Cam timing question

[chrishewett]

Can anyone tell me what the TDC lift should be?
Chris

[rgh0]

Chris
For what cam do you want it for. I have a spare set of sprint cams and could set them up in a spare head to measure it up if no one else has the data.

If its for standard or SE cams I dont have a sample of these.

You could try someone like Kent cams or QED they may have the data

regards
Rohan

[john.p.clegg]

TDC lift?,are we talking TDC of the crank or total lift of the cam,if the latter surely it's the difference between the base circle and lobe circle,easy to find with a micrometer....
John

[chrishewett]

The story is:-
Not happy with the performance of my rebuilt big valve I took it to a rolling road tuner yesterday. Having set and balanced the new carbs, run a leak down test ( result good) and set ignition timing we only had a maximum of 83 bhp at the wheels. The tuner checked the cam timing and found at TDC the inlet was at .065" and the exhaust at .010"( they should be the same?). He removed the sprocket and set them both at .065". He said that he didn't have info on the twincam TDC settings but thought that would be about right. After that setting we had a maximum of 89bhp at 5,500revs. This is obviously low. The engine has been totally rebuilt. It has a new dizzy with Aldon ignitor ignition. Brand new weber 40dcoe151 carbs. New facet fuel pump. The tuner recons that 89bhp at the wheels is about 110 at the crank. I really don't know what is happening.
P****d off!
Chris

[mark030358]

Ermmm who built bit??? So I don't go there....

[rgh0]

John

I would have expected a little more out of a standard big valve than 89hp but not to much so dont be to p**sed off. Typically 90 to 95 hp at the back wheels is what I have seen and it should be peaking at around 6000 rpm not 5500rpm. So there should be a little more with fine tuning but not a huge amount.

A few of the things that could be affecting it.

Cam timing is certainly one of them. 0.065 inch valve lift at top dead centre is in the ball park but without the exact number you dont know how close you are. You may need vernier sprockets or offset dowels to get the timing right once you know the numbers.

Ignition performance is another. Did the tuner have an ignition scope attached and was he satisified there was no issues at high revs with the system, slight missfires can be hard to feel and hear at high revs on a dyno or on the road but can cost a significant horse power. During one dyno run I did a number of years ago the ignition scope detected one faulty plug wire and replacing that gave another 5 hp, you could not hear the miss it was causing at high revs

I presume the A/F ratio at maximum power was OK and the dyno guy was satisfied with his tuning.

Tyre types and pressures affect rolling road output signficantly a low tyre pressure leads to higher losses and lower measured output.

What is the state of your exhaust system and inlet to carbs. small blockages or misalignment at the joins can cause a few hp loss at the top end.

Finally dyno calibration, atmospheric conditions and air flow in the dyno cell all affect indicated power output

[hatman]

If it's any help and, as a guide, my big-valve S4/Sprint produced 109 bhp at the wheels - and that was with a semi-knackered top end (which I have since had re-furbished - although I haven't bothered with a new RR check).

[kenny]

Very interesting thread this and one that used to cause a great deal of debate on the Caterham site when I was a member.
I totally agree with Rohan re. the parameters for the test and the diffences this can cause in results, just the rolling road itself is a main cause........we've all heard of generous ones and mean ones
I had a Vauxhall Caterham to which I fitted a 225bhp kit consisting of cams, solid lifters, ecu etc. etc......I was not happy with the differences after the conversion but a rolling road test showed just on 200bhp @ wheels, which seemed about right
I was still not happy with the car after tweaks the guy did at that session and took it to a bloke renown in these circles. He re mapped ECU and did a few other fuelling tweeks and car was like a rocket 100 times better, yet it only showed 195 @wheels on his rollers proving the two rolling roads were miles apart on consistancy. Thats when I realised that rolling roads are a great set-up aid but not the bible in learning what actual power your car has.

Another point is I don't in all honesty think a BV does give a pukka 130.
Many years back myself and 2 local lads all built Westfields for sprinting etc. 2 of us went the standard x/flow route, i.e. +.090's", BVH, Kent 244, duplex,roller rockers,etc. etc. to give the normal 135bhp spec, the other lad used a BVH 130 twink which in all honesty could not touch the x/flows for power, even though their bhp specs. were supposedly similar.

