View original topic: The making of cylinderheads for an Okrasa engine on steroids Page: 1, 2  Next
Alstrup Sat Jul 17, 2021 1:10 pm

I hardly ever share direct info about what I do, but this time I decided that I will make an exception. So this will be a thread about making the Okrasa heads better and suitable for some good street power out of an Okrasa engine.
The engine will ultimately be a 69,5 X 83 mm (1503 cc)
Carburetion will be Solex 40PII on Speedwell manifolds. The rest is not determined yet.

To get a baseline on just how much is needed to make the heads support 100-110 hp with peak around 5500 rpm I wantyed to see how they flowed - almost - out of the box. I say this because the customer already had a competion 3 angle valve job performed on them. As at least some of you will notice the improved valve seats improved with about 4 cfm flow up to about 8 mm (0,315") After that it reduces significance. The last point in the graph is without a valve to see how much the port will support as is. Interestingly enough the multi angle valve job almost makes the port flow its maximum. Only about 8 Cfm reserve. Another interesting thing is that the small 30 mm exhaust valve actually outflows the intake at 0,150" - 0,250" lift with the multi angle seat cut.
This also explains why even a bone stock Okrasa engine wakes up significantly by just increasing displacement from 77 to 80 mm bore.
This test is made with an 80 mm bore cylinder.

All flow numbers are at 28"

The Okrasa engines are somewhat limited wrt cam lift. The valve lift can be handled with ratio rockers. But knowing the behaviure of an unwelded Okrasa port there is not much idea in lifting the valve more than about 0,450 - max 0500". Also, in order to get the engine to flow enough for 105 110ish hp at that lift and approx 5500 rpm we need the intake to flow around 110 cfm @ 0,450" with good port velocity and port energy. That should be possible with a 36 mm valve. We may go to 37 mm as I may have a set of such on the shelf from a Lotus engine that I can convert. More on that later.
So this is where we start from.

Erik G Sat Jul 17, 2021 1:37 pm

awesome. looking forward to this

henry roberts Sat Jul 17, 2021 7:04 pm

this will be good.

any welding on the cards?

Alstrup Sat Jul 17, 2021 11:56 pm

No, this one will be without welding. When the time comes we will design a piston that protrudes into the chamber to get the CR we want.

evanfrucht Sun Jul 18, 2021 10:55 am

Alstrup wrote: No, this one will be without welding. When the time comes we will design a piston that protrudes into the chamber to get the CR we want.
Sounds like a fun project. Cast or forged?

Alstrup Sun Jul 18, 2021 11:47 am

Not sure yet. I am debating with myself about making a small series of say 10 set hyper cast pistons as they would be more than strong enough. But as mentioned, no decissions are made.

sled Sun Jul 18, 2021 12:06 pm

given that there is little to no quench/squish left in that chamber, are you going to open the chambers to the cylinder walls and treat is more as a "hemi" chamber with a fully domed piston?

Alstrup Sun Jul 18, 2021 12:19 pm

Basicly yes.

Bengt H Sun Jul 18, 2021 1:03 pm

This will be interesting :D

oldslow Mon Jul 19, 2021 5:08 am

Looking forward to this information 👍

Alstrup Mon Jul 19, 2021 8:19 am

One of the issues with Okrasa heads from a tuning standpoint is, that due to the plug location you canŽt flycut them much.
Personally I much prefer plugs with protruded electrodes, but that is basicly a no go. Even with a non flycut head you will only have the piston to cylinderhead clearance. With a std type plug it helps somewhat with about 2,5 mm clearance. So even with 83 mm bore and 69,5 mm stroke other measues have to be done to increase CR to anything resembling a more modern engine configuration. The chamber volume in an untouched WW Okrasa head is approx 46 cc. So with a 1503 cc displament and 1 mm deck height that results in a wimpy 8,3 - 1. So when I have finished shaping the chamber for larger valves and improving wall side flow it will be even lower. So something has to be done.

