Topic: Maximum compression PSI for 98ron fuel

[Yoshimatic]

Does anybody know what the maximum PSI compression to use for 98ron fuel? I realize that compression ratio is affected by different performance camshafts etc and was wondering if anyone knew the maximum PSI to use for 98ron fuel when doing a compression test? I ask this because I will be fitting high compression pistons of an unknown ratio and will adjust the PSI by copper spacer at the bore base gasket area. I am thinking that somewhere around 200PSI to 210PSI would be ok but not real sure.
Any help will be much appreciated.

[turboguzzi]

i would put the camshaft importance as secondary to squish. if you build real good squish area, i.e. no more than 1mm thick, you could run above 11:1 and be ok, my gpz750/810 has 12:1 and runs on 98-100 "super+" available  here in italy without any issues even in very hot days
but high compression  with no squish and it's detonation day and just regulating it through gasket thickness at the base is a bit old school
not so easy to do with a 750 deep head, easier on a 500/550 which is flatter, but your not saying what motor you are building

[PeWe]

Turboguzzi.... about squish....
Head gasket is available in different thicknesses. Like MLS 0.030" and 0.040".
So if pistons outer edge is about 0.3mm under the gasket surface, the thinner head gasket is the choice then? This with thinner RCS base gasket. This is more important than the total CR? Closer to 11:1 than 10.5:1

[turboguzzi]

you need to always do a dry build, put Plasticine at the edges, turn engine past TDC and measure the end result... the combustion doesn't care if you do it through an thinner RSC gasket, a milled block, whatever

[PeWe]

I have used model clay, a lot of it to verify where valve and pockets in pistons meet and the outer edge of pistons vs head.
With the DP315 cam to be used.
To get max 1mm squish, the thinner head gasket is needed. This will increase CR but squish is more important, right?

[turboguzzi]

well, if to get 1mm of squish you end up with 14:1, id say, calm down...
maybe machine areas in the center of the piston crown to bring CR down. Just give you an idea, latest version of ducati's 800 air cooled twin, essentially the last perfomance two valve design that you can still buy (Scrambler) is up to 11:1 stock and thats with a 88mm bore.... and indeed those heads have a nice, wide squish band.
squish band needs to be at least 5mm wide to do some work, more is better,
Am not an expert on 750's but just from heads i saw you'd need a piston with VERY steep crown edges that fits tightly into the head. the kawi piston head you see in the pic is totally different in that sense, the chamber in a KZ is super shallow compared to a CB's   

[PeWe]

OK, the actual squish band is less since chambers are widened to 65mm. Piston crowns made for std chambers of 61mm. Pistons 71.2mm, squish band is about 3mm wide.
Dynoman 1000cc pistons, probably 12.5:1 pistons that are machined to give 10.5:1 with 24.4cc chambers. (std K-head ~22.5cc)
Weights are 190 grams, exact in the middle of the 10.5:1 (185g) and 12.5:1 pistons (195g)

Chambers 24.4cc, pistons made for that chamber and recommended the thicker gasket. I guess I let the narrow squish band to be 1.3mm thick then. (gasket 0.0040" = 1mm + 0.3mm piston squish area under the cyl/head mating surface).


About PSI that this thread started with:
My 836cc had around 190 PSI WITH cam (Action Fours SS-1 or maybe RC295 copy). No problem with Shell V-Power fuel. Not that close to ping either when carbs delivered enough fuel. Too small pilot jets/fuel screws too closed caused pinging, same with too small main jets.

[Yoshimatic]

Thanks for all the replies, I have since found that 200 psi is ok with 93ron fuel and for each ron number increase you can go 5psi higher after that. Therefore with 98ron being 5ron higher than the 93ron I can go to 225psi max.
I will aim at 215psi to make it a bit more forgiving.

[Ilja]

you need to always do a dry build, put Plasticine at the edges, turn engine past TDC and measure the end result... the combustion doesn't care if you do it through an thinner RSC gasket, a milled block, whatever

I fully agree with TurboGuzzi! 

But mind you, these (kawasaki gpz) bikes do not ping easily... the camshafts are MUCH wilder than most CB camshafts and the engine has a shorter stroke which also influences peak pressure following from ignition timing.
The Cb's start to ping sooner.. I found out by turbo charging both bikes.   

Knowing cranking psi is a good guideline, but the engine stroke combined with ignition timing also plays a big role.
With a longer stroke the mixture is ignited when the piston is still further from the head causing a higher peak pressure.

[PeWe]

Reduce pinging by retarding will reduce power, right?  I had pinging when driving away from a stop, twisting throttle from 4000-5000 rpms on 5:th gear. Fixed it with retarded ignition and lost a lot of power. Killed all modifications
Fixed the real problem by richer pilot circuit and richer main, advanced the ignition and got the power.

CB750 can handle CR like 11-12.5:1 without race gas, methanol...? Cam with plenty of overlap is one way? Fuel consumption....

I like the DP315 cam so I have to keep CR away from pinging. Plan B will be a shiny welded Megacycle 125-20

I'm struggling with the oil rings on my 71.25mm pistons. When I have got them to please me and pistons I'll measure the PSI that might be over 200. Maybe less when not started and new pistons, bores.

[turboguzzi]

peewee, guess you need to start your own thread.... we totally hacked the poor guy's original thread...

[PeWe]

I think all posts are related. If keeping the thread boiling others might chime in.
I have seen posts where 200 something PSI were mentioned.

[turboguzzi]

psi... my humble opinion (surely others will disagree). measuring psi at electric start cranking speed has too many variables to be a guidance number. a cam with lots of overlap will have lower numbers thant a mild cam. The only way of measuring CR is by actually measuring it with a burette

by the same train of thought, knock happens at max torque when cylinder filing is best, in your engine it's around 5K, with a wilder cam it will happen here, but in both cases what counts is the static CR. 

lets not forget that if you use psi as measure of compression, you have to assemble the complete motor just to measure and dissassemble, with a burette you just mount head and can easily correct.

[teebee67]

Surely measuring with a burette only gives a theoretical CR. You can have a measured CR of say 10:1 but if volumetric efficiency was poor the actual compression ratio would be much lower?

[turboguzzi]

well, i said people will disagree
measuring psi at 600 rpm starter cranking speed is also theoretical, tells you little about psi at torque peak rpm.

[Ilja]

Its simple: If you change to a cam which closes your inlet valve later during the compression stroke (more duration), your dynamic compression ratio will be lower. This means you can up your static compression ratio to compensate.