Thread: Cr,cranking Pressure,and Power

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One of you more experienced guys,Denny,Pat,Eric,Dave,Techinspector,nitrious,,,OTHERS,,,please explain something to me and others I'm sure would like to know,,,why does a high CR and cylinder pressure make power in some motors but yet other motors with lower CR and cyl.pressure are just as fast. Example,,,,Joe Domecrome is running 11:1 with well over 200 cranking pressure but Jack flattops has 9:1 with 160 cranking pressure and is just as quick in the quarter???
Each car has differences but if nitrious and superchargers make that difference they make because of cylinder pressures why isn't high CR the thing to build for,,,i.e.,smallest combustion chambers,highest domed pistons.I mean,,,excessive pressure on that rotating assembly is what makes power,,,push it harder,push it faster,,,someone please give insight on this.

There are just too many variables to make it a sure thing to say high comp. will beat low comp. With all else equal, it's a bit easier, but you can build two engines absolutely identical and one will beat the other. Voodoo, or something, at work!

Originally Posted by FAYLUR

One of you more experienced guys,Denny,Pat,Eric,Dave,Techinspector,nitrious,,,OTHERS,,,please explain something to me and others I'm sure would like to know,,,why does a high CR and cylinder pressure make power in some motors but yet other motors with lower CR and cyl.pressure are just as fast. Example,,,,Joe Domecrome is running 11:1 with well over 200 cranking pressure but Jack flattops has 9:1 with 160 cranking pressure and is just as quick in the quarter???
Each car has differences but if nitrious and superchargers make that difference they make because of cylinder pressures why isn't high CR the thing to build for,,,i.e.,smallest combustion chambers,highest domed pistons.I mean,,,excessive pressure on that rotating assembly is what makes power,,,push it harder,push it faster,,,someone please give insight on this.

You are thinking like a true engine builder a person that does this for a living.
If you shove a very large dome in a combustion chamber sometimes it does work sometimes it does not.
Some problems are cam timing, L/S etc.
When you put the large dome in the combustion chamber you divide up the combustion chamber into different area's.
You will have lean area's and rich area's.
If you have a small combustion area you run the risk of "shrouding" the valves.
On the SBC 23 degree heads I will try and "roll the heads" meaning I will angle mill the heads from 23 degree's down to 20 3/4 degree's.
Not only does this increase my comp. ratio it also will help my heads flow better.
A 64 cc SBC head will be down in the 58 cc area.
In a 383 SBC this will put me up in the 12 to 1 comp. area.
A small 3 cc dome will put you at 13.3 comp. ratio which is enough for the averege bracket racer on gas.
The comp. ratio's have gotten crazy.
On a 410 down nozzle sprint car engine with a 13 degree ALL PRO head they are now running the comp. ratio at the 16.5 to 1 area.
When will it stop??

there are a ton of variables that can affect a motor and how it performs against another motor..... a high compression motor doesn't mean its properly tuned, or even part matched. all that squeezing of pressure doesn't help much if everything else isn't letting the pressure move freely in and out.....

besides that the weight of a car, transmissions, gear ratios, wheel/tire size, aerodynamics and driver affect everything as well

Originally Posted by thesals

there are a ton of variables that can affect a motor and how it performs against another motor..... a high compression motor doesn't mean its properly tuned, or even part matched. all that squeezing of pressure doesn't help much if everything else isn't letting the pressure move freely in and out.....

besides that the weight of a car, transmissions, gear ratios, wheel/tire size, aerodynamics and driver affect everything as well

yes that is why i will not add to much to it all has been said .i will say that the engines i build are not world beater. i do spend more time were other do not? every thing adds up. i like to see them stay together .i am in to building the best for the money that my customer has to spend. i have some very hi cr engine out there .my customer never say much on how well they do .all i know is there still out there running

Originally Posted by erik erikson

You are thinking like a true engine builder a person that does this for a living.
If you shove a very large dome in a combustion chamber sometimes it does work sometimes it does not.
Some problems are cam timing, L/S etc.
When you put the large dome in the combustion chamber you divide up the combustion chamber into different area's.
You will have lean area's and rich area's.
If you have a small combustion area you run the risk of "shrouding" the valves.
On the SBC 23 degree heads I will try and "roll the heads" meaning I will angle mill the heads from 23 degree's down to 20 3/4 degree's.
Not only does this increase my comp. ratio it also will help my heads flow better.
A 64 cc SBC head will be down in the 58 cc area.
In a 383 SBC this will put me up in the 12 to 1 comp. area.
A small 3 cc dome will put you at 13.3 comp. ratio which is enough for the averege bracket racer on gas.
The comp. ratio's have gotten crazy.
On a 410 down nozzle sprint car engine with a 13 degree ALL PRO head they are now running the comp. ratio at the 16.5 to 1 area.
When will it stop??

Great point Eric! Combustion chamber design,,,,although I'll admit this is 'over my head',no pun intended,,,,the hobbiest,on the average has no idea how important this is to producing power.WELL,,,the 'Mopar Nationals' are in town this weekend at National Trails Raceway and some of those Himee(spelled properly thank you) guys know a thing or two about chamber design giving an advantage.
In any machinery that uses 'combustion' to do work,the chamber design can make a world of difference. I used to be a furnace man and this applies even there,especially oil furnaces.
I love to see a chamber on one of those 16:1 CR drag cars.

Here's a pic of the Pro mod Chamber I run. .252" intake & 1.90" exhaust. Kuhlman got me a break at 1500.00 a piece assembled. Naturually aspirated it will stand the 16 to 1 CR. The blown set up now is at 13.7:1 CR. 4.9" cylinder centerline spacing and the o-rings were cut after the pic was taken.

that very nice nitrowarrior they look like my 18 olds big chief darts i will be running the 2.400 /1.900 valves .what is nice about the 95cc chamber is you can run a flat top pistons and get in the 13 to 1 +with no big dome you get the best of both worlds like no dome like a low cr engine better flame travel and hi cr with out the dome getting in the way

With the chamber design, it has a beautiful flame travel. The old Big Chiefs did well and I had nothing bad to say about them. It was just time to step up when newer technology called.

Originally Posted by nitrowarrior

With the chamber design, it has a beautiful flame travel. The old Big Chiefs did well and I had nothing bad to say about them. It was just time to step up when newer technology called.

lucky you i just have the CNC old s heads

The ASA cars used to run a 9.5:1 CR rule---and still made tons of power. The engine builders just had to get better on the design of the combustion chamber. As Eric mentioned, flame travel across the top of the piston is probably one of the most important--and overlooked--areas of an engine build. In my days in an automotive machine shop, tall CR numbers were the only thing guys wanted to hear, on the alky engines, 15.0 to 16.5 was not that uncommon, but some of the combinations guys tried just didn't work, mainly because of, as Eric said, the dome configuration of the piston just wouldn't allow decent flame travel. Compressing the gas is great, but you've got to be able to fire it too!!!!! I've been out of the engine building part of it for better then 10 years now. Most of my knowledge on the CR and chamber stuff is terribly outdated.... I do know the people to get hold of when I have a question. The key, IMO to this and every aspect of building a Hot Rod is planning. Getting all the components to compliment each other instead of working against each other is critical....

We're really fortunate here at CHR to have such knowledgeable engine guys who offer the good information that works and not just throw out a bunch of theory that can't be substantiated.....