Cylinder head deliberations.

[bill308]

I agree with everything that Rohan recommends except the 1.4 exhaust valves. Peter Marcovicci of Marcovicci-Wenz Engineering, probably the top engine builder on the USA east coast, recommends 1.375 inch exhaust valves and 1.625 inch intake valves on my proposed 2-liter street engine. The exhaust ports should be blended to no more than the id of the exhaust manifold. If you don't have a John Stowe head, David Vizard's recommendations are pretty good. The idea is to make the valve seat curtain area the limiting factor. One wants to keep the air/fuel mixture velocity up throughout the intake port for best atomization and resulting torque.

Bill

[rgh0]

Hi Bill
I agree for a street engine there is probably no need to go for the bigger1.4 exhaust valves even in your 2 litre unless your focusing on ultimate top end performance rather than midrange grunt. !

The exhaust side of a twin cam head breaths much better than it really needs to in most engine configurations as long as you have a good exhaust system hooked up to it. This is why a lower lift shorter duration exhaust cam and smaller exhaust valves help in some configurations.

cheers
Rohan

[dougal9887]

Vernon,
I'll take a pic of the X1/9 seats I have fitted at the weekend and post in the Plus 2 section.
Dougal.

[john.p.clegg]

Dougal
Make the title SEATS then we can all post...?

John

[dougal9887]

Well the cat's in amongst the pigeons now! Using the .25D formula indicates that the lift of the sprint cam on the inlet side is a little low, so the 420 cam is vindicated on this side at any rate.
I've already bought the long stem valves but haven't yet sent the cams for grinding. I'll await Rohans simulation then perhaps become the real world guinea pig!
Dougal.

[david.g.chapman]

I second Rohan's comment about sticking to the standard valve seat width. I had a 3 angle valve grind on my Stromberg head 12 years ago. The exhaust valves lasted about 15000 miles before burning out.

I would go for maximum valve cooling and forsake the small increase in power that 3 angles gives you.

Cheers,

Dave Chapman

[dougal9887]

Standard width 45deg seat on the exhaust and 3 angles on the inlet then?
Dougal.

[rgh0]

I have been playing with my simulation of a 1690 cc tall block twin cam with 10.5 comp ratio and a standard big valve head flow but different cams.

Results as follows. Note the absolute values are OK based on comparisons with actual published dyno data. The differences tend to be even better from an accuracy perspective. However the data is directional only in terms of deciding what you want to build as there is no real data except lap times !!!

2 x QED 420 optimised for maximum torque
max torque - 150 ft lbs at 4000 to 4500 rpm
max power - 150 hp at 5500 to 6000 rpm
timing inlet MOP 105 degrees ATDC exhaust 112 BTDC

QED 420 inlet Standard Sprint Exhaust optimised for maximum torque
max torque 150 ft lbs at 4000 rpm
max power 150 hp at 5500 rpm
timing inlet MOP 103 degrees ATDC exhaust 107 BTDC

With two QED 420 the power and torque peaks are the same but spread over a larger RPM range so a more drivable engine compared to using one QED 420 and standard sprint cam on exhaust and one that pulls harder to the 6500 rpm limit on a standard bottom end.

If you tune for maximum power you can gain some power but move the torque peak higher in the rev range

2 x QED 420 optimised for maximum power
max torque - 138 ft lbs at 5000 rpm
max power - 160 hp at 6500 rpm 165 hp at 7500 rpm if you have a steel bottom end!
timing inlet MOP 115 degrees ATDC exhaust 110 BTDC. Note the inlet MOP is outside normal ranges used in practice and should be treated with caution, I would explore this further if building an engine for max power to this spec.

QED 420 inlet Standard Sprint Exhaust optimised for maximum HP
max torque 148 ft lbs at 4500 to 5000 rpm
max power 157 hp at 6500 rpm
timing inlet MOP 113 degrees ATDC exhaust 111 BTDC

you can see how the more restricted exhaust affects power if you have an engine that can rev beyond the 6500 limit of a standard bottom end


Lots of data and you can bury yourself in playing with simulation options but for a tall block road car with a standard big valve head using two QED 420 tuned for maximum torque looks the best option

cheers
Rohan

[dougal9887]

Thanks for the input Rohan. I'll press on as planned!
Dougal.

[CBUEB1771]

Rohan,
Thanks for your modeling efforts, as always your posts are very interesting. In this case when you "tune" for power or torque is the cam timing the only variable? Which simulation software package is your starting point?

[vernon.taylor]

Vernon,
I'll take a pic of the X1/9 seats I have fitted at the weekend and post in the Plus 2 section.

