Ran a friends 383 SB Camaro TH 400 on weekend and lost mph from last year ..........over winter freshend motor , new pistons CR up about one point and advanced cam from straight up to 4*advanced new QA1 " R" coilovers in front .helped 60' otherwise everything the same

last years 12.18 -.30/ 111-112 mph 1.85 60'
This year 12.30 / 106mph 1/8 mph 87 60' 1.70

Timing / jetting similar, air was typical for where we run . I know not much info but looking for opinions on where to look for lost mph ........ valve springs ? fuel supply ?

Lot's of places this car could be upgraded but just did not expect to lose mph with only these changes curious about your thoughts.

Bill

Well, if those are the only changes, I would look at two areas: Loss of cylinder pressure because the rings did not seat correctly when the piston change was done and while advancing the cam will increase bottom end, you will lose top end and so it could simply be a matter of the cam being straight up is right for your combo.

Ok your point on cam phrase taken ........ forgot one more change 1.7 ratio rockers on intake only ........ did compression check all Ok there . Only reason engine freshed is bearing check revealed bad crank ........ new crank did not allow use of old pistons so new SRP's bot

What was the purpose of advancing? Even if you made no other changes, its true that advancing the camshaft alone will shift the power curve to a lower rpm.......losing a little bit of top end rpm and horspower might have been enough to take away some of the top end charge. Plus, changing the pistons coulda had something to do with it as well. Were the bores finished right(good ring seal), are the pistons significantly heavier than the old ones, did you experiment with timing at all with the change in compression(ya never know).....? That can effect things for sure.

And finally, even if you changed nothing at all, its very hard to compare results from last season to this season cause your results can change based on the weather and track conditons from day to day. Race friday in cool air with a tail wind, then race 24 hours later on saturday in hot air in a headwind and that will obviously greatly change results. It could be nothing more than weather or a combination of everything including weather.

I understand the effect of weather etcetc but 6mph is alot for that or maybe it is not ......the block was machined properly ring seal is good, comp test checks out .....SRP piston are lighter than TRW that came out. We played with jetting and timing ..... cam was advanced to get more bottom end to launch better .......... maybe loss as simple as advancing was not the thing to do

I understand the effect of weather etcetc but 6mph is alot for that or maybe it is not ......the block was machined properly ring seal is good, comp test checks out .....SRP piston are lighter than TRW that came out. We played with jetting and timing ..... cam was advanced to get more bottom end to launch better .......... maybe loss as simple as advancing was not the thing to do

Since you haven't really explained your whole combo, I don't have a good feel for the kind of results you would get from advancing/retarding the cam.

What I can tell you is that a high powered engine where the whole induction/exhaust system is well designed will run best in a specific RPM range, and messing with the cam timing can throw the whole thing off...as can making rocker arm changes. You might have landed on a really good combo and making the cam timing change you did just killed its rhythm.

Personally, I'd start back again with the cam in the upright position and replace the intake rockers and start from there.

Of course, not discounting weather, because the touchier the combo, the more it can effect it, did your driving change and you just didn't notice your launches weren't quite as clean or your timing was a little off.

Also, could you have a dying fuel pump, something plugging the lines or, since you changed the compression and timing, are you experiencing lean problems?

It costs money, but putting it on a chassis dyno with a Air/Fuel meter can save you a lot of time, money and headache.

I would definately look at the fuel system! Same thing happened to me this year with more pronounced effects...

All I did last winter was change heads and a cam swap.
What was a sufficient fuel delivery system with last year's setup, could no way supply enough fuel to this year's motor demands.
I was gaining only 6-8 mph on the back half of the 1/4mi the first time out this spring. The car would pull strong out of the hole but nose over hard at the 600' mark.
But...After a fuel system revamp, all is good again! :thumbsup:

Most of the time, arent the increased ratio rockers put on the exhaust to help scavenge the cylinders better???? JMHO ;) The bigger tube headers sure brought my mph back to what it was in spring air, now I need the good air so the e.t.s will drop again.

Stupid thought but, could the higher rocker ratio cause valve float or spring bind.

what about shift points? did you lower the shift points to compensate for advancing the cam? and actually the thought about higher rocker ratio causing spring bind or float isn't stupid at all. it's something fairly simple to check and doesn't cost anything.

