Can anyone confirm the technical advantage of utilizing mixed RR combos like 1.6/1.5 versus 1.6 all for both int/exh?

 

There was a discussion along these lines on another board. It made me curious about a few things. I am curious how 1.8 rockers on the intake side, and staying with 1.7 rockers on my exhaust side would perform with a GM Hotcam.

 

I too, have seen this discussed before... Did a quick search, couldnt find... I have read that some carb engines have benefit with higher ratios on the 4 corners(1,2,7,8)... Not sure about fuel injected..

 

Swaping ratio's is a trial and error procedure. Usually if you up the intake ratio and decrease the exhaust ratio you may see some gains.If the cam is spot on then you won't.It takes a lot of time to do and a lot of passes,not to mention the cost of the different rocker's.In a LT-1 type intake with all the runner's being the same the only thing it will tell you is, if the cam is close to right for your set up.IMO it is not worth doing if you have less than an all out drag car and looking for the last .01th.You can get a cam made a lot cheaper and less brain damage.

 

Good point, I've been running 1.7's on mine for a year on stock heads, with my cam it was wayyyyy more lift than needed with stock lt1 heads only flowing to about 500-550 or so, but they were brand new for 100 bucks from a friend so I checked the gemoetry real good and went for it. Year later broke 3 springs in a week and following week collapsed a CompR lifter. So I stepped down to a 1.6/1.5 RR combo by recommendation from a local race eng builder. I have AFR heads coming next Friday the 8th with better springs, all setup and I just want what's best for it. I realize the 1.7s would shine with these particular heads but I want longevity as well.

I've heard the mix combo was something stroker guys used but still I've found little on the subject. Thanks for the feedback everyone.

 

If you have top of the line spring's the 1.7 will not cause a problem.Make sure your Valve-Piston clearance is OK and make sure you have quality lock's and retainer's.Make sure your coil bind is OK to,should be no less than .050 when at full lift.

 

Springs are topnotch, 10 deg locks, hardened retains, in fact the entire valvetrain is top quality. the 1.7s are Comp HiTech Stainless, coil bind is good to go and PtoV is fine. Thanks for the input...think I'll just hang on to both sets of rockers and just experiment both ways at the track.

 

Good idea.

 

Oops. I was talking about an LS1.

 

Small Block Chevy engines using the tradtional port layout (not like a SB2 or Ford head) have shown HP increases at 7000+rpm with different rocker ratios on different cylinders. Adding .5 a point of ratio on the intake and exhaust rockers of the outer 4 cylinders on a single plane motor shows some good HP gains. This is mostly due to the longer runners of the intake manifold for those cylinders.

Does that apply to a LT1? Probably not like LR said since they are all equal length. You could also change the camshaft to make up for this unequal length on the outer cylinders too.

Bret

 

Bret-
Care to clarify "good" when speaking on the single plane intakes? Are we talking 5, 10, 15 hp? Just peak or across the board? Never seen a combo as such, but makes sense since the runners are unequal length.

 

Yeah 10-20hp, but we are talking only showing gains over 7K

Bret