Engine build advice needed.

[rgh0]

You will struggle to get a QED 420 profile out of your existing cams I would think. You would need to build them up by welding to get an acceptable base circle and the cost of that is more than a new cam blank.

If you have a head that has been heavily modified it really needs to be carefully measured up before you start spending money on a high cam setup to ensure it does not have any unexpected surprises and the parts you plan buying will work.

For a 150 hp road engine 40 DCOE with 34 mm chokes will be OK. But if you have a set of 45 DCOE and the inlets are suitably ported for them then 36 mm chokes in them will give you a little more. Its just a little more challenging to tune the bigger chokes to avoid flat spots

There is another potential issue with the bottom end of your engine if the 711m block has been decked by 8 to 10 mm to suit the shorter stroke 1500 crank. This does not leave much metal in the top deck so the piston bore castings can become unstable and crack. Also the head bolts must have been shortened to fit the remaining thread in the block. Pushing this to 150 hp may start to reveal some weaknesses in this modification.

cheers
Rohan

[Bunky Twincam]

Rohan,

If my compression ratio is about 10.5 :1 and I use the 420 cams I would be very happy with something around 120 at the wheels, I am not chasing anything more at the moment and if I was I would go down the route of steel crank and new rods and run avgas. I am not into that just yet, just want a nice torquey motor and to have some fun on the track and a few hill climbs.

[rgh0]

120 at wheels and 140 to 150 at the flywheel is putting a lot more stress on the engine than you are giving it now. Just trying to highlight what breaks and why - I have broken twin cams in most ways possible in the last 40 years

cheers
Rohan

[Davidb]

And yet Rohan continues to give sage advice despite all!

I have to say, as one who has used/abused his generous help, that he is a major attribute of this forum. And a pretty good bloke too!

[1owner69Elan]

Just to add my experience, consistent with the excellent points made by Rohan:

My engine build:

.440" lift, 250 deg duration (@.050 lift) Kent Cams. So the high lift, short duration as recommended by Rohan
~1700 cc stroked engine
1.625 intake valves, 1.375 exhaust (Manley, US made). Springs, retainers all US made.
Weber 40 DCOE, 32mm chokes
Compression: 10.25 , pump gas

Setup is for the street. Broad torque. 7000 redline.

Dyno sheet below. I think what may be striking is getting this level of output with 40's Webers and 32mm chokes and not at a high racing level rpm, differing from what seems to be usually needed for this hp (45's and larger chokes, revs into 8000 range). Maybe this is attributable to a better breathing modern SAS head being used and skilled porting by the engine builder.

Dyno results.

(Note: I am about a month away from getting this on the street to really evidence the hoped for tractability).

[john.p.clegg]

Do you have any data for below 4000 ?

John

[1owner69Elan]

First, I am no dyno expert. I asked the same question about getting numbers below 4000.

The engine builder said the dyno probably wouldn't work but he tried it anyway to please me. The dyno couldn't really handle my engine much below 4000 rpm - just started "bucking" as it couldn't hold the torque at low rpms.

Most of his work is on race engines, optimizing high hp at high rpms and thus, I suppose, his test equipment works in that range.

Going to have to see how the car works in the real world below 4000 rpm when I get it on the street.

Looking at the plot below, which looks rather linear for both torque and hp, one could perhaps extrapolate below 4000 (at least for a little) to get some idea of lower rpm values.

Looking at hp data at lower rpm from Robinshaw indicates rough linearity between 2000 and 6000. Torque, after being somewhat horizontal, drops off 33% from 4000 to 2000. If I do a similar extrapolation on the above dyno data I get:

The above is just a conjecture, someone on the forum may have "real" data below 4000 that behaves differently.

[Bunky Twincam]

So I am guessing then that you may have about 140HP at the wheels? My engine build is very similar with milder cams and only made 110HP at the wheels.

When I pull the head off to fix the exhaust valve guide oil leek I am definitely going to re-check the compression ratio as I thought it was about 10.5:1 but my dyno figures don't really support that.

I am thinking of sending my other pair of standard cams to QED for a Q420 regrind which all up is just under $500 AUS sent back to me.

I did fit race high lift springs when I rebuilt the head so with some luck all the other clearances might work out.

I would be happy with 125HP at the wheels.

Cheers

[1owner69Elan]

Looking at an old post from Rohan, he indicated 140 whp from a race tune 170 at the flywheel.

So I would expect 140+ whp on my car.

I also have a lightened flywheel which will be a new experience as well.

[rgh0]

So I am guessing then that you may have about 140HP at the wheels? My engine build is very similar with milder cams and only made 110HP at the wheels.

Cheers

With the cams you have your lucky to have 110 hp at the wheels. The porting and big valves are helping but with a long duration low lift cam they don't really achieve that much. You probably also have a dip in the torque curve in the 3000 rpm to 4000 rpm range. Which makes the engine feel flat in road driving.

The QED 420 cams will bring your engine alive.

Just be careful that the rest of the valve train will fit with them. Just because someone sell springs called "race springs" does not mean they are suitable for high lift cams. In addition valve position and stem length, cam base circle and follower thickness all become critical when building a high lift cam engine. A .420 lift cam is not that radical and its easier to fit in than when you push it to .440 and above and .500 plus lift cam gets really challenging to fit in But you still need to ensure you have a set of components and installation details that will work.

cheers
Rohan