as title really, in the post about DENNIS' engine speccing, a few people (including martin @ reyland) mentioned that a pair of bd14's would be no good and make for a pretty laggy setup. now my question is why?

i've heard loads of people suggest a bd14/bd10 setup for good low down driveability, or a bd14/std. the bd14 has only 3 degrees shorter duration than the bd10 and has an extra 14 thou lift, surely it wouldn't make the two cams perform too differently to eachother? if anything, wouldn't the extra lift (over the extra duration of the 10) make getting rid of the gasses easier, so therefore moving more air out of the cylinder at any one opening (and in turn having more air to push the exhaust wheel around)?

or am i thinking about this the wrong way? would less lift on a bd10 cause the gas speeds out of the cylinder to increase (due to them being pushed through a smaller hole), therefore spooling the turbo faster that way?

ideas and comments please people?

 

anyone?

 

I think that one bd14 is fine. i cant really give away what martins cam choices are going to be and i doubt that any of the tuners will give you that info either...

Wish someone would clear it up for all of us though because its an intresting point.

Surge has something to do with it aswell i think on big turbos.

Please remeber im still on a learning curve though.

 

Maybe a good topic for one of Stu's fastFord articles (if he's not already covered it?)

 

The best way I can explain it in simple terms is that the more wild the exhaust cam, then the faster it lets the gasses out - which means that at low speeds, this happens too quickly for the full ideal burn process to take place. The milder cam retains the charge in the cylinder longer which gives a better burn, which spins the turbo faster. Obviously from this description, you can see that this also has certain trade-offs and compromises once high rpms are reached... Again it's all swings and roundabouts and is about speccing the engine to be compromised in how the customer wants it to drive / it's intended use .

 

How does staggering the cams work like some of SCS's choices such as BD15 inlet and 14 exhaust. Is it trial and error or what is the theory behind it?

 

SCS choices are borne out of much experience on the dyno of what 'works' and what doesn't.

 

I know that Bill, I'm wondering what the theory is that ties up to it. Cams is something I know very very little about.

 

...i went bd16in and std exh on my t4 engine after bd10s were shit with a t4 but great on a t34 .48 ....on my gt35 i went bd16in bd14 out..no surge!

 

You and me both, lol.

I guess it starts with the 'theory' - as per what Mike was saying about low end vs high end response, then 'trial and error' (albeit the error = poor result, should never make it back to the customer with proper R&D) - all depending on what the car will be used for.

Ultimately, it doesn't actually matter what the cams are, as long as the engine performs as intended.

As Johhny 5 would say - (this thread) needs more input

 

As mike have said, its all up to the rest of the engine, but simple put is because of the flow caracteristics of the cossie head then it doesent need a wild exhaust cam because the flow in the exhaust chanals is so much better then the inlet chanels that you can use a milder cam to get the same power up high in the revs, but the same time if u want some low down power you have to "hold" on the exhaust gasses to get the speed/temp up so the engine uses the gases better.
some sweedes have actually gotten over 600bhp out of an cossie with std exhaust cam, but if u upgrade it on the same engine to a bd-14 u loose a little low revs but get a really big kick over 4000rpm.
Basicly it´s up to the person to find out if he wants top end power or midrange, then the head porting and cams can be specced after that.

 

Thnaks for the info mike...

Ive had a couple of people (not tuners) tell me that a standard exhaust cam is the way to go.... Dunno how thats justified though?

 

Thanks cossie manden that answers my exhaust question.

 

maybe ask yourself,,, whats the job of the exhaust cam!

 

...i will let you into a secret a std exhaust cam is known by many as a secret cam

 

LOL

Well it actually WAS until you revilled it

 

i was planning to use a bd16 inlet & bd14 exhaust cam on my engine with a ported head & t38. not to sure now. what power could be expected with bd16 inlet & std exhaust & t38?

 

t38 i reckon a 14in and std out

 

for a gt3076 setup for 450 - 500 horses I was told to get a BD16 INlet and a BD15 EXh cam by Stu, I didn't ask either, all that was told is that I want to have a red line not higher as 7500 rpm.

 

Hey sim.
that is also a good setup for your car, as it´s primarely used at the ring, and that track needs the output to be at high rpm.
I would use an bd14 exh tho, to get a little more bottom, but that also depends on how your top is ported and HOW the cams are adjusted (factory or custom)

 

I wouldnt go for more then bd-14 in and maybe bd-10 exh.
this would easy make enough air to max out the t38

 

PORTED PORTED PORTED PORTED

shall be a fast gas change

looking forward to get the engine ready till mid of next month.... just waiting for pistons