Changing the cam 4 degrees should not make a 6 MPH difference. Is there any chance the carb is not fully opening? Any chance you put a looser converter in it? Just some thoughts

Changing the cam 4 degrees should not make a 6 MPH difference. Is there any chance the carb is not fully opening? Any chance you put a looser converter in it? Just some thoughtsyou notice the change of shocks too right? difference in 60 ft times? if he was spinning before with the cam retarded 4* from where it is now, that could easily cause a significant change in mph. don't know about 6mph, but it could be a good bit of it.

No why? I'm not a racer but have a 'basic' understanding of the systems design here, mph is strictly engine bay related, i.e. power output. Raising comp ratio builds torque across the board, don't know if I would then also adv the cam. Agree with junkie once again, I'd undo those changes and see, then hit the fuel system.

Unless the tranny is ready to take a dump, you could easily lose mph through mechanical losses there.

mph is engine bay related, but traction or lack of can also cause mph to vary, as well as a number of other mechanical things such as gear ratio, tire height, convertor slippage....

Put the 1.7's on the exhaust side and try it again, you're letting more in, but it can't get out with the 1.6's. My mph doesnt vary that much when I spin and stay after it, it might vary 1 mph at the most. You've gained on the bottom end and lost it on the top end, swap rockers first, if that doesnt bring it back, put the cam back where it was.

My mph doesnt vary that much when I spin and stay after it, it might vary 1 mph at the most.

I agree with Bob. :thumbsup: I've found the very same to be true.

For example...when 2nd gear blew in my th400 a month ago during a run, I ran 85mph in the 1/8th (low for the car) BUT I stayed in it after finally getting it into 3rd and ran 129+mph at the end of the 1/4.
HP=MPH!

i didn't mean that it would cause a 6 mph difference, but that there is a difference there. i have had it vary almost 2mph before with street radials. i can't see one single factor causing a 6mph difference without noticing what the heck was wrong, so it must be a combination of factors, imo.

I'd back the timing down 4 degrees and make one more run. You've lost "horsepower" somewhere. Occasionally someone runs timing a little bit overadvanced and it doesn't matter much, but then increasing cylinder pressures through both static compression with the pistons, and closing the intake valve earlier could have brought the engine into detonation and cause noising over.

I don't understand about the cam timing. Did you degree in the cam 4 degrees advanced, or just advanced it another 4 degrees?

Guys i appreciate the info without alot more info i can't expect anyone to pinpoint the problem I was just curious about the effect of the cam advance and rocker ratio on mph will get my LM 1 out and check the A/F this weekend ..... , it does not really nose over on the top end that you notice ..... I can see the loss of 1-2 mph with a better 60' but not 6mph.......... timing...... we started out with 34 total and bumped to 36 and got better so detonation not a problem , next track day 2 weeks away so after checking the A/F will pull the 1.7 rockers and go back to 1.5 and see if that an issue then consider putting the cam back to straight up, yes the cam degreed in straight up upon initual install my guess is fuel supply and or spring pressure but just a guess

Guys Thanks for all you advice .......went back to the track on SAT armed to make a ton of changes ( new electric fuel pump, rockers, new coil etcetc. )....... first change was to replace the highly mod'd 780vs Holley with Proform main body with my out of the box 950HP jetted down to the old 780 numbers " Bingo " from a best of 12.1/112mph before rocker mods and cam advance to 12.3 /106 mph after to a NEW PB !!! 11.83 / 115mph with horrible 60' 1.79 so as this car tweaked with minor mods the carb was just not getting it done ...fuel was the problem !! ....... kinda the message we continue to see here , fuel supply is the first place to go if you slow down after more mods ...... not sure if I will ever see my carb back and now my friend a beleiver and will upgrade his fuel line size which i bugged him to do long ago , pbly 11.70's in this car ,

Thanks again for your replys

Good to see you worked out the problem. I had a similar problem that frustrated the heck out of me. My car was running mid 12's (at 4k ft ) , upgraded to a proform main body on a 750 holley and I start running low 13s to high 12's . I naturally suspected the change to the holley was the problem so I switched old main body back and it continued. It turned out being the fuel mech. fuel pump which I never thought would be the culprit. After installing a fuel pressure gauge(which should of been there to begin with) I noticed the fuel pump had only 3psi of pressure. I immediately went to a holley blue electrical pump. While I was at it I installed 1/2 fuel line . After all this I put my proform main body back on and got my low 12's . 11's here I come!