Heads for my 888: 748 or 916?

jakepeewee

Hi guys,

I want to prepare some heads with bigger valves for my 888SP3.
A while ago I ran into a couple of 748 heads. I recently bought 36 mm and 30 mm intake and exhaust valves that I want to use in these 748 heads.

However the mechanic told me it was way easier to fit these valves in 916 heads than in these 748 heads, because he would be able to use the current seats in the 916 heads.

I was under the impression that the 748 heads have a more compact design so this should result in a higher CR.

Can anybody tell me what the difference will be between the 916 and 748 heads? Are the 748 heads worth the trouble?

TIA,
Jacco

marvin

heads

jakepeewee wroteHi guys, I want to prepare some heads with bigger valves for my 888SP3. A while ago I ran into a couple of 748 heads. I recently bought 36 mm and 30 mm intake and exhaust valves that I want to use in these 748 heads. However the mechanic told me it was way easier to fit these valves in 916 heads than in these 748 heads, because he would be able to use the current seats in the 916 heads. I was under the impression that the 748 heads have a more compact design so this should result in a higher CR. Can anybody tell me what the difference will be between the 916 and 748 heads? Are the 748 heads worth the trouble? TIA, Jacco

Eons ago I used 748 heads on my '92, 851. I used 95mm cylinders and Pistal pistons which made a 907CC engine. The 748 heads worked well because the valves are not sunk in the heads as far so a higher compression ratio is made However the heads are designed/cut for a much smaller bore so one has to "unshroud" around the valves ad match the heads to the larger bore. I don't know about the seats as 916 and 748 have the same size valve heads. FWIW, after unshrouding and with the 95mm bore I CC'd the heads and came up with 12.3-1 CR That CR is nice but I don't know if it is worth all of the extra work.??

got2Lev8

Jacco

FYI The 851/888/916 etc. valves will not work in 748 heads as the 748 valves are longer.

Robert

marvin

got2Lev8 wroteJacco FYI The 851/888/916 etc. valves will not work in 748 heads as the 748 valves are longer. Robert

Yes, same diameter, different length.

brad black

i'd use the 748 heads. seats are the same in both, both will take 35mm valves without seat replacement.

marvin

748 head

Here's a pic of one of my 748 heads after removing material around valves and opening combustion chamber up to match 95mm bore

griff851

I used 748 drop cam heads on the 888. Gave me 12.2:1 static comp. Seems to go hard enough to put the willies up me. Runs out of puff at about 260kmph and bangs through to 11500 in all gears, with ability to power lift the front wheel in 1st, 2nd and 3rd. 34mm inlets. I didnt remove the chamber triangular bits as much as Marvin has, just cleaned them up.

jakepeewee

got2Lev8 wroteJacco FYI The 851/888/916 etc. valves will not work in 748 heads as the 748 valves are longer. Robert

Now I am a little worried and confused!
The valves I bought are 916SPS ones, I understand these won't fit in the 748 heads.
I spoke with the tuner to make sure the 916 heads are the way to go, but he now told me that 748R heads are perhaps even better to use. These heads are allready prepared for 36/30 valves and require just lappeing the valves and of course the port job. But these heads require different valves??? He should know that, shouldn't he?

Thanks for the info!

Addition:
Just found out that the 916SPS valves are also suited for the 748R heads.
Heard that some 748 heads have an altered design (stepped design), these heads require different valves (intake or outlet, don't know which ones).

jakepeewee

marvin wroteEons ago I used 748 heads on my '92, 851. I used 95mm cylinders and Pistal pistons which made a 907CC engine. The 748 heads worked well because the valves are not sunk in the heads as far so a higher compression ratio is made However the heads are designed/cut for a much smaller bore so one has to "unshroud" around the valves ad match the heads to the larger bore. I don't know about the seats as 916 and 748 have the same size valve heads. FWIW, after unshrouding and with the 95mm bore I CC'd the heads and came up with 12.3-1 CR That CR is nice but I don't know if it is worth all of the extra work.??

