cranking compression
 

Does high cranking compression slow a car down?

Re: cranking compression
 

Maybe.. need alot more info to get a better answer.

Re: cranking compression
 

X2
Need specifics about the engine such as type of cylinder head, static compression, cam type, etc...

Re: cranking compression
 

depends on why you have high cranking compression? Cam advanced will raise cranking
compression as one example,duration, overlap changes cranking compression,valve lash can change it.
compression works both ways like high spring pressure you have one cyl. compressing have another that just fired and is pushing another piston down. just like on valve springs people claim strong springs use power,but when you are opening one valve another is closing it evens it out. proof of that is turn engine with one valve adjusted it will be all you can do to turn engine each one you set engine gets easier to turn when all are set engine turns easy .
have you raced much this year? I got to go about 4 times set IHRA F/PS record at Mountain Park Pro Am 4th July weekend went 7.997. May go to UMTR race at Owingsville Ky. this weekend maybe Doorslammers at Ohio Valley.
Maybe give more info on what you've got going on might be able to help give me call if need to.
Mike Taylor 3601

Re: cranking compression
 

Another weird note is that 4 cyl DOHC motors can get awful high and still like it. I run 290 pounds. Got crazy when I found out tried to lower it and ran a lot slower.
Now I just accept it as one of those things I don't understand. Like how they get those ships in the bottles. LOL

Re: cranking compression
 

Thanks guys, here's some details, advanced cam 1 degree, widened valve lash .003, went from 213 to 256 cranking compression, picked up 19 lbs torque, 21 hp with single 4, 51 hp 39 lbs. torque with duels, slowed down a tenth and 3 mph, doesn't make sense to me. The reason we don't go back is we tightened up compression, and that's where everything has to be set. Maybe we just haven't found the sweet spot yet. Will be running at eddyville this weekend in the last modified lbs. per cube, no break-out race this weekend, and we need to have our ducks in a row.

Re: cranking compression
 

Compaired to your original combination, did the new power happen at the same spot in the torque curve? We had a similar situation but the combination of the engine on the dyno did not match what the car liked, we had to change the car to fit the new curve of the engine. In our case we had to change low gear and shift points. That is a big change with 1 deg of cam movement. Good luck.

Re: cranking compression
 

Pretty much the same, peak torque at 7900, peak power at 9400. The cam movement was only part of it, tightened clearances for more compression, and different ring package. Actually, it surprised me how much the cylinder pressure went up. Same dyno, same operator.

Re: cranking compression
 

Jeff, i agree, i've never had a motor slow down after a gain on the dyno. We always have ran more speed than ET, not uncommon. Ring seal is 1 percent or less. Last year was at 6 percent. Vac went from 17.89 inches to 20.13. Star vac pump. Don't know all the answers, or the car would be running 5.70, 120 mph. But something is not jiving, and we will see this weekend.
All these are 1/8 mile times.

Re: cranking compression
 

Are you just looking at increase at peaks? Look at hp&tq average #s over whole pull if you are'nt ,if you gain @ peaks but averaged #'s are less it will be slower to be faster you have raise average #.
Mike Taylor 3601

Re: cranking compression
 

IF dyno numbers / comparison is accurate AND it picked up HP/TQ with no dips that were not present before, AND the peaks are at the same RPM, then it should be a quicker car.
If it is not, it must be something external of the engine. Does anybody disagree?
And a -3 MPH should be more than .100 increase on ET. Right?

What is different about the ring package and how did that show a benefit to you on the dyno? Less leak-down? More / less vacuum in the engine? How much difference to the old engine?
Is the extra ring seal creating more HP at the same time creating more demand from the carb(s) & the fuel curve or float / needles & seats, now under par in the car as opposed to the dyno?
Does this engine have a vacuum pump?

Re: cranking compression
 

Sorry Mike, i didn't explain myself real well. The numbers were better all the way from last year. I know, how can that be and slow down. I'm at a loss.

Re: cranking compression
 

We are up 3 sizes since dyno. It was real slow before that.

Re: cranking compression
 

Mike Taylor is correct. If the average is better, then look at the jetting. When my Brake Specific # looked good on the dyno and didn't perform on the race track thats where I head. I have found on a car with a hood scoop needed to increase the jet size. On my single carb 750 I had to increase the jets 3-4 #'s.

Re: cranking compression
 

It's not always as simple as jetting. Are the bowls staying full?
Your MPH loss is not in the right relationship to the ET loss. I'm not sure what that means, but it means something.

Re: cranking compression
 

Then keep going bigger until it slows

Re: cranking compression
 

what happens when you have no scoop and not much hood to carb clearance?

Re: cranking compression
 

You have reduced airflow.

Re: cranking compression
 

We have been bigger on the jets, this is what it likes. Thanks guys, we will figure it out.

Re: cranking compression
 

Randy,
As far as cam advance goes all I've seen on dyno is when advanced gain a few pounds tq lose few hp lower peak 100 or 200 rpm ,retard exact opposite lose few# tq gain couple hp raise peaks 100-200 rpm. IMO cam timing changes are better tested on track
my experience with stick cars is advanced cam timing does'nt seem to work well they sound better better response but just don't want to go anywhere,also when you opened lash you lost a little duration and overlap that can tell you some info about your cam,usaully it takes .005 increments to show try tighten intake .005 loosen exhaust.005 from recommened settings if that works I promise you have HP&TQ in cam change PM me if you do I tell you who will fix that cam for you.
Hood Scoops will sure mess with jetting,speaking of jets if useing holley jets DON'T they are not accurate you can change them and get no results or opposite of what you should.
Convert to Max Jet they are precise when you make change you get results.
One other thing dyno numbers are corrected hp &tq., second,do you know for sure #'s are'nt adjusted for desired results. PS manipulated #'s from correction factor usaully gain lots of tq.
Mike Taylor 3601

Re: cranking compression
 

Thanks for the reply, but what I was getting at is: well I guess less airflow means less fuel! I have had similar results before in the past.

Sorry not trying to hijack this thread but how much carb clearance should there be and where to measure from off the carb??

Re: cranking compression
 

Thanks Mike, I am going to put cam back and tighten lash after we are done racing. Do i think numbers are bogus? No. I have been with this man 13 years, and if he said we gained, we gained. Ran this weekend at Eddyville, went 5.90 116.70. Had the best mid track numbers of our car ever, but still down 3 mph and .007. I think it's as simple as you have stated, clutch cars do not like the cam advanced sometimes. Now, on the valve lash, i have seen .002 make a difference. As for the jets, we are using holley, but will change to max jets. We were no. 2 qualifier, at 2 under, Rick Parrot ripped everyones throat out at 32 under. So we have some work to do.