| Author | Message | ||
     Kinder |
So I have a spare set of stock heads for my CR that I was going to modify to the 1125RR specs (good luck finding those  ) and am now thinking I may shave the heads. Anyone done it? Is this an interference engine? How much clearance is there? Did you need a new timing chain? What challenges can I expect? Thanks in advance for replies. | ||
| Stevel |
There is no advantage to be gained......none. You will create serious fitment issues depending on the amount removed. What is an interference engine? What clearance are you talking about? | ||
| Kinder |
An interference engine is one where the piston hits the valves if the timing goes off. If you are unaware of that then I doubt you have ever modified an engine's compression for performance gains. | ||
| 651lance |
The only real way to answer the clearance questions is to clay the top end of the engine. To shave the heads correctly you should cc the heads before and after to make sure both heads have the same displacement. Personally I don’t think the dollar vs horsepower gains is going to be there. I’m sure you will gain some power but not enough for the cost. What other cost will there be? Starting issues, over heating, pinging, or what? | ||
| Kinder |
Thanks Lance. Hoping someone else here may have already done it to save time and see if they found it worth it. I recognize other issues may arise. The heating would be my primary concern as these engines run hot. If I could find the specs for the original 1125RR that Buell did I'd be going that route. I know they put in bigger valves and changed the cam profile. In the end this is more of a keep busy project. 😉 | ||
| Joe7bros |
The bride didn't like the look. | ||
| Joe7bros |
And a real answer: have you checked if there are pistons that would provide a higher compression ratio, since that would be the purpose of shaving the heads; unless it overheated and the heads are warped. | ||
| Stimbrell |
Given this is a no longer supported platform with several parts known to fail becoming no longer available you may want to keep hold of the heads, someone may well be willing to pay a premium for them in the future. Move to a bike that is made by a manufacturer that is still in business if you enjoy pushing the limits, you will save a fortune. I do not know of any modern, or even not modern performance engine that is not "interference" It is of course your choice, I do not presume to tell anyone what they do or do not do with their own stuff. Above all have fun. | ||
| Stevel |
Kinder, I never heard the term "interference engine". I suspect you mean "overhead valve engine". Valves and pistons occupy common space on all overhead valve engines. No performance gains can be achieved removing metal on this Rotax engine.......anywhere. I have written many posts on this forum about this engine, including specifications and measurements unposted elsewhere. In brief, the posted 12.4 to 1 compression ratio is the physical relationship of swept verses unswept cylinder space. Changing that by removing metal from the head deck will have very little influence. The Head volume is 25cc. The piston volume is 16cc. Head gasket is 8cc. Deckspace is 8cc with standard cylinder base gasket, but the real dynamic compression ration is a product of the inlet valve closure point. The inlet cam timing of both the stock and racing cam of the 1125RR opens at 15 degrees before TDC and closes 60 degrees ABDC. Of course compression cannot start until all valves are closed. The real effective compression ratio is 8.5 to 1 because of this. That is why you can run regular pump gas with this engine without detonation. Changing the physical compression space by itself will only cause problems without any benefit. It is that cam timing that causes the lack of power below 4000 rpm and will limit any perceived advantage of shaving the heads. There are many design faults with this engine. In most cases they are fixed in following design iterations, but unfortunately that never occurred with this engine. No re-design ever occurred by HD nor EBR. If you want to improve this motor, be prepared to spent thousands. There are no manufacturers making parts for this motor. There are no oversize bearing shells, no oversize pistons, no selection of cams available. Save your money, go hot rod something else. (Message edited by steve-l on July 14, 2019) | ||
| Joe7bros |
Here you go Steve: https://en.wikipedia.org/wiki/Interference_engine | ||
| 34nineteen |
Both OHC and OHV engines have valves and pistons that occupy the same space. The question is, do the pistons hit the valves if timing jumps or a belt break. If yes, interference engine. | ||
| 1313 |
Jalopnik chimes in... | ||
| Coastrambler |
A thought occurs. If the intake valve opens at 15 degrees BTDC and stays open till 60 degrees ABDC that valve is going to be open while the piston goes through top dead center. This implies piston and valve will not occupy the same space and interfere with each other. Or am I missing something? | ||
| Stevel |
