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compression ratios and pump gas???

114K views 82 replies 20 participants last post by  Joe Clarke 
#1 ·
on a chevy small block - 383 what is the limit on compression when i have to stop using pump gas and go with the high dollar juice? i know some people might have different opinions im just looking for a rough number so i know my limit on this build.
 
#2 ·
Compression

I haven't seen any cars sold with higher than 11:1 so I would say it's the limit
baring improved engine materials, which are costly. Beyond that I doubt you get better performance out of higher octane even. But I don't know anything about race builds, there maybe different setups in those engines.
 
#3 ·
see, im building a street car and with the setup i have on mind ill have about 11:1 maybe a little less. eagle rotating assembly all forged 6" rods flat top pistons and a set of dart iron eagle platinums for the head 2.02 valve 64cc combustion chamber. that makes about 11:1 right there. i just dont want to run that octane fuel in a street truck. but who knows i might.
 
#7 ·
it all depends on the build. compression relate to not just pistons, but stroke and cc of heads. if you'r motor is 11 to 1 total with stroke, cc in head, and pistons you will be "ok" with 91 octane.but no higher. thats the max. it will run better with higher octane if its 11 to 1. so better to go with 10 3/4 to 1 total.
 
#9 ·
Mechanical compression isn't the most important factor. Cylinder pressure is. Contrary to popular thinking, the two aren't connected in a linear fashion.

A larger camshaft with higher compression is better can be just as happy with 93 octane as an engine with a smaller cam and lower compression.

Dynamic compression, look it up.



Larry
 
#10 ·
coldknock said:
Mechanical compression isn't the most important factor. Cylinder pressure is. Contrary to popular thinking, the two aren't connected in a linear fashion.

A larger camshaft with higher compression is better can be just as happy with 93 octane as an engine with a smaller cam and lower compression.

Dynamic compression, look it up.



Larry
Static compression is STILL an important factor. most people that learn about dynamic compression think that its the end all when in reality static is just as important if not more so. Sure dynamic compression give you an idea of how you can squeeze a little more out of an engine, but it has its limits.
 
#11 ·
F-BIRD'88 said:
If that was actually true then racing motors with long durations cams would not require racing gas with a high octane. They do...so....

Its just not that simple. You're ignoring the effect of the "heat of compression".
If you compress a volume of gas x amount, it is heated up. Compress it more, it gets hotter. Once it exceeds the temp that allows self ignition all hell breaks loose. (detonation, preignition and piston holes isn't far behind) Its the net temperature of the charge during combustion that determines the auto ignition (spark knock limit) threshold more than the (cylinder) pressure. "Gas Laws" "heat of compression" Look it up.

You're right about compression causing heat, and its simply stated in the gas law PV=nRT, or PV/T(1)=PV/T(2), but that kind of proves the dynamic compression theory. the actaul reason the dynamic theory doesn't work is actaully much simpler. dynamic compression is only effective where there is no intake or exhaust scavenging- at REALLY low engine speeds (idle or lower). As soon as you put these effects into the mix the cylinder pressure go up tremendously inspite of the extended cam timing. What extending the cam timing does do is push their effects higher in the RPM range where fuel atomization and distribution is much better. There i still a limit as to the cylinder pressure a fuel can take, but when you improve the delivery of the fuel you delay detonation, to a certain point.

A race engine with a high compression ratio and large cam can run to 2,000 RPM on 87 octane all day- because the cylinder pressure is still low- but as soon as the "cam comes on" (scavenging begins to take effect) it will rattle like a can of beans.
 
#12 ·
You're not paying attention to what I said.

When you start playing with cam timing to lower cylinder pressure you have to pay attention to volumetric efficiency so that you can control the point at which detonation would theoretically occur. It's best to have this happen at a higher rpm than it would've otherwise. There's less real time for it to happen and therefore not happen. It's a careful balancing act.

Combustion efficiency is a factor, the longer it takes to complete a burn cycle the greater the chance of detonation occurring. Better efficiency = lower timing = more tolerance for higher cylinder pressures using a given fuel. This is especially important at lower engine speeds.

As for the comment alluding to competition engines......any competitive engine is running on the ragged edge of detonation, for maximum combustion efficiency. That almost always amounts to more power. So the argument is a moot point.

The maximum mechanical compression ratio for a given combination is directly tied to where maximum cylinder pressure occurs in the rpm range, combustion efficiency, volumetric efficiency, the type of fuel used and the application in which the engine will be used. It's not simply a number arbitrarily given from a text book based on another person's experience.

