Reversion

zefarelly · 12 Apr 2008, 15:48 (Ref:2175903)

spent a very frustrating day on the dyno today, ironed out horrible torque glitches by taking off the Carlos fandango exhaust manifold and putting an original 60's small bore Geoff Howe one, actually increased torque considerably and kept the same power, I knew the manifold was crap!

still very dissapointed with the numbers its giving, BUT, the big problem is reversion, we get maximum power at around 6800-7000 depending on how we had it set up but at 7100 it drops like a lead baloon, didn't sound right, you could see the fuel clouding by the trumpets, cleared slightly by 7500 and power/torque came up again slightly but its not right . . . . any ideas ?

possibly something not right with the cam gear drive ? shouldn't be valve bounce at those revs, or points bounce even . . .

AU N EGL · 12 Apr 2008, 16:52 (Ref:2175923)

When you have large torque numbers under the curve, HP drops quickly after peak. Put back on your old exhuast and have lower torque numbers under the curve but high HP numbers.

Not always able to have both.

Torque gets out of corners and UP to speed. HP gives you higher top end speed.

we all know corner exit speed and acceleration makes the straight. Question now is, do the extra hp up top make up for the loss of under the curve lower end torque? I say NO it does not.

zefarelly · 12 Apr 2008, 17:50 (Ref:2175951)

Valve float . . . . thats the thought at the moment . . .

AU N EGL · 12 Apr 2008, 19:09 (Ref:2176000)

Valve float:

- too high of rpm
- warn out or not strong enough springs poor valve seat.
- valves too heavy for rpm range

Good luck with sorting it out

zefarelly · 12 Apr 2008, 19:21 (Ref:2176005)

7100 I don't think so
new head, new springs, BUT, I think the cam ramps/profile might like more spring. Never had any problems with Serdi cutting valve seats
valves aren't the lightest, but 7k isn't that much, and I've run the same engine to 8k in the past without the problem, albeit different springs and cam

AU N EGL · 12 Apr 2008, 22:57 (Ref:2176115)

Then it most likely is not valve float.

What are you running? I had to put sodium filled valves and titanium retainers for safe running above 6300. But then again it is 5.8L V8 too, My red line is 6750

R59 · 13 Apr 2008, 00:10 (Ref:2176139)

You say fuel clouding by the trumpets? Have you got a standoff problem? Longer, or full-radius trumpets may cure it. It's all to do with the resonant length of your inlet tract.

Standoff is a phenomenon where fuel droplets are suspended over the carburetor inlet due to induction-system pulsations.

I did see a Dave Vizard design known as the "saucepan" which was a 'stepped' parallel trumpet, with a full radius edge. They produced good figures, and cured most standoff issues.

Rob.

terence · 13 Apr 2008, 06:45 (Ref:2176219)

I'll agree with that,it's something I've seen with a few engines, as R59 says,different ,longer trumpets could cure it.A lot of the original F5000 cars suffered the same problem!

zefarelly · 13 Apr 2008, 07:00 (Ref:2176222)

I think I have a longer set, but the inner wing isn't far away when we get it back in the car!

the reason I'm guessing at valve float is the noise, something sounds a bit harsh as well.

I'm going to play with anopther head today and seeif I can get away with a spring shim, we can probably do that on the engine without taking it off the dyno, got to be worth a try before pulling everything apart

phoenix · 14 Apr 2008, 08:47 (Ref:2176919)

Quote:

Originally Posted by zefarelly

I think I have a longer set, but the inner wing isn't far away when we get it back in the car!

the reason I'm guessing at valve float is the noise, something sounds a bit harsh as well.

I'm going to play with anopther head today and seeif I can get away with a spring shim, we can probably do that on the engine without taking it off the dyno, got to be worth a try before pulling everything apart

As I understand it, as engine revs increase, the length of the inlet tract needs to be shorter, not longer. For best wave ram tuning at 7100 rpm the length from the back of the inlet valve to the tip of the trumpet should be around 350mm (reference: Heinz Heisler, Advanced Engine Technology ISBN 0 340 56822 4 page 262) and for 7500 rpm 330mm, 8000 rpm 310mm.

If your inlet is around that length then the problem is probably somewhere else, particularly as you have run this inlet arrangement before with success upto 8000 rpm.

On the dyno, what length exhaust are you running after the manifold? If it is too short, or too large in diameter, this could cause this kind of problem as you could have fairly large positive pressure waves reaching the exhaust valve while it is still open in the overlap period.

If it was valve bounce at 7100 rpm, wouldn't it get worse as the engine revs went beyond 7100 rpm, not better? I don't know the answer, but I imagine it would get worse.

GORDON STREETER · 14 Apr 2008, 11:49 (Ref:2177107)

Quote:

Originally Posted by phoenix

If it was valve bounce at 7100 rpm, wouldn't it get worse as the engine revs went beyond 7100 rpm, not better? I don't know the answer, but I imagine it would get worse.

Not necessarily Spring harmonics do funny things !

Robyn Slater · 26 Apr 2008, 14:45 (Ref:2187469)

[QUOTE][/I've just spent 4 hours on the old head ( Goodwood one) re porting to get to 80% port/valve size . . QUOTE]

Anyone know where this theory comes from?

I,m intending to fit twink valves in a 1300 pre-xflow head.
The std. inlet throats are already larger than the 80% rule!!!!!!

