Unequal camshaft duration 4AGE

[YoShImUrA]

What info is the cross over telling you? I think the HP and tq scale is different, so not really telling you much here?

No flowbench results, sorry.

[YoShImUrA]

Updated with good quality comparison of both dyno runs:

[jondee86]

This question has been around for a long time. The interwebz are long on opinion and short on hard facts. This http://www.wheelsjamaica.com/wheels_...?topic=44599.0 from another site covers a lot of the ground that you have already covered yourself... but there is some wheat mixed in with the chaff If you have nothing better to do for 15 minutes, have a read.

Cheers... jondee86

EDIT: Yeah... some kind of screw up in the wraparound link cleverness. But you got it right

[73GTV]

and old Ad for 1983 HKS Cams just to add to the conversation.

[YoShImUrA]

Hmm, can't seem to load that page. I'll try again later. Cheers, Jondee!

Edit: link was wrong, somehow Tried this and works! http://www.wheelsjamaica.com/wheels_...?topic=44599.0

[Just1n_mc]

I'm currently on the lookout for cams for my Silvertop in my track car and was just about to pull the trigger on a set of 272 9.2mm cams but after reading your findings I'm not sure now.

It's seems a common complaint with people changing cams in the 20v and power dropping off at high rpm. I had toyed with the idea of a larger exhaust cam myself but can't get my head around what does and doesn't work...

[trdee]

i think the issue with 20v is the fact that the vvt phasing is optimised for the stock cams and to get the best performance from aftermarket cams you need to ditch the vvt

[Just1n_mc]

That may well be true but Toda list their intake cams to be set up with a 125 degree centreline which is the same as VVT off. If they listed them at say 110 degrees I could understand how VVT would be an issue but at 125 degrees your still using the stock range just with slightly wider intake valve opening/closing points.

Even their 288's which they say not to use VVT with has a 125 centreline.

[trdee]

the centreline is one thing but the "dynamic range" between vvt on and off is the issue iirc. my brain is gonna struggle to put it into words but i have read elsewhere in the past from someone playing with vvt on a dyno that as you go up in camshaft aggressiveness, you need to have less and less movement between VVT on and VVT off to stay in the ideal range for the cams. this led to people hacking up vvt pulleys and putting in stoppers to limit the range of movement in the pulley and whatnot. with the 272 it probably still "works" but the range between vvt on and off means the cam is never truly in it's ideal spot which is why you dont get the top end punch you would expect from the cam?

[Just1n_mc]

If VVT is off at high RPM then why would it effect the top end? I can understand what your saying may effect midrange with VVT on but surely Toda designed the cam to work best when set at 125 degrees.

Even with an adjustable gear your still going to set it at 125 and make minor adjustments from there. Most people who fit adjustable gears seem to find that they don't really gain alot adjusting it from what Toda recommends.

I still can't get my head around why some people who fit aftermarket cams to 20v's always seem to be disappointed with the power at high rpm with or without VVT. The power seems to fall away from 7000rpm. I still think there may be something in running a larger exhaust cam as some people have had success with that but I'm not sure why. The whole 3 inlet valves being more efficient than the 2 exhaust valves? From what I understand a larger exhaust cam will help move power higher up, which may help with what people who've changed cams and had poor results.

[101gze]

Not sure who you speak of... who was disappointed? why were they? were they expecting 300hp? did they put it on a dyno and get upset at reality?

The concept of a larger exhaust comes from the exhaust flowing less than inlet. A larger margin of flow offered by a larger cam on the EX could offer benefits.

The industry rule of thumb that EX should flow a certain % less than IN on an N/A application is lost on these people.

It is quite possible to over exhaust the 4age and or any engine.

The whole concept is completely experimental, and that is ok, without it there is no developmental gains, but to base your understandings on others experiments, may be less than advantageous.

[Just1n_mc]

You only have to do a google search to see people have had issues with aftermarket cams and 20v's making decent power especially when trying to retain VVT.

As far as a larger exhaust cam, if you have 3 inlet valves at 26.5mm each and 2 exhaust at 26mm then the exhaust is only 65.4% of the inlet. Surely you'd want it to be upwards of 80% or am I missing something? I know there is more to it than just the size of the valves but I thought you'l could run a longer duration to try and close the difference.

[knightrous]

A bit of googlefuu seems to suggest the ideal intake to exhaust valve (or is it flow?) ratio is 70-75%.
I also vaguely remember it being mention in a Engine Analyser Pro output.

[101gze]

You only have to do a google search to see people have had issues with aftermarket cams and 20v's making decent power especially when trying to retain VVT.

As far as a larger exhaust cam, if you have 3 inlet valves at 26.5mm each and 2 exhaust at 26mm then the exhaust is only 65.4% of the inlet. Surely you'd want it to be upwards of 80% or am I missing something? I know there is more to it than just the size of the valves but I thought you'l could run a longer duration to try and close the difference.

Iam talking IN/EX flow ratio, not valve area. That is, the amount of exhaust flow relative to intake flow.

In my experience, the 20v runs at around 70 ish % stock.You could ho the bejezus out of the EX on a stock head and may be up it to 78% IN/EX ratio.

Meaning, all I did was make the EX port flow more on the same cam spec relative to the inlet.

Keep in mind that with all engines, its easier to get it out, than to get it in (works the same with women really) so, is such an increase going to help?

Possibly, with the same cam as it may assist in port scavenging, but, with a longer duration cam, it may (will)? cause pumping losses by way of releasing the combusting charge too early and for to long.

Not only have you lost your pressure on the piston (torque) by opening the valve earlier and longer than needed, you have lost your scavenging effect assisting the next intake charge.

An engine doing this will certainly run out of puff early in the rev range.

You can never have too much inlet charge, but you can have too much exhaust.

So, if you think the problems you read about is all IN vvt, it may well be too much EX vt.

[Just1n_mc]

Thanks for explaining it clearly, I've been really struggling getting my head around cams and there relation to flow, let alone trying to work out how long and when valves should be opening and closing.

The Poncams seem to be a cam that people have success with and they are staggered with a larger intake. The only issue I see with those is the valve timing is 5 degrees out for a Silvertop and I'd like a bigger cam than those. Even Toda recommend a larger intake with their cams from 264/256 up to 288/272's and I'm sure both companies have spent ample time testing what does and doesn't want work. Maybe id be better off with a 272/264 setup.