But first I need to find out how well I can make the ports flow, and how much valve size I need to use. The challenge with the intake ports are that they are very steep, sort of like in 356 heads. So next step is to make a dummy intake seat for a 35 mm valve and make a port and correct the manifolds and see where that gets us. - Some people will say "Why dont you just install a 37 mm valve and be done with it?" 1 reason really. The easiest thing is to make a big valve flow fair. The downside is, as always, that the port velocity and thereby port energy reduces dramaticly at part load resulting in decreased driveability. If I can get the port flow and characteristics I want with a 35 mm valve I will stay with that. If I need to increase valve size to get it, so be it. Here we are going to make a set of heads for a usable street engine that will see a vintage streetrace event every once in a while, so a wide powerband is the goal.
Now it will be a couple of weeks until my next step is ready. I will post results of the valve size increase and corrected port.

PeteSC Mon Jul 19, 2021 12:59 pm

I'm curious to follow along on this. I had Joe Ruiz enlarge the valve size of my WW heads when he built my engine (80x69.5), going with 35/33 for intake/exhaust. I've never dyno'd the engine so I'd be curious to see what levels of improvement you get. I can tell you that it's immensely fun to drive, WAY more so than the stock 36.

Alstrup Mon Jul 19, 2021 2:47 pm

IMHO that is too much exhaust valve area if you are chasing power. Just look at how this little 30 mm valve and how close it is to the intake up to about 10 mm lift.
But we will know the potential of a 35 mm valve in these heads when I get to it.

5500 Tue Jul 20, 2021 11:49 am

Thank you Alstrup for sharing your information publicly. I read somewhere awhile back that the WW head exhaust port flows much better than the intake right out of the box. Not wanting to take any risk at hurting midrange performance, I stuck with WW valves and ports as delivered with a 3 angle valve job (Combo WW/DPR 1397, 8.3:1 CR,WW cam). I mildly cleaned up the insides of SGs 32 NDIX manifolds and port matched them to the heads. Can the intake flow be improved with manifold tuning alone?

Very curious to see what you do to improve breathing, and what kind of numbers you make. Again, very generous of you to share your information.

Alstrup Tue Jul 20, 2021 1:26 pm

You can almost always do - some - manifold tuning, but as long as the manifolds are reasonably matched to the intake port, and has a minimum of 100 mm (4") from the head flange to the carb flange along with a fair taper it is minimal.
The intake ports in the Okrasa heads are basicly shaped as the ports in a 356 head only in a miniature version, so my approach will be basicly as you would port a 356 head. YouŽll see when we get to that, in a while.

You can of course play with the intake length to find what your engine likes the best.

gimmesomeshelter Fri Jul 23, 2021 9:37 am


May I ask what you plan on using for valve springs? When I looked at springs, it appeared to me that it would be tough to get much more than .4" worth of lift with out installing longer valves or boring recesses into the head. Have you figured this issue out?



Alstrup Fri Jul 23, 2021 4:38 pm

As it is right now I have Coil bind at 0,450" This is with Lotus Coventry springs. When IŽm done the valves will sit 0,040" lower in the heads and the valves will be 0,040" longer and finally, when I chose retainers I can easily find another 0,060"
Valve lift will not be determined until I know how well I can make the heads and manifolds work, but it will most likely end up in the 0,460" area.
As for the final decission on springs, there are Several options, but it will probably be some motorcycle springs from either Honda or Yamaha
More on that as we go along.

scotty timmerman Fri Jul 23, 2021 9:25 pm

What valves and retainers are you using? Are you staying with 7mm stems?

Alstrup Sat Jul 24, 2021 1:59 am

Right now I still have Okrasa valves in them, but the final product will most likely be Ferrea custom valves. Yes I will stasy with the 7 mm stem. I dont want more weight up there than necessary, but I will not sacrifice stability either.
Ti valves Is also in the mix, but I have not discussed it with the end user. With the Ti valves it looks like I stay around 180 psi over the nose and still rev about 7 grand safely.

[email protected] Sat Jul 24, 2021 5:59 pm

Why dont you try 5.5 intake stems and 7mm exhaust stems

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