Max SAFE Compression Ratio?

Brian Redondo · Post by **Brian Redondo** » Fri Mar 08, 2002 11:33 pm

Hello all,.. I have had a 73 914 1.7 since 94. I drove it for 2 years and then stored it in a nice warm warehouse wrapped in sheets(I visit it and clean it often) i am starting a complete resto on it and was wondering about maximum safe compression ratios. Most people are very conservative and say 8-8.5 to 1. BTW i want to build a 2270 or so looking for 160hp. I am an aircraft mech by trade and do not mind a shorter than normal engine life. 50,000 would be about right. i have also heard others say they use 10.5 to 11 :1 on their track cars. i don't plan on tracking it and want to use it as a sunny day driver. Any advice would be appreciated.

Brian Redondo

p.s. I have been lurking around this group for a while and have been pretty impressed with the info i have found. I do alot of electronics work for a living so please don't hesitate to ask me any questions on wiring or electronics. Happy to help.

Ryan72 · Post by **Ryan72** » Sat Mar 09, 2002 9:26 am

9:1 should not be a problem, its a "bit higher then the norm" but could still be used as a daily driver. 9.5:1 or higher isnt out of the question, it really depends alot on the cam and heads you will be using, with a more agressive cam, you can get away with a higher C/R. Me and a friend have a type 1 motor running a bit over 9.6:1, crappy california 91 octane gas, stock heads, 125 engle cam, with no problems, but we dont want/expect any sort of engine life either....

Bleyseng · Post by **Bleyseng** » Sat Mar 09, 2002 12:23 pm

I reread your post and if you want to go with a 2270 get a Jake Raby motor or motor kit. The 1.7 is a good start to build a 2270 with. This set up will get you to the 160hp.
To me a 2.0l that is used is a "core" motor unless it was rebuilt by a "known" rebuilder like Fat or Otto's,Jake Raby.
Geoff

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76 914 2.0L

regis101 · Post by **regis101** » Sat Mar 09, 2002 5:13 pm

Greetings. Grassroots Motorsport magazine is doing a project 73 914. Reliable daily driver. 2.0, ported heads, nice exhaust, basically stock D-Jet FI, web 73, K&N. They averaged 113 hp. It uses 9.2 compression. Builder states that it runs cool and all day. Just more data for ya.

Brian Redondo · Post by **Brian Redondo** » Sat Mar 09, 2002 7:05 pm

I really appreciate all the responses and i will check the magazine. I have heard people talking about the supersquish pistons and was wondering where i could get more information. I have also been looking at dirrerent pistons like JE and LNengineering (biral babies not the nickies) and i was wondering which were good to use considering i don't want to 6 stud my cylinders to my case.
Thanks
Brian

regis101 · Post by **regis101** » Sat Mar 09, 2002 7:43 pm

Yeah, cool. Treat yourself to J. Raby's site. Dyno charts don't lie.

Bleyseng · Post by **Bleyseng** » Sat Mar 09, 2002 11:08 pm

12 to 1 is the highest I have heard of for a daily driver. John Connely at AirCooled is offering new "SuperSquish" pistons. These will will work with standard heads.
Standard pistons figure about 9 to 1.
Geoff

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76 914 2.0L

MASSIVE TYPE IV · Post by **MASSIVE TYPE IV** » Sun Mar 10, 2002 12:56 pm

If you want 160BHP you gotta have over 9.2:1, and nice heads.

Getting this much power is not easy, the engine has be able to still be in it's powerband after 5500RPM to make 160 horse.

The absolute best thing you can do is buy my kit for the 2270, then it is really easy to bolt it all together, not even a clearance issue.....

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Jake Raby
Raby's Aircooled Technology
www.aircooledtechnology.com

Brian Redondo · Post by **Brian Redondo** » Sun Mar 10, 2002 4:50 pm

Thanks for all the info. I am shopping around prices and if i can find a good 3.0 donor engine then i might go the 6 route. Seeing as how you are in georgia i should come visit at have a look at least as the time comes closer.
Thanks all

Brian

Dave_Darling · Post by **Dave_Darling** » Mon Mar 11, 2002 10:32 am

Brian, the "max safe compression ratio" depends on a buncha things.

What kind of fuel can you get/do you want to pay for? Going up to 12:1 is fine on race gas, but don't try it on "regular unleaded".

What kind of cam are you going to use? A big "wild" lumpy cam will allow some of the compression to "leak out", which allows (and darned near requires!) higher static compression. The stock cam is very mild and you don't lose much if any compression due to it, so you can't run as much compression without detonation.

For someone running a stock mild cam on the crap fuel we get in CA (91 octane with lots of oxygenates), over 8.5:1 is pushing it, though I have heard from some people that 9:1 does work, at least somewhat. In some areas of the east coast you can get 94 octane on a regular basis at the pump--so you can go higher. 9.2:1 (per GRM's article) doesn't seem to be any problem at all. A big lumpy cam would probably let you get near 10:1 if you really want to.

The new "super squish" pistons, if they work as advertised, seem to be doing an "end run" around the old rules. If you go that route, then you need to have a nice talk with John C. about engine setup, because he knows the most about those pistons.

The usual rule of thumb is that 96mm cylinders do not need five- or six-studding. (The 103 & 105 cast-iron cylinders do.) That rule might change if you're trying to keep a 12:1 compressed charge in the chamber, though. Again, talk to John C.

--DD

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1974 VW-Porsche 914 2.0 (Type IV powered!)

Pelican Parts' 914 Tech Geek http://www.pelicanparts.com

ray greenwood · Post by **ray greenwood** » Wed Mar 13, 2002 11:12 pm

With the compression that grassroots is running on that 2.0....I would expect just a little better than 113 HP. I would bet that they have done no real tuning to the injection. There is at least another 10 HP that can be realized easily with the stock D-jet, just through tuning it better. Ray

Dave_Darling · Post by **Dave_Darling** » Thu Mar 14, 2002 3:24 pm

Hey, Ray--you might send an e-mail off to Tim at GRM. I'd bet he would love to get a handful more ponies out of that 2.0 and write it up in the mag. [email protected]

It would be pretty cool if they let you loose on the motor and you squeezed another 10 HP out of it for them. If they do it right, it wouldn't cost you anything but time...

--DD

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1974 VW-Porsche 914 2.0 (Type IV powered!)

Pelican Parts' 914 Tech Geek http://www.pelicanparts.com

ray greenwood · Post by **ray greenwood** » Fri Mar 15, 2002 7:35 pm

Cool...I'll see if I can get something off and strike up a conversation. I noticed that it was listed as basically stock D-jet. Its surprising what seemingly trivial tweeks can do for D-jet. Ray

regis101 · Post by **regis101** » Fri Mar 15, 2002 9:30 pm

Cool. I'll respond. Since I brought it up. The article stated an average of 113 with peaks of 120. Let us not forget that this is (just) a daily driver that started at 95 horses. They bumped it with some engine mods and some bolt-ons. User friendly, drive it all day horsepower. As per application, that's not bad?

Ryan72 · Post by **Ryan72** » Fri Mar 15, 2002 10:52 pm

Not bad at all, and thats a rompin' good time in a 914 im sure. Hell I had fun with my 1.7l(80HP from the factory!) that had worn rings and really low compression, so anything breaking 100 ponies would be loads of fun no doubt.... I will now go sit back and daydream for another hour about the installation day of my 2270... just a few more weeks...