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  #16 (permalink)  
Old 11-09-2008, 08:28 PM
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Ok, AP72's got it and I'm getting more of it.

These are precisely the kinds of discussions that need to take place here. I learn more, others learn more and we all go faster!



Larry

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  #17 (permalink)  
Old 11-10-2008, 06:32 AM
How fast is fast enough?
 
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Quote:
Originally Posted by F-BIRD'88
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 220 @.050" that so many people assume. If you set your car up right you can go all the way to 11:1 and 250 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.
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  #18 (permalink)  
Old 11-10-2008, 06:43 AM
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You just have to limit yourself.

ya you cannot open the throttle all the way without detonation onless you crutch it with less than optimum timing.

You can do a lot of things. Doesn;t mean it builds a good motor.

11:1 is a little much.
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  #19 (permalink)  
Old 11-10-2008, 03:25 PM
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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
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  #20 (permalink)  
Old 11-10-2008, 08:39 PM
How fast is fast enough?
 
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Quote:
Originally Posted by coldknock
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.
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Old 11-10-2008, 09:15 PM
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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.
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  #22 (permalink)  
Old 11-11-2008, 01:18 AM
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In my opinion, if it's your daily driver, leave it alone. Big mistake trying to hotrod your daily driver. If you want to hotrod something, start with the lightest body and biggest motor you can get your hands on.
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  #23 (permalink)  
Old 11-11-2008, 07:07 AM
How fast is fast enough?
 
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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.
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Old 11-11-2008, 11:05 AM
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Quote:
Originally Posted by ap72
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.

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  #25 (permalink)  
Old 11-11-2008, 02:31 PM
How fast is fast enough?
 
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Quote:
Originally Posted by techinspector1
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).
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Old 11-11-2008, 02:32 PM
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There are reasons to my madness



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 ?
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Old 11-11-2008, 02:39 PM
How fast is fast enough?
 
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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.
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  #28 (permalink)  
Old 11-11-2008, 02:41 PM
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Why are you using 91/93 octane fuel in a motor with 8.2-8.5:1 SCR? It should run great on 87.

And by the way, your thinking is skewed. If you raise the SCR, you'll need higher octane fuel, not lower.
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  #29 (permalink)  
Old 11-11-2008, 02:42 PM
How fast is fast enough?
 
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Quote:
Originally Posted by Cyberats


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.
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  #30 (permalink)  
Old 11-11-2008, 02:50 PM
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Quote:
Originally Posted by Cyberats


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 ?
If you don;t have money, get a better paying job.
Raising the compression ratio of a stock engine will require money and will require runing a higher octane fuel. A motor that is detonating will soon destroy it self. = more money spent.
if you have a stock motor with around around 8.5:1 cr and it is getting significantly better real fuel mileage with 91- or high fuel, the spark timing is wrong or the EFI has a fault.
Raising the compression ratio will not help.
If you want to go further on a tank of gas drive like ther is a egg under the gas petal and don't push the car in front of you.
This will save you much much more gas and money than raising the cr of a stock engine.
You cannot save your way out of being poor.
Get a better paying job and make more money.
Pretty simple.
And get a smaller car with a smaller engine.
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