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how octane affects streetable compression ratio?

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MTribe

MTribe

Jedi Hopeful
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Hey guys,

I'm rebuilding my '71 TR6 motor and cutting the head down to increase compression ratio. I'll be running a Good Parts GP2 cam, and Richard at Goodparts can tell me what compression ratio to run given my goals for the motor, but they're all for 93 octane.

Here in CA, we generally only get 91 octane, not 93, so we're wondering how to effectively de-rate compression ratio.

That is, say we want 9.5 CR on 93 octane. What's the equivalent in terms of drivability on 91 octane?

Any ideas on how to determine that, or first-hand experience?

Thanks!
 
Re: how octane affects streetable compression rati

I think that running 9.5:1 cr and using 91 Octane fuel is a safe bet. At worse you might experience an occasional ping but I doubt it. Otherwise, what choice do you have?

There are no additives that will put lead in you car, especially in California.

Or, use AvGas.

I run 91 Octane in my car with the same CR and cam with trips. No problems.
 
Re: how octane affects streetable compression rati

How far are you above sea level?

It's not uncommon for premium fuel to be lower octane (93 vs. 91) in higher altitude locations, because at that elevation it comes out pretty much the same.
 
Re: how octane affects streetable compression rati

For the most part, anything under 9:1 is 87 safe. Anything under 10:1 is 91 safe, and anything under 11:1 (with Aluminum heads, about 10.5:1 with Iron) is 93 safe. At least those are the numbers I've used when building small block chevys.
 
Re: how octane affects streetable compression rati

Hey welcome to the group. Lots of cajonnes to show your mug in this group. I am sure most of us have faces for radio!
 
Re: how octane affects streetable compression rati

I hope that you're done with small block Chevy's and into Triumphs now!!

Welcome to the forum.
 
Re: how octane affects streetable compression rati

HE DOES HAVE A GREAT FACE FOR RADIO!!!
 
Re: how octane affects streetable compression rati

rotoflex said:
How far are you above sea level?
Click to expand...

The last I looked, Palo Alto was a mountainous 30 feet above sea level. /bcforum/images/%%GRAEMLIN_URL%%/grin.gif
Jeff
 
Re: how octane affects streetable compression rati

So that's why I can only get 91 around here? I'm at 5550' above sea level here. Gas grades are usually 85 - 87 - 91 at most pumps.

How exactly does the altitude effect things? I've been wondering the same thing as MTribe, as 91 is the best there is around here.
 
Re: how octane affects streetable compression rati

High altitude means lower air pressure, means lower combustion chamber pressure (and temperature) when the spark happens. Effectively, it lowers your compression ratio.
 
Re: how octane affects streetable compression rati

Isn't there less oxygen in the higher altitudes? That might affect the combustion, too.
 
Re: how octane affects streetable compression rati

Hey, thanks for all the tips! Love the board. I was on the autox email list for years, but couldn't keep up with it, and just discovered this forum. How long has it been around? And this seems to be the most popular british one?

Anyhow, sounds like 9.75:1, 91 octane, and Goodparts GP2 cam should work?

If I go too high on CR is pinging the symptom? That is, residual gas detonates on the non-spark compression cycle? Or does drivability suffer or both?

Thanks!
 
Re: how octane affects streetable compression rati

Well, less oxygen only in the sense that lower pressure means fewer molecules of all types per cubic foot or whatever. The ratio of oxygen to nitrogen (etc) remains pretty much the same, but the density is reduced.

But since carbs (and some fuel injection systems) mix by volume rather than mass, they are fooled by the lower density into running too rich. "Constant depression" carbs (like SU and ZS) are somewhat less fooled than fixed venturi carbs (like Weber and Holley), but still fooled to some extent. The over-rich mixture can cause some tailpipe soot (and greatly increased CO emissions) but oddly enough doesn't affect power very much (since the excess fuel doesn't take up much space in the cylinder).
 
Re: how octane affects streetable compression rati

The CD in the ZS carbs compensates for thinner air because the vacuume or CD will be lower and not pull up the needle, so how (why) does it get tricked in the higher sea levels?
 
Re: how octane affects streetable compression rati

Randall said: ""Constant depression" carbs (like SU and ZS) are somewhat less fooled than fixed venturi carbs"

IMO the depression vacuum is fairly constant due to piston swept volume, the needles will allow somewhat the same amount of fuel past 'em: Atmospheric density doesn't have much effect on what the vacuum created by the piston is, at the carb dashpot... thereby causing carb piston rise to produce an over-rich mix in "thinner" air. Being more sensitive to the differential of CD/ambient density the ZS and SU carbs would be less affected than fixed venturi units...

Or at least that'd be my understanding of it.
 
Re: how octane affects streetable compression rati

MTribe said:
If I go too high on CR is pinging the symptom? That is, residual gas detonates on the non-spark compression cycle? Or does drivability suffer or both?

Thanks!
Click to expand...
Pinging is caused by the fuel air mixture igniting (normal) ahead of time and spontaneously. It can be caused by excessively high CR, it is also to some extent affected by engine temperature, design of the piston / combustion chamber etc.
In order to allow slightly higher compression ratio's safely, in the hot rod world, they will use a lower temperature thermostat and polish the combustion chambers (to eliminate sharp points which can become very hot and ignite the mixture ect. also timing can cause pinging, and modern engines all include knock sensors which will among other things change the timing (retard)

Pinging will cause engine damage since it causes extremely high pressure on the connecting rod bearings.
 
Re: how octane affects streetable compression rati

<div class="ubbcode-block"><div class="ubbcode-header">Quote:]depression vacuum is fairly constant due to piston swept volume [/QUOTE]
My ignorance is being amplified. I don't know the relationships (or even if there is one) with depression and cylinder pressure that would be sensed with a compression gauge. But it seems that cylinder pressure at a mile high would be less than at sea level and the intake manifold vacuum would be less at a mile high vs sea level while piston swept area is constant.

I think "less sensitive" is understating the miraculous effect of the CD carburetor concerning atmospheric changes in pressure.

I am not trying to be argumentative, just trying to understand.
 
Re: how octane affects streetable compression rati

My physics isn't good enough to be able to explain just what is going on. The venturi 'knows' nothing of what is happening downstream from it, so I doubt that swept area is relevant.

But I was part of a (loose) group that drove a bunch of Triumphs from basically sea level to over 11,000 feet; and what I saw was that the cars with original CD carbs only smoked a little or not at all at the top, while the ones with Webers either smoked a lot or got readjusted. On my TR3A with SU carbs, 1.5 flats leaner seemed about right (but didn't make much difference to power). Friend of mine with DCOE's stopped not once but twice on the way up, to swap out jets.
 
Re: how octane affects streetable compression rati

Go to 9.5:1, that leaves a little margin for error, carbon buildup, adverse temperatures, slightly out-of-tune condition, having to skim another 10 thou 5 years from now if the head gets warped or gouged when removing for valve work etc.

Just a suggestion, of course..
 
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