Help on Camshaft choice please, 3.8, ITB's, std bottom end

[sn31496]

Hello All,

I'd be grateful for some discussion, advice, pointers on cam selection for my build please. I thought I knew where i was headed but now got somewhat confused.

What car? 1989 964 stripped out, just for fun and maybe occasional track days with close ratio 6 speed and failry tight diff. Plan is to make something fairly raucous and a blast to drive the doesn't need to be that tolerable day to day. Putting it together in California but might not use it much here....might take it home to the UK (fuel related point!).

Engine so far: I have a 70k mile motor which I want to use as basis without splitting case. Parts I have: mahle 3.8 pistons and cylinders (11.4 CR) (might have to lower that a bit, thoughts?), was planning on getting my standard heads reworked by Xtreme, have some AT Power ITBs (45mm), want to use 996 GT3 early intake plenum / resonance chambers, no PAS pump, doesn't have to meet smog etc, have SCS Delta ECU (similar spec to Motec I believe), GT3 muffler. 7k rev limit.

So good parts I think but not very ambitious redline because I'm not ready to split engine open and upgrade bottom end. Wanted ITBs as found good used set, throttle response and to widen choice of cams.

I need to spec CAMS (and headers).

Me: Suspension engineer....not an engine person hence advice welcomed!


I have tryed to research as much as possible so as not to waste forum time but now bit stuck. I find so many respected builders of these engines have very different philosophys and do well evolving them into impressive engines. I get the value of developing the engine holistically with one builder but I can't afford to send it off to a shop so have assembled a collection of what i think are complemntary parts and am doing it myself.

I got as far discussing and picking a cam with John at Dougherty Racing Camshafts but having talked to others having some second thoughts.

I have a set of DC35-24 cams from John.


Duraton at 1mm Duraton at .050" Valve lift Lobe seperation angle

258/244 252/238 .490"/.460" 113


Other builders / suppliers of cams I have spoken to, when I have outlined my build, swear by their cams something more like:

Duration: over 270 on inlet and a LSA of well under 110
~

Which seems like great departure.


Recognise the cam defines the engine so any thoughts would be appreciated.

Thanks!

[jpnovak]

my $0.02. I think you will want a custom grind. The process goes like this.

If you have Extreme do the heads to support airflow for your extra displacement... Then use the flow data to set the lift limit for your cam.

You mention that 7K is your limit (agree with stock bottom end). Then pick your exhaust duration to set this limit. The ramp rates would then be chosen based on lift and duration profiles.

With EFI and ITBs I typically like about 108 deg lobe center. If more narrow your vacuum signals become noisy. Not insurmountable but something that can be avoided up front. Some ECUs are better than others at dealing with speed-density vs alpha-n algorithms and blends of load detection.

[winders]

sn,

First, I would not do what you are doing without rebuilding the bottom end.

If you want to run pump gas in California I would go no more than a real 10.5:1 unless you want to track down a gas station that sell VP101.

AT Power ITBs come in 42mm and 45mm sizes. Well, you can special order 48mm but I don't think any of those are on the used market yet. 42mm are too small for your build so let's hope they 45mm. The 45mm come in either 42mm and 44mm inlet port versions. Maybe that is what you meant.

Are those DC35-24 cams Turbo cams? Not the greatest for the motor you are putting together.

If that "other builder" is William Knight, you should listen to him. Because his cam recommendations are fantastic. That cam you mention, if it is from William, it would be great in the engine your are building. I suspect you weren't supposed to post those specs. Anyway, I have those cams in my race 3.6L with with AT Power ITBs, Xtreme heads, 11.4:1 comp pistons, GT3 Cup Motorsports crank, Carrillo rods, etc. The motor rocks.

Oh, I would look at getting Injector Dynamics ID1050x fuel injectors.

[sn31496]

Thanks both for replies. Appreciate you taking time to comment.

First impressions on the first reply, was surprised and daunted by idea of speccing custom camshaft.

I would have thought that almost every iteration of likely cam was commercially available these days. Maybe you suggest this because of unusual use of ITB's but only a standard rev range? Either way as i said....daunted! But I found an interesting article on supertec website about speccing a camshaft....which im working my way through! I hope I can get all the data i need eg will Extreme give me the flow data etc. We'll see.

I do worry though that I won't catch everything that needs to be factored in and so lack the balls to commit to my own design. I will do the exercise for interest and comparison but then will probably pick a recipe / parts someone else has proved to work and buy the key elements from them.

As I've looked into cams and engine architecture the variables and the relationship between the variables seem almost infinite and without being able to simulate the engine accurately or the years of expensive development and experience I concede and recognise my limits. (Makes my head spin: CR, static CR, volumetric efficiency, reversion, overlap, fuel type, intake velocity exhaust velocity rpm range etc etc) I also think not all successful builders fully understand / need to understand absolutely all this either and have discovered combinations that work and developed from there.

Hence I'm drawn now to endorsements such as Winders. Thank you for you comments I may follow up in that direction. Good to hear of success stories and great engines.

Decide will probably revisit bottom end....though CR is still not clear. Even with long duration cams (and I'll have knock sensing at reasonable rate) people are advocating mid 10s for this in the US even with long duration cams. If I build it here but only run it in the UK on true 98 RON feel like I'd be leaving performance on table unless its 11.1 or so (seems favored by UK builders) with comments like even this is low for the UK but leaves headroom to tune with timing advance etc. In the US is it caution with respect to detonation or because they've found lower CR works with their parts / how they like to tune?

Sorry if I'm over thinking this! Eventually I'll just shut up and buy some parts and get on with it!

[winders]

I would never dream of designing my own cams for a 911. There is more to it than just knowing the data points. My race engine would have much different cams if I followed a cookbook recipe.

William Knight is much more than an engine builder. He is an engine architect. He knows what makes air-cooled 911 engines work best and understands the nuances of cam design. Especially when operating with unusual parameters.

98 Ron fuel is equivalent to US 93 premium. If you want to keep your engine safe, don't go past a real 10.5:1. Also, a 0.5 point increase in compression will hardly be noticeable anyway.