turbo cam - with more inlet than exhaust duration

[gb500]

general question - there is a cam in australia listed as a turbo cam for a loghead (pre crossflow (or non xflow)in aust speak).

it has adv duration as 270 deg inlet and 260 deg exhaust.

why the extra inlet duration for a turbo apllication ?-

i thought the exhausts of the ford 6 benefitted from extra duration and with a turbo pushing more in ,the exhaust would use some extra duration - or have i got it all wrong ?

specs are ( no lobe centres quoted)

VALVE TIMING SPECIFICATIONS

INLET EXHAUST

VALVE TIMING 25 - 65 60 - 20

ADVERTISED DURATION 270 260

DURATION @.50" 208 200

CAM LIFT .278 .274

VALVE LIFT .417 .411

INLET LIFT @TDC .046"


Brett
melbourne

 

[Does10s]

Hehehehe....It's good to see that I'm not the only crazy one on this planet!

Our "custom cam" that I had made up by Clay Smith is using 4* less exhaust duration. Unfortunatly I told Clay Smith I wanted 6* less, but for some reason they haven't been able to get that to me and we need it.
I'm having to increase the exhaust valve lash up to .022" just to keep the car from surging on the big end.

The reason for less exhuast duration on a turbo is for spooling. A turbocharger increases the exhaust system resistance (backpressure). At low boost pressures this backpressure may exceed that of the intake boost and you'll get "reversion". The exhaust wants to go up the intake. This also the reason for little or no valve overlap.
We've had a heck of time learning this.

I'm sure Addo will chime in here.....
Later,
Will

 

[StrangeRanger]

Isky also grinds a dual pattern turbo cam with more intake than exhaust duration

 

[Stubby]

So, I guess you have to be serious before you discover this need.
All engines, are all about compromises. I have always enjoyed the small discoveries and solutions that come with pushing the limits.

 

[gb500]

thanks guys - so no good as a supercharged cam then

brett

 

[Does10s]

thanks guys - so no good as a supercharged cam then

brett

NO! Definatly not.
S/C's don't have the exhaust restriction that a turbo does.
So I think you still need to minimize valve overlap, but the exhaust duration will be more like a N/A cam.

Will

 

[JGTurbo]

Here's what Ken Duttweiler has to say about this:

http://www.hotrod.com/techarticles/hrdp ... index.html

 

[XPC66]

This one of those arguments that always results in different viewpoints and industry oracles throwing out one size fits all figures like in that article.

Anyone who has played with a DOHC complete with vernier sprocket hubs knows that small changes in valve timing make big differences in behaviour.

I'll give you a first hand example. Figures are advertised, because that's more related to valve events (remember this is a bucket engine = no rockers RR=1) :

1987 Jap turbo engine standard:

1)
23/51 49/17
that calcs out to 252°I/246°E, 105.5° LSA. 105° LCAI, 106° LCAE, 38°OL, In adv 0.5°

With 0.012" lash the true centre lines were 110.42°I & 109.43°E
and @ 0.010" lift the duration 263°I, 250°E

Gross lift 0.2983"I, 0.2822"E
CR = 7.8
kW/Litre = 74@ 10psi (~40 @ 0 psi)
DCR = 6.84

Adjusting the verniers for less overlap, did nothing to improve performance, even with the Motec remapped.... it was markedly worse.

2)

We replaced the cams with:

0.012" lash the true centre lines were 110.12°I & 110.25°E
and @ 0.010" lift the duration 276°I, 279°E

Gross lift 0.3204"I, 0.3204"E
CR = 7.8
kW/Litre = 150 @ 23psi
DCR = 6.31.( the later inlet closing, lowered the DCR and allowed us to use the higher boost).

So from this example you can see we didn't go to 115° LSA, we did make the exhaust duration greater than the intake, we definitely increased the lift, but more importantly we kept BMEP in check by paying close attention to the inlet closing angle. Compression ratios are of little use in any of it.

Of course the boost figures are pretty uselss too unless you multiply them by the DCR and divide by the CR to get an actual cylinder index (ignoring runner losses) :-

eg. our stock engine was 74kW/litre on 10 x 6.84/7.8 = 8.77 psi

and our reworked engine was 150kW/litre on 23 x 6.31/7.8 = 18.61psi

That means we got a 100% increase in power with a 42% increase in boost.

And as always and most importantly we kept cylinder temps down with AFRs around 11 or lower on boost

 

[blue beast]

how extreme to you have to be before these kind of problems are realized? im thinking of running a turbo but im clueless when it comes to this kind of stuff

 

[StrangeRanger]

You can put a small turbo on an otherwise stock engine with a stock camshaft, set the wastegate at around 6 PSI and see a nice improvement.

But if you're building an engine from the block up specifically for a turbo application, there are things you are going to want: prepped rods and crank, better pistons, headwork and a turbo-specific cam. You'll end up with a larger turbo and a higher wastegate setting and significantly more power and reliability.

It all depends on what you want to end up with and how much you're willing to spend to get it.