i searched and found alot of info on dynamic compression but not really the specific stuff im looking for. I'm in the process of buying parts to rebuild my motor. im going for a 355 with stock rods and crank. im going to buy le1 heads. i bought a cam kit from combintaion motorsports. its a 224/230 xfi cam. im trying to do this on a budget and i wanted to try comps new cam. id like to run as much compression as i can while still using 91 octane gas. any help would be appreciated. thanks alot -dave


the specs on the cam are
adv duration
int exh
274 282
dur @ .050
int exh
224 230
lift
int exh
574 569
valve timing
open close
int 4 btdc 40 abdc
exh 51 bbdc -1 atdc

108 icl 112 lsa

 

That intake closing point is pretty early. Are you sure that right?

Rich

 

judging by the durations and LSA, the overlap is 3* @ .05 which is right in line with the valve timing of the events. The durations also match up, so it sounds right. You might be thinking @.006

 

i copied that info from the cam card comp sent with the cam, that intake closing point is @.050. the card lists it at 1- actually but it seemed more right to be -1.

 

well, using this compression calculator http://kb-silvolite.com/calc.php?action=comp ive detrmined with a .030 overbore and felpro lt1 head gaskets i would probably need something around 7 - 10 cc piston. thats assuming they are .025 in the hole, like my stock lt1 is supposed to be. that would give me 8.5 - 8.7 dynamic compression. can i go higher than that on pump gas? i was thinking of going with a 100 - 125 shot of nitrous also. any help is appreciated. thanks -dave

 

I would like to know the actual intake closing point, but you are in the ball park. I woild go with a piston in the ~10cc range for a static CR of ~10.4 and a DCR of ~8.25.

Rich

 

the card lists the intake closing point as 40 abdc. if you dont think that sounds right ill talk to combination and see what they say about it. thanks -dave

 

It seems too early.

Rich

 

The car will run MUCH better with a "zero" deck and flattops, Pops and I ran 11.2 to1 static on the last 355 ci motor, on pumpgas. zero deck, 6 cc flattops(I think), .039 compressed and a 56 cc chamber. With a little 224/228@.050..112 LSA cam the car ran 90 MPH in the 1/8 mile at 3760 lbs in FULL street trim, with a mailorder tune.


David

 

yeah but what was your dynamic compression. your cam probly had alot more overlap than mine does

 

i talked to comp and the intake closing point seems to be correct. the cam uses 3015s intake lobes and 3036s exhaust lobes. i couldnt find those lobes on comps online lobe list.

 

I think the overlap was around 21 degrees or so, dynamic was close to or at 9.0....car ran good, I never dynoed it..but guessed around 380 rwhp or so from the cars I raced and its 1/8 mile MPH at that weight.

 

those numbers are at .050 and thats why they dont sound right. because they are in fact not the right ones for figuring dcr. and while it may make a difference in real world application overlap is not relevant to any simple mathematical model of dcr. the only thing that matters in simple models is advertised ivc. problem being most cams are rated at .006 or .004 not actual close but by that point the amount of air that will escape is marginal. you dont need a cam card to tell you where your events happen at either, i see this time and time again. all you need is the durations and the lsa then you choose your desired icl. and you can locate all other points from that just a for instance i'll use your cam card. they say your ivo is at 4btdc your icl is 108atdc so in theory half the lobe is ahead of that point so 224/2 = 112 and 108-112=-4 so you end up with 4btdc. cam events are sort of confusing in that they operate in relation to tdc and bdc. now for something a little more helpfull to you. your advertised intake duration is 274 so 274/2 = 137. so add that to 108 and you get 245 which is the advertised ivc relative to tdc and then if you subtract 180 to make it relative to bdc you end up with 65 and that number is abdc. so 65 is the point you should use to determine your dcr. to even further complicate things sometimes you can run a little more than you calculate based on air density at your location, also actual cr will make a difference, also your cooling system will have some effect here too on how much you can actually run. i know your not trying to run on the ragged edge or anything but an lt1 with good cooling and proper precautions can easily run a dcr sometimes up to 9:1 if you REALLY good and careful. but be warned pushing it assumes you can ALWAYS get very good high quality gas.

fastfaboy already brought up a good point. you need to make sure your quench hieght is good. .040 is the distance that i like to run on sbc's. his example 0 deck + .039 gasket gave him a .039 quench this is also acceptable. the cold quench you can run is really dependant on material expansions and temps but thats a different thread. on an lt1 i'd run the piston down in the hole a bit and opt for thinner gaskets assuming my surfaces were all very flat after machining. i feel that the thinner gasket and the piston in the hole is a stronger setup.

and jmho you could have gotten away with a lot less lsa on that cam and still been perfectly drivable. those xfi suckers realy do a good job of keeping vacuum up. especially on a high comp lt1 setup. and i hope you get the comp 918 springs to handle that setup because thats what it was designed for. otherwise it may go into valve float and you wont make the higher rpm hp you're after. anyways enough rambling and telling you what to do, i just hope some of my rambling was helpful thats all i try for.

 

nice post, it looks like ill run the .039 fel pro gaskets then, pistons that have 0 deck clearance. im going to need a bigger cc piston then. im gonna have to do the math again to see how much bigger im going to go. i do have 918's though. ill see how this works out, thanks -dave

 

ok, so i used 65 as the intake closing point, zero deck clearance, all other variables the same and it looks like i need a 10cc piston. i dont wanna push this too far lol. that was also with the felpro gaskets. does that sound good? -dave