Tried posting this on my own countries board but didnt get any real insights from posts. See what you blokes think.

1. Calculate engine CFM

CFM = (L x RPM x VE x PR) / 5660

(assume 85% VE @ max rpm)
(assume displacement of 13BT rotary engine displacement x2)
= (2.616 X 7500 X 85 X 2) / 5660
= 3335400 / 5660
= 589.3

2. Convert CFM to LB/MIN

LB/MIN = CFM x 0.0756

(no ideal gas law)
= 44.5 lb/min

3. Verify no surging

Confirm no surge 50% airflow

(assume max boost by 50% rpm)
= 44.5 x 0.5
= 22.3 lb/min

Confirm no surge 20% airflow

= 44.5 x 0.2
= 8.9lb/min



A freind of mine helped me with the figures for VE and displacement of the 13BT.

Seems the T66 is a good compressor for a pressure ratio of 2 but is there a problem with surge at higher altitudes with the thinner air?

Someone advise me if I have this wrong please.

 

an interesting post....

i match my turbos using the following formula and it has worked really well.

1.92 cfm = 1 rwhp

for example, a 450 rwhp 13b requires 864 cfm or 59.7 lbs per minute. the map of a t66 shows 72 lbs/minute max which would produce 1041 cfm and 542 rwhp.

44.5 lbs/min 644 cfm (10lbs/min = 144.17 cfm) by my figures would make 335 rwhp

i did not derive the formula but have found it to work well.

howard coleman

 

I am curious where the 85% VE was derived from.

I thought I had read that rotaries were lower, but don't remember where I had read that.

Again...just curious

 

Let's also keep in mind that at wide open throttle, your normally aspirated VE may be say... 85% but at 20% throttle at the same RPM, do you really think your VE is that good with the pumping losses from the mostly closed throttle? No, so the actual VE will be a lot lower.

 

I made a spreadsheet to easily play around with graphing on the maps like you did there... pretty handy...

but in the end, guess what matters? If it works!

Honestly on a frequently-modded car like the RX-7, it makes more sense to look at actual dyno results from other people... and then you can get an idea of what works. Doing the math of it is good too, and it might give you an idea of what to try next... "hmm if turbo A works well, and I do the math there... and then I look at Mystery Turbo B and it has a little better map... that might be worth a try...."

So all the calculating in the world is great, but don't forget to look at actual dynos to see what works and what doesn't.

B