853, R and Stuff - Written 07/09
Summary: 748 and 748R with 94mm and 96mm big bore kits and various mods
We did an 853 conversion to a 748R late in 2008 the first 853 kit I’d been involved with. Funny how it’s a popular mod in lots of places, but not Melbourne, Australia.
We used Pistal pistons, the specific 748R ones as the 748R uses small end located Titanium rods. If you fit normal 748 style pistons meant for big end located rods the rods are not held at either end and can move sideways on the crank up to just over 2mm. Chris @ CJS said he pulled apart a 748SPS (Ti rods) that someone else had fitted with std 748/916 big end located rod pistons and although it had good compression, it had a heap of blowby when running and was a bit of a dud. Correct small end located pistons fixed it. The big end located rod pistons have quite a bit of side clearance between the piston and rod at the small end usually 1 to 3mm depending on the piston manufacturer. Whereas small end located rod pistons have about 0.40mm side clearance.
We sent the cylinders out for boring and replating locally and the heads went out for port work. Similar to the last 748R we had ported, except this time we also had some exhaust port work done. Still nothing too major (or expensive).
It went back together with the usual inlet trumpet extensions, throttle body outlet boring and the same 104/108 cam timing we’ve been running on std capacity 748R. I wasn’t sure if that cam timing was going to hold it back at all, but the owner was happy for it to be more midrange than top end orientated.
Before we got it together I did some projections for how much power it might make using the previous 748R with port work results. You can export run data out of the Dynojet software as a text file and play with it in a spreadsheet. So I just multiplied the previous curve by 853/748 and got 130hp. Given that you usually don’t get the full proportional increase in power due to power being an air flow thing and not capacity I was thinking a bit over 120 was realistic. Although the torque should increase from 59 to 68 ft-lb, as torque is almost totally capacity based. And generally the Desmoquattro engines tend to produce more torque per cc the greater the capacity, so it wouldn’t surprise me if it made more than that.
So we got it going and went to the dyno and did some runs to develop a custom eprom to suit, with the WOT runs being fuelling +/- to see what it liked where. Again I exported the data out into a spreadsheet to compare it to the graphs I’d already generated, and found the result to be pretty good. The graph below shows this bike in red, the predicted output in blue, the previous ported 748R in orange, a non ported cam timed, extended trumpet, bored throttle 748R in green and an untouched 748R in yellow. Power first, then torque.
As you can see, the result was a bit more than projected increase in torque and a less than projected increase in power, peaking at 69 ft-lb and 123hp. The extra capacity has also moved the torque peaks down a little. The fall off of torque over 10,000 rpm is quite likely related to some extent to the cam timing we were running, and pulling the inlets back a few degrees to 110 or so should have held the torque better for more power. I discussed this with the owner who was much more midrange orientated and so happy to leave it the way it was. Generally lowering the peak power rpm will help reduce the rpm it’s likely to see in use too and that’s a good thing.
And given he’d ridden in an as delivered 748R and ridden out on this, the change he was going to feel was bigger than a few more hp over 10,000 rpm. The graph below shows the change from one previous as delivered 748R I’d dynod compared to this one. The owner didn’t actually think it felt a lot faster, but was quite surprised by how fast it was going when he looked down at the speedo and by how fast he found himself going into corners. This is often the response you get when you deliver a flat torque curve, even though it now has at least 30% more torque for much of the rpm range.
It also makes surprising torque for its capacity. Compare it to a cam timed 996 or 996SPS or 998S which usually make 75 76 ft-lb, giving them around 76 ft-lb per litre, this 853R makes 69 ft-lb, giving it 81 ft-lb per litre. I think this has a lot to do with the longer duration exhaust cam, but also typically of the other longer exhaust cam motors it has the midrange hole from 4,000 to 7,000-ish. The graph below compares it to a cam timed 996. It doesn’t catch the 996 until after the 996’s power peak, but it’s still pretty impressive.
One of my customers has a 748 Strada he fitted an 853 kit to. He set the cam timing at 108/108 and had fitted one of Doug Lofgren’s 853 eproms as sold by Ultimap the UM097DL. Apart from that it was std with slip ons, in this case the old Skorpian pipes which are now know as Akropovic. We ran it on the dyno to see how the fuelling worked for this cam timing, but due to a couple of issues only gave it one WOT run. So it may have made a little more power than this as it often takes 3 or 4 runs for the power to settle. Also the fuelling was pretty good for an eprom that was developed on a bike with std cam timing. Graphs below show power then torque and air/fuel.
The owner of the above bike has a mate who had a 748 that was 890’d years ago. 890cc being 748 with 96mm bore, which usually requires the cases to be split and the holes the cylinders go into being bored out 2mm to take the 96mm bore cylinders. Some people say you don’t need to use the thicker cylinders, but it’s a debate I have no interest in getting into. It had Vee Two heads with 37mm inlet and 31mm exhaust valves and Vee Two grind 431 inlet and G exhaust cams. I worked on it years ago and in my opinion it was a bit of a dud. It had fairly low compression even with reasonable leak down and the valve reliefs in the pistons were so big you could wind the exhaust cam all the way around with the piston at TDC. I was thinking it might have had 955 pistons in it (shorter to suit the 916’s longer stroke), but the squish was apparently right so that wasn’t it. The owner had reset the cam timing after someone else had had a go at it at some point, using the Vee Two specs of 108/106 (or close to).
I dynod it the same day as the above 853 just to see if there was any point getting into tuning it, as it had been running either of a couple of Vee Two supplied FIM compatible eproms that were 916 Strada based. I set it up and then added fuel via the FIM HHT as even with the trimmer at full rich it still needed 10% more to get to 5% CO. We ran it on the dyno like that and it was too rich in the midrange, but about right-ish up top. And still a dud, unfortunately. It didn’t make a whole lot more midrange leaned out like I thought it might. Like most big cam things it wanted to rev and clearly would have held the power well over 11,000 rpm, but the owner rarely goes past 10,000 as he just track days it for fun. The graph below shows the various fuelling runs. We didn’t go past 7,000 with it leaned out as it was getting very lean.
Compared to the 853R and 853 above it’s not good for all the effort put into it. 890 is blue, 853 red and 853R green.
Although that’s now changed. After we’d done these runs the 890 owner rang to ask about making it 955cc and I said that if you were going to bother going 955 (fitting a 66mm stroke crank) I’d just use a 900 2V 68mm stroke crank to give 984cc as with the valves and cams he had the thing he needed most was airflow and airflow comes from displacement. And he doesn’t rev it anyway so there’s no point worrying about the increased piston speed if you’re only going to 10 or so and that will also help the valve train survive and it’s a good idea all round.
So he bought a 900 crank and set about modifying his 890 pistons by machining about 3.25mm off the top of them. While this may sound a bit drastic, the 890 pistons were so thick in the crowns to allow the very deep valve reliefs that they’re probably now down to around the 955 piston size they’re most likely patterned from. And it also has some compression now ending up around 13:1 with some crown mods as well to reduce it from the 14:1 or so it would have been with the modified pistons and 68mm crank. All of which should make it something like it should have been originally, but with another 94cc. I’ll let you know how it goes.