn]

Here is my read on this.

[madmike]

Bullet proof it
build it with the best parts
I think about the 'stock rods" in my 1915cc ,,, sometimes (8-12lbs)
I don't know how their holding up so far so good
I have a set of Carillo's in the 2276 and no worry's 15-18 lbs with higher octane, "Your fuel Quality availability will determine your boost levels "
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[Splitdog]

Go water injection. Huge gains for the money. I'm running 15 lbs on pump gas and can drive the car 100 miles in 90* heat. Stock head studs-8mm-static CR=7.1. More boost, less compression is better I think than more compression, less boost. More boost= more fuel and air. Good luck.
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#notacallooker

[Blasted]

Thanks again, plenty of stuff for me to think about.

[Dauz]

What gearbox?

[Blasted]

[Dauz]

[mark tucker]

I wood think it's one of 2 things, deeper pocket or butt dyno getting used to it. well you asked.