boost .vs CR

[earlbrown]

http://www.modularfords.com/f17/increasing-cr-vs-boost-pressure-51059/index3.html


found this link posted on one of my boat boards. It's odd how he says low CR with more boost is better, but the math verifies why I like my higher CR with less boost.

[Steve Wood]

for peak hp, lower cr may be better, but I like mine at a bit over 9-1.  Conley built all his cars that way and I see most of the class cars are being build that way

[daveismissing]

If you've seen the entertaining Norb's thread everybody but Donnie Wang agrees with you.

[Steve Wood]

Donnie is one of the smart guys, but, his car is a purpose built race car running in a narrow rpm band last time I saw.

[Steve Wood]

One thing that many often forget is the effect of camshaft design on static compression ratios as well.

[larrym]

One thing that many often forget is the effect of camshaft design on static compression ratios as well.

What sort of cam would be best for a higher compression street engine? something that would never see more than 25psi of boost.

[Steve Wood]

Good question.  I have two cars.  One is a stock shortblock/cam and the other has forged pistons and a 210 roller.

Hidden in the wood pile is the subject of timing.  Most would probably agree that boost makes more power than does timing, I think.

On the car with the stock cam, I run 25 psi at 18-19 degs of timing whereas with the 210 cam, I have run as high (by accident) as 29-30 psi with, I think 19 degs.  On both cars, I have a problem with running more than 20 degs in 1/2 because I cannot plant the tires on the street. 

In either case, I cannot run as much timing on alky as some do.  It would probably be better on the alky control kit as it sprays more than the Chlupsa kits that I have.   

When I add timing to the high compression, I get more torque than I can handle on the street with 275/60's.  I firmly believe that more boost is better than more timing as the additional timing builds too much heat too early in my case.  Higher compression restores some of the bottom end that the longer cams take away without having to add more slip in the converter which often cuts off the bottom end of the power band.

I would guess that something around 208 degs is a good match for 9+-1 cr for a street car.

Race cars spend their lives in a very narrow power band and are a whole 'nother subject.  Race cars are much simpler.

[larrym]

thanks Steve


""Race cars spend their lives in a very narrow power band and are a whole 'nother subject.  Race cars are much simpler.

the guy i had work on my transmission said the exact same thing

[motorhead]

There is some merit to the high boost + high CR concept... but, it also requires high rpm (and a very narrow power window as Steve has pointed out). There are some imports out there using this method now. It also requires uber-octane to keep combustion under control.

[larrym]

This is a fascinating subject for me, I plan to have a short block sitting on a stand waiting and would not mind playing with a few things in the process.


9to1 sounds like a good place to start and with progressive alky I'm sure it could be tuned to start spraying sooner if octane becomes an issue at lower boost levels.


Here are the specs for my current cam I'm quite happy with how it performs with my current set up.


http://photoshare.shaw.ca/image/a/4/f/119991/camcard-0.jpg?rev=0


Its a street car with a trip to the track once or twice a year.

[Steve Wood]

That cam is a pretty good cam when used with ported heads and a low pressure loss IC/bigger turbo

It is a bit soft on the low end, but, it will work with a good combination... .I would want more compression with it.

[larrym]

my heads have minor work throats have been opened up and the valves have been back cut, CAS V4 intercooler and a  Turbonetcis BB CPT 6165. My converter is a little tight from a dig but I can spool it to about 8lbs on the foot brake with 255 60 15 MT. My BFGs for the street are kinda fun once I get to about 15psi all hell breaks loose.


They can't hold much boost on the foot brake and there isn't much point in trying.......

[Steve Wood]

should work okay if you have enuf spool at launch.  BB units seem to need a lot less converter

[larrym]

So now for some new questions did a little snooping and came up with Wiseco as a manufacture of higher compression pistons. Are they a good choice or are the other selections I should be considering I was thinking the 5120 series I have a NA 109 block as a starting point so I'm not positive of the bore just yet. .

[Steve Wood]

Wiseco, Ross, JE, and Diamond are all well regarded racing pistons.

Sealed Power is a very popular, cheaper brand that is a good long life piston because it does not expand as much, and therefore can be run tigher given longer ring life.  Many Buicks, including one of mine, have used them