slate blue

Hi to all, well my new valves turned up today, the sizes we are experimenting with are 2.02'' or 51.31 mm on the intake and 1.65'' or 41.91 mm on the exhaust. These are Nascar Valves and have 7mm stems. I have had them on a digital scales that reads in 2 gram increments. Standard size is 45mm intake and 40 mm exhaust with 9 mm stems.

First the old weights Intake 107 grams
Exhaust 108 grams
Retainer 26 grams
Spring 96 grams

The spring pressures were 60 seat pressure and 220pounds on the intake @ 0.500'' and 60 seat pressure on the exhaust but 240 pounds on the exhaust side.

The new springs will allow 0.515 lift on the intake and have 260 pounds at that lift.

The new weights are Intake 100 grams
Exhaust 80 grams
Retainer 12 grams
Spring 42 grams

N.B the retainers were weighed with the collets, I have been told you count 1/3 of the spring weight in the valvetrain weight. So this means I save;

Spring 18 grams
Retainer 14 grams
Intake valve 7 grams
Exhaust valve 28 grams

So a net saving on the intake side of 39 grams and 59 grams on the exhaust side. The other factor is the slightly higher spring pressures, what I am trying to achieve is a safe rev limit of 7500 rpm. Any thoughts? I will be using the stock lifters except they will be DLC coated. The valve lift we are looking at is 0.515'' on the intake and less than 0.500'' on the exhaust.

Cheers Greg

John Speake

Hi Greg,
As far as wear goes, if that is a concern with higher poundage, it's the rate of change of lift that is the important factor ?

So as I assume you are going for a modified cam, you may want to keep that in mind. What are your plans fo rthe camshaft ? Will it have some overlap ?

slate blue

Hi John, as far as the cam goes, Piper is grinding it. we will supply the headflow figures and assess it after that. My guys doing the heads will also come up with their own cam selection figures, but they do want a bit more lift. The heads certainly pick up flow with the higher lift, standard is 0.472 so 0.515 is only a bit more than a mm, that is not that big a jump. As far as wear goes, there should be very little, DLC coated lifters with Castrol synthetic R 10w - 60 should see to that. The coating people say a reduction in friction upto 40%

Certainly the cam will have overlap, it will fall somewhere between 260 and 280 degrees of total cam duration.

Cheers Greg

John Speake

Hi Greg,
So I assume you are using a MAP sensor with an a new engine managament system ?

mark kibort

i dont think the 928 would have a float problem up top. (especially if you had stiffer springs) remember the cams only turn at 1/2 the rpm of the crank. so we are talking about 3700rpm . however, the bottom is critical to oiling and balance

i think the main thing is the bottom end and oiling. GT3 porsches rev to 8900rpm, and have great oiling.

mk

slate blue

Hi John, yes I will use a map sensor, the system will most likely be the Motec M800.

Hi Mark, I will completely reroute the oiling like a chevy. I have bought my short motor, it has great wall thickness and the piston squirters. My engine will have reduced friction in many areas. One of the areas is the bottom end, it will have the new Mahle slipper type pistons with forced pin oiling and the rods are the Honda style rods. These required less oil, as they have less bearing area and reduced bearing speeds. These rods are said to be worth around 7 hp at 7000 rpm. This engine will also have a rod to stroke ratio of 1.84 to 1, which again should load the skirts less and result in less friction. Infact my bottom end is very much like a Nascar.

Nascar engine speeds are over 9000 rpm for the race, my pistons are very similar in design and so are the rods, with respective weights of 550 grams and 546 grams. So remembering that the old pistons weighed 722 grams and the rods were 850 grams and the crank will also be very light, I think that will get down to 22 kgs or 50 pound, it should spin it real quick.

One thing about the oiling, I have a 85 model crank, which is the Euro, I was thinking about getting an early one as the holes in the crank are smaller. Given that most of these holes wont be used when the oil is rerouted, I figured it is better to have smaller holes rather than larger ones. Also I have bought my Barnes Dry sump pump and it will have 4 scavenge stages all of this should ensure that I have plenty of oil available for lubrication and not power robbing windage.

Here's a pic of the rods.



Cheers Greg.

mark kibort

sounds very trick!!! cant wait to hear what a 928 engine sounds like at 7500rpm!!

mk

Normy

Greg- I'm sure you know more about it than I do, but I remember from my VW Golf days that increased valve size on the 2 valve GTI motors = cracked heads. On those engines, you cannot increase the intake valve size past the stock 40 mm without cracks eventually forming. I guess this is not a problem with the completely different M28 2 valve heads?

N?

m21sniper

"sounds very trick!!! cant wait to hear what a 928 engine sounds like at 7500rpm!!"

I know what mine sounds like at 7k rpm.

Angry.

Very, very angry.

Just for the record, i only revved it that high once, and it was a 'whoopsie'. By the sounds of it the motor Greg is building should easily be capable of 8,000+ rpm.

slate blue

Hi Normy, I think everything needs to be assessed on its own merits, I haven't heard of these heads cracking. Not from the early days of Devek or Mark Anderson, just the bore sealing and the crankshaft problems. The heads are solid between where the valves go. We have assesed the thicknesses in the head by using a spare we have sacraficed. So I hope there is no issue, remember on the LS1 and the like the valves go right up against eachother, with no issue.

Like another LS1 this engine is also 5785 cc, it has the exact dimensions of the early CR-5 Corvette LeMans engine, that engine made 585 hp through two 34.2 mm air restrictors, It will have everything that that engine had, with the two main variables being headflow (similar sized valves at least) and camshaft timing. On the camshaft timing probably pretty similar also, as air restricted engines don't have a lot of duration and their engine made peak power at 7200 rpm which is where I also should come out.

The guy doing the heads wants 1 7/8 inch primaries but has agreed there is packaging problems and 1 3/4 is better for the street, maybe stepped headers? I better stop, now we are going into a fantasy world.

Cheers Greg