Hi Gareth

No, not really - I guess if I was going to do that, I would replace the pistons with low comp pistons.

I wanted to understand more about what makes a good engine good, particularly when building a very high power 2.0l cosworth engine. I dont mind admitting that when I next get bored, I may look for the next step, but mainly I want to understand what does what! (a big topic indeed i suspect! LOL)

So people say that Low Comp can be made to be very good indeed, I am just curious as to what makes a good one good, and what makes a bad one bad, given that Std comp is not necessarily better than low.

JJ

what makes a good low compression engine, is head porting/cam profiles and timing!,,,(and of cause turbo selection)

everything must work as a package,,, alot of home built low compression engines could be make ALOT better by just having there cam timing optomised
:top:

agree totally.

most people dont seem to have a clue about cams and the effects of lift and duration on boost threshold, powerband, and the engine spec they would work best in, never mind timing.

most people are like "yeah, sling BD10/BD14/BD5895607834593657 in it" but they look at you blankly if you asked why or what lift and duration they are.

This makes perfect sense - so really the matter is down to idiot user rather than mechanicals then.

Mind you, I wouldnt mind a crack with a pair of BD5895607834593657's. Do you recommend both inlet and exhaust? :cry:

So it sounds like the technical stuff relates to the cylinder head and what lies within. This makes sense.

But lets turn back to the turbo. Given that we know that we can create the 500bhp realtively easily by simply bolting on a massive turbo, and that by putting a small turbo on the engine it restricts the flow of air, how do the improvements on the cylinder head get over this obstacle?

JJ

Don't forget that the turbo's exhaust housing has a massive effect on the engines Volumetric efficiency ;)

the cylinder head will flow more because of less resistance, but it would never completely get over the problem of a too small exhaust housing

of cause before of a rise in TIP (turbine inlet pressure) :top:

Ok - basically we are saying that the Turbo is a restriction, but not the only one!

For example, if the engine was in fact a straight piece of tube connecting the inlet manifold to the exhaust housing of the turbo, then at 2bar of pressure for a given turbo, the air flow would be substantially improved over that of a cylinder head.

As above, therefore, when you increase the duration of the valve opening on the cams, you are increasing the amount of time that air can be flowed, and thus the volume efficiency. Likewise, as the lift increases, same deal.

Is this right?

If so, what is the flip side of the coin? Clearly if it was one way traffic, we would all have valves the size of dustbin lids :cry:

This is a useful topic for me - I am beginning to understand a good deal more about this subject, so thanks to all for contributing :top:

...you want a high lift cam in....and a std (secret :cry: ) cam out ;) ..then timed to perfection....also you want bigger inlet ports than exhaust ports :top:

there is a fine balance between valve duration, go too much and you'll be pumping nice air straight into the exhaust because your valve overlap has increased! (both valves open at the same time) of cause you can increase cam seperation angle, but this also has downfalls, (moving peak power up higher)

cams are a twat in a 4 stroke engine :cry:

Hi Phil - Yes I had heard that this was the correct combination of cams, and clearly the std setup of inlet being bigger than exh is as you suggest, so one would assume that you would increase this kinda proportionally, if at all.


Hi Gareth

So if the cam separation angle is increased, this leads to the peak power moving higher up the rev range. I guess this is why people then need to prep the engines to be able to rev higher to allow a more useable power band. So what sets the width of the power band out of curiosity?

I guess that the reason the inlet has to be bigger is that it does not benefit from the combustion to assist with the movement of the air and fuel into the cylinder? Would this be accurate?

JJ

increased valve overlap will lead to a high boost threshold, you sort of got what i was trying to say slightly wrong!
wide cam profile = more valve overlap= raised boost threshold,
to get over the raised boost threshold, which could increase lobe seperation, but then this equals raised peak power

the inlet benefit by having the cylinder creating a higher pressure difference across the inlet valve! ;)

:oops: You may have lost me - sorry.

If I understand right.....

Wide cam profile = how long the valve us open (??). Is this the same as duration?
When wider, it means that the time when both valves are open is greater (?).
This raises the boost threshold - which is bad.

Boost threshold? :oops:


Must admit, I didnt get the bit about the inlet benefiting from the cylinder creating pressure difference accross the inlet valve.

Sorry for being thick :oops:

JJ

you get less fuel to release after the combustion take's place,neat fuel is burnt so less fuel to pass after the combustion cycle....hence exhaust is smaller :top:

like ive said before,, im not the best at explaining myself,, sorry,, its not you being thick :top:

http://auto.howstuffworks.com/camshaft.htm


a picture can speak a 1000 (bad :cry: ) words :top:


http://www.compcams.com/Base/Images/...ration-002.gif

yes,,,, boost threshold, is the point at which the turbo makes boost (what most people call lag,, but it isnt)

you increase lobe seperation by timing full lift further appart (advancing the cam timing) so where normally 110 degree full lift happens you time it at 115 degrees maybe :top:

...turbo lag is actually the time it takes to regain full boost after you change gear! :DD: ...hence anti lag is a good thing :DD:

Hi Phil

Thanks matey - the diff in exhaust and inlet now makes sense to me now.

Hi Gareth

That graph is very useful indeed :top:. I will have a read of the article right now.

