Increasing the Compression Ratio

[xj11john]

All I can say is WOW!

I wished I'd payed more attention in math class.

Nice work Turbopete.

[Dennyz]

What grade of fuel are you "big bore" owners running (Premium, Regularl, and/or octane boosters)?

What are the valve dressing options with these heads (grinding, lapping, etc ….)?

[turbopete]

Originally posted by Dennyz
What grade of fuel are you "big bore" owners running (Premium, Regularl, and/or octane boosters)?

What are the valve dressing options with these heads (grinding, lapping, etc ….)?

You shouldn't need any special fuel at all for 10:1. My 'busa ran flat-out flying miles at 11:1 (stock compression) on regular unleaded with no ill-effects (even when I inadvertantly added 6 degrees of advance at the top-end).

[Dan Hodges]

Compression Ratio & How

Originally posted by GNEPIG
.. great info Pete!, you should butt in more often

I was meandering on a windy day and saw this.Pete I believe has the math figured out but left out some variables,the head gasket thickness, deck height which varies from .0025 above the deck to .005 below deck (piston in hole) and the piston dome volumn.The stock gasket is .050 thick and is the same size as the bore and measures 5.08 cc's. All head gasket's differ in volumn depending on the brand.The combustion chamber on a stock 81 eleven measures 36.5cc's and the stock head gasket measure's 5.08 cc's. and the dome volumn on a stock piston measure's 8 cc's.A cut of .020 from the head surface will reduce the chamber volumn by 2.03 cc's and will yield a change of .52 in the compression ratio.When you port the heads and do a competition valve job you will gain a couple cc's of volumn due to the valve's being unshrouded.To bring the chamber's back to the original size,I opted to cut them .020.When you cut the heads more than about .015 you need to slot the cam sprocket's and degree the cam's to adjust for the additional cam chain length from cutting the head's.The sides of the head's are scalloped and if you know what you are doing and can keep the head cool while you are welding it you can weld up part of the scallops and then resurface the head and in this way you do not have to cut it.It doesn't take much welding to reduce it by 3-5 cc's.I might make mention here,that Wiseco's piston's have a 6.6 cc dome volumn and were based on the early head which had a smaller chamber along with smaller valve's than the the 80-82 head's and consquently do not make as much compression in the 80-82 motors as they will on the 78-79 models.Actually the hot set up is to configure the chamber's like the 83 Kawasaki Gpz 1100 head and use a set of Venolia piston's for that application.Getting back to basics,if you have your motor apart and are looking at every thing, then you can consider reducing the head volumn but just reducing the head volumn 2-3 cc's is not cost effective because a new stock head gasket is about a hundred buck's.The so called advertised compression ratio on a 81 Eleven was 9.00 to one but the number is almost meaninglesss because of the variation's mentioned above.Car motor's are the same way and hence the term blueprinting which is nothing more than bringing it into the relm of factory specifications.All of this is meaningless actually unless a person is being pursued by demon's and that demon is named perfection.If you are one of these people,then you are a hopeless case and I know of no cure yet devised.Good luck and don't worry, it's only money.

[turbopete]

I accept those are variables Dan, and I left them out deliberately. My maths stand true I believe, and I deliberately mentioned taking the .020" off the cylinders rather than the head to avoid going though the process of cc'ing the gasket, head dome and volume below the deck. I also didin't figure that .020 or 030" was enough to warrant modifying the cam timing, since many a hayabusa turbo is happily fitted with an 080" spacer with stock timing setup (you can't slot 'busa cam sprockets), albeit slightly mis-aligned marks. I just wanted to keep it simple to understand. You info is good and far more model-specific, which is knowledge I lack never having had the top off my XJ.

[Dan Hodges]

Too Much About Nothing

Originally posted by turbopete
I accept those are variables Dan, and I left them out deliberately. My maths stand true I believe, and I deliberately mentioned taking the .020" off the cylinders rather than the head to avoid going though the process of cc'ing the gasket, head dome and volume below the deck. I also didin't figure that .020 or 030" was enough to warrant modifying the cam timing, since many a hayabusa turbo is happily fitted with an 080" spacer with stock timing setup (you can't slot 'busa cam sprockets), albeit slightly mis-aligned marks. I just wanted to keep it simple to understand. You info is good and far more model-specific, which is knowledge I lack never having had the top off my XJ.

If you are going to degree the cam's, you have to slot the sprocket's and all aftermarket cam's should be degreed per the manufactures recomendation.You say Busa sprocket's cannot be slotted,okay how do you degree the cam's?If you cut the head's .030 the cam chain get's longer and changes the cam timing so the cam's must be degreed in at a couple degrees advanced to get your stock setting back.If you cut the block .020-.030,the pistons will be sticking out of the holes by that much because they are generally at .000 to - .0025 stock and unless you profile the piston domes they will hit the head's.Can the domes be profiled,yes and I know of at least half a dozen way's of altering the deck height of the piston,but that's another story. If you have your motor apart,you take these measurements to determine what you have rather than guess at it. A motor is a motor,whether it be a 500cc single cylinder Yamaha,a Busa motor or my 950 hp 565 C.I. Rat Motor in my drag race Camaro.Certain principles apply to all of them and I make every effort to get rid of every variable that's possible in all the thing's I do.

