How much CR on pump gas?

billd · 05-11-2006, 04:47 PM

I'm currently running 9.5:1 CR pistons on my 3.2L short-stroke engine. The engine has twin-plug heads and I'm planning to put in a high overlap cam GE60 to GE80 range.

I'm debating switching to 10.5:1 or 11:1 CR pistons. Does anyone have any experience on how high I can go with the CR on 91-octane pump gas before I run into detonation problems?

How much HP and torque gain do you think is gained moving from 9.5:1 to 10.5:1?

1fastredsc · 05-11-2006, 05:23 PM

Don't know about the power, but i think the general word is to use 10.5 on twin plug, 9.5 on single. That's probably why they are the standard order pistons on pelican (either 9.5 or 10.5).

kenikh · 05-11-2006, 07:07 PM

Depends on piston size, too. My 2L S runs 9.9:1 on 91 octane on single plug. With a cam as hot as a GE80, 11:1 isn't totally out of the question with the right ignition system; you margin for error is indeed much less than comfortable. With a system that can detect load and adjust ignition accordingly, you could probably pull it off if you did a diligent dyno tune.

Jeff Alton · 05-11-2006, 07:12 PM

A cam with more overlap will allow a little more CR as well..... Like Kenik said.

Cheers

Steve@Rennsport · 05-11-2006, 10:34 PM

Don't go over 10:1 with a 98mm motor on 91 octane fuel; even with twin-ignition. Smaller displacement engines are more forgiving.

Unless you have knock-sensing (which 3.2-3.4's do not), detonation will occur in hot weather that breaks rings. Remember; detonation thresholds are NOT static, they are dynamic and change with outside air temperature, cylinder head temperature, ignition timing point & air/fuel mixture.

Relying on one's ears to detect knocking will hurt your pocketbook.

KobaltBlau · 05-12-2006, 06:19 AM

Well said Steve, as always. Is 93 available in Oregon and Washington? would you recommend bringing it up a bit (I suppose still not over 10.5) for the same engine with 93?

Cheers,

kenikh · 05-12-2006, 07:35 AM

92 in Oregon and WA.

dd74 · 05-12-2006, 09:02 AM

Quote:

Originally posted by catca
A cam with more overlap will allow a little more CR as well..... Like Kenik said.

Cheers

I've never been clear on what exactly is cam overlap. Is it different from another term I've heard with cams which is "spacing?"

tadd · 05-12-2006, 10:55 AM

If you were to 'map out' the motion of the valves they would have a pesudo 'bell curve' shape to them. Now those humps are placed on a cylinder which means they are somewhere between 0 and 360*. Typically the tail of the intake hump 'overlaps' with the start of the exhaust hump. The more overlap, the longer that both valves are open at the same time. This lowers the effective CR since the fuel-air charge can leak out either.

tadd

Jeff Alton · 05-12-2006, 10:55 AM

Lobe spacing in the number of degrees between the lobe centerlines. The closer together, the more overlap a cam will have, generally speaking. Overlap is when both the intake and exhaust valves are open to some degree.

Cheers

dd74 · 05-12-2006, 01:05 PM

So with high compression engines, it's better to have more overlap and closer spaced lobes?

kenikh · 05-12-2006, 01:12 PM

Higher static compression engines can take advantage of more overlap, which results in less dynamic compression (which is the real CR of your engine).

tadd · 05-12-2006, 01:12 PM

per se. Porsche heads are considerd 'lazy'. That is they don't impart much turbulance on the incoming air-fuel mix. Turbulance is good because it homogenizes the burn, reducing detonation. Now once you start spinning the engine, you get the air stream with some velocity and some turbulance is generated. So at high revs you get a higher threshold for improper burning. Now with a large overlap where actual real compression of the air fuel mix starts later (since the valves are open longer - especially the exhaust) you loose some compression.

tadd

tadd · 05-12-2006, 01:13 PM

Damn!!

Beat me to it... Argh. Must type faster!

tadd

kenikh · 05-12-2006, 01:17 PM

I think I said it better.

camgrinder · 05-12-2006, 01:48 PM

As the exhaust valve is closing, the intake valve opens. At low speeds when the intake charge is at low velocity, a cam with high overlap will cause exhaust gasses to enter the intake port. This can reduce cylinder filling below the torque peak, making the engine more tolerable to low octane.
When the engine is in the powerband the exhaust gasses are flowing at a high rate out the exhaust port and can actually pull the intake gasses into the cylinder.
Long duration, high overlap cams tend to close the intake valve later during the compression stroke. This reduces the dynamic compression ratio. If you notice where the intake closes in this chart, about 30% after bottom dead center, the 10-1 static compression ratio is now a 7-1 dynamic compression ratio.

tadd · 05-12-2006, 01:59 PM

with a graphic even!

To bad, so so sad Kenikh!

tadd

kenikh · 05-12-2006, 02:01 PM

Duh, it's camgrinder.

camgrinder · 05-12-2006, 02:05 PM

LOL

kenikh · 05-12-2006, 02:57 PM

This begs an interesting question: Is there a magic spreadsheet/app that can take camshaft inputs and apply them toward and engine config (bore/stroke/combustion chamber volume) and calculate dynamic compression ratio in steps throughout the powerband? It seems like it would be pretty straightforward to calculate (although I have no idea what the formulas are).

Since the real number you want to know is dynamic compression, calculating the right cam to achieve n maximum CR would be pretty cool.

Yeah, I'm a total geek.