It's a lot more complicated that my simplified explanation, and your rebuttal. My point was that cylinder pressure can be manipulated so that higher static ratios can be used with lower than normal octane ratings. You just have to do your homework to make it happen.


Larry
 
#14 ·
F-BIRD'88 said:
What extending the cam timing does do is push their effects higher in the RPM range where fuel atomization and distribution is much better.

But always at the expense of low end and mid range torque. Think of how you actually use a "street motor". You end up with a motor that is lazy thru most of the rpm band you want to use and then at high rpm (where cylinder pressure is now high) you cannot run optimum spark timing to avoid detonation.

You end up with a motor that has no torque, doesn;t accelerate the car or truck as well and is very heavy on fuel consumption and does not go as fast as it could.
You do not end up ahead of the game or even equal. You end up with a poor, inefficient engine design. It just doesn't work well in the real world.

The most common mistake that people do when designing a "performance motor" is over camming it.

but that kind of proves the dynamic compression theory.

No, it proves the real world limitation and over emphasis that people put on "dynamic compression theory and calc", in fooling them selves that they can compenstate for excessive mechanical compression ratio (with X fuel octane) with (over) extended valve timing. by picking a bigger cam than the motor actually needs.

Higher compression engines (that actually make real power) need higher fuel octane, than lower compression engines do.
It does lower your effective displacement in the lower RPM, which does cause you to loose some torque, but it may not be "lazy" at all. You just have to limit yourself. I agree that you cannot go overboard on the cam just to lower cranking compression. You do need to limit yourself, HOWEVER you can run 11:1 on the street in a properly built engine matched to the right drivetrain in the right car.

Another thing to consider is that to take advantage of the power upstairs you'll run steeper gearing, which often time reduces the amount of low end laziness that you feel. Also, increasing the stall helps this. Sure you're getting away from a freeway cruiser and closer to a track bruiser, but the whole intention of building a higher compression street engine is to make it more "race car-esque"

I'm not trying to disprove you, I agree with you that there are limitations- but the limitations are not set at 9.5:1 compression and 220º @.050" that so many people assume. If you set your car up right you can go all the way to 11:1 and 250º and still be very street friendly. And no those numbers are not the exact limit, its a range. I'm sure you understand that F'bird but I don't want someone else taking it as the gospel.
 
#15 ·
Not really, GM stuffed 10.8-1 compression into the LT4 engine, 10.5 in the LT1, with a short cam. They tamed the combustion with reverse cooling and a wide lsa, it was a nice engine. It really woke up with a better cam and ported heads too.

Like I said, nothing's written in stone. Many engine builders prove it on a regular basis. I've done it twice myself. Neither of the engines were what you would call docile though. Not even a daily bruiser kinda engine. In fact, they both ran better with 110 gas and a bit more timing. The point is that they can, and do, run on 93 octane on a regular basis.

I understand your point of view though. Once upon a time I would told me that I was a nutjob, but I learned.



Larry
 
#16 ·
coldknock said:
Not really, GM stuffed 10.8-1 compression into the LT4 engine, 10.5 in the LT1, with a short cam. They tamed the combustion with reverse cooling and a wide lsa, it was a nice engine. It really woke up with a better cam and ported heads too.

Like I said, nothing's written in stone. Many engine builders prove it on a regular basis. I've done it twice myself. Neither of the engines were what you would call docile though. Not even a daily bruiser kinda engine. In fact, they both ran better with 110 gas and a bit more timing. The point is that they can, and do, run on 93 octane on a regular basis.

I understand your point of view though. Once upon a time I would told me that I was a nutjob, but I learned.


Larry

Yea, so many modern engines are running around 11:1. It used to not be possible, and I wouldn't dare to try it on an old open chamber head with domed pistons, but with modern heads, intakes, cams, ignitions, and open exhaust systems the engines can take alot more than they used to. I think the LT1 is a great example becasue its not all that different from a Vortec engine.
 
#17 ·
Ok, I'm learning a few things...

So I still don't know how total compression is measured in order not to screw up the engine, long or short run.
I have a daily driver 305cu.in. Ford engine, stock compression is 8.2-8.5:1.
Question is what is the cheapest way to up the compression and to how much ?
I read many hotrod mags. and I like the roller lifter arms myself.
 
