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Old 12-24-2004, 03:43 AM
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Octane, Ignition, Cr & Detonation

just finished reading the article "design for the times" in the winter 2004 issue of Engine Masters & its got me asking some questions.

in the article they are building a 383 to run on low octane gas. as such, they are limiting the compression to 9.75-1 so as to avoid detonation. question: what would happen in the same engine if you were to start using high test? would it alter the compression ratio further, add hp, or just further discourage detonation?

next, they mentioned that by adding a few degrees of timing this will also add to detonation avoidance. how?

lastly, & this question is all my own because i am getting ready to build a 383 & would like to leave open the possibility of a supercharger, if the aforementioned engine was built, would the compression be low enough for the supercharger to work? would it neccesitate a change in cam? if you switched to premium at the pump would it help the engine better receive the charger?

thanks for the help

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Old 12-24-2004, 05:24 AM
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Lets put it like this, if you are currently running 87 octane, then you switch to 117 octane race fuel and made no other changes in a non computer controlled vehicle, there would be no change, no change in power, no noticeable difference. If you advanced the spark timing to take advantage of the higher octane fuel you would notice a little change, the difference in the octane is it`s burn rate, the higher the octane number the slower the burn. Back in the day when high horsepower high compression cars rolled off the line, 117 octane was avalible from the corner gas station for next to nothing, and it had anti detonation additive in it also. You can run higher compression than the 9:1 area if it`s did right. Such as the Quench distance on the tight side, the mixture slightly on the rich side, no sharp points in the combustion chamber or piston top. in most cases you can scrape by on 10:1 like this. combustion efficientcy is not only a power aide but it`s also a anti detonation aide, the more efficient the combustion the less worry of detonation. if you went with a engine with compression in the high 9`s and added a supercharger, the compression would need to be lowered or the boost of the blower set to a rather low level. A camshaft change you wouldn`t have to do, but it would likely be wise since there are blower cams on the market that help the blower do it`s job.
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Old 12-24-2004, 08:44 AM
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If you are planning on adding a blower, I would keep CR at 9:1. Any lower and it is going to start getting mushy, but it is high enough to make good power and run on cheap gas. With 9:1 and premium fuel, you should be able to run 10psi or so. This is very dependant on the tune of the engine and the combustion efficiency as mentioned earlier. You will want to pay attention to quench and keep it in the .050 or below range to help aid in detonation resistance if you add a blower.

As for the cam, you can get a cam that will make good power in NA or boosted form. The main factor is getting a cam with a wide lobe separation angle(lsa), which is always listed with the cam specs. You want 112-114 lsa. Duration numbers will depend on the RPM range you want to use the cam. With a wide lsa in a big cube small block, I would look for a cam with around 220-240 duration. Again, it depends on the RPM range you are looking for. 220 will put you around 1500-5500 and 240 would be more like 2500-6500.

Chris
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Old 12-24-2004, 04:11 PM
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i see...
would a 9:1 ratio make good power in a NA engine without a blower, assuming everything is set up right with regards to the combustion chamber & whatnot? would it be a good place to start?
also, can somebody explain quench?
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Old 12-24-2004, 04:38 PM
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If you go with the larger cam range the 9:1 would be a little hindering in the power department, but with a 220 range cam it would work very well. Quench is the distance from the flat part of the piston to the flat part of the head. Typically the piston sits a few to 20 thousandths in the hole. Then you have the head gasket thickness. The idea is to keep this distance .050 or less. The main problem comes into play when you use cheap rebuild pistons designed for a decked block that sit down in the hole too far. If you can get the piston within .010 of the top of the block and use a .040 gasket, then you are good.

Whay is this important? Because quench is directly related to detonation which is uncontrolled combustion. With forced induction detonation resistance becomes very important. It is even more important when you are talking about higher than optimal compression ratio.

Chris
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Old 12-24-2004, 04:58 PM
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gotcha.
so when you get your block zero decked, does this mean that the piston tops will be even with the block at tdc, thereby reducing the quench? would you use a thicker gasket in this situation to make up for it?
when you say you want to keep the quench within .050, is it preferable to have it lower, ie , is this the MAX amount you want?
thanks for the schooling!
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Old 12-24-2004, 09:47 PM
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Yes on the zero decking. You really dont want to go under .035(I think) for quench just because things grow a little when they get warm. .050 is the max to shoot for, and .040 is even better. With a zero deck and standard .040 gaskets you are right on the money.

