Duratec porting, a few thoughts.

[MikeMolloy]

Sorry, wasn't meaning to imply "SBD are better than Raceco" or anything, just thought it was interesting that, as per your previous prediction, they had to rev the 2 litre to 9K5 to get their headline 300+ figure.

Reading the linked articles seems to indicate it is not without its challenges though...

E.g. valve springs:

"...had to develop special valve springs in order to cope with over 13mm of valve lift, regularly rev above 9500rpm."

"...had a custom spring made at great expense, being made from the highest spec material, made in a dual rate which is also put through many treatments to extend the life of the spring."

And camchains:

"if you've got an extremely lairy set of cams spnning at massive speeds, then they have the effect of trying to push the chain off the pulley's teeth - not what you want when dealing with extremes and minute adjustments in race conditions. SBD has already made a manual cam chain tensioner as the hydraulic ones were causing problems, but it's not stopping there; "We're developing a roller chain for the next level of cams," says Steve, which hopefully won't try and hop off the pulleys at extreme revs."

 

[AMMO]

Hi Fred

 

No need to say sorry. I didn't interpret your post as “SBD are better than Raceco”. Such a thing never entered my head, so don't worry.

 

Looking again at the graph here of the 280 bhp 2 litre it's pretty obvious that power is still rising and will peak in the region of 9,000 + rpm. In fact that engine is probably already a 290 something engine if we revved it. The 9,500 rpm figure comes from what I consider the safe mean piston speed in the region 26 metres per second. This is already a higher piston speed than a lot of high rpm engines. It would be nice to have another 500 rpm past peak power for gear shifting. I'd really like to go to 10,000 pm if I could but I'm not sure I would be comfortable with it.

 

Regarding valve springs I also differ in opinion. Valve springs already exist to do the job. Since my first trip to Daytona in 1989 I have owned an American made Rimac valve spring tester. I have done a lot of spring tests and have tested most springs available for the Duratec and other springs available from elsewhere. Other engine builders often used to send me springs for testing. The involvement with motorcycles means that I am used to running higher rpm. I had a 2 valve Guzzi with a 13mm valve lift running to 10,000 rpm in the late 80s. The 4 valve Guzzi ran to 9,200. I've run a 13.5mm cam in a Duratec with my own spring package. What amazes me is that many engine builders put valve springs in engines that the cam supplier suggests without actually knowing the full lift, over the nose poundage. This and the distance to coil bind are the two of the most important parameters. I know the on the seat and full lift poundages for all the engines I build. My guess is that a lot of people don't bother doing this and just bung in the springs they are given. I also had a tool made for checking valve spring coil bind on the engine. There is a critical full lift to coil bind measurement I use. When you look at the engine from the outside it looks like any other Duratec. You can't see the extra hours that have been put in doing all the checks that will give you some sort of fighting chance to make the engine survive.

 

The Duratec is fitted with a high velocity Hy-Vo chain as standard. The same type of chain that is fitted to thousands of racing motorcycles that turn more rpm than the Duratec ever will. There are two different sorts of Hy-Vo chain fitted to Duratecs. An 8mm and a 6mm pitch chain. The 8mm chain is the one I use. Old Guzzis ran a roller chain and it was pretty useless as it would stretch and rattle in no time. Replacing the Duratec chain isn't on my list. If the chain is properly tensioned there is no room for it to jump off the sprocket. For the manual tensioner conversion I have been fitting for quite a while there is a tensioning procedure that I developed that ensures the tension is correct when the engine is hot. It we don't want the cam timing to fluctuate is is essential that the chain tension is correct. The only problem with the stock tensioner that I have actually witnessed was that the ratcheting mechanism allowed the chain to over tension. The mechanical conversion stops this happening. In one case of a chain over tensioning I heard of a chain snapping on the dyno. Glad it wasn't my engine as it made a terrible mess apparently.

 

Different engine builders and tuners will have different findings and experience with engines. Their opinions will differ. That's OK. Nothing wrong with having a different point of view or approach. It is only right that they should persue their own courses and find out for themselves what works and what doesn't. If everyone ended up building the same engine it would be very boring.

 

 

[MikeMolloy]