The figure of 90bhp on the original question may not be a million miles out. When my engine rebuild is finished (that's a laugh) on a (so called ) 112bhp SE motor I was expexting to see maybe 80-85 at wheels in reality on a proper/mean road.

Good topic this

Kenny

[rgh0]

109hp at the back wheels at 6500 rpm normally takes some significant modification to achieve. It equates to around 135 - 140 hp at the flywheel between 6500 and 7000rpm. Rolling road peak rpm is always a little lower than engine peak power rpm as drive train losses are rising faster than engine power near the peak of the power curve

The sorts of mods I would expect to see with this output

34 or 36 mm carb chokes
cleaning up of the inlet and exhaust ports with careful alignment of joins
higher compression ratio
big bore exhaust system
higher lift cam ( around .390 lift) and maybe longer duration
plus a very careful blueprinted build and dyno setting of carbs and ignition

IMHO if you got it from a standard big valve its either a very unusual engine or a very optimistic dyno. Not saying it did not happen just that its not the norm in my experience or maybe its just that all the rolling road dynos I have seen used are reading low, who knows !

cheers
Rohan

[rt6810]

Been following this one with interest and will throw my hat into the ring. Firstly I have to declare an interest as I recomended the guy who rebuilt this engine to Chris (who incidently I consider to be a friend of mine) in the 1st palce. He has done a lot of work for me in the past and for people who I have steered his way. Without exception (Chris feel free to dive in here!) all have been delighted with both the results and cost of his work. I must stress that I have no connection to the guy other than as a satisfied customer. I do feel more than a little defensive when people post messages such as "Ermmm who built bit??? So I don't go there.... ". Chris, am I right in thinking that this is not the first time the car has been on rollers? Have further changes been made? Did the 1st guy that RR'd it not say it was one of the nicest twincs he had come across?
The carbs, dizzy and Aldon etc may all be new but if they have not been set up correctly they may as well still be on the shelf at Deamon Tweeks.
There has been a lot of sense written here (thank you Rohan) in that we may only be chasing 6 bhp and given that the RR guy only guessed the cam timing perhaps that is not so much to be getting het up about... I would also question why the power generated was quoted at 5,500 rpm when there would be at least another 500+ worthwhile rpm to go? Did the RR guy know you wanted an absolute 'balls out' max power run?
The engine was after all built as a nice road engine so cam timing was set using the std sprockets in their std positions. As I recall the engine may not even have Sprint Cams in it... As most of the power of a Sprint comes from the cams not the "big valves" perhaps that is the root of the problem??
Rant rant rant but FWIW my next 2 Lotus twincs and race Alfa engine will all go to this guy and I am still VERY happy to recomend him to anyone so there!
Happy horsepower hunting
Rob

[types26/36]

Am I missing something here? Full valve lift is usually quoted @ XXX number of crankshaft degrees ATDC for the inlet valve and XXX number of degrees BTDC for the exhaust. Why is the valve lift @ TDC being discussed? surely that is immaterial if the full lift is correct as per cam spec (and thereby the valve timing should be correct)
I'm sure there is a figure for lift at TDC but I have never seen TDC lift quoted even by Piper or Kent or is this now a new way to set the valve timing?
Sprint Cams (to the best of my knowledge) are CPL2 cams and these have a duration of 272 degrees and both have a full lift @ 110 degrees.

[davewalker]

Ok, I've had enough of this engine / customer and have registered on this forum to put my side. Thanks for your support Rob, Rohan, Kenny and the rest. It seems I've found an internet forum where at least some people talk sense.

My name is Dave Walker. A full time, self employed engine builder, primarily concerned with competition engines but also happy to work on classics etc. I have a comprehensive workshop/machine shop (do Stuart Rolts machining for example), I work alone and am generally happy. My workload varies between busy and totally overloaded. I do not advertise - ever.

