Race Engine Forum UK

Good morning Guy, i hope you're well.

My question regards Exhaust/Intake ratios.

On the cylinder heads im currently modifying, i have intake valves at 34mm and exhausts sized at 27.5mm, and its a 4 valve per cylinder type design.
If my math is up to par, i calculate the size ratios for the exhaust at 80% the size of the intake.
Reading a book by Mr David Vizard (quite an old book) the reason given for the valve sizes being different is due to the density of the gases and the fact that exhaust gases tend to be forcibly expelled by the piston, general practice dictating a size of between 0.8 to 0.9 the size of the intake.
My valves obviously fall on that 80% figure.

However, measuring bare port flows reveals the intakes ports flow 121cfm@10" and the exhaust ports 93cfm@10".
The ratio for the exhaust to intake flows then would be around 76% in an unmodified form.

Modification of the intake ports can yield 145cfm@10" and exhaust ports 107cfm@10".
So the flow ratios for the modified ports would then be around 73%.

My question (finally) is would there be a benefit to ensuring that the flow ratios of the bare ports match with the valve size ratios? In this case 80%?
Obviously there are a few ways to do this, either by raising the exhaust flows or reducing the intake flows ( bad i would think).
Is there an optimum ratio i should be aiming for in terms of flows?
My current practice has been to improve the flows of the ports and keep them as per stock ratios as measured in bare port flow, but im uncertain as to whether or not this is the correct route to be taking and i may be wasting possible improvement.

As ever, any light you may be able to shed on this question would be gratefully appreciated, thank you for your time.

Tony.

You can definitely assume 75% BPF E/I is fine for normally aspirated. Less (certainly below 68%) and you might well tend to struggle to get high power/rpm and more will make no difference. This is based on a huge amount of flowbench testing (by me) correlated to very accurate dyno feedback. To a certain extent a weak ex flow charateristic can be compensated for by longer duration ex cam with more lift at tdc - but this narrows the narrow the powerband very substantially.

The ex gas vents itself so long as the cyl pressure is above atmospheric (or above pipe static pressure) and beyond that the piston has to pump it out. The more it has to do (ie: the higher the 'pumping loss' ) the less power the engine will produce as power is used up doing it and the more residuals (ex gas contamination) there will be to dilute the incoming charge near the end of the ex stroke.

I have to say that with turbocharged engines more ex flow capacity 'seems' to be a good thing but I very much doubt that you'd need to aim for more than 80%.

G

Many thanks for your response Guy.

It turns out that I can just about get to 75% E/I with these exhaust ports anyway and since the head (Toyota) is a bit of an odd design with two unequal length exhaust ports and one straight shot they're hardly optimal out of the factory, with at least 10cfm difference between them due to that design.

I've found that I can attain 75% on the two odd length ports but easily exceed it on the other straight port so it's a case of not developing that one quite as highly so they'll all be as close to equal as I can get them.

Once again, thank you.

Tony.

good-oh!

I wouldn't waste too much time trying to equalise them though, you'd be astonished how littlde difference it makes to an engine's behaviour when there are variances cylinder to cylinder. I've even run odd bore sizes!

G

This topic has seen me drift into day dreaming many times, just random thought as I've still to organise a flow bench for myself but as and when that time comes I'd like to plot the ratio across the valve lift range. To fuel my thoughts would you Guy be able to offer some experience of how the ratio might behave over a valve lift? Do the ratio's mentioned above apply to a bare port? The usual lower exhaust lift maybe taylors the ratio to an ideal? I've read how an exhaust with a flow of less than 76% can be a reason to increase the exhaust duration, maybe this ties in with the higher % to suit FI engines, keeping the overlap in check?

Getting close to another topic so enough of my random thought,too late this evening to be straining my feeble brain!

Nik

You cannot correlate ex to inlet valve-in flow at all and the E/I ratio using bpf only is a rule of thumb based on my own exp. I have not seen it published anywhere else but it seems to hold true.

All you can do with the ex side - as I have frequently demonstrated with inlet ports - is optimise the ex seat to ensure that at some sensible lift, ex valve-in flow near intersects with ex bare port flow (bpf). If it does not it will be because of a problem with shrouding, ex SSR, valve or seat shape.

G