Re-timing stock camshafts to benefit track useage

[pedrosixfour]

Hey guys.

 

Thinking about camshafts and adjustable pulleys today and my mind wandered a bit too far into the darker (dimmer) recesses of my tired mind.

 

Just wondering if anyone could speculate whether re-timing either or indeed both of the stock camshafts in relation to the crank might have a positive effect on mid range and top-end in a BPU'd 1JZ without leading to engine destruction.

 

I'm basically asking if retarding or advancing one or both camshafts by a tooth or two could free up some more power at the expense of a smooth idle or very low end torque which is of little consequence on my track car.

 

The basic details of the stock 1JZ cams are as follows

 

 

lift (Intake) 7.69 (Exhaust) 7.95

 

duration (adv) (Intake) 224 (Exhaust) 228

 

duration @ .050" (Intake) 189.1 (Exhaust) 194.1

 

Sorry if this is a truly idiotic question but I can't see any harm in asking as I know the engine is non-interference and, being mass produced, had all the added gravity of being responsible for drowning polar bears and choking trees when it left the factory.

 

Thanks for any input. constructive or otherwise.

Edited November 26, 2013 by pedrosixfour (see edit history)

[Swampy442]

Yeah of course, thats why you have adjustable cam gears, but its only a matter of a couple of degrees I believe. Someone like Chris Wilson, Lee, Dude etc would be able to give you better info but thats what VVTI does, changing the cam timing can give improvement in torque/power at different point in the rev range.

[pedrosixfour]

Yeah, adjustable pulleys would certainly allow for precision, but I just thought it would be interesting to know if something positive could be achieved without the need for new parts and dialling-in procedures.

 

All this, in a perfect world, would then mean the next time someone changes a timing belt they have the choice to alter the power delivery of their engine by simply rotating a shaft by a tooth from the recommended position.

 

From what I can gather rotating a camshaft a single tooth on a stock pulley would alter the timing of that shaft by 7.5 degrees.

 

Too much to be of any use?

 

And is it even worth considering if duration and lift are unchanged on the shaft itself?

 

I assume the VVTi engines switch to a different cam lobe profile altogether when the system engages, as opposed to just advancing or retarding the same lobe profile that's used before the system kicks in (yo)

[Tricky-Ricky]

I am pretty sure the VVTI is quite primitive in that it just advances and retards the cam depending on load/RPM signals from the ECU, so duration and lift is the same,

All I can tell you with regards to changing the std valve timing is that on the TT advancing the exhaust side about 3-4 degrees will help with quicker spool up and keep EGTs lower, and feels like it gives a slight mid range torque boost.

[pedrosixfour]

I'm such a tool.

 

I completely forgot that 1 degree of change on a camshaft is 2 degrees on the crankshaft.

 

So moving a camshaft by a tooth would alter the timing by 15 degrees in relation to the crank.

 

Ah well.

[Guest SupraV8]

Why don't you just remove the woodruff key and dial the new settings in? It's the bolt that locks the cams in place on the taper, not the locating key.

[Chris Wilson]

You will move the torque peak about in a very limited band before power falls off an unacceptable amount somewhere. Carried to extremes with none stock cams, or even just a tad, with race spec cams, can result in valve to piston contact and many tears. With interference cams you *MUST* find the safe limits of cam timing changes at the build stage, with soft valve springs and either a VERY gentle touch, or proper measuring. Cam pulleys on a none taper (usually Woodruff keyed) shaft, with no key in place are a recipe for disaster if you have interference cams. Use either offset keys once dialled in, or drill accurately and use dowel pins. Preferably solid silver stell, but if you are a wobbly driller, roll pins, doubled up, one inside another.

[Ian C]

You'll only get more power by increasing the duration, which you can't do without buying bigger grind cams. Yes, an NA engine would see minor gains up top (in exchange for major losses everywhere else) if you increased overlap but even then without longer duration cams to start with it's all a bit hopeless.

 

You can tune the powerband position with adjustable gears, but reducing overlap is really what you want to do with this ability on forced induction motors. When you're forcing the air in the last thing you need is a lot of overlap as it'll all go out the other 'ole into the exhaust

[Digsy]

I would have thought that a bit of overlap was OK at the top end on a straight 6 engine with two 3-into-1 exhaust manifolds as the chance of reversion is low. If you can keep blowing down the cylinder against the pre-turbine pressure then you'll increase your cylinder fill and reduce trapped exhaust gas, which will reduce the likelihood of knock. The problem with doing this on port fueled engines is that you can end up chucking a lot of fuel straight into the exhaust as well.

[Mk4Gaz]

I am pretty sure the VVTI is quite primitive in that it just advances and retards the cam depending on load/RPM signals from the ECU, so duration and lift is the same,

All I can tell you with regards to changing the std valve timing is that on the TT advancing the exhaust side about 3-4 degrees will help with quicker spool up and keep EGTs lower, and feels like it gives a slight mid range torque boost.

 

So on a non vvti, would it be as simple as adding an adjustable cam gear to the exhaust side, and advancing it to 3 degrees, leaving the crank and inlet cam at tdc?

[Ian C]

I would have thought that a bit of overlap was OK at the top end on a straight 6 engine with two 3-into-1 exhaust manifolds as the chance of reversion is low. If you can keep blowing down the cylinder against the pre-turbine pressure then you'll increase your cylinder fill and reduce trapped exhaust gas, which will reduce the likelihood of knock. The problem with doing this on port fueled engines is that you can end up chucking a lot of fuel straight into the exhaust as well.

 

Reversion is at low rpm and idle isn't it? I was definitely getting that, when I retarded the intake and advanced the exhaust (I think that's the right way around, pardon me it's 4am ) the idle cleaned up really nicely and it seemed happier low down. I didn't notice any loss up top from a lack of overlap

[Tricky-Ricky]

So on a non vvti, would it be as simple as adding an adjustable cam gear to the exhaust side, and advancing it to 3 degrees, leaving the crank and inlet cam at tdc?

 

Depends on your setup as to how much advance you give it but yes it worked well for me both in TTC and big single mode.

[Digsy]

Reversion is at low rpm and idle isn't it? I was definitely getting that, when I retarded the intake and advanced the exhaust (I think that's the right way around, pardon me it's 4am ) the idle cleaned up really nicely and it seemed happier low down. I didn't notice any loss up top from a lack of overlap

Yes, that's the right way round :-) Reversion on the overlap is probably less of an issue on a blown engine where the in cylinder pressure might be higher than the pre-turbine pressure during the overlap. This might not be the case on, say, an I4 where the exhaust valve events overlap (assuming exhaust cams of greater than 180 degrees duration).