Compression ratios

[zefarelly]

that might be why it runs out of steam at 8k . . .34mm chokes !

right . .wheres those 36mm ones :-)

generally speaking the exhaust tends to look after itself, or at least I don't wake up thinking about exhaust flow ! we rework it obviously and have a decentish manifold.

more torques are obviously the way to go if the breathings going to start restricting the revs

[phoenix]

Originally Posted by zefarelly

that might be why it runs out of steam at 8k . . .34mm chokes !

right . .wheres those 36mm ones :-)

generally speaking the exhaust tends to look after itself, or at least I don't wake up thinking about exhaust flow ! we rework it obviously and have a decentish manifold.

more torques are obviously the way to go if the breathings going to start restricting the revs

I wasn't so much thinking about exhaust flow as tuned length. At peak torque you want the pressure wave created by the exhaust valve opening to reach the collector and return (as a negative pulse) at about TDC to help scavenge remaining exhaust gasses and pull in the new charge.

The length this requires is obviously different for different RPM. Most exhausts seem to be tuned for about 4000 - 4500 rpm. You MAY want something tuned for a higher engine speed.

Same goes for the inlet length which can be tuned to help fill the cylinder. As the inlet valve opens a negative pulse runs up the inlet tract to the end of the trumpets. This is reflected back as a positive pressure wave which you can use to improve cylinder filling. To help you want this positive pulse to arrive at the inlet valve at around 110 - 120 degrees after TDC - though anytime before BDC will help!

[zefarelly]

ah I see, well I have a 4 into 1 exhaust manifold which was designed length wise and should be goof for 7-7500 max power, I've modified it from a 105speed one, next one I'll probably make myself.

never tunes inlet length though, although I do have different length trumpets if needs be, I figured longer for torque and shorter for higher revs etc, but that was something I read a long time ago

engine design/tuning by committee this is . . . B O T . . . can we get to the perfect CR . . .12:1 I think, maybe 12.5:1 if the casting can take it and remain reliable . . . . . looks like I'm sticking with what I've got for Goodwood so pressures off for the new build . . .at least until October !

[phoenix]

Originally Posted by zefarelly

ah I see, well I have a 4 into 1 exhaust manifold which was designed length wise and should be goof for 7-7500 max power, I've modified it from a 105speed one, next one I'll probably make myself.

never tunes inlet length though, although I do have different length trumpets if needs be, I figured longer for torque and shorter for higher revs etc, but that was something I read a long time ago

engine design/tuning by committee this is . . . B O T . . . can we get to the perfect CR . . .12:1 I think, maybe 12.5:1 if the casting can take it and remain reliable . . . . . looks like I'm sticking with what I've got for Goodwood so pressures off for the new build . . .at least until October !

Back on thread - a static CR of 12.5:1 won't cause stress on it's own and as your cam people suggested you will get a useful increase in dynamic CR over what you would get from 12:1 static.

If there isn't enough meat on the head then I wouldn't bother as that could lead to reliability issues. Is there clearance so that you could fit pistons with a dome on them?

On the inlet side, what is the length from the back of the valve to the tip of the trumpets, and what is the trumpet length you are running?

[zefarelly]

this is the next thing to look at, inlet tract length . . . I have a collection of trumpets, currently theyre 40mm but I think shorter will help

need to measure inlet length, its about 100mm, so is inlet manifild.