Porting/valve size question wrt my 2.7RS+ spec motor
 

i have a 2.7 in my RS rep race car with the following basic specs:

10.5:1 JE pistons
2.7RS MFI
2.7RS valves/ports
GE80 cams
Bosch twin plug/twin CDI
early heat exchangers
stock muffler converted to 2 out (2.5" outlets)

the car is mainly used for historic racing and thus i have to keep the 2.7 capacity and the period style induction and ignition. also, as we often run tarmac rallies, i'm stuck with pump fuel and the 10.5:1 comp ratio.

keeping the above limitations in mind, i'm wondering what else i could do to this motor.

notwithstanding the GE80 cams, the power peaks about 7000rpm (i thought it would be higher). i'm wondering if the current valves and ports are the limiting factor.

has anyone used 2.8RSR sized valves/ports on a 2.7?

all suggestions welcome.

There are others way more qualified to offer suggestions but I would think the particular circuits you race on would be the real determining factor for you. Each one being unique and having it's own requirements what is the best compromise for you?

Is it in your best interests to trade off your present torque curve for one that moves up the RPM band for bigger HP numbers?

Could your needs be better served with tranny gearing?

Considering that being a period race car is your limiting factor I would look to Porsches past experience in the matter inre. valve and port size.

We know that they went to the larger valves (I49 E41.5) with the 2.8RSR which used 43mm ports. Before this the largest port size was 41mm with your present (I46 E40) valve sizes. So I think it's safe to assume they felt 41mm was the largest port that the (I46 E40) valves could efficiently support.

41mm is a substantial increase to your present 36mm ports so it seems there is plenty of room for peak HP increase with your present valves. This is good considering the cost of RSR heads. We know per BAs book that Porsche made at least two race motors with the 41mm ports. 911/70 a longstroke 2.492 liter which made 270hp@8000 and 911/73 a shortstroke 2.464 liter which made 275hp@8000. Both motors used 906 cams, I46 E40 valves and 41mm in. and ex. ports. So realistically you could build yourself a 2.7 liter 906 race motor probably good for atleast 285HP with your current I46 E40 heads.

Don't forget the 2.1 RSR turbo... IIRC from P Frere's book (the porsche 911 story) it used the 2.x heads and used a 47 mm intake and a 40.5 mm exhaust valve. It is listed with 43 mm ports. I figure this was as big as it got with the 2.x heads. If porsche had used 2.8RSR heads for this motor, it just would have had the 'big valves' - but as it doesn't...

As I have been chasing up this tree myself, it really seems to be the only option short of trying to use 3.0+L parts.

So my next 'cataloge search' was going to be for 47 mm valves with 1mm thinner stems (a al 935) if I can find them.

tadd

Ryan, G-day mate!

My suggestion is

38mm ports

Bottom of throttle bodies opened up to match the port. Right now 38/36mm. you want 38/38mm

Open your stacks up to 38mm bottom 46mm at the top. It may be in your best interest to sell the S stacks and bore out some T stacks due to the value of the S stuff.

You will keep some important mid-range torque to get out of the corners and get a bit more for the straights.


If you decide to go bigger, you will need bigger butterflies in your throttle bodies.

the max you can take out of the plastic stacks are about 40mm on the bottom and 46mm on the top. Maybe 1/2 a mm more but you are getting very thin!!




Bobby,Excellent post my friend!SmileWavy

thanks guys - and extra thanks for the throttle body comments aaron, i'd almost forgotten about them (and have a spare T set...)

My 2.7 track motor is about like yours, only Mahle 10.3/1s, carbs (46s) and an Elgin 315 cam (don't know just how that compares). My HP peak is at 7100. I've got 38mm ports in and ex. At about 250 hp (220 RWHP) I'm well shy of factory 110 hp/L figures, but it pulls nicely.

The rap I hear about the factory huge ports is that those worked well for high speed European tracks where the peak HP was needed to muscle the air aside on long straights, but that for the tracks of interest to me here in the middle of the US I needed a wider power band more than that top end. Don't know what kind of tracks you run on down under. May be complete BS, but something to consider. At 38mm I still have a fine power band, so naturally that would be my first thought as I know that is not too large. You could always open up the heads more later, but from Burn-Bro's post it sounds like you aren't going to get to 41mm anyway.

You can add about 1mm to your valves' effective opening by moving the seat mating surface out. That was how the factory did its race motors (even mentioned as an option in the 2.0 era factory race parts catalog), and I suspect is what the 2.1 little turbo had, with bores less than 84mm. Not apt to aid longevity nor lend itself to valve/seat regrinds. Because with 90mm (less for the little turbo) bores there just isn't room for bigger seats and valves. To make bigger valves work on the 92s they had to change the valve angle so they didn't bump into each other.

I second the gears suggestion, and add another way to spend your money that you must have thought of - weight.

Have you had the engine on a dyno? Even a chassis dyno would be good, as you are looking at the shape and location of the torque curve. Perhaps a different cam might move yours up. I figured if I could shift mine up (to the right on the graph) 500 rpm I'd be making 100 hp/L or more. Don't ask me which cam, if any, might work such magic.

Walt Fricke

thanks walt

a new gear chart is being prepared as we speak - harder than it sounds when you mainly do 4 or 5 rally events with 8 or 9 completely different stages a day.

i'm also tossing up moving the car up a class where we can run lightweight panels etc.

the slope is slippery...

My thoughts are in line with Bobboloo and Walt's. I wouldn't pay too much attention to the example of the turbo motor within the context of what you're trying to do since the mixture going throught a Turbo's ports and valves is pressurized, that's a whole 'nother ball of wax.

My research on porting a few years ago suggested that the 36 mm ports on the stock 2.7 RS are a bit restrictive for generating max HP, and is one of the reasons why a 2.7 RS feels more like a "big E" rather then a "big S" engine.

What sort of rev range do you want with your engine? Once that's defined, the HP targets should fall into place from there.

john,

i have Ti valve retainers and my rev limiter is presently set at 7300.

on a circuit i'll hit it fairly often. during a rally, i normally shift around 7000.

the car pulls pretty cleanly from about 4000 onwards.

walt

what size headers do you use with your 38mm exhaust ports? (38mm = 1.5")

i'm currently using early style heat exchagers but part of the car's weight loss program should see the removal of all the vent gear so it wouldn't be a problem to convert to headers.

i can't recall the ID of the stock h/e's.

Headers
 

I just happen to have a set of stock '74s on my work bench. 34mm ID at the flange.

The Bursch headers I use on my race motor measure 39mm minus at the flange. The tube is 1.6" plus OD, which I take to make these nominal 1 5/8 headers (which is what sticks in my mind from when I bought them some good while back. I measured my spare set from under the work bench.

Walt

Very good thread.

About the headers - I always thought heat exchangers and headers like the Bursch headers were the same diameter. What difference would there be in performance with a header .5mm in diameter larger than a heat exchanger, but going to a standard two in one out muffler?

Thanks.

SmileWavy

In all honesty, I don't get a sense that header diameter is a big constraint with most 911 motors. Having a slight step from the exhaust port to a slighty larger primary tube isn't a bad thing either. But going too big in the header size results in the loss of exhaust velocity which makes scavenging harder.