from what he has my guess is it is happiest in the 4500 to 7000 RPM range but it will run down to 3000 RPM. I much would rather have a motor that is happy in the 2500 to 5000 RPM range for every day use. to compair a lighty hotted up 850 to a hotted up 750 is chalk and cheese.
750 engine tune.
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That is where my 850 is happiest, above 4k. But i dont ride so much in the city anymore. Mostly twisty country roads.
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How it runs at low rpm can be important if you’re trying to get across a town with 30 mph and 20 mph speed limits and traffic lights.
And saying you don’t ride through towns wouldn’t be a very helpful answer.
And saying you don’t ride through towns wouldn’t be a very helpful answer.
Chris
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Martyn
I know Jim says 4500 revs & I know I've never burbled through town but through the gears from standstill my version of your engine accelerates well. I would try to bump up the compression with a set of pistons first before contemplating sleeving the intakes.
Chris
I know Jim says 4500 revs & I know I've never burbled through town but through the gears from standstill my version of your engine accelerates well. I would try to bump up the compression with a set of pistons first before contemplating sleeving the intakes.
Chris
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The OP has a hot cam and Stage 1 Maney head, and is complainig about lo-mid RPM performance.
I have a hot cam and a FA head with big valves and conical springs that Comstock flow ported, and 9.5:1 pistons.
My motor spins up well thru to 4k, but comes alive above that. It may have just as much oomph below that that it used to have and maybe more, but without a dyno on both, who knows?
But a stage 1 motor should like to be opened up more than a stock one, by design. He has 2/3 of that motor, just not the compression to support it.
So, if riding around at 20mph is the goal (unknown) then his motor as built probably aint it.
That said, i do fine when i am in town under 50kph. But i dont usually get out of first gear.
Fast Eddie
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The OPs issue is lack of performance below 3,000rpm and not poor running etc. So negotiating slow traffic etc isn’t the topic.
IMHO I’d start with a compression boost and re evaluate then as the cam, and probably head work, are all designed with high ish CR in mind. But with that tune, and the light crank, it ain’t gonna potato-potato along like a HD!
Of course, it could simply be that the OPs hot snot 920 has screwed up his personal calibration!
IMHO I’d start with a compression boost and re evaluate then as the cam, and probably head work, are all designed with high ish CR in mind. But with that tune, and the light crank, it ain’t gonna potato-potato along like a HD!
Of course, it could simply be that the OPs hot snot 920 has screwed up his personal calibration!
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Hallelujah, brother.
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"My motor spins up well thru to 4k, but comes alive above that. It may have just as much oomph below that that it used to have and maybe more, but without a dyno on both, who knows?"
On a conventional engine (no computer control/vario valve timing/vario ignition timing, etc), a hot cam/ported heads, etc will have less low/mid power than a stock setup. There is no way around that. The only engine change that can increase power everywhere is additional compression. Re that, it's not as much as many people think. A one point compression increase (no other changes) is generally going to provide around 3% more power on any 4-stroke motor.
3% more based on 60HP (per the factory
) is ...well...not much.
On a conventional engine (no computer control/vario valve timing/vario ignition timing, etc), a hot cam/ported heads, etc will have less low/mid power than a stock setup. There is no way around that. The only engine change that can increase power everywhere is additional compression. Re that, it's not as much as many people think. A one point compression increase (no other changes) is generally going to provide around 3% more power on any 4-stroke motor.
3% more based on 60HP (per the factory
) is ...well...not much.
Fast Eddie
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That 1 point compression increase is a flawed argument IMHO.
Sure, add 1 point to an engine that is optimal at the previous CR and you ‘might’ be right.
But take 2 points away from a hot motor that’s designed for a certain CR and you will most definitely notice the difference!
FWIW, I just reduced the deck height on a 774cc T140 by .030” which, I estimate, has raised the CR by 0.7 points and tightened up the squish. The difference is definitely noticeable.
Sure, add 1 point to an engine that is optimal at the previous CR and you ‘might’ be right.
But take 2 points away from a hot motor that’s designed for a certain CR and you will most definitely notice the difference!
FWIW, I just reduced the deck height on a 774cc T140 by .030” which, I estimate, has raised the CR by 0.7 points and tightened up the squish. The difference is definitely noticeable.
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Fast Eddie said:
OK, but towns and other slow places are where I notice lack of performance at low rpm.