You are missing some basic geometry. a full crank revolution is 360 degrees. Valve motion is in relation to the crank position. The crank motion in this case is broken down with 2 starting points each 180 degrees apart, TDC and BDC. In the case of the Rotax, the intake valve closes at 60 degrees after BDC leaving only 120 degrees of the compression stroke for compression. In comparison, a Harley motor closes the inlet valve with most cams between 15 and 30 degrees after BDC. If you are asking this question it probably means you do not understand why this late closing is done. In short it is because a gas column has inertia and its motion change is not instantaneous. This inertia is used to sort of supercharge the cylinder. of course the exact closure point it is a balance thing. In depends on the column speed. Too slow or too late and you have reversion, which you can feel if you hold your hand over the intake or exhaust at engine idle. Too fast you have too much restriction. Too early you have less power. It is actually very complex mathematically. The timing of the stock cams is stupid. It would work if the engine could spin to 14,000 rpm, but it can't. Read my previous posts for more clarity or use Google. | ||
| Kinder |
For those who do not understand the difference between a non-interference engine and an interference one.  } | ||
| Shoggin |
If the valve stays open through TDC and the piston is high enough in the chamber in that particular engine to contact the valve, there are reliefs machined in the piston ,shaped like the edge of the valve. Put there for clearance to make it a non-interference engine. A non-interference engine can rotate the crank 360* with both the valves open. Break a timing chain, belt, cam whatever and you're 'ok'. An interference engine cannot rotate the crank with the valves open. Break a timing belt, chain, whatever and engine parts catastrophically smash together. So, the obvious next question is: Why isn't every single engine non-interference? Most are, but sometimes performance, packaging, and even simplicity play part in how they are made. You will even find interference engines made with valve reliefs in the pistons to ensure things like high RPM loss of valve train control "valve float" doesn't make one redline miss a catastrophic issue. | ||
| Shoggin |
IMO, shaving the heads just because they are off is a bad idea/can of worms and won't provide any performance gain. This ain't like takin' the restrictor plates off the Red Dragon! https://www.youtube.com/watch?v=bzQaQIQRBxk | ||
| Stimbrell |
Just a thought, can anyone give an example of a modern non-interference engine in anything other than perhaps a lawnmower or similar? | ||
| Kinder |
http://yourcarangel.com/2014/07/interference-engin es-complete-list/ Old but gives you an idea. | ||
| Shoggin |
Great resource! Makes sense they are all 'cheap' tiny engines though. | ||
| Coastrambler |
Steve, your own words say the intake valve is open as the piston goes through TDC - "The inlet cam timing of both the stock and racing cam of the 1125RR opens at 15 degrees before TDC and closes 60 degrees ABDC." Thus we have reliefs cut into the piston top. And I do appreciate the complexities of moving air through a motor. All is a compromise. | ||
| Stevel |
Shoggin. You are misinformed. All overhead poppet valve engines, no matter how the valves are actuated, are interference engines. Only flat head/side valve and port fed (two stroke) engines are not. The reason these overhead valve engines are popular is because of their better volumetric efficiency and efficient combustion chamber designs. | ||
| Kinder |
Not sure if you are just trolling at this point Steve or just stubborn. Joe posted a wiki explaining the difference. 1313 posted a posted a simple article explaining the difference that has the same pic I posted later. The engine list I posted, while a bit dated, includes ohv engines that are not interference engines. What gives? | ||
| Shoggin |
Stevel, I have read previous posts from you and know you are familiar with engine architecture. But you do not understand the definitions of "Interference engine" and "Non-Interference engine" as explained by us. Since you didn't click it last time...here ya go: https://en.wikipedia.org/wiki/Interference_engine (Message edited by shoggin on July 18, 2019) | ||
| Froggy |
From past threads I am sure Stevel knows what he is talking about, but this sounds like a language barrier issue and mixing up terminology. | ||
| Stevel |
Coastrambler, Please read my posts carefully. The inlet valve does not stay open past TDC. It stays open AFTER BOTTOM DEAD CENTER (ABDC) In a perfect world where inertia does not exist, the intake stroke would open at TDC and close at BDC sucking in the fresh fuel/air charge. The exhaust valve would open at BDC and close at TDC purging the cylinder of exhaust gases. In the real world where inertia exists, nothing happen instantaneously. In fact the opening and closing points of valves are stated at .050" in the US and at 1 mm valve lift elsewhere. This is because the valve motion at the beginning and end of the cam lobe's lift cycle is designed to be gentle to prevent part failure due to shock loads. Add that fact to gas column inertia and in the real world the intake valve opens before TDC and closes after BDC to compensate for and take advantage of this. | ||