So it seems i,d be fitting valves which are going to cause more shrouding and also weigh more.

Any thoughts?

phoenix · 27 Apr 2008, 17:10 (Ref:2188389)

[QUOTE=Robyn Slater]

Quote:

[/I've just spent 4 hours on the old head ( Goodwood one) re porting to get to 80% port/valve size . . QUOTE]

Anyone know where this theory comes from?

I,m intending to fit twink valves in a 1300 pre-xflow head.
The std. inlet throats are already larger than the 80% rule!!!!!!

So it seems i,d be fitting valves which are going to cause more shrouding and also weigh more.

Any thoughts?

The theory comes from the velocity at which the air coming through the inlet tract is stable - i.e. there are no losses due to turbulence etc. - and refers the ration between the valve diameter and the port diameter - note NOT the valve diameter and port throat diameter.

I believe these values have largely been found by experiment and are only a guide, because each port and each valve in individual engine designs will behave slightly differently, because of interference to the flow from things like valve guides and stems, port geometry, the proximity of valves to the cylinder wall or head (shrouding) etc.

Even in the most perfect aerodynamic environment, airflow become turbulent at about mach 0.6. As ports tend to fall short of perfection, because they have things like vale guides and valves in the way and bends in them - all of which do their best to cause disruption to the airflow - turbulence sets in at a lower speed - usually somewhere around mach 0.5 in a 'good' port.

Air flow exiting a poppet valve becomes turbulent at an even lower speed - somewhere between mach 0.29 and mach 0.32 depending on the shape of the valve, valve seat, port angle, port throat and the proximity of the valve to the cylinder wall or the combustion chamber.

If you check the ratio between speed in a 'good' port and a 'good' valve, given above, you will see that the port can be 64% of the area of the valve area (or the port 80% the diameter of the valve) though on some engines the port can be stable at only 60% of the area of the valve (about 77.5% of the valve diameter).

Large valves definitely give the possibility of making more power, so I think you should not dismiss them too readily. I think standard Lotus TC valves are 38.862 mm diameter and the 'Big Valve' ones are 39.751 mm. Ports for these would need to be 30-31 mm and 31-32 mm respectively. Surely your ports are not that large already?

GORDON STREETER · 13 Apr 2008, 11:41 (Ref:2176354)

Zef. what poundage are you running at the fitted spring length ?

zefarelly · 13 Apr 2008, 15:05 (Ref:2176470)

If I had a spring tester I'd tell you . . . I'm going to try and test it with some bathroom scales, but from finger feel its not as high as my old head which had VS37's @34mm . . .

I've just spent 4 hours on the old head ( Goodwood one) re porting to get to 80% port/valve size . . . I was using some other old dudes head as a guide

still got 4 ports to go, and half filled a takeaway tub with swarf and dust! . . . we'll swap the heads over in the week and see what happens

head on the engine at the mo has 34mm installed Iskeys, idea being 11.6mm lift, 21mm bound and 1.4 to spare . . . . obviously not right.

GORDON STREETER · 13 Apr 2008, 18:57 (Ref:2176590)

You really want 80--90lbs otherwise they will actually bounce on the seat (especially if the valves are heavy) Its always a compromise getting the fitted height vs poundage on full lift, and as you know it takes a fair bit of time to get it right. I do run them close to coil bound ! but don't forget I don't use loads of lift.
I wonder where you found that old dude's head !

zefarelly · 13 Apr 2008, 19:00 (Ref:2176595)

Its al adding up to the old 'pre crossflow' rule, monster duration/overlap. average lift . . .although I'm currently experimenting with 6 of one and half a dozen of the other !

zefarelly · 14 Apr 2008, 12:35 (Ref:2177169)

I can see this is going to turn to guitars soon Gordon

going to play with the bathroom scales and see what I can decifer in a mo. . . .

terence · 14 Apr 2008, 13:48 (Ref:2177230)

They wont bring the ball back after youve thrown it Zef,think you might need a rest.

zefarelly · 14 Apr 2008, 13:53 (Ref:2177236)

81lb on the nose according to our digital scales . . . . a 2.5mm washers makes 100lb on the nose, too much, so I'm making some 1.5mm shims, hopefully that'll be as near as damn it 90lb and we'll go from there.

If only I knew what I was doing in the first place

terence · 14 Apr 2008, 14:03 (Ref:2177245)

Ever thought of doing this full time Zef,seems like your getting pretty good at it.

zefarelly · 14 Apr 2008, 16:02 (Ref:2177312)

I need to earn a living Terry

you should know that messing about in garages and experiments on engines doesn't earn you a living

unless by sum sheer fluke it works

hopefully problem solved so back on the dyno next week now

terence · 14 Apr 2008, 16:35 (Ref:2177339)

Only too well Zef,I should have gone into I.T, but they had'nt been invented then!! Good luck next week,I'm sure it'll be fine,got to give at least 190 to be "Up there" amongst the Drabbles etc.

zefarelly · 14 Apr 2008, 18:03 (Ref:2177406)

the more I learn, the more everyone else is fibbing, or more politely put, believing the twaddle they're told!

terence · 14 Apr 2008, 18:39 (Ref:2177428)

Quote:

Originally Posted by zefarelly

the more I learn, the more everyone else is fibbing, or more politely put, believing the twaddle they're told!

The secrets out.