I carried out this rebuild for Chris Hewitt. A previously poorly rebuilt std twincam. It is/was intended for road use only and was built on a budget. I charged him ?1190 all up for an engine build that included fitting 4 liners! Yes you read the price right. He messed it up (I think ran it without oil, but don't know), he wrecked the crank, shells etc during the first few minutes of commissioning and I rebuilt it totally for ?460, yes ?460.

It went to Dave Bogg's rollers, a 71 year old rolling road operator who I use who has a modern SUN rolling road. He will in the course of a year tune/retune maybe 200 of my engines. He ran it up gently, rang me and said it was sweet although the carbs were poor. He invited said customer back when he had some miles on the clock. He didn't go back. This was nearly TWO YEARS AGO.

Since then he decided to change the jets and chokes himself. Didn't run right (suprise, suprise) so he took it to Corten Miller rolling road in Lincs. Immediately diagnosed as overfuelling badly - oil full of petrol etc. He buys new carbs and is supposed to take it back - he doesn't.

Months pass by and he now visits PRP rolling road in Lincs, another good friend of mine who sees several of my engines every year. Paul sets his car up again.

I wasn't aware of this thread but earlier today I was speaking to Paul at PRP about a rally car that I was sending to him next week. He relates to me the TC story - sweet engine, no cylinder leakage (despite the abuse it has had) all OK. Because Chris was grumbling about its poor performance (numerically that is). He checks the cam timing and by filing the dowel hole in the exhaust sprocket, achieves a lift SIMILAR to the inlet (65 thou at tdc). The car makes 89 bhp at the wheels. I don't recall what cams were in this engine (which was a mish mash of parts) but PRP state they are NOT Sprint cams.

Ok you may say why didn't you frig the cam timing? I now post the exact extract from the build report issued to him:

Cylinder head fitted with copper head gasket (all gaskets by Payen) and torqued accordingly. Chain fitted to timing marks on cam pulleys and valve timing checks made. Like all twincams timing marks are vague (you can easily be one tooth out). Valve timing checks made were to confirm best position only. If performance camshafts were to be fitted at a later date then vernier sprockets or the use of offset dowels would be necessary

So, how much power should it be making at 5500 revs? You tell me. The engine cylinder leakage is excellent, it doesn't spew oil everywhere like most rebuilt engines do, it was a budget build that few of you could match....

Sorry but I don't need the hassle, or implied critisism and the man who wanted to know who to avoid, you now know, go to QED pal.

[stuartgb100]

Hi Dave,

Welcome to Elan.net.

It's always a benefit to hear another side of the story. Context and balance is important.

I hope you'll decide to stay a while and check us out. Your knowledge would be valuable. I think you'll find us an enthusiastic, and basically friendly/helpful bunch of guys and gals.

I'm sure you'd be most welcome.

Regards,
Stuart.

[john.p.clegg]

Brian
Looks like we're on the same track here,maybe Dave can shed some light on it?

John

[davewalker]

John and Stuart thankyou. To the rest of the forum users, sorry if I went on a bit.

The reason that rolling road operators like to use cam lift at TDC is that it is a lot easier to set the cams when the engine is in the car. It is always difficult and in most FWD installations it's impossible to satisfactorily fix a protractor to the crankshaft pulley and read it with any accuracy. You either can't get one down there and if you can, you can't see it properly.

The TDC lift method which Paul was using is undertaken as follows:
Fix dial gauge down number one plug hole to note when piston comes to TDC. Have another dial guage on number 1 inlet valve to monitor its movement. Rotate engine and note both when at TDC is reached and the reading on the 2nd dial gauge showing amount of valve movement at the point the piston comes to TDC. This amount of valve movement is what Piper and Kent etc quote as "Valve lift @ TDC" in their catalogues. Repeat for No 1 exhaust valve.

When the valve lift has been found to be in error, turn engine to TDC, slacken vernier locking screws and rotate the cam slowly until the desired lift is achieved. Lock the vernier etc....

Simple isn't it!

Obviously lift at TDC is different for every cam profile in every engine. Look in one of the catalogues. It underlines what a ridiculous question was being asked in the first instance and the lack of technical understanding accompanying it!