Chris
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Old 12-24-2004, 09:51 PM
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Quote:
Originally posted by DoubleVision
Lets put it like this, if you are currently running 87 octane, then you switch to 117 octane race fuel and made no other changes in a non computer controlled vehicle, there would be no change, no change in power, no noticeable difference.
I think you would loose power with th3 117 octane gas. The higher the octane, the longer the fuel takes to burn. That is why it is important to only use as much octane as you need.
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Old 12-24-2004, 11:09 PM
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so would it be reasonable to build a 383 that has been zero decked, with 0.040 quench, 9:1 CR, a suitable cam & running on 87 octane?
with this setup would it then be possible at a later date to add a supercharger with a minimum of modification? could i get away with just running premium once the charger is on & maybe changing the cam if needed? im gonna build the bottom end properly from the beginning, so that the components can handle the increased HP of the charger.
i wanna do it this way because chargers are hellu expensive! one step at a time.
also, what kind of power would it make at first, without the charger? i wanna make some real power, so would it be best to run a cam that optimizes the non charger motor, then switch it when i charge it?
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Old 12-25-2004, 12:43 AM
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That should be no problem normally aspirated.

Forced induction is another story.

If you really want to make forced induction levels of power you have to build the everything in the bottom end for that purpose. If you do build it correctly, it will not run well without the supercharger.
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Old 12-25-2004, 02:25 AM
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to build it up especially for a charger do you mean the pistons & cam should be specifically for it? should i just build the NA 383 as big & bad as possible & forget the charger? cant i have my cake, then later when i can afford it, eat it too?
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Old 12-25-2004, 09:11 AM
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You are going to give up a few ponies by going with lower CR and smaller cam. It will still make good power if you use the right cam and make sure CR is actually at 9:1. It all depends on if you are really serious about the procharger. If you are serious about it, build a very stout bottom end with everything forged and block studded top to bottom. I would also suggest a short fill with hard block to help stabilize the cylinders when you go with the procharger.

There is not doubt that for high HP pump gas drivers, forced induction is THE best way to go. You can make 600HP on pump gas and it will idle in traffic at 800RPM without gassing you out of the car. My turbo engine will idle at 800RPM and with the new heads should make between 600-700HP on pump gas when I get it back together. The trade off for the best setup is what it always is......money.

Chris
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Old 12-25-2004, 07:24 PM
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yes the money issue is why id like to build & drive the engine first, then switch to a charger down the road. it wont be too bad if its spread out. as long as im making over 300hp to start (although id prefer to have closer to 400hp!)
when getting a block zero decked, is it done with a combination of rod & piston to achieve the proper length, or is the deck itself machined down to match what the piston rod combo will come up to?
what about a big block? would i still be shooting for the same quench? would it be better to build a 454 instead of a 383? could i build a milder setup this way but still get the big numbers that im looking for? perhaps with better reliablilty?
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Old 12-25-2004, 10:04 PM
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Sure a big block will make more power. It is not without expense, though. If you are going to go with a big block, you dont need the boost to make the power you are looking for. A healthy 502 will make 600HP without alot of problem and still be fairly drivable.

Zero decking is accomplished my measuring the distance from the crank centerline to the top of the deck. Most engines have some extra meat there to work with. The machinst simply removes this extra material. I think that quench numebers are the same for any engine combination, but maybe someone else will chime in on that.

Chris
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Old 12-25-2004, 11:08 PM
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good stuff.
maybe ill have to do a bit more research on big blocks before i make my decision. ive got another month at work anyways...
can anyone give me their opinions on small block vs big block? pros & cons? i realize price is a factor, but then, isnt it always?
also, there is a lot of info on the sbc. lt1, ls1, get a vortech out of a 96+ truck, etc. what about the big block? i cant even name one other than the zl1 (or is that lz1?) where would a likely donor come from?
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