Why I get bad gas mileage, Now I know why

[heliolps2]

Yes I'm keeping my car , thankfully , As some of you remember my trip to TN from DC the best gas mileage could mange was 15.7 MPG at 14.7AFR , yes I could of adjusted my SP to get 15.3 AFR and my MPG would increased a little , by how much, I will test that next time I do a road trip.

The other day I was working on a buddy's stock 930 with the 4 speed trans and noticed that at hwy speed of 70mph the rpms was at 2200. I have G50-01 with 8;32 ring and pinion , at 70 mph i'm at 3000 rpms , I love how close my gears are, but I could use a taller 5th gear , At the drag races I'm just redlining 4th at 7300 rpms at 124mph at theend of the 1/4 mile, I'm good with first thru 4th, my 0 to 60 on a good day is 3.98 , saying that, My city driving I only get at best 11 mpg but sometimes as low as 7mpg and at the track I get 4-5mpg

Right my top speed is 163 at 7300 rpms But I can get there in about a half a mile from a dead stop, but with a taller 5th mile top speed should increase and get better gas mileage, and it would be quieter in the car also at hwy speeds, I guess now I need to find the right gear for my car , I running 285-35-18 on the rears on the street and 315-30-17 on the track, what would be a good 5th gear ? and how much would cost me?

thanks for your support
Helio

[RarlyL8]

Helio, while tall gears will help that is not the major problem. My first 915 had short gears and ran 3400rpm @70mph. That setup achieved 20mpg on the highway.
To tune the fuel useage you need a vacuum gage. When at cruize the engine must not just remain in vacuum but you need to tune it as deep into vacuum as possible. The sweet spot will rocket instantly from vac to boost with the move of your big toe.

[willtel]

People here always seem to hate on the the factory 4 speed but for a street car I really like them. Once you get the power levels up the gears don't seem as long and you can stay under boost while the guy next to you loses ground stirring the shifter.

You could also put a taller tire\wheel combo out back to bring the final drive ratio up. You can't lose with 22's!

[Tippy]

Wow, redlining in 4th gear at 124 MPH. My car did not redline (I too have done 124 MPH in the 1/4 mile) and we have the same trans and by your pictures, you have taller tires making this seem even weirder.

It sounds like your trans is geared lower than a stock G-50.

[heliolps2]

Hey Brian, how have been, say hi to the family for me,
What you said sounds interesting , your gonna have to explain this better to me , I have not heard of this before, but I am willing to learn. we can do it by phone or here on this thread, and if you have pics or diagrams that would be awesome.how your your doing, have had any more progress since the last time I saw it.

Helio

[heliolps2]

Hey Tippy,
I have a 8:32 ring and pinion that lowers the gearing a bit, what rpms are you at in 5th at 70 mph? since the only difference of our trans is the R & P that would be good to know. stock tires are 25 to 25.5 inches my new 18s are 26 inches ( but I haven't used them for any racing just for show) and my 17 are 25.7 inches,

thanks

[Tippy]

I don't remember off hand what I do @ 70 MPH (my car is not on the road at the moment to test ), but I remember it too was high like yours.

Honestly, I thought it was higher RPM than yours in 5th but 4th in the quarter sounded a lot lower.

Weird.

[RarlyL8]

Family is great thank you. The car hasn't moved in weeks as I have no time for it right now.
As I stated above if your boost gage does not also show vacuum you need to get a vac gage that goes to ~15psi. Note what it reads when you are cruising down a flat road at 70mph. My guess is that you are probably on a very low level of boost which is robbing you of fuel. Another robber is having fuel delivery volume that exceeds what is required, aka flow modification. Safe AFR's can be achieved with stock delivery volume up to ~375WHP when used in conjunction with an adjustable WUR.

[911nut]

Originally Posted by RarlyL8

Helio, while tall gears will help that is not the major problem. My first 915 had short gears and ran 3400rpm @70mph. That setup achieved 20mpg on the highway.
To tune the fuel useage you need a vacuum gage. When at cruize the engine must not just remain in vacuum but you need to tune it as deep into vacuum as possible. The sweet spot will rocket instantly from vac to boost with the move of your big toe.

Brian, I'm not aware of a way to do that. Please explain the process. Thanks.

[RarlyL8]

Timing and fuel, just get out the hotrodder handbook. Same principals that work on a carberated HEI Chebby work on a Porsh.

[willtel]

How can you turn 3400 rpm under throttle and not be making boost? It would seem that you can't push that much air over the hot side impeller without having an effect on the cold side.

[smurfbus]

It must be also turbine size dependant. Good amount of timing advance helps to keep the heat low but not sure about the best AFR. Over 15 is colder that stoich so if the spark is good enough you can even try 15.5AFR with a lot of advance.

IIRC best economy is around 16AFR but not sure if any 930 runs without bucking that lean?

When I had my timing too retarded I would cruise on boost even at 65mph.

[IMR-Merlin]

This is what I use for my research when looking for a G50. they have all the gear charts and the have all the gears you will need. I was thinking of going with a taller ring and pinion.

California Motorsports Porsche G50 5 speed transmission section

Glad to hear you're keeping your car.

Brendon

[heliolps2]

Me too Brendon, Ya i'm familiar with them, Roger at CM in stalled my R & P and sold me KEP clutch , back when I lived in the west coast, I'll have get together with them again. they great to work with,

[drmatera]

heliolps2 - is your car EFI or CIS I don't remember. It does sound like maybe you are not running enough timing @ cruise.

[mooney265]

Originally Posted by willtel

How can you turn 3400 rpm under throttle and not be making boost? It would seem that you can't push that much air over the hot side impeller without having an effect on the cold side.

Mine would be steady at 0.4bar at 3,400rpm. Unless going downhill. Currently, mine is 2,800rpm = 80mph and 0.2bar.

I used to have 8:42 and 80mph was 3,800rpm!! Don't miss that...

Last month, I got 19.5mpg on a 1,200 mile trip at 80mph with stock gearing.

[heliolps2]

Yep, CIS

[vascott01]

helio..I have seen you drive.....Lift up on the throttle a bit and you will get better gas mileage..Everything cant be 0-100 all the time and expect good gas mileage... ;-)

Happy New year !

[heliolps2]

Hey , Scott
Very funny man, I do love to see the boost gauge move, ��