castiron exhaust and proper camshaft

[L89 chevelle]

Dave maybe you can answer this. We see cam makers most always making cams with bigger exhaust lobes for engines with poor exhaust systems. I see some now making them with smaller ( 4 to 10* ) exhaust lobes.

When do you need a bigger exhaust lobe then the intake?
When do you need a smaller exhaust lobe then the intake?

We still have race tracks that mandate stock cast iron exhaust some with 2" and some with 2 1/2" manifolds and 500 2brl carbs. Do you go bigger exhaust dur or smaller to get rid of overlap. These engines seem to have the biggest restricton in the exhaust side even with a 2brl carb.

Looking over GM's cams with performance engines I see they favor smaller ( up to 12* ) exhaust lobes on there LT-1 engines with factory manifolds.

Like to here your thoughts and experiance on this.

Thanks

[L89 chevelle]

Originally Posted by L89 chevelle

Dave maybe you can answer this. We see cam makers most always making cams with bigger exhaust lobes for engines with poor exhaust systems. I see some now making them with smaller ( 4 to 10* ) exhaust lobes.

When do you need a bigger exhaust lobe then the intake?
When do you need a smaller exhaust lobe then the intake?

We still have race tracks that mandate stock cast iron exhaust some with 2" and some with 2 1/2" manifolds and 500 2brl carbs. Do you go bigger exhaust dur or smaller to get rid of overlap. These engines seem to have the biggest restricton in the exhaust side even with a 2brl carb.

Looking over GM's cams with performance engines I see they favor smaller ( up to 12* ) exhaust lobes on there LT-1 engines with factory manifolds.

Like to here your thoughts and experiance on this.

Thanks

anyone's thoughts???

[old blue 75]

This may help.

Restricter Race Motors -make more power than your opposition

[L89 chevelle]

I read that but I was looking for more on the reverse lobe's being used now.

Thanks

[old blue 75]

Originally Posted by L89 chevelle

I read that but I was looking for more on the reverse lobe's being used now.

Thanks

I'm not sure what you mean by Reverse lobes?

[L89 chevelle]

Bigger intake lobe ( more dur. @ .050 ) then the exhaust.

ex: 250in @ .050
240ex @ .050
108 l/s
This is more common on blower engines but I see some nat asp engines running them also with restricted exhaust systems.

ex: google cam specs for a 95 LT-1 engine. GM used 12* less exhaust lobe then intake lobe.

[DavidVizard-GFN]

Originally Posted by L89 chevelle

Bigger intake lobe ( more dur. @ .050 ) then the exhaust.

ex: 250in @ .050
240ex @ .050
108 l/s
This is more common on blower engines but I see some nat asp engines running them also with restricted exhaust systems.

ex: google cam specs for a 95 LT-1 engine. GM used 12* less exhaust lobe then intake lobe.

Psss - it's all (well mostly all) to do with fuel economy and clean air --
DV

[L89 chevelle]

Originally Posted by DavidVizard-GFN

Psss - it's all (well mostly all) to do with fuel economy and clean air --
DV

Thanks for looking at my post Dave. So this has nothing to do with the fack they use stock exhaust manifolds and they are getting rid of overlap and reversion?

How does this affect fuel economy any clean air? Can you explain more?

Thanks

[Ol' CW]

Exhaust cycle actually starts BEFORE the exhaust stroke. Restricted exhaust needs to open SOONER in the power stroke to let more pressure "blow down", so you would see this as a longer exh duration.

OEM's probably were using the smaller exh dur to keep from over-scavenging the cylinder putting unburned fuel into the exhaust

It probably also was used to make the car idle smoother. For some reason, detroit thinks people want cars that idle smooth??

[DavidVizard-GFN]

Guys,

Hopefully this answers points brought up be several of you.

First priority on any car manufactures list is emissions closely followed by fuel economy. At part throttle cruise opening the exhust valve any sooner than absolutly necessary cuts mileage. At 2000 rpm part throtle a milage cam will need to open the exhaust valve 20 - 10 degrees BBDC. Overlap is also an issue. The less overlap there is the better the mileage. My tests to date here (and they won't be anything like as extensive as GM's for sure) are that an overlap of about 10-15 degrees gets best economy (generally). Also overlap effects emissions. At idle some residual exhaust in the chamber is good as it cuts emmissions.

On the subject of power the increased presence of intake vacuum and exhaust back pressure really move the optimum cam events around a bunch. The big problem here is without some figures as to the back pressure and the vacuum involved it is very difficult to home in on a cam spec. If I had some numbers here I might be able to shed some light on the situation.

DV