Maximum runable compression Ratio, on 87 octane gas |
|
Porsche, and the Porsche crest are registered trademarks of Dr. Ing. h.c. F. Porsche AG.
This site is not affiliated with Porsche in any way. Its only purpose is to provide an online forum for car enthusiasts. All other trademarks are property of their respective owners. |
|
Maximum runable compression Ratio, on 87 octane gas |
'73-914kid |
Jun 18 2011, 09:39 AM
Post
#1
|
Senior Member Group: Members Posts: 1,473 Joined: 1-November 08 From: Vista, CA Member No.: 9,714 Region Association: Southern California |
Well, after collecting parts and doing all the compression ratio calculations, My old engine was at 7.8:1 with flycut heads and bus deep dish pistons on a 74mm crank. I'm ready to get some more umph out of this engine, and figured while it was apart, my as well get some more compression out of this motor.
This begs the question... what is the maximum compression ratio you can run on 87 octane (cheap gas) with carbs? I'm using this as my daily driver and cannot afford to be putting premium or even midgrade in my car when it's constantly being driven around on a poor college kid's budget. Thanks, Ethan |
messix |
Jun 18 2011, 09:44 AM
Post
#2
|
AKA "CLUTCH KILLER"! Group: Members Posts: 6,995 Joined: 14-April 05 From: between shit kickers and pinky lifters/ puget sound wa.north of Seattle south of Canada Member No.: 3,931 Region Association: Pacific Northwest |
i would think 8.5 is safe on a stock cam.
|
'73-914kid |
Jun 18 2011, 09:50 AM
Post
#3
|
Senior Member Group: Members Posts: 1,473 Joined: 1-November 08 From: Vista, CA Member No.: 9,714 Region Association: Southern California |
The cam is not stock. Its an almost identical grind to the webcam 86 from Schneider cams here in san diego
|
Mike Bellis |
Jun 18 2011, 09:51 AM
Post
#4
|
Resident Electrician Group: Members Posts: 8,345 Joined: 22-June 09 From: Midlothian TX Member No.: 10,496 Region Association: None |
10.5:1 can be run on a watercooled with aluminum heads. A lot depends on timing too. 87 octane has more combustion energy than 91octane. 91 octane helps to reduce preignition. if you install a modern EFI system, the computer does all the timing for you and reduces pinging. Most modern cars that say "must use premium gas" is all BS devised by the gas companies and vehicle makers. I will run any car on 87 and not have any problems. The exception is a high comp race engine.
|
VaccaRabite |
Jun 18 2011, 09:51 AM
Post
#5
|
En Garde! Group: Admin Posts: 13,444 Joined: 15-December 03 From: Dallastown, PA Member No.: 1,435 Region Association: MidAtlantic Region |
for 87 octane in a 914 I would not go higher then 8.
By 9 you need premium or you will ping - which is bad bad bad. Even at 8 you MAY need mid grade. Zach |
messix |
Jun 18 2011, 10:13 AM
Post
#6
|
AKA "CLUTCH KILLER"! Group: Members Posts: 6,995 Joined: 14-April 05 From: between shit kickers and pinky lifters/ puget sound wa.north of Seattle south of Canada Member No.: 3,931 Region Association: Pacific Northwest |
10.5:1 can be run on a watercooled with aluminum heads. A lot depends on timing too. 87 octane has more combustion energy than 91octane. 91 octane helps to reduce preignition. if you install a modern EFI system, the computer does all the timing for you and reduces pinging. Most modern cars that say "must use premium gas" is all BS devised by the gas companies and vehicle makers. I will run any car on 87 and not have any problems. The exception is a high comp race engine. 10.5 on modern engines is due to advances in combustion chamber design and computer controlled fuel and ignition management. there is no way a 1970 engine will with stand that kind of compression. and yes the modern high compression, high octane engines will run on 87 but will do so at a reduced power and efficiency. running lower octane fuel will cause the ecm to dial back timing and richen the fuel mix trying to keep preigition from happening. running high octane will not improve the profomance a engine designed to run on 87 fuel, it will likely hurt proformance. |
brant |
Jun 18 2011, 10:50 AM
Post
#7
|
914 Wizard Group: Members Posts: 11,623 Joined: 30-December 02 From: Colorado Member No.: 47 Region Association: Rocky Mountains |
I'm no engine expert
but I've had to use race fuel over 9.0 so your probably pretty much there for low grade pump gas with your 7.8 you already have. at higher altitude you might go to 8.0 maybe... but with the quality of gas that is out there these days you may want to leave it alone. brant |
J P Stein |
Jun 18 2011, 11:04 AM
Post
#8
|
Irrelevant old fart Group: Members Posts: 8,797 Joined: 30-December 02 From: Vancouver, WA Member No.: 45 Region Association: None |
I used 93 pump for 9.7:1, 110 for 10.5.....apparently I'm not as smart as Mike.
|
andys |
Jun 18 2011, 11:59 AM
Post
#9
|
Advanced Member Group: Members Posts: 2,165 Joined: 21-May 03 From: Valencia, CA Member No.: 721 Region Association: None |
A lot would depend on that cam you have (timint events), and how it affects your dynamic compression ratio. Not an authority on type 4, so I'll defer a recommendation to the experts.
