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What Vacuum Gauge Reading @rb25 With Hks In/ex 256/8.8 Cams?

Stronzo

By Stronzo,
in Engines & Forced Induction

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Stronzo Collaborator

Stronzo

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Hi,

Just installed new rb25 engine (after rebuild) with rb26 bottom end, wiseco forgies and HKS 256/8.8 in/ex camshafts.

The car idles with vacuum gauge reading ~ -0.43...-0.45 kpa which seems a bit low (usually ~0.6). I know that aftermarket cams can bring the vacuum down -- is this figures ok?

The second problem was that after initial ignition and fuel pressure tuning the car idles quite lean 17-18 AFR. Vacuum leakes where not found... Can cams cause this?

Stronzo Collaborator

Stronzo

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Have the cams been dialled in, that vacuum isnt too bad, as it sounds like it hasnt been tuned yet. When you say ignition and fuel pressure tuning, what do you mean exactly? Stock ecu?

the cams was installed with stock camgears -- so they have not been dialed in with a special procedures. as i read in this forums tomei poncams give the best result in default position. hks have very similar to tomei figures so i assumed it`s true for hks too.

ignition tuning is rotating the CAS with a ignition light to get 15 degrees on the crank pulley at idle.

fuel pressure tuning was setting the base fuel pressure at sard fpr (it was brand new and was set to 4 bar instead of 3bar -- but with 4 bar the idle was better).

ecu is power fc.

Dobz Experienced

Dobz

Posted
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Tomei grind their cams at a lobe centre angle that is optimum for their bigger duration to give the right amount of overlap, this allows you to use stock cam pulleys. The HKS cams are ground to standard lobe centre angles, with increased duration the HKS items will be causing too much overlap hence the worse vacuum and the need to dial them in with adjustable cam pulleys.

Stronzo Collaborator

Stronzo

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ok, i understand how to dial in exhaust cam as it is possible to install adjustable cam gear on it, but what to do with intake camshaft -- it has nvcs gear so it can not be adjusted...

lachlanw Veteran

lachlanw

Posted
Posted

intake is adjustable fron the factory . take it out and look at the back of the cam gear or get one of these

http://www.trademe.co.nz/Trade-Me-Motors/C...n-293718049.htm

lotsalag Contributor

lotsalag

Posted
Posted

i read somewhere years ago it can be difficult to set mixtures at idle with big cams.

however, ur cams arent too aggressive, it may need further checking with the afr's u mentioned.

if its a stock computer ur kidding urself expecting it to run like this :thumbsup:

rb26 bottom end? changed stroke/cc etc? this will also not help the poor old stock ecu if thats whats still running(trying to run) it.

Stronzo Collaborator

Stronzo

Posted
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if its a stock computer ur kidding urself expecting it to run like this :thumbsup:

rb26 bottom end? changed stroke/cc etc? this will also not help the poor old stock ecu if thats whats still running(trying to run) it.

it`s not stock, it`s power fc as i mentioned before, so it`s not a big problem.

frozenwaffles Mentor

frozenwaffles

Posted
Posted
Tomei grind their cams at a lobe centre angle that is optimum for their bigger duration to give the right amount of overlap, this allows you to use stock cam pulleys. The HKS cams are ground to standard lobe centre angles, with increased duration the HKS items will be causing too much overlap hence the worse vacuum and the need to dial them in with adjustable cam pulleys.

this is not true, please see the follwoing links for comparison:

Tomei Factory Camshaft Specs

HKS Camshaft Specs

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