General Category > Quadrajet Carb Talk and Tips
vacuum at WOT
Miles:
I'm getting intermittent detonation, worse with the vacuum advance and part throttle pulls. Has 210 psi cranking pressure I may install a larger cam to reduce the pressure and up the high rpm power. Always used vacuum but can't here.
BSFC is .397-.465, air fuel was 10.5 to 11.5 we leaned it to 11.5 to 12.5 Idles and cruises at 13.5-14.5
Thanks for your response.
Cliff Ruggles:
It's not really my style to be critical of what others are doing but I don't like the cam choice for that build. It's 463cid and 10.5 to 1 compression, and no mention of the actual seat timing or duration at .002 or .006" tappet lift, but that cam isn't nearly enough to see full potential from the engine. With that much head flow and the good cylinder filling abilities of a roller profile no doubt cranking compression with be high and it will not want a lot of timing and prone to detonation on pump gas.
I've ran into this many times over the years helping folks out with this sort of thing. Big engines with relatively high compression ratios and excellent flowing heads need a lot of cam to make them happy.
When you don't give an engine enough cam it responds by acting like it's a lot higher static compression ratio than it really is. If your dynamic compression is too high, it's not going to like a lot of timing from the vacuum advance at light load either.
I had a 455 Pontiac in here a few years back with an XR276HR cam from Comp, 224/230 on a 110LSA. The engine was only 9.5 to 1 compression but acted more like 11.5 to 1. It POUNDED like sledgehammers at heavy and full throttle and wouldn't take any vacuum advance at all w/o light throttle pinging. It was also WAY down on power from what it should have been. I returned it to the owner not being able to fix it because he didn't want to change the cam.
Jump ahead till last year, a customer built a 455 Pontiac engine with that same XR276HR cam, it was only 9.3 to 1 compression and topped with ported 250cfm heads. It didn't make nearly the power they were expecting and pinged on the dyno hard enough to knock the rod bearings out of it.
The owner/builder called me, of course blaming the Q-jet for all the issues. Even the dyno operator said the Quadrajunk was at fault. I advised that they change the camshaft and provided specs for different one.
The XR276HR cam only made 428hp at 4800rpms and peak tq.
The cam I spec'd out was 289/308 seat timing, 236/245 @ .050" on a 114LSA. I used .380" lobes for a lot more lift as well. They installed the cam and no other changes.
The custom cam made 514hp @ 5800rpms and torque jumped up to 569ft lbs peak and a BUTTLOAD more average power.
Even better the new cam idled better and managed pump gas w/o any issues anyplace.......Cliff
Cliff Ruggles:
PS: I'd add here that those Edelbrock carburetors are pure JUNK! I've exploited them over the years, dyno, street and at the track and consider them hopeless. They show up all over the place simply because they are cheap, and do OK at powering an engine well enough to move a vehicle back and forth to car cruises and shows.
The weighted secondary air door is HOPELESS, and the factory ditched that design WAY back in the 1968 when they couldn't get the thru emission testing put in place at that time......FWIW.....
Miles:
Thank you for your reply.
Timing at 0.006 is 286/289.
Don't be shy about giving me advice, that's why I logged on to this forum.
I've given up on the EPS carb and trying to make up my mind what size replacement to use, thus the question about vacuum drop at WOT.
I also agree about the camshaft. When I built this thing I was more concerned about torque than HP, wanted to be able to go over the Rockies without changing down to fourth gear. Must have had in my mind a Chieftain or a Tropedo, not a Firebird. I only need fourth and fifth to move this thing around. I'm also not worried about actual HP numbers, nice to say one has 540 HP but how often would I drive it at 6500 rpm? I have a street car not a race car. I used the dyno more for break in and tuning than getting a number.
Didn't really want to admit the poor choice of camshaft but realized this summer I'd have to change for a larger one. The intake valve closes at 43* abdc at 50 which is giving me a rather high effective compression ratio. I calculated the cranking cylinder pressure to be around 190 but it's 210 and I've tested the gauge against two vacuum gauges on a vacuum pump. I've had the distributor in and out a couple of dozen times welding and grinding the plate trying to get the timing where it would give me the most power and not knock. Got most HP and torque on the dyno at 29 degrees.
What intake was on that 455 which switched to the 236/245 cam? Was it a roller or flat? Those specs are close to Dave's O.F. but a LSA of 112 instead of 114.
Would you be willing to suggest a camshaft for mine if I gave you all the parameters of the car and usage?
Once again,
Thank you.
Cliff Ruggles:
For sure some changes are in order as 210 cranking pressure is pretty high and tells you that it's closing the intake valve very early and dynamic compression will also be very high and at low rpm's.
The detonation thing happens much easier below and at peak VE because the events are happening much slower and in any N/A engine cylinder filling is more effective at low rpm's. Roller camshafts also contribute more to that deal as they will by design improve cylinder filling at relatively low engine speeds. Hence why the XR276HR cam is a HORRIBLE choice for a 455 Pontiac engine build with much past about 8.5 to 1 compression in it, and even then it's not enough cam to effectively use good flowing heads or make acceptable upper mid-range and top end power.
Nearly everyone in this hobby looks at everything "static", especially cam specs. One has to realize that as the rpms increase the time in milliseconds the valves are open decreases. So what we need to do is to concentrate our efforts to keep cylinder pressure low enough thru peak torque so we can manage pump gas with a "normal" tune.
This is best done with camshafts that have a lot of off seat timing, wide LSA and later intake closing. What this does to the power curve is to make it broad, flat and smooth. Camshafts with short seat timing events, tight LSA and earlier closing intake do just the opposite, they narrow up the power curve, higher peak torque and it will occur earlier in the rpm range. As you are finding out you also get the same result with a smaller cam even if it's out on a 112LSA. So two things work against us here as far as building a moderate compression ratio engine for pump gas, cam too small, and/or LSA too tight.
To date I've never used Dave's "Stump Puller" camshaft simply because most of the 455 engine builds we do here are at least 10.5 to 1 compression and the larger Old Faithful cam suits them much better.
There are other players in the "recipe" to effectively manage pump fuel than just the camshaft, but it is a "key" ingredient and when it's too small the engine really never has a fighting chance to make optimum power or effectively manage pump gas so we're shot down twice before we even get started.
I've also found that quench distance is a BIG deal here as well, as is LSA. For the big Pontiac engines we've found two things to be true in recent years. The wider we go with LSA the more power we are rewarded with, and the tighter the quench the less timing is needed to make best power and they manage pump fuel better. For this reason our street engine builds these days use higher compression, lots of off seat timing, wide LSA and quench is NEVER over .039" for any reason.....Cliff
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