can one have too much exhaust flow?

[SirYun]

can one have too much exhaust flow?

I have just tested both my newest project a siamese port 12G202 head (as fitted to 1098cc mini engines) and a spare ''stage 3 for what it is worth '' 1275cc head i ported a few years ago.

I have worked a lot on the exhaust ports (as these are quite restricitive) and was kind of surpised that compared to my 1275 fully ported head the exhaust would flow about the same. I just tested at full lift (11mm) as the seat of the new head is not yet cut properly. more or less like a bare port flow measurement.

ergo. the 1275 flow numbers are about on par (within 2CFM and i retested it 10 times to be shure) with what is published by Vizard. and the small bore head is..if the first impressions can be relied upon, rather quite efficient. ( and 99% of the porting was done in the 25mm before and after the seat

Now here is the catch. the intake on the small head is 30.9mm (a nice waisted stem item) and the one on the big head 35.7mm.

I'm pretty shure i can get enough intake flow with the big head, but i have to get the intake into 75 CFM@10'' regions to keep it at Intake:0.75 Exhaust ratio.

did I overdo the exhaust ?

thank you

Joost

[Guy Croft]

Joost,

please tell me both inlet and valve sizes for the head you're working on and the (equivalent) bpf for the ex that you have achieved (and state depression please, eg: 10").

GC

[SirYun]

Thank you Guy. these are the numbers


Intake big head : 35.7mm exhaust valve: 29.2mm

Exhaust flow 11mm lift @ 10' depression 60.1 CFM


Intake small head : 30.93mm exhaust valve: 26.5mm

Exhaust flow 11mm lift @ 10' depression 57 CFM

The numbers are not corrected to 10 inch (as the flow quik will do quite effectively) but depression was set to within 0.10 inch of the 10' goal and averaged over a series of measurements ( very low variance btw)

chamber and entire port are worked quite extensively on both heads ( ports have not been enlarged in the runners by much, other than was needed to clean up and radius) but i have a feeling that the extra height (about 3 to 4mm) of the short side radius of the small head helps quite a bit..


thank you

btw the port is just been sprayed using brake cleaner so it looks wavvy..it is not in real life but has a full flat finish using a 80 grit roll.

Joost

202 center exhaust

IMG_0976.jpg (65.6 KiB) Viewed 6762 times

[Guy Croft]

I assume you mean 'pressure' for testing the ex port, because that is what you're supposed to do, you quote 'depression'.

Assume you did use vacuum for the ex testing, just go get an idea of what might be possible from the ports I usually have a look at the ratio of inlet-ex valve area before starting a new head, just square the diameters and ratio them ie:

Big valve head:
(in 35.7mm) 35.7 x 35.7 = 1275 (appr)
the ex at 29.2mm gives 852.6

so the ratio of inlet to ex valve area is 1275/852.6 which is 1.5. It doesn't really matter whether you use the ratio of the valve head or the throat diameter for this it will work out much the same and the ratio of valve size is a good indicator of whether the valve match is right. So one might expect that the inlet valve (and potentially the port too) could flow 1.5 x as much as the ex. If the ex port flowed 50 cfm the inlet would flow 75 etc. And the E/I ratio will be 50/75 which is 67%, a bit marginal in fact, but whether you end up with an E/I bare port flow ratio of that low value really depends on how much flow your really get from the inlet. You can do the same numbers on any head..

However you've started with the ex port we know you've got 60.1cfm at near-max-lift. The inlet valve could flow 1.5 x 60.1 which is 90 cfm. But you're never going to get that kind of flow from a sidedraft port with a 35.7mm valve, os yes, it does rather sound as if you've over-ported the exhaust. By my reckoning the most you're likely to get from that valve size on one of those heads (at 10") is about 77cfm.

However it doesn't matter if you 'overport' the ex side, except that you are more likely to break thru the casting.

GC

[SirYun]

well, my setup at this moment ( it will be converted to two way flow) is vacuum only so I tested by applying vacuum to the port (at the manifold side) with a bore fitted, hence depression not pressure.

At least this keeps upstream (atmospheric) pressure constant as well as temperature, which in theory should be good.

I will see what the intake flows when I get the head back from the engineers where I send it for seat inserts.

All very interesting this but I'm allready more confused than I was before.. as I anticipated..

[Guy Croft]

Nothing wrong with applying vacuum to the ex port outer face, rather than blowing from the cylinder end, never done it but I guess the result will be more or less the same.

You're confused? Well, next time do the inlets first and optimise them as best you can, then apply the rule of thumb that in terms of bpf (bare port flow) the ex needs - ideally 70-75% of inlet. This will tell you how much ex work you have to do and stop you overporting..

GC

[SirYun]

...some feedback.

The guy I send the head to (I did the head work trade for a 1275 short block with bad pistons..he got a very good deal) had it finished and after a gearbox failure and rebuild got it set up at the rolling road.

It delivered 69BHP at the rollers. Quite good (dare I say very good) for a 998cc A+series with +0.060" overbore with a Swiftune SW5-07 cam which does not normally rev beyond 6500 rpm.

A gain of over 29BHP from the 40-ish stock BHP.

[Guy Croft]

well done!

Nice to see you again. Can't argue with 29bhp!


GC