Tuning on srx engine

[shaggy]

The engine of my srx needs a hone or rebore due to light scuffing of the back of the bore.
The option is to keep it standard or overbore it and tidy the head.

Looking at the wiseco site there are option up to 671cc, alought i'm guessing that a bit to far due to needing to replace the rod and no doubt other weaker item due to the extra power/torque?

What will the 635cc and 621cc kits feel like and how far can you go with the head or is just a case of clearing round the vavle seats?

[shaggy]

sorry the 620cc and 634cc wiseco pistons

[Steve H]

My understanding is that anything much bigger than 621 can cause problems. On its own the 621 will increase mid-range torque. I cant comment on how much because I'm still in the process of building mine. You will need to watch compression ratios. I beleive Wiseco do one which has the stock ratio. If you dont want to make other changes then this would probabaly be the best bet. The Higher compressions pistons tend to have quite a high compression ratio, Ive also found they tend to be higher than the quoted CR, so something quoted as 10.5:1 can be up around 11.0:1 when measured. This sort of CR would either need the crown machining down or a longer duration camshaft to reduce the dynamic CR and avoiding pinking/detonation.
If you want to retain the reliability of the stock engine then the best bet is a first or second oversize piston.  A free flowing exhaust and Keihin CR's carbs can brighten up an otherwise stock SRX.

[Steve Lake]

Well, my #1 SRX6 runs 634 with a wiseco 11;1 piston, I spent £200 at the cylinder head shop which got me lovely inlet and exhaust tracts plus profiled (bluddy near invisible, valve guides) and slightly waisted valve stems, profiles valve seats. I currently run twin 34mm dellorto's into it. Oh...and a handmade stainless exhaust system terminating in a questionable (noise wise) megaphone style 'silencer'
Still running a standard cam......but i do have a yoshimura stage 2 cam to try, might have a go at it this year.

in this form it has done about 15k reliable miles, is tractable, and with a larger front sprocket and a rev limit of about 6.5k (it's happiest, and developes most of it's power between 4-5.5)..has excellent acceleration and a good top speed.

Now then.....you mention scuff marks at the back of the bore.....this is a known fault....it is a hot spot. you'll find it lines up with the back of the camchain tunnel. Nothing you can do about  the hot spot...and heat (if not too excessive) is not a problem....as long as everything is working ok......but what you have is a tired and possibly scored oil pump, the lower pressure is not giving enough splash to the bore......this can also be caused by failure of the two pump seals (which cannot be obtained separately as far as i'm aware), In any event....expensive as it is (£80+) I would ALWAYS fit a new pump when undertaking any tuning on these engines....

Somewhere on this forum, some time ago......we had a chat about these pumps, and there is a way of machining the seal seats out and fitting seals from a different pump....I've not done it so cannot comment further on this.....but as the new pumps are of a higher capacity, i still think a new pump is the way to go.........I know from expensive experience...the one srx6 engine i bored and fitted oversized piston in (rebore £40 piston £100) and didn't change the pump , and run in properly....picked up at the back very badly (ie rebore and piston AGAIN) at 1500 miles on my way back from cadwell....not doing anything excessive.
And I know its overkill, but i change my oil VERY regularly (1500 miles) with a new filter...and use a magnetic sump plug
(i don't waste the oil...it's used in my chainsaws and mowers)
good luck.....Denver Motorcycles (sadly no more.....but Slipstream Racing do a great job on SRX's)....were getting near 70bhp out of a race tuned srx before moving on to the more robust bmw/rotax engine for their supermono racers.  And of course our Andy230 now has his srx racer ready to roll....and he has met and overcome all the snags and pitfalls of tuning these lumps....

[andy230]

Hi shaggy

Andy230 now has his srx racer ready to roll....and he has met and overcome all the snags and pitfalls of tuning these lumps....

Ahem!  Well, lets not speak too soon Steve.  The 640 hasnt turned a wheel yet!  Hoping to test at Brands on the 22nd.  This newest one is a 98mm bore (640cc), aircooled 4v top end built onto a beefed up 660cc (XTZ660) bottom end.  For the boring details look at the discussions in "Andys Racer", or www.team-thumper.blogspot.com  You will have to be a total anorak to be able to wade thru this tho!

In about 2001 or 2002 I got an SRX motor off the bench straight from slipstream.  It had a stage 2 cam (dialled), 2 CR33 carbs, flowed + ported head, (and bored inlet manifolds) standard valves, 621cc Arias forged piston.  New standard rod when built.  The bottom end had undercut gears and all new bearings.  Standard Yam headers, 2-1 supertrapp unit (Y-piece), terminating in a renegade can.  there are a few pics about in the photos section.

I used it on trips to france, commuted to work, raced it, rode it london-glasgow.  All in a brilliant bike.  Last I heard it was still going well, testament to Martin sweet (silpstream) build.

Max torque was at 5.5k, peak power was 52 rwbhp.  Not quite enough to race(or am I just too slow?!), plenty of grunt for the road tho.  And more importantly absolutely reliable.  Revved to 8k easily, 8.5k+ in moments of madness, but that is really asking for bother, especially with a standard rod.

I think putting more tuning on top of this will lead to problems.  You can physically put in a 102mm (690cc) piston, but you'll need to open out the crankcase mouth.  Heed SteveL's adice the oil pumps!  (mine had a new one fitted)

I reckon with singles generally its about getting them breathing.  they are hard to silence, so if you are bothered about this tuning may not be the best idea.  So I think its not just a case of more capacity.  The YDIS carbs are complicated and restrictive, but make the bike easy to ride and forgiving.  Get a pair of CR33s if you can, and use in conjunction with head work.  If boring, a 621cc should be ok, but not a lot of point without doing cam carbs and head (and of course a pipe).  Its a case of doing a lot of little things, otherwise you may just end up with a noisy, lumpy, unreliable bike, which isnt is good as standard, and upon which you have spent a fortune!

I'd talk to martin sweet, he's spent 20 years tuning these, and knows his onions.  However, his work is not cheap.  In 2001/2002, the rebuild above was £888, and the guy gave him some parts.  Think 1500 for the full monty now.  The problem is that then you are starting to stress the bottom end... and if its not been done, there's a good chance that your 5th gear is maybe a bit knackered anyway?  Have a look with a torch when you get the barrel off.

cheers mate, good luck.  Dont let the above put you off, I'm not wanting to sound like I'm pissing on the campfire, and they can be great motors if done right.  Tho it may well be cheaper to try to buy either a race bike and rob the motor, or a motor which has been done and you know is good.  Its quite a lot to spend for what is after all only another 10 horses. 

a