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Vacuum for big cam NA Engine in Idle

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Hi Guys,

i have built a 2 Liter high compression Engine (~14:1) with a 292° intake Cam and a 285° exhaust Cam from Cat Cams


What numbers in vacuum do i need to expect ?

Currently at idle (1200 rpm) the Map sensor reads between 80 and 85 kPa.

The Baro sensor reads 101 kPa.

I think thats quite poor. What do you guys think for this kind of Setup ?

If this is an very unusual number even for big cam egines then, i might need to adjust my cams again.

I would expect you should be able to see 50-60 kPa at idle.

Can you close the throttle more? Are you sure you don't have any intake leaks?

Thanks, i will doublecheck all the gaskets.

Throttle is about 0,5-1,5% open (mostly 1%)

Maybe the Gasket directly behind the throttlebody should be exchanged.

If this does not solve the problem i need to adjust my cams right?

Do a smoke test of the manifold to look for leaks. There are low-cost smoke machines available:


What ignition advance are you using? That makes a BIG difference and you can run a lot more under no, or light, loads compared to the full load timing!

@David, Thanks for the Hint.

@Gord, currently 42° BTDC (Stock Engine and Software it is 47° BTDC). It tought thats a good start :)

That should be plenty, but trying +/- 5 degrees may point towards an improvement.

Manually adjusting the AFR for best idle may also help, if you're using a 'target' the big cam's may be letting unused air through on overlap and, if so, the ECU may be enriching the mixture to try and compensate for what it sees as a 'lean' mixture?

If you have the camshafts timed correctly you shouldn't really need to adjust them. [edit] Oh, just had a thought, you did check the indicated and true TDC crankshaft positions aligned, and the corrected ignition 0 value also co-incided? That's a bit granny-egg, but sometimes missed.

Yes you are right with indicated and true TDC position. Unfortunately i did not check with a timing gun. Currently the TDC position for the ECU was calculated by me in ° , and teeth offset.

By the end of this week i will have it checked with a timing gun and can confirm if the timing was correct or if my calculation was off.

After that i will check with varying the timing in idle by +-5° and check the trends.

When i´m still way off 60kPa i´ll check the cam positions

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