Matchless
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If anyone knows of a 750 Fullauto head for sale or part ex. for the Maney please let me know. The problem with raising the CR is the available petrol where I tend to ride. I live in Cumbria & many of the smaller petrol stations do not stock super dooper unleaded. When I go out for a ride it is usually 200 miles+ so I can't always fill up with the best stuff. Also, I have gone down the high CR route a couple of times before with a 930 Triple & a 1000 BMW airhead. The result.......PINKING, especially when the weather is on the warm side. I spent a lot of time & money on the Triumph even having pazon build a custom ignition unit all to no avail. 9 to 1 maybe okay but I can't see it making that much difference.
Please don't misunderstand my motives for wanting the motor to pull better low down as I am not a dawdler, but nobody can ride around flat out all the time, & whilst both my Commandos see the redline every time I go out on them riding like that all the time gets a little tiring.
Please don't misunderstand my motives for wanting the motor to pull better low down as I am not a dawdler, but nobody can ride around flat out all the time, & whilst both my Commandos see the redline every time I go out on them riding like that all the time gets a little tiring.
Matchless
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I wonder if you might get relief by going to a 19th gearbox sprocket.. I would also check Steves timing for the 4s as against factory spec. They may be identical cams but its possible Steve pushed the envelope by retarding the valve timing . Certainly try advancing it slightlysubject to the usual checks.
I would have thought you would be ok on 95 unleaded on CR below 10to1 and, if not, witches brew !
Surely there is someone out there who would swap you an RM1 head foryours!!!
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maylar
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I do, regularly. Stock 850 with 20T sprocket is 40 MPH at 2500. It's a sweet spot in my ISO's, the vibration drops to near zero. Twist the throttle from there and the MKII responds instantly.
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Sleeving the ports is fine and it can always be undone - you can flair the end near the valve. A radius on the lifters will reduce duration and add low end (with a correspondingly lowered RPM valve float limit).
An easy test would be to sleeve the ports (slide in a couple tubes by hand and hold in place with a small dab of silicone). Then open the tappet clearance up to .020" or more. This way you wouldn't have to disassemble the motor.
An easy test would be to sleeve the ports (slide in a couple tubes by hand and hold in place with a small dab of silicone). Then open the tappet clearance up to .020" or more. This way you wouldn't have to disassemble the motor.
Matchless
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When I get time next week I will be doing both of these things. At this stage I don't want to start pulling it apart, but if these two mods don't give any improement I will then pull the top end.
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I have many years of engine building dyno work when a compression change was a normal part of an engine build. The laws of physics being what they are, a three percent increase is all you are going to get on a one point compression increase with no other changes. You are right, of course, that the synergy of all the changes can be greater than the sum of the parts. And, frankly, I don't recall a situation where the only change to an engine was a compression increase. By the same token, I have seen compression increases that lost HP because the other "high performance" changes were not thought out at all, producing less power than the stock engine did.
I'm not speaking against raising compression, I've done it on a LOT of engines, including many of my own. It's a worthwhile thing to do but it's not going to produce the big HP gains that are often pronounced. BUT, if your vehicle has a couple more HP than a competitor's vehicle of the same class/weight, assuming the driver skills are equal, you'll win the race every time!
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the decisive factor for port bore is surely the diameter of the inlet valve. The cam is a different issue.. There is nothing wrong with a 4s cam for road use . The issue here is not whether CR should be raised per se but whether it should be restored so as to enable valve over lapand dwell to do their thing. The negative pressure wave that arrives back at the exhaust port is crucialin pulling mixture into the cylinder against a rising piston. Lower Cr and some of the effect will be lost.
Sleeving the ports willnot IMHO make much difference except give carburation problems at higher revs .
Sleeving the ports willnot IMHO make much difference except give carburation problems at higher revs .
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The reason for smaller ports is to increase the air velocity and cylinder filling charge at lower RPM. I would go with 30mm ID and cut the tube ends near the valve at a 45 deg angle so the floor stays high closer to the valve seat (and the short side will clear the valve guide).
Chamfer the ID of the tubes on the valve side end to streamline the flow just in case it works so well you decide to leave them in.
I would try .030" tappet clearance if its not too noisy. Shouldn't hurt for a short while.
Chamfer the ID of the tubes on the valve side end to streamline the flow just in case it works so well you decide to leave them in.
I would try .030" tappet clearance if its not too noisy. Shouldn't hurt for a short while.
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From the first page he states he has fitted a Manley head with massive 34mm inlet ports, no matter what compression raito is in the engine, you will always suffer gutless bottom end power if left at this size, and indeed mid range/revs.
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