C/R VS MAX BOOST ?
Turbo Mini Forums

looking into the complexly of static cylinder pressure and the affects of boosting
what should i call the max affective cylinder pressure? 250psi on cast iron head and block?
looking for a low C/R high BOOST build so was thinking 7:1 with 30psi boost giving 210psi in the cylinders given no losses and cam choice valve timing stuff not conceded lol

what do some of you much more experienced guys/girls think and advise
planing on a track/sprint car that will be road reg (weekend toy when i have time to go for a blast) im happy to have a pig off boost ill just keep the thing moving to stay on boost lol most of the time you only start the car from standing still once in a race..

from what ive read and under stood? there is no point in saying i have x sweped volume and x non sweped = said CR as the cam plays a big part in the real CR
that said how do i find the affective (true) CR of a motor and cam combo?
after that how do i then find the max boost i can add with out doing damage given 98 octain fuel?

8.21cr
25psi was max without wi
Most i succesfully ran with wi was 28psi
I had shot for 35psi but couldnt get enough fuel in there
My figures seem to back up your 7:1cr / 30psi goal

Yes i moved to the darkside

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You say that you can just keep it "on boost" - well, I doubt that would be achievable on a lot of tracks. It doesn't even matter if you had a low-inertia turbo, 7:1 is really low and will take a lot of effort to keep it on-boost when cornering, when your concentration would best be applied to, well, cornering...

If you just want to be overtaken on corners, only to blast past someone on the straightaway, then be overtaken out of corner again, then what you are proposing will probbly be a lot of fun and really cool. I think it'll get boring pretty quickly though.

Edited by turbodave16v on 21st Jun, 2013.

brett what cam did you have at the time also how much displacement also you say you couldn't get enough fuel in was that efi or carb (HIF44)
dave the work truck is a turbo disel it donent come on bost till 2500 and all in about 3000 kinder like the metro chalange t3 on an 1150cc as you say its a pain in the ass to keep on boost in the corners and when your just cruseing and stab the gass it just dosent go till 2.5-3k but when it gets some bost its pulling hard so i think i might move up in CR to say 7.5-8.25: to gain some dirveablity im sure the long stroke will help the boost spool also some good maniflods etc i plan to build the motor as tough as i can then boost it to the max
safely..

the yellow book only has NA info on CR vs cam so is it right to think that my cam of choi ce 247 has only 124degrese to get stadic compresion so how do i go about working out a cylender presure at a said CR to find therical max boost lol or do you guys jut build it turn it up till it knocks then back it off a touch?

or find some aviation fuel and just go nuts lol

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

Yes i moved to the darkside

Instagram @jdm_brett

Lowering the CR into the 7s is a MASSIVE compromise over the overall characteristics of the engine. You'll find it gutless at the bottom end and will restrict the power band to a narrower area (where boost comes in). Going for a smaller turbo to reduce lag, will result in more power lower end, but it may not have enough flow for higher boost at the top end.

If you're aim is high boost, then you need to be looking at an efficient turbo at high boost, and a very effective intercooler setup to keep the inlet temps down. If it were me I'd be looking at keeping the CR at no lower than around 8.4:1 and if det is an issue at your desired boost and its a track car then water injection would be my next route. If you spec the engine right then high boost isn't needed for high power as robert has proved.

Edited by matty on 22nd Jun, 2013.

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1/4mile in 13.2sec @ 111 terminal on 15psi

sounds like im being silly in thinking low cr high boost would be a good setup for anything more than fooling around in..
how dose WI work i have never looked into it as i thought id never run a track car lol is if a function in MS3X like an injector but it sprays water in the port? or is it to do with the IC

Ms3x can control it like fuel injection have a search on here i have posted some links in the past

Yes i moved to the darkside

Instagram @jdm_brett