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I have about 350 hp,355 ci sb,9:8:1 compression and it runs on 87 without run on's.It hase run on a mixture of 87,93,111,and 116:eek: :eek: :eek: :eek: :eek: :eek: with no promblems.This is what a chevy can do not [email protected]:D :D :D Hope this helps.BTW i'm not lying eather.
 

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Yes, I am trying real hard to get Kev's engine done by the end of the month.

anyway that rule of thumb applies to a typical 350 SBC with quench heads.

Car Craft did a test with a 440 Chrysler (non squish heads) where they went from 8:1 to 11:1 and recorded an 8% increase per point of CR, but the engine was "overcammed" at 8:1, just right at 10:1 and under cammed at 11:1. There's an important interaction between the cam specs and CR so you can't just raise the CR without changing other factors.

It isn't a one to one increase, meaning if you go from 10:1 to 14:1 it isn't automatically worth 100 hp. The increase in power is not linear.
Car Craft also did a test by changing head gaskets on a 355 and the hp chart shows a non linear rise, dip and rise in HP as the quench changed along with the CR. The got about 30 hp per point at power peak but the average gains were:
9.62:1 avg 390.5hp avg VE 99.6
9.86:1 avg 396.1hp avg VE 100.1
10.14:1 avg 394.6hp avg VE 100.3
10.49:1 avg 400.4hp avg VE 100.5

The dip in power at 10.14 is strange. the VE peak was higher than peak torque on this test run also. I'm not sure why this test was different but it goes to show that you can't count on anything to happen the way you think it will.

You also can't raise CR forever even if you could get octane high enough. HP gains fall off as you get near 15:1, though some restrictor engines have run CR's in the 16-17:1 range because the manifold air density is very low.


Here's an example of predicting HP increases just from CR changes:

HP= Intake pressure x SCR x VE X CID x rpm/5252/150.8

A 350 with 9:1 can make 350.79 Hp at 6,000 rpm assuming 100% VE at sea level.
Same 350 350 with 10:1 can make 389.8Hp (+39),
11:1 can make 428.74 hp (+38.94)
12:1 can make 467.72 (+38.98)

This is just a mathematical equation using best case parameters. In the real world it's more like 25-30 hp for every point of CR increase, however the cam must be compatible with the CR. As noted, on a SBC the squish distance is important.

You mentioned milling your heads. If you mill more than .010" you'll have intake manifild fit and port alingment problems. It's important to mill the intake to fit the new stack up height.
There are benefits to angle milling the heads. There are airflow gains possible from rolling the heads to a lower angle than 23 degrees but the cost and skill level for this project is "expert" category.
 

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Paul Wright said:
This is just a mathematical equation using best case parameters. In the real world it's more like 25-30 hp for every point of CR increase, however the cam must be compatible with the CR. As noted, on a SBC the squish distance is important.
I don't have much quench right now at .053".

Paul Wright said:
You mentioned milling your heads. If you mill more than .010" you'll have intake manifild fit and port alingment problems. It's important to mill the intake to fit the new stack up height.
There are benefits to angle milling the heads. There are airflow gains possible from rolling the heads to a lower angle than 23 degrees but the cost and skill level for this project is "expert" category.
Ok, good to know. Money may be better spent on a new short-block.



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Discussion Starter #166 (Edited)
Paul Wright said:
As far as the octane booster comment above, I think that gets very expensive to keep a case of booster handy. Not very practical for a street car. Jet fuel? Yeah, right.
If you are talking about JP5 Jet fuel (as in airplanes) here's a quote I found:

Diesel and Jet fuel (along with kerosene) have, indeed, terrible octane numbers; typically about 15-25 "octane". They tend to ignite easily from high compression. Their use in a gasoline engine will quickly destroy the engine.
Paul; I was just rereading this tread today and came across the above quote you had posted(pg. 6 post #88) that I find very interesting.


Not trying to be a jerk here but I'm just wondering how this can be true? I'm confused because as far as I know most, if not all, diesels have very high compression ratios as that is what causes the fuel to combust in the cylinder (as they dont have spark plugs though they do have glow plugs but they serve a different purpose than spark plugs)

Most diesels, I believe, have like 16:1cr or better. Very few cars will ever have compression ratios in that range. You can actually run diesel in a gas engine..it wont run well and will smoke like crazy but it will work...you cannot however run gas in a diesel motor...you get massive detenation going on if you do that...or at least that is my understanding...

Just trying to understand things:p
 

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Hmmm, that does sound backwards, doesn't it!

Diesel fuel is rated with a Cetane number not Octane. Centane is backwards from Octane. The higher the Cetane #, the less resistant it is to detonation. However, Diesel fuel isn't just a low grade gasoline and it won't self ignite with the relatively low compression of a gas engine. Diesel simply isn't a good fuel for gas engines.

Just goes to show when you cut and paste from the internet, you don't always get a good answer.

Added:
Found a tech article on Keith Black Silvolite site:

Compression Ratio vs compression pressure
 
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