Valve timing

[TeamCAM]

I race a Prokart, using 2 Honda 5.5HP Generator type engines. The modifications limited to replacing valve springs/removing part or all of the airfilter. I've been told that changing the position of the crankshaft timing gear will shift the power band one way or another It's just how far/advance or retard?.
I want to increase to power at the lower end of the rev range.
Any advice greatly appreciated...Thanks.

[graham bahr]

normally you just shift the cam a couple of degrees but as to which way , that all depends on the engine type, no two different engines will respond the same way, it wil be down to trail and error.

surely adjusting the cam timing would be considered a modification????

[TeamCAM]

regulations say we are not allowed to add/remove or machine parts of the engine.We can adjust anything we like i.e fuel mixture/jet.. only the ignition is fixed, the camshaft is driven from a timing gear on the crank (interferance fit) we are allowed to adjust the position of the gear which will adjust the valve timing.(we are allowed to do this)

[zefarelly]

from my 4 stroke tuning book, and the way I've been doing it, set the valve timing spot on, 1 or 2 degrees advance will have little effect, 4-6 should show some improvement in mid range power at the slight expense of top end power. the only reason for retarding valve timing is if you have an exccess of power and are sstruggling for grip . . .not something I suspect any of us can afford to suffer from !!!

I set mine 1 degree advanced on a new engine to allow a little settling of the timing chain, seems to work ok

[graham bahr]

i'd argree with the degree advance to allow for chain stretch.

the rule of thumb advance for top end power doesn't always work, infact with most of the beemer cams i've used they make the most topend power slightly retarded, and we have played with the cam timing on the rollers and proved it.

which basically goes back to what i said earlier you got to experiment

[zefarelly]

so basically set it spot on as a universal setting (afterall the cam companies do know something !) with an allowance depending on timing control (belt/chain etc) and then the only real way is to test it

[dtype38]

I'm told that other effects can be gained by changing the relationship of the cam timing. ie., retarding or advancing one relative to the other will change the torque/power relationship to give better bottom end torque or better top end power.For my engine, one race parts manufacturer has produced a cam timing tool for exactly this sort of setting up. Don't know which way is which tho, and if the prokart has a single cam it won't apply under your rules. :-)

[zefarelly]

stop showing off . . .some of us have to make do with a single cam !

but I have read the same, you can tweak by advancing or retarding one cam in relation to the other.

[graham bahr]

Originally Posted by zefarelly

so basically set it spot on as a universal setting (afterall the cam companies do know something !) with an allowance depending on timing control (belt/chain etc) and then the only real way is to test it

100% spot on i'd say,

the crane/kent 260 profile in an x/flow will make a good full race cam, but retime it several degrees differently it makes a good single gear grass track cam infact if you have an old old kent cams catalog it will give both timing settings.

my friend who was building lots of rally engines back in the 70's used mainly A3,A6 and A8 cams and apparently most of them worked best retimed, although it should be born in mind that a good spread of power is more important on a rally car than the ultimate output when compaired to a circuit racer

with twin cams your laughing as its the lift on overlap that has the biggest effect on the power curve and your adjusting that as much as the overal timing on a twincam, basically the more the better in terms of topend, usually you will want to retard the inlet and advance the exhaust by the same amount, although if your engine doesn't have the usual 80% inlet exhaust flow ratio you may well benifit from moving one cam more than the other, unless you end up with bent valves that is!!!!

[Al Weyman]

Don't retard it too much or you will have the exhaust glowing like a fireball, I have seen that problem before.

[R59]

Rule of thumb time again.

Advance timing to move power band down the rev range. Increases mid-range.
Retard cam timing to move power UP the rev range.

A classic example of this was on the Kent Cams spec sheets for the Ford crossflow, or Pinto, which listed a couple of the cams having different capabilities when set at different timings.

I ran one of my 1600 Nova engines with avanced timing once, which gave it massive mid range grunt at the expense of outright top end. The cam was supposed to give a 4500 to 8500 band, with peak at 7800. I ran it to give a peak at 6700, and it worked really well. Another cam worked very well when retarded by 6 degrees, moving it's 2800 to 7500 band to become 3400 to 8000, peak again around 7200.

Rob.

[zefarelly]

I'm running an A6 which peaks just after 7k and will pull to 7500-7800 the other thing which sems to effect that is compression, the A series cam seem to like higher comp ratios . . . .if you dare skim the head that much . . . now thats another thread entirely !

