Down on power

[Robbie693]

Hello all,

For as long as I've owned my Plus 2 I've felt it was a bit lacking in the horses department, it certainly doesn't feel as quick as the previous one I had at least. Initially I put that down to a high rev misfire it was suffering from for a long time, meaning the car wouldn't rev past 5000 rpm and sometimes less. As I seem to have cured this and now have access to the full rev range it still doesn't feel quick!

I took it to Aldon for a rolling road session a while ago and got the results as shown in the attached; 74 bhp at the wheels. Having read in a few posts here recently that an S/130 should be more like 95 bhp I guess my feeling about the power output is correct. The engine does seem to be ok for torque though.

I know that the head is going to need work at some point as it's running out of clearance on a few of the valves and is down to the thinnest shims but I also get a bit of oil in in the airbox.

The engine was rebuilt about 10,000 miles and 17 years ago but I don't have much detail on what head work was done. When I last had the head off (about 7000 miles ago) the bores were still showing signs of the honing from the rebore.

My question is, does the fact that the torque curve looks ok and the power low point to the cause being the head needing work or the bottom end not being up to scratch too?

Or is not as simple as that...?

Robbie

[rgh0]

Assuming that the ignition advance and timing is working correctly and that the mixtures are correct which should have been checked during the dyno run. Also assuming that compression is OK then the issue is most likely to be a problem with cam timing which could result in a good torque curve but loss of power above around 4000 rpm.

I would do a check to verify cam timing and lift (checking for lobe wear) based on direct measurement of TDC and measurement of degrees for maximum opening points

cheers
Rohan

[Robbie693]

Thanks Rohan,

I recall how to do that is covered in the Wilkins book, I'll have to get some kit to do it though (dial gauge, degree wheel etc) unless I can find someone to do it for me. The timing is aligned with the original sprocket marks but I've never measured the head to ascertain how much it's been skimmed by (I should do that!)

I do remember I had the compressions checked a few years ago by Paul Matty and although they thought it was ok they weren't the 160psi they should be. The memory is dim but possibly they were around the 120psi mark. I intend to get it checked again and do a proper wet and dry test.

Cheers

Robbie

[rgh0]

It is some what challenging to do an accurate timing check with the engine in the car as a normal timing wheel fouls on the chassis if you try to fit it to the front pulley.

I have made small timing wheels using CD disks printed with the photo from my larger timing wheel. These are small enough to fit on the front pulley with the engine in the car or to the camshaft sprocket.

cheers
Rohan

[pauljones]

Robbie,

Search for "on the rollers"

Theres my dyno print out on there. Well it was a while ago so may need some looking for. It should be good for a comparison.

Cheers.Paul.

[Robbie693]

Hi Paul, I found the post but the attachment/photo seems to be missing.

It is some what challenging to do an accurate timing check with the engine in the car as a normal timing wheel fouls on the chassis if you try to fit it to the front pulley.

I have made small timing wheels using CD disks printed with the photo from my larger timing wheel. These are small enough to fit on the front pulley with the engine in the car or to the camshaft sprocket.

cheers
Rohan

I've been looking through some old price lists for engine work and found one from QED when they were still doing it; it says they supply the rebuilt cylinder head "timed", how do you do the valve timing with the head off the car?

Robbie

[rgh0]

I presume they set up the cams on a dummy block and marked the sprockets and installed offset dowels for correct timing allowing for the amount that had been shaved off the head and any other variations in the head build so that the head could be bolted to a block , the marks lined up and the timing be correct.

It would work assuming your block timing chain was new, your block was standard dimensions and your crank sprocket was not worn -- all of which are fairly hopeful assumptions Not many people understand assembly tolerances and it is becoming a lost art among current mechanical engineers unfortunately also

cheers
Rohan

[Robbie693]

Thanks very much Rohan, i think this is looking like I should do the head rebuild and the timing setup at the same time incase it needs another skim.

I guess I'm now looking for recommendations on good head reconditioners in the Midlands!

Cheers

Robbie

[Chancer]

Not many people understand assembly tolerances and it is becoming a lost art among current mechanical engineers unfortunately also

cheers
Rohan

That is the understatement of the 21st century.

I remember when I started tuning and machining my first engine back in the 80's, I was a mechanical design engineer then and had done a 5 year apprenticeship, 8 years of college etc.

I went to a supposed specialist in the Midlands for alleged dynamic engine balancing, I should have followed my gut instinct and walked straight back out the door when I saw a post it note behind the counter to remind them that 1" = 25.4mm and 1mm = 40 thou"

They completely butchered my lightened (already balanced) and shot peened con rods by giving them the good news with a big feck off angle grinder right on the part of maximum stress (they already had the bosses removed and the cap thinned down and radius shaped) the crankshaft was returned looking like it had been through a crushing mill.

During the latter years I became more and more disallusioned with machining and fabrication suppliers always run by a non technical salesman who had no perception of limits and fits and tol?rances and would answer any query with "we make everything spot on"

[pauljones]

Robbie

I cant find mine either, but result was 94.7 bhp at wheels, and i think torque was around 85 lbs/ft. Bhp ran out and dropped off just over 6500rpm but what impressed me was the torque was faily flat across 90% of rev range.
Off memory it also looked similar to yours.

While i wanted more and still do,the torque being flat makes for a great drive, im really starting to love her.

Paul

[Robbie693]

Ah ok, that is interesting. Thanks Paul

Robbie

[nmauduit]

did you see that recent thread and consider your engine might have a similar issue regarding valve timing ? if the compressions are indeed as low as 120 and consistently on an otherwise recent engine (if engine not that recent you may add a leak down test to the compression test), I would investigate in that direction.

lotus-twincam-f39/low-compression-t39266.html

[Robbie693]

did you see that recent thread and consider your engine might have a similar issue regarding valve timing ? if the compressions are indeed as low as 120 and consistently on an otherwise recent engine (if engine not that recent you may add a leak down test to the compression test), I would investigate in that direction.

lotus-twincam-f39/low-compression-t39266.html

I did and have just read it again, thanks for the reminder. The valve timing has been set and/or checked 4 times in the time I've owned the car - twice by myself and twice by Paul Matty so I'm sure it's set as per the Lotus timing marks but wether they are now accurate I don't know.

I'm really not confident I remember the compression readings correctly so I think the priority is to get them re-checked and do a wet and dry test. I do remember that the compression test was done just after I'd done a head gasket change and I'd cleaned all the carbon off the head, bores, combustion chambers etc. Wether that would affect the outcome I don't know...

Cheers

Robbie

[pauljones]

Robbie

Theres some timing verniers up in the for sale section, may be worth it for you.

Paul.

Edited to remove top half which was meant for different post. Due to old age i made a booboo.

[Robbie693]

I think you're mixing me up with another poster Paul - I don't have an ECU, still running on distributor. I do have electronic ignition though.

Yes I saw the verniers and thought it a timely ad, thanks.

Robbie