#20 · (Edited)
ap72 said:
that's not the whole story, oterwise you'd never see a hotrodded Chevelle with a 350, its a heavy car and a smaller displacement sbc. And that setup may be the most common one out there.
It may be the most common, but that doesn't mean it makes the most sense. I, also, had a different mindset when I was your age.

Peer pressure and the opinions of friends is a strong motivating factor when you're a young man. If 2 dozen of your friends are urging you to change the cam in your 350 Chevelle daily driver and 1 old guy is urging you to leave the Chevelle alone and stuff a 500 inch BBC into a Vega, it's pretty clear who is going to sway you.
 
#21 ·
techinspector1 said:
It may be the most common, but that doesn't mean it makes the most sense. I, also, had a different mindset when I was your age.

Peer pressure and the opinions of friends is a strong motivating factor when you're a young man. If 2 dozen of your friends are urging you to change the cam in your 350 Chevelle daily driver and 1 old guy is urging you to leave the Chevelle alone and stuff a 500 inch BBC into a Vega, it's pretty clear who is going to sway you.
well the vega would be faster, but the chevelle looks better, and you'd be hard pressed to argue that one. My favorite car is still my Olds Cutlass which was my first car and I still drive regularly. Aftr that its a toss up, I've drove some real fun cars, hell I had a 4cylinder S10 with a manual that was a blast to drive. (it was the iron duke for the 4cyl dirt track guys) Having the best power to weight ratio does not make the best hotrod- otherwise we'd all be driving motorcycles (whihc I also used to hotrod until I wrecked one).
 
#22 ·
There are reasons to my madness

:cool:

I want higher compression so I can use 87 octane and get the same or better mpgs (without knocking or choking) as I get at 91/93.

I know there's more stress on engine parts, it's why I asked how much compression can stock engine withstand without excessive wear.
I don't have the money or the will to change vehicles to an econo model. This 1991 V8 gets about 15c - 23h mpgs with good oil and high octane.
Wider is Better and so is longer wheel base, far as comfy ride.
BTW if you never tried, the oil usually used for truck engines - Rotella - is both cheap and up and above any oil you can buy.

Is there a compromise or am I stuck ?
 
#23 ·
I'm toying with the idea of going slightly over 11:1 with a Vortec engine. The heads have been completely reworked and the chambers will be polished (they're on my work bench right now). I'm thinking of doing a short fill on the block, running an oversized oil cooler, and a 170 T stat. I think with all of that my cooling will be pretty close to the LT1's system. The cams I'm trying to decide between are the 60122 from lunati or the cc503 from Comp. both of these cams have been run in LT1's with 11.5:1. I may just page through the lobe catalogs and spec one out myself though.

I was planing on running a solid cam setup, but I just purchased a fully machined unassembled long block last night. Since its a roller block I figured I'll just go with a roller cam for easier upkeep and break in.

For my piston I'll be running a 2VR coated hyper from Speed Pro, and .040" quench. That should put me right around 11:1 depending on where my chambers finish up at.

And I know what F'bird is going to say, "Oh my God! you'll destroy a piston!" To which I'll respond, this has all been done before and proven.
 
#25 ·
Cyberats said:
:cool:

I want higher compression so I can use 87 octane and get the same or better mpgs (without knocking or choking) as I get at 91/93.

I know there's more stress on engine parts, it's why I asked how much compression can stock engine withstand without excessive wear.
I don't have the money or the will to change vehicles to an econo model. This 1991 V8 gets about 15c - 23h mpgs with good oil and high octane.
Wider is Better and so is longer wheel base, far as comfy ride.
BTW if you never tried, the oil usually used for truck engines - Rotella - is both cheap and up and above any oil you can buy.

Is there a compromise or am I stuck ?
Get a set of AFR 195's, cam with short duration, and compression at about 9.7:1. It should run real well on 87. You can choose other head options too, and some may work better. 195's are pretty much a failsafe head though, they work pretty well in just about every application.
 
#26 ·
ap72 said:
Get a set of AFR 195's, cam with short duration, and compression at about 9.7:1. It should run real well on 87. You can choose other head options too, and some may work better. 195's are pretty much a failsafe head though, they work pretty well in just about every application.
Once again, I am in disagreement with you mon ami. 195 heads won't support a short duration cam. And they are not a good match for a 305 motor on the street. Now, if this was a race only motor buzzed to 7500-8000, then fine. And 9.7:1 won't support a short duration cam either. Too much cylinder pressure for anything this side of E85 or race gas.
 
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