[graham bahr]

oh yes cams and compression are well linked, the more cam you have the more compression to use, i would be looking to use at the very least 11.1 on a race engine with an A6 probably more on the pre x as it should burn better having a proper combustion chamber, the only real limit as to how much you can skim x/flow and i'm sure pre x is the same, is to the size of the triangluar water ways, the more you skim the head the bigger the waterway gets eventually the head gasket has nothing to seal on

[graham bahr]

one reason the a series likes very high compressions is its poor cylinder filling, the less you have in the the more you have to sqwash it

[zefarelly]

I've got 11:1 . . .I bottled it . . . I'm working on a new hea dalready to see if I can get 12 and a few more horses, I'm also thinking about getting figures together for a special cam with slightly more lift as well as the porting flows very well at up to 1/2" lift (A6 is 0.4") not sure I want anything as wild as an A8 though

[graham bahr]

if i remember right kent cams have an A6 profile with extra lift for engines that cannot for regulation purposes have high lift rockers

[Al Weyman]

Originally Posted by zefarelly

I've got 11:1 . . .I bottled it . . . I'm working on a new hea dalready to see if I can get 12 and a few more horses, I'm also thinking about getting figures together for a special cam with slightly more lift as well as the porting flows very well at up to 1/2" lift (A6 is 0.4") not sure I want anything as wild as an A8 though

Blimey what fuel are you running to enable a 12 to 1 compression engine not to detonate even with a killer cam. You are getting into the realms of diesel engine compression there

[graham bahr]

arh the benifits of not having a gallon sized engine, small engines small combustion chambers.
if you have a small chamber that burns well and plenty of squish its not a problem if the timing and fueling are spot on.

my old non turbo beemer engine ran 11.75, it would of been more but i couldn't get it higher, it was quite happy on normal super unleaded pump fuel, optimax which is generall said to be the best wasn't needed, it had to run very tight sqwish clearances though like close enough that when you removed the head there wasn't a carbon build up in the sqwish areas, due to no burning taking place in that area,

it was very senstive to ignition timing, i got to the point that 2 dgrees either way knocked off 4 brake, i have to say it was fortunate that i ran full engine managment so the timing could be accuratly mapped everywhere and unless the triggerwheel feel off the crankshaft it was impossible for the timing to wander and slip.

i doubt that zef has mapped ignition and fuelling, but with the pre x/flow having smaller chambers than my beemer i cant see 12:1 being a problem provided its very carefully set up, and he has the swish clearances nice and tight.

keeping the engine running on the cool side will also help

[Al Weyman]

I used to be very friendly with Roland Hayes of HT Racing (we were going in to business together but got shafted by some unmentionable motor sport 'face') and he told me he built Pinto engines with the piston virtually physically touching the cylinder head ar the squish area and relied on the compressed gases as a buffer from doing any damage, I know he was an advocate of extremelly high compression ratios but we are going back abit now and I would have thought that the 'Standard Road Fuels' as outlined by our peers the MSA are not quite what they were as we all know unleaded is no longer available apart from some very few and far between outlets at great cost.

[graham bahr]

to be honest modern super unleaded in particular optimax is better than 4 star of old, if your engine can handle the lack of lead

[Al Weyman]

Thanks for that Graham, I intend to get one or both of my cars out later this year for a run now the MSA have given me my licence at last and there was leaded about last time I raced, both cars are running heads with hardened valve seats so should be fine. I had though about seeking out some high octane leaded there is a place in Barnet that sells it but it is very expensive.

[graham bahr]

i know several drivers all running 11:1 and above on pump optimax with no probs, my own engine, all be it now turbo'd runs 8:1 with 1.5bar boost and is quite happy on optimax, without any additives or water injection, so i doubt your need anything more in the way of fuel unless your engines very very special

[Al Weyman]

Well the black car is a higher spec all roller motor runing at aroound 10.5 to one, the other is a hydraulic flat tappet motor around the same CR so should be OK as you say.

[dtype38]

As we've sort of strayed from valve timing into compression ratio, from what I understand the two can't be treated independantly...

I use high lift, but short duration, cams with very little overlap. This give me lots of torque, albeit at the expense of top end power. It also means that compression begins very early in the stroke, so with standard 9:1 pistons I get at around 190psi static compression. One of the blokes I compete against, however, uses very long duration cams to get more power, this means he has lots of overlap, and even with extra high compression pistons only gets around 120psi static compression and needs quite a lot of revs to get it working properly.

I guess, what I'm saying is that the cams you're running make just as much difference to what fuel you can run as does the calculated compression ratio. Or am just very confused??

[graham bahr]

yes your quite right, you cant treat cams and compression totally seperatly, you certainly couldn't run 11-12:1 compressions on std or fast road cams, likewise the 336 degree duration cam i used to use in my atmo engine would of been a total waste of time with std 9:1 compression.

as with lots of things ther aren't clearcut hard and fast rules as to how much cam and how much compression as cylinder filling also plays a part,

i know one engine builder who runs a 15:1 compression on his 16v vauxhall lump on normal pump fuel and mega high lift but low duration cams, BUT the engine breaths through a 26mm restricter, its runs such a high compression as the cylinders dont have much mixture to squash, its the exact opposite of why forced induction engines have low compressions, and the more boost you stuff in the lower they have to go

re cams and fuel, yes your right they do have an effect on the fuel you run, but thats got more to do with the compression and cams being matched, and in all cases i'm assuming they aren't totally mismatched

i'm suggesting zef uses a compression of way over 11:1 but only because it will work with his A6 cam, i would be saying somthing very different if he were using an A2