How much boost is to much boost??

I was talking about boost levels to mapper at Surry Rolling Road the other week and i said most of us more powerful coupes are under 2 bar of boost and some people have this image of high boost heros etc as a bad thing so my question to the forum today is

How much boost is to much boost? and what sort of power (including good power band) should people be seeing at what level.

also if you look at some of the other tuned engines out there (evo, Audi 5 pot, ford YB etc) alot of them are running up past the 3 bar mark with rev limits in some cases at 10k RPM and making high reliable power with massive broard power curves.

I may of open a can of worms but i do like a good debate

I'm mapped for 1.6 bar, but I find 1.4 bar is a decent everyday setting

A few years ago, I was mapped for 2.1 bar and race fuel and it was utterly ballistic, but violent to drive - huge fun when in the mood, but just too frantic for the road.

I now prefer the "grunty" feel of relatively modest boost, but still with no tail-off of boost at the top end, thanks to a hefty turbo

I'm guessing that 2+ bar is easier to handle when the rear wheels (or all four) are being driven....

power band is not related with turbo pressure,

some larger turbo will give more power with less boost

for example original turbo on 16v max is 250hp
with t4 compressor it goes on 380hp on same boost

on 16v engine is hard to get big power band because cams are fixed if you want more power you need more aggressive cams and than you need a turbo with bigger turbine ar and bigger compressor ar

power band can be achieved with twin scroll turbocharger witch spools up on lower revs.

not always car with more hp will be faster

if you are pushing standard turbocharger above efficiency it will just create more heat not more power and you will kill the turbo sooner, yes it can make 2,8bar boost but it will heat air so much that you want be able to cool it down

Too much boost is exactly at the level that destroys your engine
Jokes aside, as saidits all down to theturbochsrger and what its built to do.. A 250 hp turbocharger is built for good spool, midrange etc and will produce a lot of heat and backpreassure at higher boost/power. If you strap a gtx40 to the thing you could probbably boost north of 3 bar on a built motor with the right parts and the right tune.. But not really usable (or sain) on a fwd coupe.. 1,5 bar on that thing would probbably make twice the power of 1,5 bar on an 60trim T3..
Oooooh the compromises

Don't forget the block and internals are the biggest limiting factor!

How much boost you can run safely depends on the overall setup and tune of the car:

1. Turbo (standard TB28 turbo with more than 1.2 bar just kicks out heat, and wont last long - going higher does not give more power, but you are far more likely to melt a piston due to charge temps if you are pushing the car for sustained periods.

2. Compression ratio - The cosworth YB and EVO run low compression, which means more boost and less likely to detonate - more on/off feel to the boost threshold

3. Rev limit - standard rod bolts on the 20vt become the limit first, then valve springs - at 10,000 you will put a rod through the block.

4. the bigger you go on the turbo, the more boost you can run at lower temps, but then the threshold is higher, so you have to compensate with a higher rev limit to get the powerband. This still does not help you on the road, where realistically for a fast road car you should have positive boost around 3-3,500 rpm

5. The bigger the turbo, the more unrestricted flow you need, i.e. 4 inch exhaust, external wastegate, 5 stud turbo to downpipe, higher lift cams, etc.

it all comes down to how much you want to spend, and balancing driveability with power. if you you need is a 3K power band for doing 1/4's that's fine - go for that, but the car will be a pig on the road, or on most tracks.

There's a lot more info there if you choose to get really stuck in, but that is the gist of it.

I'd challenge anyone taking a stock evo or Cosworth YB and slapping 3 bar through it at 10K rpm without a lot of supporting mods...

EVO's actually run higher CR than the coupe. Most of the time it's 8.5/1 - 9/1, whereas YB's tend to go lower to 7/1. I wouldn't quite agree with the on/off threshold though. Mine is 7/1 and it's actually quite nice for spooling / threshold. Whereas the Juke RS that I've got as well, is higher CR and when left in 'sport' mode, that kicks in more on the threshold, plus, it feels laggier, but then again, I am comparing a 1.6 200bhp to a 2.0 435bhp

Begbie's a better encyclopaedia than me.

Must be that fancy Guy Croft flow work.

sorry guys been away so havent seen the replys but catching back up now

The 2+ boost can be mapped to be more easier to handle though ie boost per gear and Rpm so wouldnt that be something ever to consider think of having all that power but have the drivablity at the same time

yes begbie's 16vt is a highly worked beast but is there anything which can be changed to improve the over all power etc

this is without spending a small fortune getting stroker kits involved. unless theres a cheaper stroker kit i havent seen.

I know bigger turbos make more power with less boost etc but this is why i was saying about higher rev limits to make use of the power band of a bigger turbo which unlike my 2871 tails off after 7200rpm

I am talking about forged lumps as a base not stock because we all know roughly what there good for.

have a search on Nyssa7 on here - there have been a few threads on rev limits on the 20VT in the past, and he's pushed the limits on a track.

well Biggends was playing in the zone of 8200rpm safely so with some uprated valve springs etc should be able to push abit more.