What is the significance of the ratio of intake to exhaust flow? What is the result of heads that have a high ratio of exhaust to intake (80%) versus heads that have a lower ratio (60%).

 

A head with around 80%+ would be killer Nitrous head ,
70-75%would be a killer N/a head with a same lobes
60 % would need a cam with more exhaust durration.

this is all pretty general though

 

Lift Intake Exhaust Exhaust to Intake %
.150 94.1 73.1 78%
.200 124.3 96.4 78%
.250 168 116.6 69%
.300 206.8 133.3 64%
.350 240.8 149 62%
.400 267.5 165 62%
.450 285.9 179.5 63%
.500 289.1 191.1 66%
.550 300.2 200 67%
.600 303.6 206 68%
.650 305.3 212.9 70%
.700 303.3 216.2 71%

 

So based upon the above information, this set of heads would benefit from a traditional split duration cam. A reverse split would be wasted do to the deficient exhaust port flow. Am I interpreting this correctly?

 

Depends on how the head was flowed <img border="0" title="" alt="[Smile]" src="gr_stretch.gif" />
But those are some pretty low flow numbers on the exhaust. <img border="0" title="" alt="[Smile]" src="gr_stretch.gif" />
We always flowed with a pipe on the exhaust it will show bigger numbers,just a matter of preference.
Also you have to put an intake on the heads and this will hurt its performance alot.
Your best bet is to ask the person that ported your heads which cam to use.They will know what has worked for them in the past.
Why did they flow them to .700?
I think this is place to ask about others experience with certain heads and cams etc,But I honestly think your porter would be better to choose a cam for you as he will have more info on what your expecting and what your goals are with the heads and cam combo.As well as how his heads perform with certain cam's.
Brent
PS Good luck

 

The intake port was clayed. The exhaust port was flowed w/o an adapter. We flowed them up to .700 out of curiosity. Wanted to know how they would respond to a solid roller conversion.

<small>[ August 28, 2002, 10:17 PM: Message edited by: JP98SS ]</small>

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif"> What is the significance of the ratio of intake to exhaust flow? What is the result of heads that have a high ratio of exhaust to intake (80%) versus heads that have a lower ratio (60%). </font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">So many variable applications, but for the sake of discussion, 70% or more is recommended for N/A engines while 80% is more desirable for engines with power adding devices.

Cheers,
Richard

 

I guess I will rephrase my question. For simplicity we will stick to N/A performance. What happens, in terms of hp/tq when you have too much exhaust flow, and what happens when you don't have enough. What suffers under what condition.

 

According to some engine dyno program I have it appears as if too much exhaust flow is detrimental to low end tq. Assuming that the intake side stays the same, if the efficiency of once the exhaust port increases past a certain point you lose bottom end tq for an increasingly smaller gain in top end hp/tq. This however ignores to some extent the effect of port velocity. If the increase in flow of the exhaust port comes with decreasing velocity you will lose even more tq.

Am I totally off base here? I am trying to figure out why the aforementioned heads managed to dyno with more tq than hp on a 388. Single duration 229 cam with .579 lift was used.

 

My guess would be it did that because the intake tract is limiting the hp of the engine.That isnt a big cam for a 388

 

A proper set of headers will help the exhaust out. Remember the exhaust flow is not at 28" of water its alot more, because of the extreme heat and pressure the combusted charge is at.