The Official 356Talk Forum

91mm pistons
11.5:1 compression
elgin 7805 cam
42mm & 34mm valves
44mm Solex carbs
twin plug (but have not finished 2nd version of dual dizzy drive, so currently run as single plug)
Elva Porsche gear driven flat fan

So, assuming stock stroke, 1925cc. I think you said your class was allowed 2000cc so that should help your competitiveness. Have you been using the 7805 cam previously?

Did you or are you testing this engine or previous engines? You ought to test this one with and without the twin plugs so that you will know that twin plugs are a waste of time and money. The peak power ignition advance setting will be different but otherwise there will be no measurable difference.

My guess is that your new engine makes nearly 200hp. That should be fun! Enjoy it!

hi Greg,

11.5:1 are you running airplane gas? my 1883cc started pinging at 10.5, had to go down to 10.3 for it to run smoothly (11-25 advance, msd box), twin plug with 91 pump gas, no way to run anywhere close to that without twin plug standard 356 head geometry.

Greg,

The Devin is a race car so I use 110 octane. I also have a twin plug 2135cc in the 904. It has 10.25 compression and is pinging a bit on street gas. Still working out what it is going to need for octane.

Greg x 2,
You need to take a modern approach to your thinking about your detonation. FI engines are mapped for maximum safe performance at each rpm and load combination which is a big part of how they use higher CR and still run safely on pump gas. You also can do this to a point with a 123 Tune distributor or by a re-curve of your centrifugal advance on your dinosaur distributor.

Rules for radicals...

The cam choice must match the desired rpm range most often used by that engine.

There is no such thing as a magic cam.

Most people trying to "improve" an engine go too far with CR, cam, or carb size.

Pick your cam first then the CR.

CR and cam choice are interdependent. Early intake closing requires less static compression and vice versa.

Tune for the fuel you are using.

The squish and the other features of the piston crown and head have major influence on power and on ignition timing and fuel requirements.

Almost always a well modified engine needs less ignition advance. Sometimes a lot less.

So if you have detonation on any reasonably thought out engine you should be able to tune it out with fuel or ignition changes. For the most part an engine with carbs will be rich anyway and will not have detonation from lack of fuel. An engine must be running very rich before problems result. Ignition timing on the other hand is a big player. If you are using a vacuum sensor like on the 123 you can pull timing out when the pedal is pushed and have higher timing under light load. This is the way to go but if you don't want to do that you can still reduce the timing at the rpm where the detonation occurs under load to eliminate it. Usually detonation occurs around the point of peak torque when combustion pressures are highest, around 3500-4000rpm under load with our cars.

If you want to hot rod your car you had better be prepared to do a lot of work to have it run as nicely as a stock engine. There is no free lunch and even when you have it right your modified engine will give up some of the good features of a stock engine.

CJ, it took pretty much a year to get it to run that smooth, didn’t try to use the vacuum yet but could be wise to.
Pistons from Shasta, with no abrupt angles, cam optimized for 10:1 compression from John W, running fairly rich

911 2.8l twin plug bored up from a 2.4. Magnesium head casings, hot cam dyno’d at 270hp at 7000 rpm on 91 octane. Bigger part of power curve at 6000 rpm