Cam timing and power gains...

[Tango13]

I've just collected my bike from the dyno shop where it produced this at the wheel, 138bhp @ 9600rpm and 80lbs/ft of torque between 4,000~9,600rpm.



It's a 3 cylinder engine with 79mm bore X 71.4mm stroke giving 1050cc and is currently running the cams below with 33.5mm inlet valves and 27mm exhaust valves. It has a skimmed head, 46mm dia throttle bodies, Carrillo rods and forged J.E pistons running about 13:1 compression.


1050cc cams...

Inlet valve lift 8.75mm
Open 11.25¢ªBTDC @ 1.0mm lift
Close 41.25¢ªABDC @ 1.0mm lift
Duration 232.50¢ª

Exhaust valve lift 7.45mm
Open 34¢ªBBDC @ 1.0mm lift
Close 4¢ªATDC @ 1.0mm lift
Duration 228¢ª


Lurking in my toolbox I have these cams which will drop right in, they came out of the head I'm currently using which was bolted to a short stroke engine of 65mm stroke giving 955cc and peak power around 10,200rpm, the rest being identical to the above.


955cc cams

Inlet valve lift 10.15mm
Open 25¢ªBTDC @ 1.0mm lift
Close 53¢ªABDC @ 1.0mm lift
Duration 258¢ª

Exhaust valve lift 9.65mm
Open 39.5¢ªBBDC @ 1.0mm lift
Close 29¢ªATDC @ 1.0mm lift
Duration 249¢ª


I've read elsewhere that fitting the 1050cc inlet cam to the exhaust side and the 955 exhaust cam to the inlet gives about 8~10bhp on top of a standard 1050cc engine which produces about 115bhp at the wheel.


Anyone have opinions on whether gains like this are realistically possible? Seems a lot for a simple cam swap! Fueling is not a problem, the ECU can be re-programmed and I have some spare inlet trumpets about 35~40mm longer than the ones fitted.

[Evoluzione]

If someone has already done it then they can pass on the results, if not you'd just have to try it, but they do look promising.
Ultimately if you had very intimate and accurate knowledge of your engine then calculations can be done to make an accurate prediction, but it's rare anyone has that so it's just a case of chuck 'em in and hope for the best.

Edited by Evoluzione on Sunday 24th April 09:18

[HarryW]

As said if it's been done before then why not, if not then I'd be a bit concerned about piston to valve matters given the difference in lift and timing.

[Mr2Mike]

Isn't the 1050 rev limit something like 2000RPM lower than the 955i? Unless you can extend the RPM range then you may not see the benefit of longer duration cams, i.e. lose a bit of low/mid torque but not be able to rev high enough to see a good power gain.

[Tango13]

Mr2Mike said:

Isn't the 1050 rev limit something like 2000RPM lower than the 955i? Unless you can extend the RPM range then you may not see the benefit of longer duration cams, i.e. lose a bit of low/mid torque but not be able to rev high enough to see a good power gain.

Cam timing is a bit of a black art to me hence the initial post but as I understand it the extra lift and duration from swapping the cams would extend the rev range and allow more fuel/air into the engine equalling more power.

The 1050 is limited to 9,100rpm, I tried revving one a bit higher and the only things to survive the resulting carnage were the cams and the crank...

The 955i would go to 10,700rpm so as I'm using the 955i head I now know the valve train is safe. 10,100rpm on a 71.4mm stroke gives an average piston speed of just over 24m/s which is about as high as I'd be prepared to go on a road engine.

What I'm curious to know is, would swapping the cams give me an extra 4-500rpm and a boost in power without hurting the mid-range too much?

[HarryW]

Not sure if it's been covered off or I made myself clear earlier. I do not know the engine, but I'd be surprised if you don't gave pocketed Pistons to give clearance for the valves. You've already had a head skim, so the valve to piston clearance will be closer that standard. You are considering putting in cams with a much great lift and duration. Are you happy that you modified engine will not have issues with valves meeting Pistons. In particular clearances can reduce when coming off throttle with stretch rather than compression from on throttle iykwim.

[Tango13]

HarryW said:

Not sure if it's been covered off or I made myself clear earlier. I do not know the engine, but I'd be surprised if you don't gave pocketed Pistons to give clearance for the valves. You've already had a head skim, so the valve to piston clearance will be closer that standard. You are considering putting in cams with a much great lift and duration. Are you happy that you modified engine will not have issues with valves meeting Pistons. In particular clearances can reduce when coming off throttle with stretch rather than compression from on throttle iykwim.

I don't see it being a problem for a couple of reasons...

The inlet valves currently miss the piston so swapping the inlet cam onto the exhaust side with the same lift/duration will miss.

Also as the piston is on a longer stroke than before it will be a bit further down the bore when the cam starts doing its thing.

[HarryW]

Tango13 said:

HarryW said:

Not sure if it's been covered off or I made myself clear earlier. I do not know the engine, but I'd be surprised if you don't gave pocketed Pistons to give clearance for the valves. You've already had a head skim, so the valve to piston clearance will be closer that standard. You are considering putting in cams with a much great lift and duration. Are you happy that you modified engine will not have issues with valves meeting Pistons. In particular clearances can reduce when coming off throttle with stretch rather than compression from on throttle iykwim.

I don't see it being a problem for a couple of reasons...

The inlet valves currently miss the piston so swapping the inlet cam onto the exhaust side with the same lift/duration will miss.

Also as the piston is on a longer stroke than before it will be a bit further down the bore when the cam starts doing its thing.

Ok, as I said I do not know the engine, but increasing the lift by up to 2mm and the duration by over 20 degs could be an issue for some engines. If you have done the calcs and are happy who am I to say...

[rev-erend]

Is there a little technique like putting a little bit of plastercine or blu tack on the piston and turning the engine over by hand to check the clearance ?

[Tango13]

rev-erend said:

Is there a little technique like putting a little bit of plastercine or blu tack on the piston and turning the engine over by hand to check the clearance ?

When my brother used to build the engine for his 350LC race bike he always had a roll of 2mm diameter lead solder to hand, he would thread it down the spark plug hole and gently poke it about until it was almost touching the cylinder wall.

Then he would then gently pull the engine through on the kick start to squash the lead solder thus enabling him to measure the squish.

The 350LC had seperate barrels and a one piece head so he always added a few extra base gaskets which enabled him to torque the head down a bit tigher at a specific corner if required to get the squish even. The head gasket wasn't a problem, it didn't have one!! Just a fine smear of bathroom sealant between the head and barrels.