I'm looking for some advice and opinions on a 455 build up for my 75 G.P. I bought and read Jim Hand's book and selected a Comp Cams XE268H-10 cam 224/230 duration at .050 477/480 lift 110 LSA. I have an Edelbrock Performer intake, Summit 1 5/8" 4 tube headers and dual 2 1/4" exhaust. Heads are 6X4 with 92cc chambers. I emailed Cliff Ruggles about reworking a Q-Jet and he said the cam was a VERY POOR choice due to detionation problems he found using this cam and recommended a Crower 60919 which is close to a Ram Air IV cam but ground on a 112 LSA and Rhodes lifters. A friend of mine who has built numerous Pontiac 455's said I would need to put a higher than stock converter to make it idle properly. I'd like to know if anyone out there has run a similar cam in a 455. The car will basically be street driven at highway speeds as I live in a rural area. Could I get away with 3.08 gears with this cam and converter? I definately don't want to go any steeper than 3.42 as the car has A/C. Any advice from the Pontiac Nation would be appreciated.  

The point to understand is that you have a big inch motor that has a compression ratio in the range of 9.5-9.7:1. Your first choice in cams will produce very high cylinder pressure, which will likely cause detonation on pump 92-94 octane gas. The can Cliff recommends will bleed off pressure at low rpm and make the engine more liveable (detonation-wise), however you must still consider the octane requirements of a large bore engine over 9.5:1 cr. Sure the larger cam will bleed off psi at low rpm, but as rpm increases and the engine gets more air and fuel it requires more octane to cope with the cyl psi. Likely with the bigger cam you won't need the gears and converter, due to the size of the engine and the high cr. I would try it before changing anything else.
Steve  

Steve,
Thanks for the reply. I understand that the bigger cam will bleed off cyl pressure due to more duration and slightly wider LSA. From what I remember about Rhodes lifters they decrease duration by about 10 degrees of duration under approximately 3000 RPM. This would decrease duration down at low RPM below the other cam but with 2 more degrees LSA. My car has 2.56 gears in it and weighs approx 4200 lbs. If I were to retain these gears to keep cruise RPM down and just use standard lifters would I be correct to assume that it would produce sufficient low end torque to accelerate the car without lugging the engine?  

Hi,I am a newbie looking to find info on swapping a 73 455 into my 68 GTO.My question is I bought the car with a Chevy motor in it and I have a set of frame mounts from a 350 lemans donor car.Will the 350 frame mounts work with the 455?Also which frame mount tall or short goes on which side?Which motor mounts to use 350 or 455?Also is there a difference in output from a short shaft 350 waterpump and a 455 long shaft as I have all the pulleys from the 350 (68)motor and they are different offset than the 455 ones I have that are rusted out for the long shaft 455 pump. Thanks!  

2 answers, one for fireprix and one for hornet1.

Fireprix, you are correct about the Rhodes lifters, they basically make the cam act like it is smaller, so using them will negate the advantage of the slightly larger cam to bleed off pressure. I'm pretty sure you will have tons of torque with a 9.5+:1 455, even in a heavy car like your GP. The 2.56 gears will tame wheel spin(if you have a posi) and will be a great highway car.

Hornet1, first some advice to make the forums easier for everyone and easier to find your answer. Don't piggyback on another post, click on a forum heading (driveline restoration, for example) then click on "new post". That way you can go directly to your question, instead of searching thru other answers to find it.
Now to your question, the Lemans 350 frame brackets and block brackets will work on the 455 if the block has all the holes drilled and tapped for the early mounts (compare with the 350). If not use the adapter plates that are listed in most resto sites, they are cheap and easy to use. As for the short and tall brackets, I'm not sure which side is which, I believe the tall one is the driver's side. Maybe make a post about that to get more feedback.
The 350 pulleys, water pump and accesory brackets will work on the 455 if you use the 350 timing cover and harmonic balancer. Another member just addressed this, do a forum search for "water pump", he had all the part numbers and everything. The short pump and accessories will make the motor look more correct for your 68 GTO.
Steve  

Thanks for the good advice Barneyformula.  

Steve thanks for the advice. Sorry it took so long to get back with a much appreciated thanks but work has been hectic  

I have a 76 gp with a 455 stock and im thinking of building it up like you. what hp do think you it will have when you are done?which cam are you going with did you change it?  

I emailed Cliff Ruggles about redoing a Q-Jet to work with this combo and he said this combo would make around 450 horespower and 540 poundfeet of torque with the Crower cam. This is the cam I will go with. The core block I started with was a 76 455 with 6H heads that yield 7.6:1 compression. If you keep these heads the Comp cam would be a better choice. If you're looking for diffeent heads 6X4 heads are 92cc chamber and were found on W72 400 Trans Ams or most mid 70's 350's. 6X8 and 5C were found on mid 70's 400's and are around 100cc chambers. I think these would result in about 1/2 to 3/4 point lower compression somewhere around 9:1. If you are interested in changing rear end gears you'll want to see which rear your G.P. has. If the cover has a drain plug in it you'll have a lot of fun trying to find anything for it. I believe most factory 73-76 455 G.P.'s had this oddball rear in it. I've been looking for almost 2 years for gears and a posi for it with no success. Good luck!!!!!!!