| Stevel |
Shoggin, You are very correct, I do understand engine design and architecture. Froggy is also correct, this is a language thing. No professional would ever use the term interference engine, because it adds no engineering value. I suspect it is used by less informed folks because it makes sense to them. I have read the wiki definition and it is correct, but I assure you that all overhead valve engines meet this definition and it is in much more common use. If you think that I'm incorrect, please pick an overhead valve engine of your choice, get out your measuring tools and dial indicator and measure yourself. That does not mean that every valve train drive failure always ends in a valve to piston collision, it doesn't. You can be lucky. It depends on the stopping point of the camshaft. It is possible that the valves stop in mid lift and there just might be sufficient clearance to prevent the catastrophe. | ||
| Araignee |
Just to add fuel to the fire, I owned an early 1980s 500cc small-block Moto-Guzzi Monza that had a Herron head (not sure the spelling is correct). It had two overhead valves which were arranged side by side and nearly vertical. The combustion chamber was a dome-shaped recess in the top of the piston. The cylinder head was nearly flat; there may have been some minor smoothing or relief in it, and of course the valves were both recessed and relieved on the underside. I pulled one of the heads off to replace a leaking head gasket. The top of the piston had a shallow bathtub shape cast/machined into it. This design was probably used on other small-block Guzzis of the era, but I don't know the details. As I recall from the Cycle magazine review, the Herron head allowed a fairly high compression ratio without detonation while using 87 octane fuel, and was probably simple and economical to manufacture. It sure was fun to ride, and could indeed haul 250lbs of rider and passenger up a long climb out of the Spokane valley. The H-D XR-750 flat track racer, and to a lesser extent, it's XR-1000 bastard stepchild both managed to make ~100bhp with two valve heads, largely by exploiting the volumetric efficiency of the design. An XR-1000 modified by tuner Jerry Branch for a Motorcyclist article claimed a volumetric efficiency of 133%, aptly described as "4 quarts in a 3 quart bucket". Of course, hundreds of hours of work by H-D and many private tuners helped make this happen. Kevin Cameron describes some of it in his Sportbike Performance Handbook. Given the highly evolved engineering and tuning of many stock sportbikes, you might want to do some research, get smart about the complexities of current engine design, and keep your ego in check. Kudos to stevel for the tip on sensing reversion by feeling the pulse with a hand over the intake with the engine at idle. I will use the technique to help me choose the correct velocity stack length for my own XR-1000. | ||
| Stevel |
Stephen, If you are using a single carb on your XR , no velocity stack works well because the the intake track is shared by both cylinders. Please see this: http://www.phaedrus.me/id150.html He completely wasted his time. What does work is a dual carb setup with separate intake tracks. Back in the day, this was a standard mod on the XR 750 national mile bikes. Calculating the optimum intake runner length is just arithmetic. There are online calculators that can help as well. See: https://www.eng-tips.com/viewthread.cfm?qid=165529 Do not make the mistake of tuning the intake, cam and exhaust for the same engine speed, it will make the engine very, very peaky. They need to be offset to flatten the torque curve. By the way Jerry Branch passed away at the age of 94 last December. I run a set of his welded up EVO heads on my Road King. | ||
| Araignee |
Stevel, I should have been more specific and identified my XR-1000 as a 1984, rather than than Harley's strange attempt to create a single carb street replica in more recent years. Thus, my XR came with separate intakes for each cylinder, which included 70mm long velocity stacks. Branch tested their modified street XR with and without K&N air filters, and found no difference on the flow bench. Nicely machined alloy velocity stacks are also available in 36 and 45mm. A great history of the development and tuning of the XR-750, from flathead origins through multiple single and dual carb iron head experiments, to the last dual carb alloy racers, was written by Allan Girdler (ISBN 0-87938-510-3). Thanks for the intake length calculator. Most XR tuners seemed to use longer intakes (and often exhausts) for more midrange drive, and shorter setups for more top end. I used the Sig Erson cam, as it seemed to work well in the Branch-modified street XR. I have witnessed the complex work of cutting each of the four cam lobs and then re-welding them in new positions to modify and hopefully improve power, and I stayed out of that rabbit-hole with my engine. Few engines have been through such a lengthy and tangled refinement. |