I appreciate you taking the time to try to explain it to me at all :top:

Now you explain it, I do remember boost threshold, so that now makes sense. Clearly this is exactly the opposite of part of what we are trying to achieve.

Lets leave cams to one side for a moment.

HOw much difference does the porting of the head make on its own.

Lets take my engine for example (more so I can take as read what I have already).

Lets say that all other things generally remain equal (ie comp ratio).

Lets say that my engine produces 400bhp on std comp.

So now, I port the head (both inlet and exhaust sides) to improve the flow as much as possible. BUT at this stage, everything else remains the same.

Assume that the map is re-optimised to avoid problems, what will happen overall to the engine?

As I see it, this will effectively improve the air flow through the engine, and as such, 2 Bar from the turbo will be at a greater volume.

Correct?

JJ

you will more likely improve you peak power,,, but it could cost you bottom end torque because the air speed will be slower through the larger ports

.......ports are opened up for increased flow at high rpm and camshaft timing and lifts are increased...both of which kill off low rpm torque/power/fuel economy....If you can consume 2bar at std port sizes i'd stay std!

....but on the upside you can run a lower boost for the same bhp.


...i hope i am simplifying everything...too many big words on this forum :cry:

Went out in my 6.8:1,bd16 inlet and exhaust ,t4,8 greens good old fashioned

rs 500 touring car specced ,motor today.......................Fooooooooook me!


puddy :top: :top: :top: :top:

very similar to my spec :top:

I was the guy in the scoobie jacket @ the daddy thrash with my mate in

his focus cossie..............loved your motor ;)

puddy :top:

i remember talking to you Puddy :top:

seems to be a very popular spec ;) , and to be honest, when i went out in it with just a T34 i was supprised :eek:

:wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall: :wall:

I think that what chip is trying to say is that the 2bar of pressure that is seen does not represent the efficiency of flow at all, and that if the engine is flowed nicely, the volumetric efficiency will improve for a given level of boost, which will therefore lead to improved peak performance (providing the turbo is in fact capable of both allowing the flow and providing the power!)

Is this correct?

JJ

Lol, yes thats pretty much it.

The engines does NOT consume 2 bar of boost, EVER, thats the whole point, the 2 bar of pressure in the manifold represents the air that could NOT be consumed, and has no direct relationship at all to a measurement of how much was consumed.

At least this thread is managing to help you grasp the basics if only because Phil gets it SO badly wrong that it makes it easy to see why he is wrong, even to a relative lay-person :cry:

...Chip you have no idea mate :?

...what you KNOW i meant was if your YB can see 2bar from a T4 without surge you do not need to open the ports :top:

..i dont care where you see 2bar..on your boost gauge..on your penis...on a sign held up by a passing Granny..thats what i MEANT..stop twisting its childish :roll:

Everything i have said on this thead is CORRECT its you trying to be too technical that confuses it!

* Yawn *



Phil mate you cant even manage the one post in which you said everything was correct without typing misleading crap, let alone an entire thread





Seriously Phil, just stick to threads you actually understand mate, it really would benefit the community as a whole, you are only adding confusion in here with the way you interchange the words boost and airflow, especially when you starting making out its other people saying it in fact you actually went as far as to say:

Neither one of us said anything of the fucking sort as we are both TOTALLY aware of enormous fucking gap between boost and airflow, if you would just stay away from threads like this it would be a LOT less confusing for peopel reading them as they wouldnt have to wade through all your the misunderstood crap that you spew up onto the forum in incorrect places.

Seriously, just stick to your funny little threads about shagging mikeR and Pon and keep away from the stuff you dont understand.

*yawn*


...copy what i said :roll:

CHILDISH

Now dont ruin another very interesting thread because you cant accept i am a forum user with my own style..STOP THE HATING.

Your 'style' is to wander around and shit little nuggets of crap information randomly all over threads, and no I dont want to accept you doing it on what should have been a useful disucssion :wall:

..information from the best my friend...just cause you cant understand my meanings isnt MY problem :top:

Everything i have said in this thread and any thread is CORRECT. :p

A quick time out please guys! :mass:

Lets just agree that all people are at least trying to help a dullard like me! I dont want to lose the topic completely to old arguments ;)

So back on topic......

So we have established that the exhaust cam that Ford make in fact does a bloody good job. However, I note that a number of people will then change the inlet cam.

Given that we know that if we mess about the zorst cam we can screw up the Boost threshold, what would be the down side of fitting an inlet cam with greater lift or duration?

JJ

....you loose bottom end power and torque...its a fine balance and cam timing is critical to get the best of everything.

remember you could run the inlet cam slight advanced and the exhaust cam at standard timings , ;)

you can play about with cam phasing alot :top:

the problem with talking about cams i find is that we never look at the complete picture as we (I :cry: ) forget another part of the operation,,, sometimes its better just to trial and error :oops:

So lets say that we slightly advance the standard inlet cam. What would be the effects; both positive and negative. Deal with theory rather than practical if you like - what should happen?

JJ

Advancing the exhaust cam and retarding the inlet cam moves the peak torque down the rpm range :top:

So what if you leave the exhaust alone, and just adjust the inlet?

Trying to deal more in theory rather than practice, so that I get a grip with the principals really :top:

JJ