cylinder head theory question

Jon Kaase says that when using P-51 heads you will loose 50hp going from 30 degrees timing to 34 degrees timing because the head flows so well. Why would a better flowing head loose hp when advancing timing past 30 degees?

this is true for some heads,( THAT MORE OR TOO MUCH TIMING/ADVANCE WILL COST POWER) this is because the better the head the less lead time it needs ( less advance/total) to make max power





-RICH

cooter wrote:Jon Kaase says that when using P-51 heads you will loose 50hp going from 30 degrees timing to 34 degrees timing because the head flows so well. Why would a better flowing head loose hp when advancing timing past 30 degees?

I agree that 29 - 30* of timing works really well with P-51 heads on race engines. However, IMO it is not due to the head flowing so well (if that's what he said), but more likely due to better flame propagation due to an ideally located spark plug, and a really good chamber shape with a double quench pad design.

Charlie Evans

Impressive - 29-30* is close to the timing normally seen in a small bore engine with a 4V pent-roof head, not a 2V big bore. Are there any other 385 series heads that require such little timing advance?

71f150 wrote:Impressive - 29-30* is close to the timing normally seen in a small bore engine with a 4V pent-roof head, not a 2V big bore. Are there any other 385 series heads that require such little timing advance?

TFS Streets, Ford Motorsport small block N head just to name two.

TFS Streets require more, less or similar timing?

so what causes a power loss if the motor is not detonating?

Compression ratio has alot to do with timing requirements

cooter wrote:so what causes a power loss if the motor is not detonating?

An engine can be unhappy without being completely over the edge.

Starting the mixture to burn to soon will cause a lean condition. The better the combustion chamber/piston top design, the later you can light the fire.

Cooter - power loss occurs when the peak combustion pressure is no longer at the most effective crank angle. If your engine makes best power when the peak combustion pressure is say 18 degrees ATDC and you advance your timing such that the peak pressure occurs at 10 degrees ATDC (or even worse at 10 BTDC) you will lose power. Likewise if you retard it such that peak pressure happens later in the down stroke you will lose power.

There are a lot of variables that influence how much timing is required - compression, fuel, piston top, spark plug type and heat range, air temp, humidity, boost level, valve timing events, water temp, ring crevice volume, end gas temps - the list goes on. Having said that though, some heads really do require less timing than others to make max power because they are simply more efficient. Look at the 4v pent roof design as an example.

As you approach the limits in terms of a build and tune, the difference in degrees between max power and detonation gets close and closer; ie it might make max power at 32* and detonate at 36*.

Referring specifically to the P51s and TFS Streets (or any other 'fast burn' 385 series head for that matter) can anyone share their experiences with respect to the window between max power and detonation for any particular combo?

thanx for the replies, i am just trying to understand the finer points of the world of engines. As a Chevy guy from years gone by i have never seen timing less than 32 make max hp.

The faster the burn the less amount of advance required and the less likely it is before it'll get into detonation territory.

my old A460's like low timing,on alky.

As Jim McFarland is known to say; "the top of the piston is the bottom half of the combustion chamber". So, our BB Fords with shallow chambers with a flat-top piston tend to "mate up" together better,,, than a bigger deeper chamber, such as a 119 cc big block Chevy and then a big dome on the piston. With a somewhat centrally located plug, the flame propagation is better. The less lead time, via the amount of ignition advance, in order to have peak cylinder pressure at say 18* ATDC, then the less negative work you have done. Anytime combustion takes place while the piston is on the way up, you are doing some degree of negative work.

Charlie

alot can depend in the fuel and compression.

cooter wrote:thanx for the replies, i am just trying to understand the finer points of the world of engines. As a Chevy guy from years gone by i have never seen timing less than 32 make max hp.

I'm guessing that you haven't had much experience with the "Big Chief" heads which use 30 degrees or less ignition timing.

10 years ago i was part of a competition eliminator firebird team and we used big chief heads but i never had anything to do with timing or carburation.