1915cc engine - what is holding it?

[jaqo]

Hi
I took a 1915 cc engine on a dyno and this is the graph:


The spec is:
autolinea aluminium case, full flowed, ported inside.
stock crank, light flywheel, stock size pulley, hd flywheel bolt, 8 dowels, stage one pressure plate - balanced
scat c45 camshaft
stock Length h-beam rods
wiseco pistons  revmaster barrels
manton pushrods
mini-d scat heads 40x37.5 valves, dual valve springs, chromoly retainers
1.1:1 rockers
1 3/4 header with muffler from vw speedshop
welded and ported intake manifolds
drla 40 carbs with 34 or 36 vents - don't remember but I'm pretty sure they're 36mm

The stronger run is without the fanbelt. What can be the cause of that flat part from 5700 till the end? Carbs are too small? Maybe I should change them to 44 idf?
Another amazing thing is what the fan is doing at around 7000rpm.

[fish]

What's your compression and deck height set up, how long are your primaries and secondaries, usually when torque and hp figures drop at the point of intersect you have reached the limit of the combination between cam, heads, carbs, exhaust.

[MC Dyno Don]

The Scat C45 cam is close to an engle 120 and will best suit your needs if you use a smaller exhaust,1 5/8 would be recommended( max) and will aid in better bottom end torque value. The larger 1 3/4  is recommended for higher rpm range, higher flowing heads,larger cam,larger carbs,compression. The larger carbs will benefit the engine better and will help alleviate the flat power range when used with the smaller exhaust. Another limit is the small 40mm intake valve. Hope this helps you? good luck, Dyno

[jaqo]

Deck is 1.2, compression is 9.8:1 or 9.9 if i remember correctly. The power as it is is EXACTLY what I was aiming for with this combo, it drives well, and I don't need to push it more (its a road engine for a young guy, drives veeeery smooth, has a nice power band)  but I was just curious about the flat part - on all the others dyno graphs of different vw engines I saw, the power curve was falling after its peak - here, the flat part is huge. If its just the end of the combination - fine, I'm surprised that it wants to rev that high anyway with this cam, but if there is one thing holding it then maybe its worth a look. It will surely get a powerpulley (it has no problems with cooling now)
About the exhaust being too big - first thing I don't think that the torque values at low revs are bad - its easy to drive smoothly, it has a more modern feel. Second thing is its bought with the future modifications in mind.

Interesting thing about those mini-d heads is that they had almost the same port size as wedge ports we bought earlier. Inlet manifolds needed to be welded in order to match port them.

[Torben Alstrup]

Hello.
A couple of things:
1. Reving a stock crank to 7 grand is stretching it. But since you have built it let it run.
2. Yes the 1 3/4 header is too big for the displacement. That is part of the reason that the power curve flattens out at the top end.
3. The other part of the reason is the Mini d heads. The port volume is too big for the valve size and displacement. The heads can make power, but the cam needs to be wilder so the peak hp is going to be another 1000 rpm. What happens is that the air speed in the ports never reach the max efficiency, so both the torque and the hp figures suffers to some extend.
4. A set of 40 Dells with 34 mm venturies will keep up to 140ish hp without problems, so there is still room for improvement in those.

5. Now, it´s been a while since I have built an engine around a c45 with stock rockers. That cam works very well with 1,25 or 1,3 rockers. Usually I get about 68-70 hp/l and 95 -105 Nm per litre in similar set up, with 1,25 rockers though.

6. In your case there is no doubt that the heads are overscavenging. In order to reduce that problem you could add 1,25 rockers on intake alone. If youre lucky the intake flow will improve enough to make the heads work better, balancing the scavening so that both power and torque will increase from about 2800 rpm and up.

7. Are you running a porsche fan ? 15+ hp drag from a stock fan is very unusual (?)

T

[Chris bugster]

It is running a standard VW T1 fan housing with VW pulley. It had a CB steel pulley, but it was poorly made and leaked so went back to a 40+ year old VW pulley.
Anyway, as Jas wrote earlier, the engine was built with future upgrades in mind so that is why the heads and exhaust are somewhat on the large side. The exhaust is a stainless steel item and so rather expensive.
There is a set of 1.25 rockers on the shelf waiting for this engine and I suppose that they will be the next upgrade and at 68-70hp/litre, that equates to about 130-134hp which sounds realistic as it is already making 124hp.

[Zach Gomulka]

I agree what others have said. 1 5/8", 1.25:1 rockers, 36mm vents.

[Chris bugster]

Thanks everyone.