Right guys,

Just been having and interesting discussion with a few friends about matching turbos and cams.

Their understanding is:

If a larger less restrictive turbine is used, manifold backpressure is reduced, reducing of preventing reversion, thus allowing more duration of the exhaust cam. I.e. later closing, hence, more overlap.

Now this wasn't my think, which I won't go into yet as I don't want to "colour" anyones ideas...


What are you thoughts?


Alex

 

whoosh - straight over my head

should make interesting reading though. good topic

 

hhmmm yes i concur with that ....just wish i understood it...

 

Bollocks, as a larger turbo reduces gas velocity and a smaller overlap is needed.

 

Alex, I have done an awefull lot of work in this area and it is from this that I have my own range of cams for RS turbos and cossies.

As a basic example if we utilise a turbine housing with a larger area/radius ratio, in order to combat an increase in lag and reduction in response, its essential to decrease overlap. This doesnt mean you necessarily have to reduce the cam duration but as with all factors everything has a knock on effect.

I'm not going to quote specific figures for obvious reasons, but lets take for example a cvh turbo.

I have a specifc cam profile I use when running a 0.48 exhaust housing. When I upsize this to a 0.63 I regrind the cam with an increase in lobe separation. (i.e. decreasing overlap)

This results in an engine which drives equally as well as the 0.48 with similar lag between gears. The difference then is simply a later spool up time with the 0.63 but of course you get a corresponding increase in top end power.

If I left the overlap the same the 0.63 exhaust housing simply would not work at all!!! The engine would literally take several seconds to come on boost when changing gear!!

It's a complex issue and the best way to develop a camshaft is by real experience on the road NOT on a dyno. A camshaft CANNOT be developed on a dyno as it does not give any driver "feel" feedback.

 

Yea! I was right

 

what has gas velocity got to do with it? is it thought that the gas velocity is what drives the turbine at speed?

surely it is the volume flow of air through the turbine that determines how fast it turns? obviously if you have the same volume of air flowing through a larger turbine, the gas speed will be slower, but does that mean that the turbine will spin slower? is it not related to the pressure differential either side of the turbine?

i hope to learn something from this post, so thanks alex

 

thanks for the interesting reading karl

 

I agree with Rick n Karl, jus wish idve typed it when i thought it as idve looked clever.

People wonder why im not using wild mega duration cams, but i am using a stupidly big turbo, well there u go...

Whats everyones views on lift and port size and compared to turbo size in the same way duration has on this thread?

Great thread

 

Phew...

Glad we agree Karl!

 

Steve,

Maximum valve lift is not the issue. Its the actual flow rate you achieve for the time the valve is off its seat. The factors that have a primary effect on this are valve seat profile design, valve head design and port shape/angle.

Its a VERY complex area and one that really must be left to the pro's as its NOT within the capability of any DIY'ers.

 

are all of these things not about getting a compromise between on and off boost and low and high engine speed driveability on a road car?

 

ForeignRS, of course any engine is about compromises.

Nothing is for free. We simply move our parameters around to obtain a torque curve/response that suits our engine/driving needs.

The only way you can have your cake and eat it, is by using a very large capacity turboed engine with 8/10+ cylinder layout. In this manner we can have an engine that makes the required power output and fullfills all the required low rpm drivability issues as well as meeting far improved NVH and design issues.

Sadly such an engine would be more heavy and of course costly, so theres the BIG problem!!!

Life is full of compromises.

 

so what youre saying is its better to have a larger capacity many-cyl engine?

shit i've been telling people for years. 2.8 V6 baby! yeah!

 

Jim If I could chose an engine layout for my cossie it would be a 4000cc V10 with a pair of turbos.

Your shonky V6 is hardly the pinnacle of engine design. PMSL

 

Its the pinnacle of boat anchor design tho