yrkesman

Is there some rough guide what the exhaust back pressure should not exceed in Cosworth YB engine? I remember seeing somewhere that it should not be more than 25% bigger than boost pressure at peak power. Is this correct?

yrkesman

Anyone from the evening guys?

GARETH T

where you reading the back pressure from? before the turbo or after?

rapidcossie

what back presure do you have?

Stavros

I take it you mean pre turbo, as post turbo you should just go for as little as possible.

IMO, PRE turbo, you shouldnt have a rule, as people get all obsessed with keeping the pressue down so go for crazy exhaust housings that makes the powerband shite.

You might not be getting all the power you could, but its about how well it drives for me.

I bet in a lot of common Cossie specs the backpressure before turbo is 250% what the boost is, never mind 25%

rapidcossie

Im willing to bet that my cossie is the only one on here with no back pressure in the exhaust.

GARETH T

NO back pressure? at peak torque?

rapidcossie

Dont think so but dont quote me on that

Stavros

In the exhaust, not in the manifold like what the thread is (i expect) about.

And thats believable depending on exhaust.

rapidcossie

sorry ? he states exhaust back pressure in the first post?

Stavros

pre turbo back pressure im sure he means.

engines NEED backpressure to run right, turbos dont.

but many turbo cars run stupid amounts or pre turbo pressure, though for a good powerband it may be the best option.

i bet your motor has a ton of it, most road going big boost cossies prob do.

GARETH T

i bet both TIP and exhaust pressure are present

yrkesman

Yes, I meant before turbo, sorry for my confusing text I haven't measured the pressure from my new engine since it is still in pieces, but I will monitor 4ch egt's and back pressure when the engine is been tested.

foreigneRS

i'll take that bet for any amount of money you like

simply because i understand physics and where you have a massflow you must have a pressure difference

think of it like this: in nature, how is the wind created? it's not from all those new white electric wind generators it's from the pressure difference between one place and another that causes the air to rush from the place of high pressure to the place of low pressure. just like water will always flow from high to low

yrkesman

So there is no other option to just try and monitor exhaust gas temperature and the pre turbo pressure and see how they go together and see how far power rises when upping boost? So what would indicate that the turbine side is too small and needs to be changed to bigger one, those things I mentioned above?

Stavros

Well thing is, different people reccomend different things, from a max of 25% more like you say, up to double the boost or more (IIRC ive seen 3times boost pressure on a number of cars).

The small exhaust housing may be restrictive as fook, BUT a bigger one will most likely reduce low down power, so its always a compromise.

I think its more effective (for drivabilitys sake anyhow) to just test the effects of different size exhaust housings if you think you want to improve on the original setup.