Andys |
larryM |
Jun 18 2011, 12:31 PM
Post
#10
|
emoze Group: Members Posts: 891 Joined: 1-January 03 From: mid- California Member No.: 65 Region Association: Northern California |
L-O-O-ng thread on this subjuct on the Pelican 911 forum
general consensus in the racing industry is that 8.5 is max for our aircooled engines, and 9.0 for iron motors on 91 octane yup - some folks run higher - and some folks hole pistons cam overlap & duration make a big difference - re: prior comment about dynamic CR - but even if ya have a wild cam NEVER "lug" the motor or you will sure kill a piston - ya maybe will hear it ping or detonate just b4 that, but maybe no - eroded edges of the piston top are the first consquence - and you will usually not know it is going on until enuf of that aluminum decides to melt down onto the sidewall and then the engine will just seize - or break big-time if ya are going real fast |
brant |
Jun 18 2011, 01:26 PM
Post
#11
|
914 Wizard Group: Members Posts: 11,623 Joined: 30-December 02 From: Colorado Member No.: 47 Region Association: Rocky Mountains |
|
J P Stein |
Jun 18 2011, 02:07 PM
Post
#12
|
Irrelevant old fart Group: Members Posts: 8,797 Joined: 30-December 02 From: Vancouver, WA Member No.: 45 Region Association: None |
I used 93 pump for 9.7:1, 110 for 10.5.....apparently I'm not as smart as Mike. was that on your 6 ? different combustion chamber shape? Yes, the 6. No, standard shape, single plug. I did carefully set the timing to 35 total. After 7 years those 9.7 pistons looked nearly brand new. You seem a bit conserative, but there is nothing wrong with that. I never had a problem with 87 on the stock 2.0L 4. |
J P Stein |
Jun 18 2011, 02:09 PM
Post
#13
|
Irrelevant old fart Group: Members Posts: 8,797 Joined: 30-December 02 From: Vancouver, WA Member No.: 45 Region Association: None |
I used 93 pump for 9.7:1, 110 for 10.5.....apparently I'm not as smart as Mike. was that on your 6 ? different combustion chamber shape? Yes, the 6. No, standard shape, single plug. I did carefully set the timing to 35 total. After 7 years those 9.7 pistons looked nearly brand new. You seem a bit conserative, but there is nothing wrong with that. I never had a problem with 87 on the stock 2.0L 4. Attached thumbnail(s) |
Dave_Darling |
Jun 18 2011, 05:20 PM
Post
#14
|
914 Idiot Group: Members Posts: 14,986 Joined: 9-January 03 From: Silicon Valley / Kailua-Kona Member No.: 121 Region Association: Northern California |
There are (experimental) engines out there running 14:1 on 87 octane fuel. They are obviously not 914 engines.
The cam grind, combustion chamber shape, bore size, location and number of spark plugs, altitude, boost level (if any), ignition timing, mixture control, and many more things determine if an engine pings on a given fuel. Not just compression ratio. Two data points for you: The stock 2.0 engines were spec'ed for regular-grade fuel. (91 RON, 87 AKI which is what the US pumps are labeled in.) The European-spec 2.0 914 engines were spec'ed for mid-grade fuel (I think it was 94 RON, 89-91 AKI). The only real difference was that the US-spec engine was 7.6:1 and the European-spec engine ran 8.0:1 compression. Obviously the cam, chamber shape, bore, and so on were all stock. I know of people who have run the European pistons and used regular unleaded without apparent problems in the short term. It is possible that the factory left generous safety margins on their fuel recommendations, or it is possible that they got lucky. --DD |
messix |
Jun 18 2011, 05:46 PM
Post
#15
|
AKA "CLUTCH KILLER"! Group: Members Posts: 6,995 Joined: 14-April 05 From: between shit kickers and pinky lifters/ puget sound wa.north of Seattle south of Canada Member No.: 3,931 Region Association: Pacific Northwest |
There are (experimental) engines out there running 14:1 on 87 octane fuel. They are obviously not 914 engines. The cam grind, combustion chamber shape, bore size, location and number of spark plugs, altitude, boost level (if any), ignition timing, mixture control, and many more things determine if an engine pings on a given fuel. Not just compression ratio. Two data points for you: The stock 2.0 engines were spec'ed for regular-grade fuel. (91 RON, 87 AKI which is what the US pumps are labeled in.) The European-spec 2.0 914 engines were spec'ed for mid-grade fuel (I think it was 94 RON, 89-91 AKI). The only real difference was that the US-spec engine was 7.6:1 and the European-spec engine ran 8.0:1 compression. Obviously the cam, chamber shape, bore, and so on were all stock. I know of people who have run the European pistons and used regular unleaded without apparent problems in the short term. It is possible that the factory left generous safety margins on their fuel recommendations, or it is possible that they got lucky. --DD there are gas direct injected [fuel injected directly into the combustion chamber] that are doing this. very high tech tuning here. power and economy/ emissions are outstanding and unprecedented. |
sean_v8_914 |
Jun 18 2011, 10:06 PM
Post
#16
|
Chingon 601 Group: Members Posts: 4,011 Joined: 1-February 05 From: San Diego Member No.: 3,541 |
Dave hit many points that must be considered. comp alone is not enough to make such statements. there are details that can be done to enable higher comp with 87 fuel. this level of hand work detail is not financially viable in a production engine. cam profile, port velocity, chamber shape , piston design/shape, ring gap, chamber size, valve size, shape, deck height, quinch...
guys that know dont give this stuff out on teh internet for free but their engines can do the talkin. |
Lo-Fi Version | Time is now: 17th May 2024 - 04:07 AM |
All rights reserved 914World.com © since 2002 |
914World.com is the fastest growing online 914 community! We have it all, classifieds, events, forums, vendors, parts, autocross, racing, technical articles, events calendar, newsletter, restoration, gallery, archives, history and more for your Porsche 914 ... |