Compression 205 Psi, How much Ocane needed??
 

My 2.8 RS engine has a cold compression of 205 PSI, how much octane do I need to run this engine??

thanks, harold

Harold:

I need more information as there is a LOT more to this than cold compression figures,..... :)

What P/C's are you using?

What is your compression ratio?

What cams are you using?

Single or twin-ignition?


The answers to the above questions will define what grade of fuel you'll need.

Pistons and cylinder are 2.8 rs, cams are RSR, single ignition, weber 46, bursch muffler on heaterboxes.. more about that engine under www.911works.

Once the engine was rebuild and fresh we had compression 150 and all was running fine.
Now the problem is on coasting between 1500 and 2800 rpm, engine hesistates, under full throttle all ok.
Checked ignition valves, carburettors - all that is not the reason.
will get racing gas and see what happens.

thanks , harold

You have a tuning issue that race gas probably will not 'fix'. It will only prevent detonation on loaded acceleration.

Big time tuning issue, might even include cam timing, cam selection, leaking gaskets, carb jetting...... But if it did run ok once that makes it more likely a something loose or leaking gasket issue.

Harold:

The very first thing I'll tell you is that unless you have the VERY VERY rare 2.8 RSR heads, your compression ratio is over 11:1 with any 2.4 or 2.7 head and you MUST use twin-ignition or 108 octane race gas.

Next, those heaterboxes and muffler are killing the motor with those RSR cams. Either use less aggressive cams or install some 1 5/8" headers and a proper race muffler.

After all that, then and only then can you jet the carbs accurately.

Will install a header system with bursch muffler, readjust valves and then go with better gas.
What gets me is that in the beginning, engine was running great but not anymore. I thought that with rising compresion - new rings are seated now - that compression has someting to do with engine not wanting to coast. have installed other carbs, distributor, coil and box - no success.

thanks, harold

Harold:

Lose the Bursch muffler. Those are no good even for "S" cammed motors. Use a Series 80 Flowmaster and make sure you have 15" of pipe between the header collectors and the muffler inlets.

As long as you are using the proper fuel and that the carbs are not starving (check those float levels with the engine running), you will be fine. Trust me,......:)

I'd have to piggyback with Steve's observations about float level; but you mentioned you changed carbs (??). So maybe it's the exhaust; but you mentioned it ran fine before.

I'd step back and check the fuel delivery system; check fuel pressure, fuel level.

Can you get onto a chassis dyno and replicate the conditions while monitoring air/fuel and ignition?

Sherwood

Steve,

Why would it run ok at WOT, and assuming it really didi run ok to begin with, why would more compression (rings broke in) cause hesitation at low rpm? I would think that cams, mufflers, octane would have shown up immediately.

I cannot imagine the RSR cams in a 2.8 litre making much power until 5000 rpms. Especially with 46mm carbs.

Reversion????

With the throttles (mostly) closed you could be running some degree of vacuum between the butterflies and the intake valve during overlap, while the exhaust (especially if in a specific range of it's "tune") may be seeing positive or ambiant pressure. The result is that exhaust is pulled back into the cylinder or even up into the exhaust. Once you open the throttles, the vacuum goes away. 500 RPM on either side and the exhaust will have a negative pulse at the exhaust valve and the engine feels fine. Once the RPM's pick up, the exhaust develops enough inertia to pull the charge into the cylinders right up to peak HP rpm.

I could picture increasing the CR will increase the amplitude of the exhaust pulse, so the vacuum pulses get stronger, but so do the pressure pulses between them.

This is a VERY complex subject and my fingers are not up to a long post here,....:)

I will unequivocally tell you that 911 engines are VERY VERY sensitive about exhaust tuning, especially with long-duration, narrow lobe center cams and that affects the motor at throttle openings less than WOT and from 3000 to peak RPM.

As Sherwood said, the carb setup & configuration as well as fuel pressures must be perfect when using race-type cams.

When troubleshooting issues like these, one must assume that the fuel pressures are at 3.5 psi with sufficient volume, the carbs are correctly jetted, and the ignition timing is correct for the particular engine's parameters. :)

John is right, RSR cams in a 2.8 do little until 5500 and pull hard to 8000+ depending on the heads.

John, Steve, I totally agree with you. I am just trying to understand why it would have run "OK" ever or at all And on top of that still ok at WOT. To me there are two possibilities, one the compression increase did it (I just don't buy this for some reason) or there is something differen't , say a gasket leak under the carbs or manifold has developed. I have personally seen the later on many occasions, so thats what I suspect. Good mechanics NEVER see this because they do not put things togather that shortly thereafter develop leaks.
But for some reason amateurs do.

Hi Jack:

You're right,......one has to also assume that the basics have been attended to and that includes a leakdown test to make sure the engine is healthy, the cams are timed correctly, valves are set, and that the intake is tight with zero vacuum leaks.

After that, its more about the configuration,.....:)