Marvin,
That sounds a bit like my bike: also 95 mm cylinders, but with JE pistons. At first this resulted for my SP3 in 103 RWHP. Later on I installed G inlet cams (SP5), bumped the peak hp just a little to 106 RWHP, but also better torque all through the rev range. It just ran out of breath at higher rpm's, hence the bigger valves and port job (and in addition to that also 748 head design). Don't know what bump to expect, hoping for 115 RWHP.

marvin

heads

It is only the later 748 heads that are droop heads.

marvin

cams/heads

FWIW, My take on it is that the G cams are not the way to go. Ducati tried this for years in their Corse bikes and they blew up like popcorn. Those G cams don't make power until revved really high,,,"poof".

I prefer the SPS (or V2 604, SPS copy but billet) which drops the power band down into tolerable rev range. Just my 2 cents!

jakepeewee

Marvin,
I know that it's better to use the later style cam with it's shorter duration en higher lift. If I run into a set op 748R cams later on, that would be nice. But in the mean time this is what I got.

Friend of mine got an SP5 which runs really sweet, love that motor. And I like to have more power on top because that is what lacking right now. Dyno curve is really flat from 8200 rpm up til 10500, I think it should be possible to add a couple of horses in that part of the rev range.

See the graph I attached

powercurve 888sp3.pdf

28.41KB

brad black

quite possible the extra fuel pressure has made it too rich. that hurts them at the top end on a dyno ime.

jakepeewee

Brad,
I don't think that is the case. See attached graph for the A/F ratio. Don't know if it's very accurate since it is done bij sticking the probe in one of the cans.

Remember, these are still my original SP3 heads so 33/29 mm valves. Timing probably a little too advanced with 48.5 and 49 degrees.

AF 888sp3.pdf

20.17KB

jakepeewee

Brad,
Can you recommend centerline settings for the G inlet and A exhaust cams?
Just read your piece about what the centerline is, up till now I just didn't understand what it was, but now I understand 😱

You recommend 108/108 for strada cams for 851, but what about these SP5 cams? And I will have to look out for valve clearance I guess if advancing further.

Jacco

vij

Not an answer to your question at all but a desmo quattro seems to like inlet VLV closing at about 60 degrees ATDC and exhaust opening about 60 degrees BBDC. This is measured at 1mm lift. This seems to be a good compromises for street use. If you go very high on compression like 14/1 you might want to retard the inlets maybe 10 degrees to reduce the cylinder pressure. This is a good guide for STD cams, 996sps cams 748r and 748rs inlets cams. If it works on long duration cams like a or g I could not tell you. It would create a massive overlap that might hurt the performance.

Jocke

Sent from my SM-G935F using Tapatalk

brad black

however they lie when you put them on the bench after you fit some 996sps.

http://www.bikeboy.org/955kitted748916.html is all i've ever done with them. well, i did an 888sp5 once, not sure what to now.

http://www.bikeboy.org/916bigbore.html was set to 93/104

jakepeewee

Jocke, Brad,
Thanks for the info!

Jacco

Mr.R

Not going as deep into tuning as the other guys but...
Instead of using 748 heads to get a smaller combustion chamber, you could use either '92 851, later 888 or 916 heads and skim 1mm off of the head gasket face, this will result in 12-1+ comp' ratio without the need to unshroud the valves and you can use the valves that you've already bought.

Steve R

jakepeewee

Mr.R wroteNot going as deep into tuning as the other guys but... Instead of using 748 heads to get a smaller combustion chamber, you could use either '92 851, later 888 or 916 heads and skim 1mm off of the head gasket face, this will result in 12-1+ comp' ratio without the need to unshroud the valves and you can use the valves that you've already bought. Steve R

Steve,
I understand but I want to keep my 888 heads as is, that's why I originally collected those 748 heads (for very little money). Thought these would be a good base for tuning. Now I am gonna use 748R heads since they are allready prepared for the 36/30 valves. The valves arrived last Friday, next saturday I am going to drop off the valves and the intake manifolds at the tuner so he can prepare the 748R heads :-)

I am also keeping an eye out for some 748R cams, but that will be for some time later (tight budget).

Thanks,
Jacco