3.0L 11 to 1 Compression on Pump Gas?

[jamesjedi]

With a 3.0L engine being twin plugged, and having "11 to 1" compression, can the engine be run on "pump gas"?

Thanks for any help!

[Steve@Rennsport]

Originally Posted by jamesjedi

With a 3.0L engine being twin plugged, and having "11 to 1" compression, can the engine be run on "pump gas"?

Thanks for any help!

Not unless you run VERY little timing and thats counterproductive on many fronts,..

[jamesjedi]

Thanks Steve. So...10.5 to 1 is the way to go?

[tctnd]

For what it's worth I went with 9.5-1 for my 3.2 ss. In a street car I'm perfectly willing to sacrifice a few HP for the ability to comfortably climb out of Death Valley on a hot day with pump gas.

regards,
Phil

[tadd]

Steve is the pelican engine mister, but Ill have to give an ignorant 'it depends'.

Cam choice is really important on what octane rating is required. Although the phrase 'dynamic compression' is tossed around, its really a term that bundles a bunch of engine parameters into one loose figure of merit. I've been scolded for using the term, but it's a don't toss the baby with the bathwater thing IMHO. It has usefullness.

Now for your question... my motor is a 2.8 using the old style RSR pistons with 2.4 heads. This gives ~11.5:1 CR. I'm told you can just squeek 12:1 with the 2.4 non peanut chambers if your careful. Im shooting for 11.5:1 on premium pump gas. I may need to toss in a few gallons of toluene (or turbo blue) in the hot summer months.

This is possible with a really healthy cam choice, basicly DC80 or better. I have two cams im deciding between. One is an 'experiment' of a 3.8 RSR cam on 102* lobe center, the other is the factory 2.8 RSR grind (906 lobes). People poo poo the 906 grind, but it has really gentle ramps which are nice for longevity at revs (again, IMHO).

I also know of two street motors that are 12:1 on pump gas (built by someone that knows their stuff) and have been that way for two years with no issues. One of the motors gets yearly 'manitice' builds and has gotten two clean bills of health.

All of the above is twin plugged of course...

t

[Henry Schmidt]

Steve is correct. Anything over 10:1 and you're at the mercy of ambient temperatures, questionable fuel and reduced [cam] ignition timing.
Build it with conservative compression and dial in advance.
It's safer and the power loss if designed to be low compression is minimal.
Compression also requires very close tolerances (piston to valve, piston to chamber) which makes driver error far more expensive.

[Steve@Rennsport]

Originally Posted by Henry Schmidt

Steve is correct. Anything over 10:1 and you're at the mercy of ambient temperatures, questionable fuel and reduced cam timing.
Build it with conservative compression and dial in advance.
It's safer and the power loss if designed to be low compression is minimal.
Compression also requires very close tolerances (piston to valve, piston to chamber) which makes driver error far more expensive.

+++++++1

A lower CR with good advance is FAR snappier than a high CR engine with retarded timing to run on the sorry state of pump gas.

Street fuel isn't very consistent and aggressive compression ratios usually result in broken rings or worse.

Detonation thresholds in air-cooled engine vary with ambient temperature as well as load and one must allow for that.

Personally, I would run 10:1 in a street car and dial in timing to match the fuel.

[jamesjedi]

Thanks! Amazing to get that kind of input. Very cool.

[lvporschepilot]

Wow, are these Porsche motors that sensitive to detonation? I suspect it's a plug placement issue?

I just finished a carbureted 2v Ferrari 308 engine running 10.5:1 compression and can dial in a considerable amount of advance.

[HawgRyder]

I remember one of the NASCAR boys running 17:1 on race gas.
I think it was Bill Elliot.
He did some work on the pistons in the ring land area to improve sealing and create a "wave effect" in the chamber to lesson the detonation factor.
Of course NASCAR banned the thing almost as soon as it got on the track.
But it does show that we don't know everything about stopping or reducing detonation.
Bob

[Henry Schmidt]

Hemi heads detonate. Same problem with the big Chrysler Hemis of the 60s.
That was the reason for our peanut chamber experiment.

Originally Posted by HawgRyder

I remember one of the NASCAR boys running 17:1 on race gas.
I think it was Bill Elliot.
He did some work on the pistons in the ring land area to improve sealing and create a "wave effect" in the chamber to lesson the detonation factor.
Of course NASCAR banned the thing almost as soon as it got on the track.
But it does show that we don't know everything about stopping or reducing detonation.
Bob

Those 17:1 NASCAR motors had to creep past maximum torque or the engines would explode. It was fun to watch the drivers nail the accelerator coming out of the pits and spray the track with engine parts. Some of those 17:1 engine were also restrictor plate engines which meant cylinder filling was never optimized.

[strokher racing]

Check out E85 in your area. Then pump gas becomes superior to race gas at a 1/3 of the price

[HawgRyder]

Henry...I remember those engines blowing up too....but had forgotton why.
Trust the man to set me straight....thanks.
Bob

[Iciclehead]

I am working on my 3.4 in the not too distant future (Mahle 10.3 P/C's, twin plugged (thanks Steve) and EFI. I guess I was of the opinion after reading other material that with premium gas and twin plugging that all would be fine....but do I read correctly that 10.3 might be too high?

I currently am running 9.8 compression single plug (my Kremer engine) using CIS along with a J&S Safeguard and while every now and again the J&S shows action (mainly just off of idle at low load), it is clean in both normal and aggressive driving conditions - the point of which being that I thought that twin plugging gave you an extra 1 point (e.g. up to 10.8) of compression.

Realize that there are many variables in this.....but thoughts would be appreciated

Dennis

[strokher racing]

Sorry i assumed this was an efi engine?

[jamesjedi]

From what Steve and Henry are saying; there is safety margin with twin plug 10.1.

The difference between 10.1 and 10.5 is small, however, it is large enough that the risk of detonation is present.

How much does this have to do with pump gas quality? I have 94 octane available locally , but perhaps the safest thing is still 10.1. Fuel at that octane may not be available for much longer.

[Steve@Rennsport]

James,

If you have 94 octane fuel available and are willing to use that exclusively, you may safely run 10.5:1 in your motor. Just be mindful of your ignition timing.

[porterdog]

What makes the 3.6 better in this regard? Is it knock sensing, or is it that the published CR of 11:1 doesn't actually exist?

[jamesjedi]

Thanks Steve. I wish your shop was local.

If 10.5 is OK, on 94 octane, would the 10.1 still be a better choice due to the ignition timing? The car will be tracked, but driven to the track. Could the 10.1 still be a better choice?

[dipso]

Mine seems to be OK at 11.1. I'm in so cal and gas is 91.

I think the cam choice has a lot to do with it. But I, in no way, know as much as the others.

Intake
Exhaust
Valve Lash (Inch):
.004"
.004"
Valve Lift (Inch):
0.475
0.440
Valve Lift (mm):
12.07
11.18
Advertised Duration:
280°
262°
Duration @ 0.050":
256°
238°
Lobe Center:
101°
101°