How much compression is safe for the street?

Have heard a few different but interesting opinions.
From folks experience in a non knock-sensor car, how much compression is safe with American pump premium?

9.7?

10?

10.5?

I've mostly seen that 10 the most compression that a pump gas street car should run.

Opinions?

 

depends on the engine and the use I'd say, our new Audi is running a mind blowing 13.5:1 ! but it does run direct injection for fuel so temps are lower and detonation is reduced.
I have a BBF with 12:1 but due to cam timing and lift it's dynamic is 10:1 and this is with iron heads no less! but barely runs safely on pump 91 93 would be min and 100 is best.

 

I went for 11 but I think it ended up at about 10.5

I messed with the timing curve so I could run on street premium (91 octane here) . . . 28 deg under full load seems to work fine. I've had a full track day at Willow Springs which has a lot of full throttle and so far so good.

One of these days I'll get a tank full of race gas and kick the timing up.

FYI, I'm using a Haltech ecu for controlling timing curve.

Sean

 

Are you refering to compression or compression ratio? If you are refering to "compression" as mentioned above, then the number above does'nt relate, but if you are talking about "compression ratio then yes it relates to your question.

 

Compression ratio, as implied by the description using the commonly accepted vernacular term.

 

The answer is, it depends. 4 valve pent roof chambers are more knock resistant than the 2 valve hemi heads. ECU metering of fuel and ignition timing gives you more margin to increase compression.

For comparisons sake you might want to look at other engines that are similar in design that will use the same carburation and ignition. Half a Ferrari V8 are the Fiats and Alfas.

Here is a David Vizard article http://www.popularhotrodding.com/tech/0311_phr_compression_ratio_tech/

Article on Dynamic Compression Ratio http://www.empirenet.com/pkelley2/DynamicCR.html

A critique of said article http://speedtalk.com/forum/viewtopic.php?t=1372

 

What JCR failed to say was that four valve motors require less cam, therefore more compression is available because of less valve overlap and hence less valve timing for a given motor. Very elementary.

 

Excellent reading - many thanks, I learned a lot I did not know. Does anyone know where to find the Ferrari variables such as rod length to calculate dynamic compression ratio?
Interesting also was the commonly held opinion that even the DCR is only a rough approximation of octane requirement.
Just to see the theory, I'd like to figure out the stock 328 DCR and vary some cam parameters to hold it optimal for the street raising the static compression ratio to 10:1.

 

Ohh, look. F-chats biggest tool is back. Considering everything you wrote about adding acetone to gasoline, using AeroShell oil in autos, and saying AeroShell and Redline had "similar characteristics", Gary you have zero credibility on this forum.

 

the WSM on the 328 I have lists the con rod at 136,9~137mm
let me know if you need anything else. also still working on getting you a set hopefully soon.

 

If all your going to do is raise hell and vengefully attack everyone you dont agree with, why not just go play somewhere else? Your technological input is far outwieghed by your rotten attitude. Gary has a alot more credibility right now than you have ever had. Outside of you I dont think anyone has ever attacked him. And sniping quotes out of other threads to harrass people, or to continuously bringing up old subjects to grind your axe against people like him simply proves the point. All you are doing is ruining yet another good thread with your blabbering BS. This is about compression ratios, if you can read the thread, not about JCR's personal vendetta. I may not be a moderator, but they have banned others for saying a lot less than you just have. Grow up. No one has any time for your 7th grade junior high games.

 

Well, hopefully my numbers might be a bit off, but I used Pat Kelley's Dynamic compression calculator and Dynamic stoke calculated from rod length & 48 degree intake valve closing. Assuming a stock US compression ratio of 9.2:1 the stock 328 has a dynamic compression ratio of 7.78.

This makes sense as from the second referenced article:
"It appears that most gas engines make the best power with a DCR between 7.5 and 8.5 on 91 or better octane."

This also would imply that there is room to go up on the static CR without as much risk of detonation, certainly if you were running cams with a bit more duration!

OBTW - let's please just talk about CR, guys.

 

As some know, I have a set of higher lift slightly longer duration intake cams currently in the car with a duration of 261 (235@050) with 105 lobe center. This moves the intake closing angle to 55.5, reducing the calculated dynamic compression ratio to approximately 7.44.

So, assuming I kept these cams, I could increase the static CR to 10.1:1 to give a Dynamic CR of about 8.54 which is on the edge, but still suggestively compatable with pump premium gas assuming the approximate upper suggested 8.5 guideline; Perhaps moving the timing just slightly to bring it down to 8.5 is also possible.

 

Russ,
It seems like the calculator you fornd is a good place to start, but is not telling you the whole story. Cylinder pressure, engine temp and cooling, combustion chamber design, ignition timing and a few other things.

In general, as you increase the flow (your new exhaust, carbs, porting), the cylinder pressure will go up significantly. The higher cylinder pressure will want slightly less ignition timing probably. My only point is that you have more variables than you may realize.

Ignition timing is pretty important. If you increase CR to a point where you need to pull the timing back to prevent knock, you probably have not done yourself any favors....you need a dyno to be sure, but you really don't want to chance having to set the timing below where it makes peak hp. you lose something like 5% for every 2 degrees you retad the timing from ideal (ideal is different for every engine).

All that said, the software Vic has does a dynamic CR taking the intake velocity into account. That's important because the intake flow strea has energy in it and it's moving into the cylinder, so even though the intake valve isn't closed, you are still beginning compression. The higher the velocity in the port, the higher the dynamic CR....and you have the the highest velocity port Vic has ever seen.

