Enlarged Exhaust Outlets?

[plasticblack]

I've just been offered a pair of Heads that have the exhaust outlets enlarged, but have had nothing done to the valves or castings apart from this? 

If I choose to buy these heads, could anyone offer a few tips on what else could be done by make to make them as productive as possible?

 Thanks  Paul 

[MC Dyno Don]

 Depending on how much material has been removed and how large the exhaust ports have been massaged they may be more suitable for a turbo application or a natural aspirated engine build with LOTS of compression to overcome the lack of decent back pressure. I have used heads with larger exhaust ports before that ran hard but had lots of TLC into the overall head. If they are opened to 1 5/8 or larger,these are NOT your ticket to ride. If opened  to 1 1/2 then they can be used with good results. In reality you will need a 42 intake to help overcome the exhaust saturation and some good compression @ 9:5 min.  If you can post a picture,that would help,also the measurement of the port will be helpful.  Good luck, Dyno

[hotrodsurplus]

Depending on how much material has been removed and how large the exhaust ports have been massaged they may be more suitable for a turbo application or a natural aspirated engine build with LOTS of compression to overcome the lack of decent back pressure. I have used heads with larger exhaust ports before that ran hard but had lots of TLC into the overall head. If they are opened to 1 5/8 or larger,these are NOT your ticket to ride. If opened  to 1 1/2 then they can be used with good results. In reality you will need a 42 intake to help overcome the exhaust saturation and some good compression @ 9:5 min.  If you can post a picture,that would help,also the measurement of the port will be helpful.  Good luck, Dyno

To be clear, backpressure is always the enemy and you want to eliminate it whenever possible. The problem with excessively large ports (or poorly designed ones) is a lack of velocity. True, an excessively large port will impose less backpressure but the detriment is the diminished velocity through the port and not the lack of backpressure.

As I understand it, the exhaust side of the Type I head generally outperforms the intake side so modifications to it without touching the intake side will create flow issues. And modifying the flange side of the port without touching the bowl side is usually a recipe for disaster. In fact many highly modified ports maintain a relatively small flange diameter--it's common that the exhaust remains larger than the port that feeds it. The entire port must maintain consistent velocity and touching one side of the port without addressing the other will impair velocity.

I agree with Don that these might not be the heads for you. I would consult a head specialist.

[modnrod]

If the heads are in good condition, if they are just down the road, if they are the right price, then just buy them, they will be fine.

Talk to most head guys about a street motor (under6000-6500) and they talk intakes, the exhausts are barely mentioned. Unless you're chasing that very last 2%-3%, unlikely in a streeter, then the exhaust ports simply aren't that important as long as they're not restrictive.
At least you know your motor won't overheat due to a restricted exhaust! 

[hotrodsurplus]

Unless you're chasing that very last 2%-3%, unlikely in a streeter, then the exhaust ports simply aren't that important as long as they're not restrictive.

If you don't mind pronounced flat spots and a marginal torque output then no, exhaust ports aren't that important. A lazy (low velocity) exhaust port has the same effect as an excessively large exhaust system. The sluggish gas flow has a similar effect as increasing cam overlap in the sense that the cylinder will not be able to purge itself of burned gases as effectively.

Not a lot of attention gets paid to the exhaust side on most street heads because it's already quite good in stock form and doesn't need a whole lot of work to match a more modified inlet side. Opening an exhaust side willy-nilly will do considerably more harm than good. It's for reasons like this that butchered heads are such 'good deals' when in fact some are poor value even if free.

The power is in the heads. It's the last place you want to cut corners to save a buck. 

[plasticblack]

Great info folks many thanks..

   Sounds as if these heads come with a lot of issues and therefore better off avoided I think?

     Thanks All.