Cam timing for XFlow

[Robster]

I am changing my cam from 234 to a 244 wink.gifand have read a bit about people using offset dowels, and altering the cam timing from that of 'standard'. Does this apply to a xflow? Should I set the new cam according to the marks on it..ie accurate to the nearest link in the timing chain?

 

Thanks

 

Robster

[oilyhands]

The reason for using offset dowels/verniers is to optimise / correct the cam timing, quite often replacement cams are not ground 100% accurately with respect to the dowel hole/keyway which can cause the cam to be mistimed when assembled using just the standard timing marks. With OHC engines , skimming the head can affect cam timing too. Generally a cam manufacturer will give a lobe centre figure which represents the point at which the cam will gives its maximum lift, usually this is between 102 and 112 degrees after TDC, another quoted statisitic is the valve lift at TDC, either of these two methods can be used to correctly time the cam, personally I prefer the latter. In order to acheive correct timing it is necessary to be able to vary the cams timing WRT to the crank position, hence the offset dowels/keyways/verniers. The big disadvantage with offset dowels and keyways is that the pulleys have to be removed to replace them when timing the cams, with a vernier this is not necessary. A 244 cam has a lobe centre of 103 degrees on the inlet lobe, so it will give maximum lift at 103 degrees after TDC.

 

Assembly to the standard timing marks will get your cam timing near, but its worth checking where maximum valve lift is by using a protractor on the bottom pulley and a dial gauge on the appropriate cam follower.

 

Dave

[Roger King]

It is by no means unusual with the 244 to find that the timing is so far out that offset dowels alone won't correct it (i.e. over 10 degrees out). In such cases you have to move the cam sprocket round one tooth. This means that the timing marks on the pullies no longer line up! Offset dowels can then be used to set the timing precisely.

 

Statistically, it is likely that you will not encounter the above scenario, but it is common enough that you must check.

 

Although offset dowels are more awkward than vernier sprockets, they are cheaper, more reliable and lighter, so they are our preferred method. This is particularly so on a pushrod engine where the sprockets are internal and cannot be checked once the engine is assembled.

 

Incidentally, there is no such thing as correct cam timing. The figure quoted by the manufacturer is their opinion about what the optimum timing should be on a typical engine. It is possible to alter the timing to achieve different effects on the shape of the power curve. The 244 is particularly good for this, whereas some other profiles are less tolerant of being set differently to the recommended figure. Ultimately, of course, you can stray so far away from the recommended figure that the cam doesn't work anywhere in the rev range.

 

Edited by - roger king on 18 Jul 2001 09:43:31

[kenny.]

Just a quick and obvious point regarding the 244 cam.

On my first 1700 x/flow 7 I changed from an unkown cam to 244 and because of the extra lift had to gash the pistons quite considerably (something like 6mm) if memory seves. Car was running 1300GT pistons.......I'm sure you know about this, but just thought I'd mention it. I'm sure Roger will know if this is needed on a 234/244 swap...it may be ok.

 

Kenny HPC

 

 

[oilyhands]

Good point Kenny, Kent recommend 100thou extra to be removed from the valve pockets for all the 2x4 range, it's likely that the pockets have already been machined for the 234, but it's definitely worth checking clearances.

 

Oily

[Roger King]

I wouldn't recommend the standard Supersprint piston with a 244 under any circumstances. The things are marginal enough with a 234, let alone the higher revving 244. This is not a piston pocket depth issue, but an issue of strength and piston ring suitability. We've seen several failures over the years.

[kenny.]

Out of interest (excuse my ignorance) what is a standard piston as used in a supersprint motor.

I thought these motors all had 135 bhp engines.

On the x'flows I've built at home, to get 135bhp, I've certainly needed the 244 cam........this usually gave 112@wheels on rolling road.

Spec I used to use was =090" pistons (1300GT type), chamberless but ported head, 244 Kent, 40's, valves as reccomended at the time, H/P oil pump,comp. block breather, duplex chain and springs, steel posts, and spacers,and uprated big end bolts.

Rev limit was usually around the 7000 mark.

Do supersprint engines really deliver 135bhp, because I was on a tight budget and struggled to get this power for less than £2000 in parts only, labour was free.

That was ten years ago...things have probably moved on, seems a lot cheaper now to get more power from Vauxhall, but I suppose it's all relative.

 

Kenny HPC

 

PS I collect Lotus Memorebilia, and I have a mounted DFV piston and rod direct from Team, and I'm sure they used the same thing in Anglia's.

 

Kenny HPC

 

 

 

Edited by - kenny on 18 Jul 2001 18:57:02

[millsn]

answer is no they didn't. If you spend some dough on a nice cylinder head (no extra compression or revs) then you'll get it though.

 

Nigel Mills - 2.0 Zetec carbs

[oilyhands]

I'm pretty sure that the standard piston is the Hepolite cast type , slotted and with steel clamps to limit skirt expansion, this are not renowned for their strength (the tops come off under duress). Certainly wouldnt recommend them for sustained high RPM.

 

Oily

 

Edited by - oilyhands on 18 Jul 2001 22:11:50

[Robster]

Thanks for all the advise. I have accralite forged pistons...the standard hepalite ones are very heavy in comparisson.

 

[Casbar]

I am just about to get my xflow re-built, but can't decide on the cam. I have a 234 at present, but may go to a 244, but have heard differing oppinions as to the drive ability of the 244 cam. Is the 244, too wild for a road engine.

 

Robert

[Roger King]

Firstly Robster,

 

You'll have no problems with Accralites and a 244. You will also gain significant power (typically around 8bhp compared to the Hepolite item) from the pistons due to vastly reduced running friction and better sealing from the ring pack.

 

Casbar,

 

The 244 is near as dammit as driveable as the 234, but with much better top end.

 

Kenny,

 

The standard Supersprint piston is a 1300ccm Mk2 Escort item on a .090" oversize. This means that it was designed for about 55bhp. We reckon that a later spec standard Supersprint is around 120bhp, but the figure will vary according to testing method, the particular dyno used, etc. Unless you test to specified standards (DIN, SAE, etc)you cannot really compare one set of figures to another. Even then you will probably get small variations from one dyno to another. Tony Martyr's knowledge on this one (see various posts in the past) is about 10 000% greater than mine if you want more details.

[Westfield]

I have 1 700 X flow running a piper 285 cam. The details are

 

full lift at 108 ATDC

 

36/72

72/36

288 duration

 

well, that was what was written on a scrap of paper in the history file!

 

I will probably be "refreshing" the engine soon so this would provide an opportunity for a cam change. Would there be any advantage in changing the cam? The engine was built in 1990 and has done 15000 miles

 

anyone advise me please

 

thanks

 

Tony