I think you want to be in the 9-10 range with 10 probably being too high unless you up the cam duration. Vic will have a recomendation for you too....it's definately a better safe than sorry situation as the small gain you get would become a loss if you end up retarding the ignition timing to work with the octane you can get.

 

Mark
Absolutely agree - In the absence of numbers (still! ) from Vic yet, I do need to be learning about this and make a decision soon about compression ratio. I think in your third paragraph, I'm sure you really mean cylinder pressure but I certainly do take home the point -- quoting the second article posted:

"Cylinder pressures change almost continuously due to many factors including RPM, intake manifold design, head port volume and efficiency, overlap, exhaust design, valve timing, throttle position, and a number of other factors. DCR is derived from measured or calculated values that are the actual dimensions of the engine. Therefore, unless variable cam timing is used, just like the static compression ratio, the Dynamic Compression Ratio, is fixed when the engine is built and never changes during the operation of the engine.

Two important points to remember:

The DCR is always lower than the SCR
The DCR does not change at any time during the operation of the engine"

Talking to Frank in Aus, he really stressed to me the real world experience of these motors needing more compression - of course that may have been prejudiced on pretty significant intake durations and overlap on his cams, but nonetheless point also well taken. Also, Vic does have the actual combustion chamber volume, which he has not sent me yet, so this figure is an approximation on the factory figure of a stock US combustion ratio of 9.2: (swept volume + CC volume) divided by (CC volume) = 9.2. So yes everyone out there in Ferrari land, this is just a mathematical approximation for a starting point. Two days ago I couldn't even spell 'dynamic compression'...

 

So will a motor with 10.5:1 with retarded timing not have as much power as a motor with 10:1 without retarded timing?

I have .400" plus lift on the intake cams but not a ton of duration . . . if there's an extra +10% power lurking down in there by running race fuel and kicking the timing back up, I really think this was a great way to go with my motor because it pulls just fine with street gas already . . . as one 328 owner commented when we were out on a drive: "shouldn't my car be faster than a 308?"

BTW, I have an extra set of 11:1 pistons but not enought valve clearnce with my cams (2V though . . . think you guys are discussing 4V).

 

I agree with the above. Given the engine controls on a 3X8 engine, the factory compression ratio is about right. I have a 440 Plymouth that is currently at about 10.25:1 and I have to back off the timing a little to keep it from knocking on 91 octane fuel. It is a different engine on 106 octane. My 308 runs very nicely on 91 with factory spec timing. I have been advised by many knowledgable people to be conservative with the compression ratio and more agressive with ignition timing.

 

I see what they’re saying, but defining dynamic CR in terms of fixed timing events isn’t very useful really. It tells you something about idle and will predict cranking compression pretty well, but will absolutely send you the wrong way up at the torque peak.

To get 450hp, I’m guessing Frank did 2 things. First he went with BIG ports and valves that flow more, he had to be up around 130+ cfm@10” to make those numbers. The problem is that the port loses velocity when you do that, which mean it becomes a top-end only engine. The second thing he had to have done was add about 20-30 degrees of .050” lift duration (unless the head flow was up closer to 150), which would also really hurt the bottom end unless the CR was bumped to around 12+:1. I don’t think that would be a good engine to look at as a model for a street car.

 

That's right....BUT other thing you did may have changed the timing the engine needs to make full power. Also, just turning the timing up until it pings then backing off is not necessarily getting you full power. It’s probably well worth your time to do a couple dyno runs going up and down on the timing to see where your engine wants to be. If you find that you had it right just turning it up until it pings….then there is almost certainly more to be gained by switching to higher octane fuel and repeating the test. Or it might be perfect already, if so, more octane and more timing won’t add any power….it’s worth doing the test.

First, I really really want .400" lift intake cams too....web cams is trying to figure out if they will fit in a 4v head.

That’s a good example of you need to know what you want from your car. A little less CR might give better street performance, but a higher CR will always make a bit more power if you have the octane to run optimal timing. Street/track is always a balancing act and pretty much everyone will think the “best” solution is something different than everyone else. It sounds like you have a pretty good set-up, but some data on pump and race gas would let you know if what you think is real or not.

 

If you go that route, however, you have to be very careful, as if you go too far, you can burn a piston...

 

That's it is with my Lotus. My CR is about 9.5:1 and the factory advance is 6 degrees. Some guys are running 10.5:1 and that's about it for the advance; I have my advance about 12 degrees and it's fine with 93 octane gas. Europa owners who have driven my car, even with Webers instead of my Strombergs have said it's the quickest Europa they've driven (with a stock engine).

Ken

 

Russ.

I'm running 10.4:1 in my Dino. I run premium 92 or 93 octane, and have no problems at all.

Stock ignition (Dinoplex), and a slightly re-jetted set of carbs. That's it!

Runs great. Extra oomph, too. Car makes about 180 RWHP US spec was 175 at the crank.

Dave

 

That is amazing hp for a 246 - 308 levels actually. Do you recall how much ignition advance you are running? I assume that these are also stock heads and cams - wow!

YET another reason I am hawking your car if you ever decide to part with it...

 

Stock heads and cams, Stebro sport exhaust.

We also swapped the stock 6 lobe US distributor for a Euro spec 3 lobe distributor.

I can get the exact details on ignition from my mechanic. I have to see him anyway, I'll ask Friday.

Dave

 

You'll be the first to know!!!

Dave