Thanks, Dougal

[rgh0]

Rohan,
Thanks for your modeling efforts, as always your posts are very interesting. In this case when you "tune" for power or torque is the cam timing the only variable? Which simulation software package is your starting point?

I use a software package is called Dynomation5

The optimisation I did was just cam timing which tends to be the key variable. The package will automatically optimise in a number of other areas also such as bore to stroke ratio and style of inlet and exhaust manifolds if you want but these are not generally relevant in a twincam. You can manually play with all the engine build variables relatively easily to see the effect also.

The software has two different methods of simulation. The first called" filling / emptying" which is a parametric model of the engine and how the cylinders fill and empty. The second is called "wave action" which does a detailed finite element analysis model of the pressure waves in the inlets and exhaust to calculate the gas flow. Technically the second way should give better results but in practice the first methods correlates better with actual real world data and its that first methods I used for the results I presented.

The results are a reasonable simulation of SAE Gross HP - i.e. they don't take into account losses due to ancillaries and only model the major internal friction losses but they do model the type of exhaust used in the car installation.

cheers
Rohan

[david.g.chapman]

Hi Dougal,

You could go for 3 angles on the inlet only - my inlet valves have never shown any signs of burning. I don't know what it gives you in the way of extra HP.

Dave Chapman.

[Plus2soon]

Hello Dougal,

I just found this thread and I'm about to do the same mode to my engine as you have. How did it all turn out? Are you satisfied with the result? What cam timing did you end up with?
My plan is to us the 711M block and crank, bore for 1700 cc, 5,23" Carrillo rods, custom pistons matching 11:1 and around 0,430 lift / 290? duration. Poring and 1,625 / 1,375 valves to suite.

Cheers Johan, Sweden

[nmauduit]

I have been playing with my simulation of a 1690 cc tall block twin cam with 10.5 comp ratio and a standard big valve head flow but different cams.

Results as follows. Note the absolute values are OK based on comparisons with actual published dyno data. The differences tend to be even better from an accuracy perspective. However the data is directional only in terms of deciding what you want to build as there is no real data except lap times !!!

2 x QED 420 optimised for maximum torque
max torque - 150 ft lbs at 4000 to 4500 rpm
max power - 150 hp at 5500 to 6000 rpm
timing inlet MOP 105 degrees ATDC exhaust 112 BTDC

QED 420 inlet Standard Sprint Exhaust optimised for maximum torque
max torque 150 ft lbs at 4000 rpm
max power 150 hp at 5500 rpm
timing inlet MOP 103 degrees ATDC exhaust 107 BTDC

With two QED 420 the power and torque peaks are the same but spread over a larger RPM range so a more drivable engine compared to using one QED 420 and standard sprint cam on exhaust and one that pulls harder to the 6500 rpm limit on a standard bottom end.

If you tune for maximum power you can gain some power but move the torque peak higher in the rev range

2 x QED 420 optimised for maximum power
max torque - 138 ft lbs at 5000 rpm
max power - 160 hp at 6500 rpm 165 hp at 7500 rpm if you have a steel bottom end!
timing inlet MOP 115 degrees ATDC exhaust 110 BTDC. Note the inlet MOP is outside normal ranges used in practice and should be treated with caution, I would explore this further if building an engine for max power to this spec.

QED 420 inlet Standard Sprint Exhaust optimised for maximum HP
max torque 148 ft lbs at 4500 to 5000 rpm
max power 157 hp at 6500 rpm
timing inlet MOP 113 degrees ATDC exhaust 111 BTDC

you can see how the more restricted exhaust affects power if you have an engine that can rev beyond the 6500 limit of a standard bottom end


Lots of data and you can bury yourself in playing with simulation options but for a tall block road car with a standard big valve head using two QED 420 tuned for maximum torque looks the best option

cheers
Rohan

very interesting discussion that I revisit still with the same interest... though I would now wonder about a slightly different angle :

I believe Q420 timing specs being presented by QED as inlet 100? ATDC and exhaust 106? BTDC
how much can one depart from manufacturer specs to one's benefits, up to 10? in your simulations (your warning duly noted), and would a similar approach lead to significant gains with other manufacturers ? the side question being will one likely need to accommodate the resulting overlap variation with some specific steps (e.g. piston pocket machining), which may be why the stock timing specs are more conservative (i.e. to improve installation ease thus fitting costs - while on second thoughts I would think that increasing lobe separation from standard would ease valve clash clearance : one need to figure out which limitation comes into play first for a given tolerance requirement)