Relationship of Comp ratio to HP

[Mn_Z_Man]

Hey all, I'm getting ready to start a 280 NA street build, and am wondering how to pick a compression ratio. Is it worth it to target, say 10:1 (head/piston swap), over the stock 9:1, all other things being equal? I know that going too high will cause problems, but aren't most high output na motors around 10.5:1?

thanks

Eric

[John Coffey]

but aren't most high output na motors around 10.5:1?

Typically they are 13 to 1 and higher. For a street car a lot depends on the head. The P90 has quench so its a little more detonation resistant. You can run those at 10.5 or even 11 to 1 on 93 octane. All others are better at 10 to 1 or even less for 91 octane.

[bob m]

Eric

IMO, Compression ratio is only one part of the puzzle. Decide what you want to do with the car, then what you need from the motor to get those results. I suggest working with your engine builder from the start to decide on the cam, compression, piston, head work and carburation before you decide on 10:1 or 10.5:1. The cam can make a big difference on the effect of your compression for instance. There are some very experienced Z engine builders that can save you a lot of time and money by building your motor to do what you want.

Bob M

[tlorber]

Does anyone know the relationship (conversion) between psi and compression ratio?

[tlorber]

Oh, and while we're at it, in California, where we can only get pump gas as high as 91 oct. (and farmers can't get water at all) what is the highest compression one can comfortably run without predetonation?

[conedodger]

I recently blew up a Porsche motor with 9.5:1 compression by exploding a piston. I was on pump premium unleaded Chevron here in California. I think I will probably back down to 9:1 on the rebuild which will start in the next month when my sponsor gets parts. If this is a street motor, just stick with half a point higher than stock. Be careful with your cam. Biggest mistakes are made there...

[Lazeum]

I'm in process of changing the head of my L28 and I'm going to reduce the comp ratio that's currently too high.

As of today, I do have a F54 / N42 head, flat tops and stage II MSA cam (Scheinder 274/274).

I also have Triples Weber, 6-2 header, 2.5" exhaust and electronic ignition.

I had to retard the timing a lot to avoid knock (28° instead of 34-36°). The comp ratio is around 10.3/10.5 which creates some bad burning condition with open chambers. As a results, the car is not so powerful (but feels & sounds nice)

I recently got a port/polish P79 head from Braap on HBZ with bigger cam from Rebello. Even with the quench chambers, we've reduced the comp ratio to around 9.3/9.5. This should bring much more power than my high comp engine with low advance. Head is still in the box, but Braap garanteed me change would be dramatic

If I'd chosen to keep my current combo, I would have had to get a much bigger cam to reduce dynamic comp ratio, this would make the engine less street friendly but it will keep it away from Knock.

Edited December 1, 2009 by Lazeum

[Derek]

I'm in process of changing the head of my L28 and I'm going to reduce the comp ratio that's currently too high.

As of today, I do have a F54 / N42 head, flat tops and stage II MSA cam (Scheinder 274/274).

I also have Triples Weber, 6-2 header, 2.5" exhaust and electronic ignition.

I had to retard the timing a lot to avoid knock (28° instead of 34-36°). The comp ratio is around 10.3/10.5 which creates some bad burning condition with open chambers. As a results, the car is not so powerful (but feels & sounds nice)

I recently got a port/polish P79 head from Braap on HBZ with bigger cam from Rebello. Even with the quench chambers, we've reduced the comp ratio to around 9.3/9.5. This should bring much more power than my high comp engine with low advance. Head is still in the box, but Braap garanteed me change would be dramatic

If I'd chosen to keep my current combo, I would have had to get a much bigger cam to reduce dynamic comp ratio, this would make the engine less street friendly but it will keep it away from Knock.

I'm starting to make some decisions on my motor set up and I'm leaning towards the lower comp higher advance theory as well. I have a P79 that I'm starting to work. Did paul shave the head to reduce the combustion chamber size?

Thanks

Derek

[Lazeum]

Paul shaved the head a little to make it clean and flat but barely, not the usual 0.08" we can usually see. I don't even think he shimmed the cam towers.

Edited December 1, 2009 by Lazeum

[mr.inline6er]

Does anyone know the relationship (conversion) between psi and compression ratio?

There is no exact formula for detirmining compression ration from PSI as the PSI is influenced by so many variables like the condition of the rings and valve seats, altitude, temp. etc.

However you can roughly guesstimate it by using CR=(PSI/14.7):1

[Mn_Z_Man]

Thanks Lazeum and John, that was exactly what I was was try to get to. Your build is even similar to what I am planning. I've got an N42 block with an N47 head I picked up used. I also have a fresh rebuilt stock E88 off my L24. I'm trying to do the mix and match thing, with flat top pistons on the table if needed, or the E88 head, but that has smaller valves. I have found several nice write-ups, was just looking for some actual experience with similar builds.

Lazeum, do you feel your triples were worth the extra cost over SU's? I've got about $2500 to spend, and triples would eat up a lot of that (new, anyway)- SU's would leave more for other goodies.

[Zedyone_kenobi]

SU's are wonderful and can be tuned to handle VERY large HP Normally aspirated engines. I would stick with SU's and build your engine using the best parts you can. Also, do not skimp on machine work. Building an engine is like painting a car. Its all in the prep and precision.

I have built over a dozen or so engines in the past, mostly V8's but the rules hold true for anything...Here are some guidelines I recommend you follow...

1) Your best bet is to take some time and be honest with yourself. Are you racing for money, are you running ITS class in an SCCA championship. Is this a street engine 99% of the time, or is it closer to 50/50 street/track. You really have to be honest here. Build the engine for what you will use it most for. Sure it is awesome to say you have 300HP, but if it only comes alive at 7600 rpm, it is not much use to you.

2) Match parts. Match parts. Match parts. We do not have the benefit of VTEC, or variable intake runners. We have a single cam and two valves per cylinder. You will only get one shot at a peak performance combination. Having properly matched parts for the driving style you do is FAR more important than having an uber built head, or a monster cam or huge carbs. Let me say that again. Its FAR more important to have all your parts in harmony with one another. Case and point, I have built many street engines in my V8 days using off the shelf GM parts that were carefully picked to work together, with a mild cam and reasonable carb and intake. I routinely beat my buddies with mega dollar aluminum heads, huge cams, monster carbs, etc. The parts must be designed to work together with not only each other, but with your car as well (gearing, driving conditions, etc)

3) Do not try to make the most powerful engine you can. The more high strung engines are the closer you need to monitor them and keep them in perfect tune. IF you like this kind of thing, then great, but if you want a streetable engine that is fun all the time, then build smart, build conservatively, and build cleanly. Put your ego aside and build the engine you want, not the engine you THINK you want. Again, it goes back to being honest with yourself.

4) Research, what is available form Datsun/Nissan. Engineers spent millions of dollars and many years researching the L series of engine. It is very much like the small block chevy of Japan. Parts interchangeability is huge and a killer combo can be picked directly form OEM parts. READ READ READ how to manuals. Read books on building the L series of engines. It will not only teach you how, but why things work and what evolution the engine took in development.

The number one thing that is needed to build a fantastic engine is not money, its knowledge. Read as many books as you can to learn the intricacies of the L series engine. You will be smarter, and you WILL make smarter choices. Check amazon for books on the subject, you will be rewarded 10 fold.