1967 cc with 40IDf's heads or carbs limiting?

[bcrazy]

I have the following engine in my 912

1967cc (74x92)
1.5/8 4-1 exhaust
9.5:1
Webcam 86A [email protected]", intake 0,435" exhaust 0,370" Advanced 2 deg.
Stock 912 heads, sligthly ported, 3 angle valve job. Back cut intake valves. Intake 38 mm exhaust 34 mm
2x40IDF with 36 mm venturis.

The engine was uprated from 1720 cc last year. Had the same specs on the 1720, and that engine pulled willingly to 7k. Now with the bigger capacity it still pulls to 7k, but drops of faster after about 5500. Mid and lowrange has been vastly improved, but top end not that much.

So I am thinking that maybe som 44 IDF will help with top end power, or is the heads and not the carbs that is the bottleneck?

[mark tucker]

I would gess heads,carbs&cam.stick some ratio rockers on it to get some more hangtime. and mildly ported leaves a lot open.or not a lot open.Im not familuar with that head.the 44's may help it, but I doubt it would do all that much as that is still a small motor,and a small cam for 44's to perform real well.if you keep trying to get iot to make power where it dosent it will scatter it self and then you will have to do some work. first try the rockers, that will tell you if the cam needs to be biger and the eads need some more work.if the carbs are already being out flowed the power should stay about the same or go lower in the power band due to the more lift&durating at a lower rpm flowing more and running out of carb sooner.but I dont think your running out of carb.then there is the fuel, is there enough fuell pressure to keep the bowels full at wot ?? or is the more cc, s using up the fuell at 5000 and not being kept up with.thus the sag in performance at higher rpm.

[Alstrup]

[bcrazy]

OK, I do not have the original 40PII's.
The 1720 was kind of weak down low (I also had 34 mm venturis in the carbs then), so that whas the reason for keeping the cam, as i wanted more low end torque.

I realise that I can not have both without some compromise, but if 44 IDF's will gain me some more top end, I will try that. The 36 mm venturis I have in the 40's also makes it a bit diffcult to tune. According to my Wideband it does not run out of fuel at high rpm's

Where can you get ratio rockers for the 912 heads?

[mharney]

36mm venturis should be rolling off around 6000 RPM with a 2 liter displacement. But an 86a isn't a 7000 RPM cam. It is about a 6000 RPM cam tops, so your power will be going away around there. Revving to 7K doesn't mean power to 7K, though you might have thought it felt like it. An 86b cam powers you to about 6500 RPM strong. Heads could have something to do with it too depending on the port work. Bigger displacement means more air and it may be the limit.

[bcrazy]

Yes I know I don't have peak power at 7k, but with the 1720 it felt like peak power at 6-6.5, now peak power feels more like 5.5-6k.

It's not like this is a big surprise to me, just asking if bigger carbs will help. Changing the cam, or more head work is not an option. (atleast not yet)

[jfats808]

The main thing I see possibly holding u back initially would be your exhaust. You need a merged set up not a 4-1 if you want good high rpm flow. The heads dependant on the port work is more than capable of flowing decently with the rest of your proposed components. Bigger carbs should give you a few hundred rpm upper range, but your right around the threshold of the heads capability. My take on it.
_________________
2276 IDA's 86C 11-1 DD !
2017 48 Trijet DRLA's W125

[bcrazy]

This is the exhaust (modified to fit the 912 heads), and muffler and not stinger.

http://www2.cip1.com/ProductDetails.asp?ProductCode=ACC%2DD%2D2008

[jfats808]

Ahh looks like its merged.
_________________
2276 IDA's 86C 11-1 DD !
2017 48 Trijet DRLA's W125

[bcrazy]

Well i just bought a set of nicely used Dellorto 45DRLA's, they came with an extra set of venturis, so i have the choice beetween 36 or 38 mm.
The 38 mm are mounted now. I'm thinking they are a bit too big, but i might try them anyway. My previous expericence with dellorto carbs is that they are much easier to tune, and more driveable than webers, but that may be just bad luck with the weber carbs i've dealt with.

The 40IDFs on the car now is for sure quite worn (loose throttle shafts) so it will be fun to see the change to the dellortos.

Btw a stock 912 head flows about 140CFM intake and 115CFM exhaust @28inch
Mine has a 3 angle valve job, and thinned downed intake guide bosses.

[bcrazy]

Mounted the 45 DRLA's today. Vent with the 36 mm venturis.
Main jet is a little lean with 140, and around 14:1 at WOT from 2500-4000
Will try with 145. Driveability has improved quite a bit below 2k. The 36 mm venturis in the webers probably did make it more difficult to tune, but I also think the 5th progression hole in the dellorto's has something to do with this.

Top end is slightly better from 5k and it pulls more willingly to 6.5 now.
Jetting 60 idle, 140main, 180air.

[mightymouse]

Valve spring pressure.

What springs are in these heads?

Dual or single?
_________________
Nothing can stop the man with the right mental attitude from achieving his goal; nothing on earth can help the man with the wrong mental attitude.
Thomas Jefferson


Note to EVERYONE.
Know your ZDDP levels or you WILL lose a cam and lifters.

[bcrazy]

Stock 912 springs(single). If i remember correctly at max lift spring pressure is about 100kg, installed pressure about 35 kg. In a stock engine they are good to around 6500. I have aluminium pushrods wich weigh 44 grams. The stock steel pushrods are 69

[mightymouse]

Not gonna work. You need to get dual springs in there asap, or some serious (pacaloy) singles.

You have about half the spring youd need with that cam.
_________________
Nothing can stop the man with the right mental attitude from achieving his goal; nothing on earth can help the man with the wrong mental attitude.
Thomas Jefferson


Note to EVERYONE.
Know your ZDDP levels or you WILL lose a cam and lifters.

[bcrazy]

Well I had the engine apart last winter. Cam looks fine, so does the lifters, valves and rockers. Only about 15.000km on the cam and lifters but they seem to hold up fine. I do not rev past 6.5k on a regular basis.

[Alstrup]

[DarthWeber]

[bcrazy]

Springs are new.

[mightymouse]

Id bet they are floating up top. Thats why the 5500 nose over. Seen it over and over in my experiences.

MY personal engine did exactly the same thing when i was like 21. I couldnt figure it out. A friend/guru stopped by to help me. stuff like 100 thou under my dual springs... wallah! no more float/low rpm nose over.

After that i was shifting that engine at 8500 on nitrous. so.... yeah.

You may have something entirely different going on and unless one of us was there to help we are just shooting in the dark.

But the cam you have is similar to engle 120... and i dont know anyone who would run any single spring with a cam that size.
_________________
Nothing can stop the man with the right mental attitude from achieving his goal; nothing on earth can help the man with the wrong mental attitude.
Thomas Jefferson


Note to EVERYONE.
Know your ZDDP levels or you WILL lose a cam and lifters.

[Alstrup]

Guys, its a 616/912 engine. And apart from the looks they vary greatly from a type 1.
With that cam it´ll rev to 7 safe. But the engine has stopped making power way before that. As earlier menthioned, the 86a is not designed to pull that high up.

T