compression ratio

[4stroker]

ok im lookin for some of the really knowledgeable engine guys on here to help me out with this!

 

ive read Graham Bells book and a whole bunch of stuff on the net, and have designed a dome for a buddy's trex motor. (im a cad technician)

 

we put a spacer plate under the barrels to lift the port timings and have drawn a new dome to compensate for the extra deck height, coz we are unsure if he will like the new power delivery, we dont want to deck the barrels so we can put it back to how it was if need be.

 

ANYWAY!! before the mods, uncorrected compression was approx 12.9:1. corrected compression was approx. 6.4:1. the old dome measured at 21cc.

 

now with the spacer plate, we have less effective stroke, so to keep the corrected compression at 6.4:1, i have designed the dome to only be approx. 18.5cc. however with this dome, uncorrected compression is around 14.5:1.

 

we dont wanna run anything higher than 100 octane, which is what we were running before. i am not sure which figures i should pay attention to, the corrected or uncorrected comp ratios?

 

i guess what we will do is get a whole bunch of domes made up in different sizes and do some testing, but it would be really cool if we got it right the first time!

 

thanks, mark

Edited December 6, 2007 by 4stroker

[blanco25]

dont the t-rex have different domes sizes than normal. i think this i due to using flat top pistons. I have a t-rex and my domes are like 10cc and i have another stet that is 7 or 8cc.

[blowit]

dont the t-rex have different domes sizes than normal. i think this i due to using flat top pistons. I have a t-rex and my domes are like 10cc and i have another stet that is 7 or 8cc.

 

 

That statment is dead on. The get closer in accuracy, the bore size and dome volume from timing edge must be calculated. The dome can change everything. Since you are in CAD work, get some measurements on the some and figure the volume of it and put that in the equation. You also should figure the gasket if using one. If you can post all the specs like port height, piston height relative to deck at TDC, bore, and so on, I will check my numbers. We always use correctly because I feel that is the only true way to do it.

 

Uncorrectly just makes little sense to me. You port your engine and comp ratios does not change? BS It sure does. This gets much tougher with thumpers but worth the efforts when building.

 

Good job doing your homework and taking time to check things out.

 

Brandon

[4stroker]

thanks for the replies guys.

 

im pretty sure i have the design of the new dome spot on.

 

it has a 10.6mm wide squish band (72.42mm bore = 50% squish ratio) and a squish height of 45 thou. i gave it a 1mm blending radius and a true hemispherical combustion chamber. the squish angle is 3 degrees relative to the piston. the trex piston is "flat top" however it does have a slight 2 degree rise from the edge to the centre.

 

the old dome had 21cc engraved on it. i measured it roughly with water but need to check it more acurately (got a burette ordered) before we machine anything.

 

i used some special formulas such as "volume of a cone frustrum" and "spherical portion" to calculate the dimensions of the new dome. the new dome should measure out at approx 18.5cc.

 

oh yeah its a 72mm bore, 7mm stroker trex!

Edited December 6, 2007 by 4stroker

[4stroker]

went back through my calculations and realised id posted in here too early lol...

 

i have managed to keep the corrected compression ratio the same (6.349:1) but when i checked my uncorrected ratios i had forgotten to take into account the squish volume for both scenarios....

 

so, in actual fact, the uncorrected compression before was 11.188:1, and now with the smaller domes it is approx. 11.8:1 (because even though the dome is smaller, i have opened the squish up 10 thou)

 

so i am confident that this will work no worries, as UCCR hasnt even risen a whole point, and CCR is the same as before!

 

thanks anyway guys!