RacerX_IU

I know there has been interest in building twin kits with the gt28r and the gt28rs. i have a stock port 13cosmo engine and i was curious as to whether or not you guys think the gt28rs turbos would be too big for it?

the_glass_man

iTrader: (2)

What are your goals for the car? You RE comes with nice intake ports, but the stock exhaust ports are a little more restrictive than a standard port REW. You need to figure out what kind of power band and hp goal you have for your car now and in the future. Any particular reason why you would prefer two turbos over a single?

Icemastr

iTrader: (2)

I would go for twin GT25Rs.

RacerX_IU

i would like to make atleast 450 on stock ports and have the potential there for a port job when the engine gives. as far as power band i dont want to make 400+hp on my last 1000rpms, the sooner the power can be made the better. i just thought doing my own twin kit would be fun and less ordinary but i havent ruled the single out...just seeing if the 28rs would be too big and making a comparision in prices with a twin kit and a single

z8cw

These are the numbers I came up with for an average street port. Keep in mind there are big differences of what an engine can flow.

These are maximum flows

10 psi 35lbs/min
15 psi 42lbs/min
20 psi 48lbs/min
30 psi 61lbs/min

at 5000 rpm the flows are
26, 31, 36, 45

2 gt28rs flow all your maximums no problem. If anything you are staying on the left side of the compressor map which isn't ideal since you don't use the turbos highest efficency level effectively. That said, if you have a really good port job you will flow more and the turbos could be ideal.

In terms of picking an A/R for your turbine it looks like your back pressure with the .64 will be at 1.67 to 1 with 10 psi of boost and 1.36 to 1 at 15 psi, 1.17 at 20 psi and 1.11 at 30 psi. Depending what boost you are running that isn't bad at all.

The larger .82 ratio will give you
1.32 at 10, 1.11 at 15, 1.02 at 20 and .98 at 30

Please take these numbers as an indication. Every engine is different. The numbers are based on an IC efficency of 80%, CP of 70%, 70 degree ambient temp. Manifold and exhaust restrictions are not considered in these calculations.

CW

z8cw

Sorry didn't see you had stock ports, or better I didn't think you could consider two of these turbos with stock ports. Stock flows will only be about 80% of the above or in other words the above flows 25% more than stock.

RacerX_IU

with ^ that said you would think the gt28rs to be too much for stock ports but good for a ported engine. would the gt28r be more in my range? and how did u come up with that stuff, i need to know how to do that

RacerX_IU

if i go single i was looking to go with the gt40r or the r85kit

z8cw---since you obviously have greater knowledge then i do what do you think would be best between my 3 ideas for good power numbers made early in the power band?

z8cw

I gave you all the flow numbers for your engine, now find the compressor maps and match up a turbo that fits. Basically you will not come up with much else than other members on the board have experienced. My numbers are derived from different dyno charts. A crude method to figure out whta our engines can flow. With a large enough sampling I just took the average HP made at certain rpms. From there you can calculate the necessary flow to make the numbers. Good thing is it matches up with the real life experience of other RX7 owners.

The basis of whatever turbo you slap on your engine is how much air can go through your ports at what RPMs. The ports are more important than the turbo, that means no turbo in the same category will give you any super magic over another. You might get a little earlier spool and different effficency range but don't expect a 50 hp difference. BBs are very popular these days for faster spool but are very sensitive to crank pressure and oil flow. Everything is a compromise.

One more hint for you. With a OEM port your max HP at 20 PSI will be 420 at best.

CW