Porting without affecting mpg-possible?

[Django91]

What kind of porting can give power gains without affecting the benefits of zero(port) overlap on the renesis engine?

I'm guessing this would be a half-bridge at the maximum, as with this the overlap occurs once higher into rev range. Just wondering if there's means of upping power without a big drop in standard mpg, with the half-bridge, shouldn't mpg's be theoretically about the same around town and even on longer trips?

Toby

[warpc0il]

A street port shouldn't impact your mpg, not in any way that's measurable, give all the variables.

However, if you're worried about mpg then you've bought the wrong car

[Django91]

Not worried-just assessing potential

[Dr. FrankenRex]

TBF a full bridgeport doesn't induce any overlap in the Renesis engine, it's only when you get freaky with the exhaust port that you can induce overlap.

That said, a half bridge would be fine as that only comes in to play beyond ~5k rpm if memory serves, so performance up until that point (and economy) would remain largely unchanged.

Also, I ran a fairly chunky street port in my old 192 and found the MPG to be marginally better in some scenarios compared to the stock porting. Only marginal, and that was with a decat.

[Django91]

TBF a full bridgeport doesn't induce any overlap in the Renesis engine, it's only when you get freaky with the exhaust port that you can induce overlap.

That said, a half bridge would be fine as that only comes in to play beyond ~5k rpm if memory serves, so performance up until that point (and economy) would remain largely unchanged.

Also, I ran a fairly chunky street port in my old 192 and found the MPG to be marginally better in some scenarios compared to the stock porting. Only marginal, and that was with a decat.

With the full bridge, wouldn't the 'bridge' on all intake ports have some overlap with the exhaust? Correct me if I'm wrong. I think that's kinda the point of the half bridge in a sense, keep the drive-ability fairly standard at low revs, by not 'opening up' the primaries.

[V8 Power]

I have a standard R3 and a S1 that has gone from standard to bridge port to peripheral port and to be honest I can’t say I have noticed any dramatic difference in fuel consumption between them.
They are all awful and get worse the heavier your right foot is.
Maybe back to back tests would show a difference but I don’t think it would have any bearing on your choice of tune.

[Django91]

Technically, yes, but the amount of overlap is so low that it doesn't really cause any of the effects and behaviours present in a tradition bridgeport on say an REW engine.

This is because of the oil and exhaust control rings on the rotors themselves, this prevents any useable amount of overlap.

That said, because of the bridge on the primary intake porting you will lose some of the lower-down 'grunt' because of the impact this has on the useable compression - as a result you will likely see a fuel consumption increase due to the need for more revs to pull away and accelerate in what would be the 'normal' rev band whilst driving normally. Essentially the same effect as only running a standard engine between 3 and 5k rpm instead of 1,500 to 3,500rpm.

With a half bridge you don't impact the low rev range as much, so the torque (lol) is still there (at least what little there is is still there), hence why fuel economy isn't anywhere near as impacted.

It's a LOT more complex than I have summarised in just this small post, and you'd be much better talking to an engine builder (either Rotary Revs or RRP would be my recommendation).

Add in to it that any kind of porting is likely to make you want to press the loud pedal more, any potential fuel economy savings are pretty much torpedoed.

That said, my R3 has a full bridge with some mentally aggressive exhaust porting and crazy light rotating assembly (compression test after run in period was 4.9 on all faces) and I have still achieved over 300 miles to a tank of petrol, so don't believe people when they say a bridgeport is bad for economy - how you drive it changes.

EDIT: beaten to the punch by V8 Power. TL;DR - porting makes little difference, your impending lead foot does.

[New Duke]

To Dan's post into perspective. My R3 engine is totally standard (but tuned) and Dan's mentally aggressive exhausting porting still gets better fuel economy than mine.

[Ollie]

To Dan's post into perspective. My R3 engine is totally standard (but tuned) and Dan's mentally aggressive exhausting porting still gets better fuel economy than mine.

Maybe its the bricks attached to the bottom of your shoes Hayden.... Tried not having them on? Heheh.

[New Duke]

To Dan's post into perspective. My R3 engine is totally standard (but tuned) and Dan's mentally aggressive exhausting porting still gets better fuel economy than mine.

Maybe its the bricks attached to the bottom of your shoes Hayden.... Tried not having them on? Heheh.

I wish. Then I'd be taller!

[Django91]

I have a standard R3 and a S1 that has gone from standard to bridge port to peripheral port and to be honest I can’t say I have noticed any dramatic difference in fuel consumption between them.
They are all awful and get worse the heavier your right foot is.
Maybe back to back tests would show a difference but I don’t think it would have any bearing on your choice of tune.

The RX8 is pretty interesting MPG wise, I get no more than 18.5 mpg on my work commute where I don't exceed 40 mph, I do a fast road run most weekends to give the car a good 'work-out' and that sees me getting around 20 mpg, that's with plenty of beeps. Most I've seen is 27 mpg on longer runs. Engine has been recently rebuilt and I have a toyo sport decat, engine is pretty standard otherwise

[Conan]

I was just about to say that porting won’t necessarily give you worse mpg.
I then saw your post about the rebuild .
Increased power can give you better mpg because you don’t have to use as much throttle .
What makes the biggest difference is the actual rebuild rather than the porting
However most people use more throttle for more smiles so don’t see the mpg increase
Regards
Pete

[casey]

I have noticed no significant mpg change with my full bridgeport. I have fairly meticulous mpg records since I bought my car new in 2004. If anything, there is a slight improvement in mpg, which was most unexpected!

[Scotty PZ]

Out of curiosity, what sort of mpg are people getting? I consistently get 180 miles to a tank of super with 125ml of premix. Mines got a large street port.

Edit: and I drove it like I stole it.

[casey]

Hugely variable mpg, but fairly consistent, depending in driving style:
25mpg best (long trip, 60-70mph, light throttle, no fun....)
19mpg average on normal driving (some fun)
7-10mpg trackday (massive fun!)

[v-rex]

Another data point, street port on mine.

I'd say 180-200 miles out of a tank around town. Most of my driving is a mixture of motorways and A roads, when I do use the RX8 for weekend trips and I'd say 220-240 a tank is easily achievable.

I'd say my mpg improved after I ported, but I think the car had been getting worse over time. I certainly remember getting 270 miles out of the tank very early on when I did a trip to Cheshire, but I'll say now with the Street port and the slightly extra torque it's alot easier to stay one gear higher and still have enough power, so it's easier to play the hypermiling game with a street port imo.

I reckon 270-300 would be very easily achievable with my streetport now with a drive of mostly motorways, if you so desired, being able to keep it in 6th all the way and not needing to change down due to the extra torque.

[delta0]

For reference a non ported R3 I can do Calais to Le Mans on one tank. That is with a passenger, a boot full of stuff and at their higher speed limit then some hard acceleration on top. A bit of town I’m driving in places but mostly motorway. This is 275 miles.

[Dr. FrankenRex]

It is worth noting that an engine that is nearing rebuild territory will be down on MPG compared to a fresh build anyway, so even if you went from an engine that was low on comp to a full bridgeport you'd likely feel an improvement in economy just because everything is fresh and running more efficiently than before the rebuild.