[Dennyz]

I had already plan to slot the cam sprockets. It is easy for me to do and I want to experiment with different cam timings on stock cams.

[turbopete]

Woah there, we're on the same side here let's not get into a pissing contest! I'm not taking anything away from your comments all I'm saying is that I know of several 'busas that happily run on altered timing due to having .080" spacers fitted. Hayabusa cam sprockets are pressed on the cams, not bolted. They do aftermarket adjustable 2-piece sprocket carries that press on the cams and have the sprockets pressed onto them, then you can degree, but since they are expensive many don't bother. Besides that, manufactuing tolerances can potentially leave it that far out. I've already conceded that I know nothing of the specifics of working the xs engine for increased compression, but I do know the procedure, and I know how to use clay for measuring.

This is me cc'ing my cylinder head last week, after 4 hours work with the die-grinder on the chambers.



One thing I will disagree with you on. Manufacturers make reccomendations on cam timing figures but they are not hard-and-fast. Cam timing is determined by the builders specific engine characteristic requirements, and can be altered accordingly. Stock is safe, but it might not be ideal for the application. Also inlet valve closing time is actually what finally determines the running engine's compression ratio, not the static calculations we are discussing, because the piston does not start compressing until the valve is closed, and that as you say can be altered.

[pggg]

C.S.L. who hot-up motors to any specs you want, told me that degreeing the cams on a stock Japanese bike will give maybe one extra horsepower, and usually a hole or two will appear somewhere else in the rev range.

[Dan Hodges]

Originally posted by turbopete
Woah there, we're on the same side here let's not get into a pissing contest! I'm not taking anything away from your comments all I'm saying is that I know of several 'busas that happily run on altered timing due to having .080" spacers fitted. Hayabusa cam sprockets are pressed on the cams, not bolted. They do aftermarket adjustable 2-piece sprocket carries that press on the cams and have the sprockets pressed onto them, then you can degree, but since they are expensive many don't bother. Besides that, manufactuing tolerances can potentially leave it that far out. I've already conceded that I know nothing of the specifics of working the xs engine for increased compression, but I do know the procedure, and I know how to use clay for measuring.

This is me cc'ing my cylinder head last week, after 4 hours work with the die-grinder on the chambers.



One thing I will disagree with you on. Manufacturers make reccomendations on cam timing figures but they are not hard-and-fast. Cam timing is determined by the builders specific engine characteristic requirements, and can be altered accordingly. Stock is safe, but it might not be ideal for the application. Also inlet valve closing time is actually what finally determines the running engine's compression ratio, not the static calculations we are discussing, because the piston does not start compressing until the valve is closed, and that as you say can be altered.

I do not get involved in so called pissin contest or endless chit chat,but I

[turbopete]

.... am able to hold a reasoned discussion and disagree with someone without having to start a fight. I'm quite happy to discuss others methods and opinions and much can be learned from it. It is clear that we don't share the same ideaology, and you are more intent on provocative rather than constructive conversation. I have no interest in this I'd much rather share information and offer constructive help and therefore I shall continue no further in this thread, since it will merely be adding fuel to your fire.

Pete.

[Dan Hodges]

Compression & More

[QUOTE]Originally posted by Dan Hodges
[B]I do not get involved in so called pissin contest or endless chit chat,but my purpose in posting this was to share information that I have amassed with someone who was seeking information regarding increasing the compression ratio by milling the head. I hope the information I posted will be helpful to someone as you will not find it anywhere else.As for as adjustable cam sprocket's for Busu's,they are available from Megacycle for $147.00,part #csaks.Lot's of motor's have pressed on cam sprocket's,an in line six cylinder Chevrolet being one of many.Six cylinder Chevies and Blown Busa's are not the subject matter here nor is 1000 hp rat motor's for that matter,so I will keep my conversation focused on the old XS Eleven motor as this is Channel Eleven.

[posthuman]

Thats way more information than I needed, its appreciated!

However I'm looking for a simple answer: is there a shorter head gasket available, or can one be made (of say... copper), that will safely increase compression, however so little, for a modest yet noticable "bolt on" improvement in performance?

Thanks again

[Crazcnuk]

I'd send pics of my pistons, but it's too much of a PITA to get photos on here!

I do have a thinner solid copper head gasket, I got from Cometic.

[pggg]

Post, if it's more power you want from the motor, the 0.5mm or so of extra compression only gives a small hardly noticeable boost in grunt down low and mid revs, without wiseco 1196 or 1179 pistons, valve job/port job, cams to suit, big carbs, piped & filtered, hot coils and other things, it won't be doing a lot on it's own(the thinner gasket) copper head gaskets can be prone to leaking under hard conditions